CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application Serial No. 62/758,485 filed November 9, 2018, and entitled“Multi-Part Projectile for Percussion Sidewall Coring and Methods for Using Same to Obtain a Core Sample,” which is hereby incorporated herein by reference in its entirety for ail purposes. This application also claims benefit of U.S. provisional patent application Serial No. 62/846,048 filed May 10, 2019, and entitled “Multi-Part Projectile for Percussion Sidewall Coring and Methods for Using Same to Obtain a Core Sample,” and U.S. provisional patent application Serial No. 62/872,725 filed July 11 , 2019, and entitled “Multi-Part Projectile for Percussion Sidewall Coring and Methods for Using Same to Obtain a Core Sample,” each of which is hereby incorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND

[0003] The disclosure relates generally to devices and methods for obtaining subterranean core samples for analysis. More particularly, the disclosure relates to devices and methods for capturing subterranean core samples with projectiles, and then extracting the core samples from the projectiles for subsequent analyses.

[0004] Physical and petrophysical properties of subterranean rock formations are useful for assessing hydrocarbon containing reservoirs and development strategies for those reservoirs. Samples or cores of the subterranean rock formations are often recovered with coring tools, and then analyzed to determine physical and petrophysical properties of the formations. For example, a percussion sidewall core (“PSWC”) may be obtained by discharging a hollow projectile or“bullet” into the sidewall of a borehole drilled in a subterranean formation. The sample of the formation material captured in the hollow barrel of the bullet is subsequently recovered at the surface for analysis. In particular, the sample is removed from the bullet barrel at the surface and subjected to physical laboratory tests to determine its physical and petrophysical properties, which are generally representative of the formation from which the sample was obtained

BRIEF SU MARY OF THE DISCLOSURE

[0005] Embodiments of percussion side wall core (PSWC) bullets are disclosed herein. In one embodiment, a percussion side wall core (PSWC) bullet has a central axis, a leading end, and a trailing end axially opposite the leading end. In addition, the bullet comprises a first portion extending axially from the leading end of the bullet. The bullet also comprises a second portion removably coupled to the first portion. The second portion extends axially from the trailing end of the bullet. Further, the bullet comprises a sleeve removably positioned in the first portion. The sleeve includes an inner cavity configured to receive a core sample.

[0008] In another embodiment, a percussion side wall core (PSWC) bullet has a central axis, a leading end, and a trailing end axially opposite the leading end. In addition, the bullet comprises a single-piece body extending axially from the leading end of the bullet to the trailing end of the bullet. The single-piece body is X-ray transparent.

[0007] In yet another embodiment, a percussion side wall core (PSWC) bullet has a central axis, a leading end, and a trailing end axially opposite the leading end. In addition, the bullet comprises a penetrating portion extending axially from the leading end of the bullet. The bullet also comprises a passage extending axially from the leading end through the penetrating portion. The penetrating portion is configured to penetrate a formation to receive a core sample in a portion of the passage. The penetrating portion has a first volume and the portion of the passage configured to receive the core sample has a second volume. The ratio of the first volume to the second volume is less than 1.00.

[0008] Embodiments of methods for obtaining core samples from a subterranean formation are disclosed herein. In one embodiment, a method for obtaining a core sample comprises (a) receiving a percussion side wail core (PSWC) bullet. The bullet includes a first portion, a second portion removably attached to the first portion, and a sleeve removably disposed in the first portion. A core sample is disposed in the sleeve. The method also comprises (b) decoupling and separating the first portion and the second portion after (a). In addition, the method comprises (c) removing the sleeve from the first portion with the core sample within the sleeve after (b). [0009] Embodiments described herein comprise a combination of features and characteristics intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical characteristics of the disclosed embodiments in order that the detailed description that follows may be better understood. The various characteristics and features described above, as well as others, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes as the disclosed embodiments. It should also be realized that such equivalent constructions do not depart from the spirit and scope of the principles disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a detailed description of various exemplary embodiments, reference will now be made to the accompanying drawings in which:

[0011] FIG. 1 is a perspective view of an embodiment of a percussion side wail core (PSWC) projectile in accordance with the principles described herein.

[0012] FIG. 2 is a cross-sectional view of the PSWC projectile of FIG. 1.

[0013] FIG. 3 is a perspective view of the first portion of the PSWC projectile of FIG 1.

[0014] FIG. 4 is a top end view of the first portion of FIG. 3.

[0015] FIG. 5 is a cross-section of the first portion of FIG. 4 taken along section 5-5 of FIG. 4.

[0016] FIG. 6 is a bottom end view of the first portion of FIG 3.

[0017] FIG. 7 is a perspective view of the second portion of the PSWC projectile of FIG 1.

[0018] FIG. 8 is a bottom end view of the second portion of FIG. 7.

[0019] FIG. 9 is a perspective view of the removable sleeve of the PSWC projectile of

FIG. 1.

[0020] FIG. 10 is a bottom end view of the removable sleeve of FIG. 9.

[0021] FIG. 11 is a cross-sectional view of an embodiment of a percussion side wall core (PSWC) projectile in accordance with the principles described herein. [0022] FIG. 12 is a graphical illustration of an embodiment of a method for assembling the PSWC projectile of FIG. 1 in accordance with principles described herein.

[0023] FIG. 13 is a graphical illustration of an embodiment of a method for obtaining a core sample with the PSWC projectile of FIG. 1 , extracting the core sample from the PSWC projectile of FIG. 1 , and imaging the core sample in accordance with principles described herein.

[0024] FIG. 14 is a cross-sectional side view of an embodiment of a first portion of a percussion side wall core (PSWC) projectile in accordance with the principles described herein and including an annular cutting edge.

[0025] FIG. 15 is a cross-sectional side view of an embodiment of a first portion of a percussion side wail core (PSWC) projectile in accordance with the principles described herein and including an annuiar serrated cutting edge.

[0026] FIG. 16 is a cross-sectional side view of an embodiment of a first portion of a percussion side wall core (PSWC) projectile in accordance with the principles described herein and including an annular sinusoidal cutting edge.

[0027] FIG. 17 is a perspective view of a first portion of a percussion side wail core (PSWC) projectile in accordance with principles described herein and including stress relief slots

[0028] FIG 18 is a perspective view of a removable sleeve of a percussion side wall core (PSWC) projectile in accordance with the principles described herein and including stress relief slots.

[0029] FIG. 19 is a side view of an embodiment of a first portion of a percussion side wail core (PSWC) projectile in accordance with the principles described herein.

[0030] FIG. 20 is an end view of the first portion of FIG. 19.

[0031] FIG. 21 is a cross-sectional perspective view of an embodiment of a percussion side wall core (PSWC) projectile in accordance with the principies described herein.

[0032] FIG. 22 is a perspective view of the first portion of the PSWC projectile of FIG. 21 .

[0033] FIG 23 is a cross-sectional perspective view of an embodiment of a percussion side wall core (PSWC) projectile in accordance with the principles described herein.

[0034] FIG 24 is a perspective view of the first portion of the PSWC projectile of FIG.

23.

FIG. 25 is an end view of the first portion of the PSWC projectile of FIG. 23. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] The following discussion is directed to various exemplary embodiments. However, one skilled in the art will understand that the examples disclosed herein have broad application, and that the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to suggest that the scope of the disclosure, including the claims, is limited to that embodiment.

[0037] Certain terms are used throughout the following description and ciaims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

[0038] Unless the context dictates the contrary, ail ranges set forth herein should be interpreted as being inclusive of their endpoints, and open-ended ranges should be interpreted to include only commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.

[0039] In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to... .” Also, the term “couple” or“couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct engagement between the two devices, or through an indirect connection that is established via other devices, components, nodes, and connections. In addition, as used herein, the terms“axial” and“axially” generally mean along or parallel to a particular axis (e.g., central axis of a body or a port), while the terms“radial” and“radially” generally mean perpendicular to a particular axis. For instance, an axial distance refers to a distance measured along or parallel to the axis, and a radial distance means a distance measured perpendicular to the axis. As used herein, the terms “approximately,” "about,”“substantially,” and the like mean within 10% (i.e., plus or minus 10%) of the recited value. Thus, for example, a recited angle of “about 80 degrees” refers to an angle ranging from 72 degrees to 88 degrees. [0040] This disclosure will be described in connection with its embodiments, which can be used in methods for obtaining and preparing a rock sample for use in digital numerical simulation analysis to determine properties of the formation from which the rock sample was acquired. However, it should be appreciated that this disclosure may be useful and beneficial in other applications beyond those expressly described in this disclosure Accordingly, if is to be understood that the following description is provided by way of example only, and is not intended to limit the true scope of this disclosure as claimed.

[0041] Embodiments disclosed herein pertain to the acquisition of samples of formation material, also referred to as“cores” or“core samples,” and analysis of same by way of direct numerical simulation. As such, it is contemplated that embodiments disclosed herein may be beneficial in the acquisition of samples from sub-surface formations of interest in the exploration and production of hydrocarbons. More specifically, the rock(s) from which the samples may be acquired are contemplated to correspond to formations accessed by terrestrial or marine drilling systems such as used to extract hydrocarbons, water, and the like from those formations The optimization of oil and gas production operations is largely influenced by the structure and physical properties of these sub-surface rock formations. The samples obtained according to embodiments of this disclosure may be useful in understanding those formation attributes.

[0042] Conventionally, percussion side wail core (PSWC) samples are obtained by discharging an open ended and generaliy hollow projectile or bullet into the sidewall of a borehole traversing a subterranean formation of interest. As the bullet penetrates the formation, some of the formation material passes through the open end of the bullet and is retained within the hollow body of the bullet. The bullet with the core sample therein captured therein is subsequently retrieved to the surface. At the surface, the core sample is typically removed or extracted from the body of the bullet by a push and/or press technique (i.e., the core sample is pushed and/or pressed out of the body of the bullet without cutting the core or the projectile). Although relatively simple and fast, this technique of core extraction does not ensure (1 ) structural integrity of the core sampme (due to potential deformation of the core); and/or (2) chemical integrity of the core sample (due to mixing of the core with drilling mud). Consequently, information derived from subsequent analysis of the extracted core sample may be limited to being qualitative in nature, such as, for example, grain statistics (i.e., Laser Particle Size Analysis, “LPSA”) and chemistry (X-Ray Diffraction,“XRD”), both of which may be biased by mud contamination.

[0043] Embodiments disclosed herein offer the potential to improve the structural and chemical integrity of core samples, thereby allowing for more accurate core analyses. In particular, embodiments disclosed herein are directed to projectiles that include an insert or sleeve that is transparent to X-rays. Such transparency allows for imaging of the PSWC with X-ray tomography without removal of the core sample from the projectile. According to embodiments disclosed herein, PSWCs may be obtained by discharging a hollow projectile or bullet into the sidewall of a subterranean borehole to capture formation material (e.g., rock) from the surrounding formation. Subsequently, the projectile Is removed to the surface, and the X-ray transparent insert containing the core sampie is removed from the projectile and subjected to direct imaging with X-ray tomography. As the core sample remains within the insert (i.e., it is not pushed and/or pressed from the insert), the structural and chemical integrity of the core sampie (due to a lack of deformation of the core and minimal mixing of the core with drilling mud) are substantially preserved. Tomographic image analysis may be performed on the core (e.g., in case of a disturbed structure) to provide a grain statistics description. The tomographic images (e.g , digital images) may also be utilized with direct numerical simulations (“DNS”) to obtain petrophysical and hydrodynamic properties of the core sampie.

[0044] DNS of material properties from digital images of rock is a technology for determining the material and petrophysical properties of rock samples. According to DNS, an X-ray tomographic image of a core sampie is taken to produce digital images representative of that sample. A computational experiment is then applied to the digital images to simulate mechanisms from which the physical and petrophysical properties of the core can be estimated or measured. Properties of the core, such as, for example, porosity, absolute permeability, relative permeability, formation factor, elastic moduli, and the like, can be determined using direct numerical simulation. In particular, DNS is capable of estimating the material properties of rock types, such as tight gas sands or carbonates, within a timeframe that is substantially shorter than that required for the corresponding physical measurement. In addition, test equipment is not occupied over long periods of time according to DNS techniques, as the analogous numerical conditions to the physical experiment can be immediately applied by computer simulation software. [0045] Referring now to FIGS. 1 and 2, an embodiment of a percussion sidewall core (PSWC) projectile or bullet 100 is shown. Bullet 100 has a central or longitudinal axis 105, a first end 100a, a second end 100b axially opposite end 100a, and an inner cavity 101 extending axially from end 100a to end 100b Thus, cavity 101 defines an opening 101 a in end 100a and an opening 101 b in end 100b. As will be described in more detail below, bullet 100 is oriented and shot into the sidewall of the borehole with axis 105 oriented perpendicular to the sidewall and with end 100a leading bullet 100 Into the formation. Accordingly, end 100a may also be described as the leading” end of bullet 100, and end 100b may be described as the“trailing” end of bullet 100.

[0046] In this embodiment, bullet 100 is an assembly of separate and distinct components. In particular, bullet 100 includes a first portion 110, a second portion 120 removably coupled to first portion 110 with screws 130, and a cylindrical sleeve 140 removably disposed between portions 110, 120. Portions 110, 120 and sleeve 140 are coaxially aligned such that each shares the same central axis 105. Portions 110, 120 and sleeve 140 are made of a rigid, durable material(s) suitable for use in a downhole environment and capable of maintaining their shape and integrity while penetrating the borehole sidewall. Examples of suitable materials for portions 110, 120 include steel, or any other metal or non-metal compound that maintains its mechanical properties during subterranean coring operations. In some embodiments, one or both portions 110, 120 are X-ray transparent. For example, in some embodiments, portion 110 is X-ray transparent while portion 120 is not X-ray transparent, and in other embodiments, both portions 110, 120 are X-ray transparent. In embodiments described herein, sleeve 140 is X-ray transparent. In general, X-ray transparent materials include materials that are suitable for conventional (non-synchrotron) X-ray source based tomography. More specifically, as used herein, the term“X-ray transparent” may be used to refer to an object or material (having a geometry and physical properties) that can be imaged in less than 24 hours via X-ray tomography (e.g., micro-computed tomography or pCT) using a conventional, standard formation core sample imaging X-ray source (non-synchrotron) to produce a meaningful image with sufficient quality for subsequent direct numerical simulations (DNS). The term“X-ray transparent” may also be used to refer to or describe an object or material (having a geometry and physical properties) that allows at least 5% of X-rays from a conventional, standard formation core sample imaging X-ray source (non-synchrotron) to transmit therethrough, more preferably at least 10% of X-rays to transmit therethrough, and even more preferably at least 20% of X-rays to transmit therethrough Examples of materials that are generally X-ray transparent include aluminum, light metal alloys (e.g. duralumin etc.), carbon fiber composites, ceramics (e.g. AI2O3), and thermopiastics (e.g., polyetherether ketone (PEEK), poiyetherketone (PEK), etc ). Heavier metais, metal alloys, and compounds may also be X-ray transparent if sufficiently thin (e.g., a metal or metal alloy having a thickness less than 1.0 mm or less than 0.5 mm). In general, the particular material composition chosen for an object to ensure X-ray transparency may depend upon a variety of factors including, without limitation, the type of the subterranean formation, the downhoie conditions and environment (e.g., pressure, temperature, types of fluids, etc.), etc. As will be described in more detail below, in some embodiments, the bullet (e.g., bullet 100) may be an X-ray transparent unitary piece, which offers the potential to eliminate the need for multiple components of the bullet (e.g., portions 110, 120, and sleeve 140).

[0047] Referring now to FIGS. 2-6, first portion 110 has a first end 110a, a second end 110b axially opposite first end 110a, and a through passage 111 extending axially from first end 110a to second end 110b. Passage 111 defines an opening 111a at end 110a and an opening 111 b at end 110b.

[0048] In this embodiment, second end 110b comprises a planar surface disposed in a plane oriented perpendicular to axis 105. As best shown in FIG. 2, first end 110a defines leading end 100a of projectile 100 and through passage 111 defines the portion of cavity 101 extending through first portion 110. In addition, first portion 110 includes an annular flange or base 112 at end 110b, an annular intermediate portion

113 extending axially from base 112, and an annular nose portion 114 extending axially from end 110a to intermediate portion 113. Thus, intermediate portion 113 extends axially from base 112 to nose portion 114.

[0049] In this embodiment, base 112 has a cylindrical shape with passage 111 extending axially therethrough and defining an opening at end 110b. Intermediate portion 113 also has a cylindrical shape with passage 111 extending therethrough. Intermediate portion 113 generally adds axial length to first portion 110 to allow recovery of a core sample of sufficient length for subsequent analysis. Nose portion

114 has a tapered shape defined by a radiaily outer surface disposed at an outer diameter that generally increases moving axially from end 110a to intermediate portion 113. Passage 111 extends axially through nose portion 114, thereby defining openings 101a, 111a at end 110a that allows for recovery of a core sample when bullet 100 propagates into the subterranean formation.

[0050] As noted above, bullet 100 is oriented and shot into the sidewall of the borehole with axis 105 oriented perpendicular to the sidewall and with ends 100a, 110a leading bullet 100 into the formation. Bullet 100 is advanced into the formation until base 112 abuts and engages the borehole sidewall. Thus, nose portion 114 and intermediate portion 113 penetrate the formation, whereas base 112 does not penetrate the formation. As used herein, the term“penetrating” portion or section may be used to refer to the portion of a PSWC or bullet that penetrates the formation. Thus, in this embodiment, nose portion 114 and intermediate portion 113 may collectively be referred to as the penetrating portion of bullet 100. Since base 112 extends radially outward relative to portions 113, 114 and generally stops the advancement of bullet 100 into the formation, base 112 may also be referred to as a stop ring or flange. As portions 113, 114 penetrate the formation, the core sample is received into passage 111 via the openings 101a, 111a at end 110a. In particular, the core sample advances axially into passage 111 from end 110a until base 112 contacts the borehole sidewall. Thus, the portion of passage 111 extending through nose portion 114 and intermediate portion 113 (i.e., the penetrating portion of bullet 100) receives the core sample, and the axial length of the core sample received by passage 111 is equal to (or substantially equal to) the length measured axially from end 110a to base 112, which is less than the axial length of passage 111 measured axially from end 110a to end 110b.

[0051] As previously described, passage 111 extends axially through first portion 110. An annular planar shoulder 115 extends radially inward along passage 111 in nose portion 114 proximal end 110a. Shoulder 115 lies in a plane oriented perpendicular to axis 105 and faces end 110b.

[0052] Referring still to FIGS. 2-6, a plurality of circumferentially-spaced holes 116 extend axially through base 112 and a plurality of uniformly circumferentiaily-spaced counterbores 117 extend axially from end 110b into but not through base 112. In this embodiment, two holes 116 are angularly spaced 180° apart and two counterbores 117 are angularly spaced 180° apart. In addition, holes 116 and counterbores 117 are uniformly spaced apart about base 112 with one hole 116 circumferentially disposed between each pair of circumferentially adjacent counterbores 117. Holes 116 are sized and positioned to receive screws 130 and counterbores 117 are sized and positioned to receive pins. Screws 130 secure portions 110, 120 together (FIG. 2) to form bullet 100, and screws 130 and the pins prevent portions 110, 120 from rotating relative to each other when coupled together. A clearance recess or slot 118 extends axially from each hole 116 along the radially outer surface of Intermediate portion 113 and nose portion 114 to allow access for inserting screws 130 into (or removing screws 130 from) holes 116. That is, clearance recesses 118 provide space for the heads of screws 130 to be accessed and rotated.

[0053] Referring now to FIGS. 2, 7, and 8, second portion 120 has a first end 120a, a second end 120b axially opposite end 120a, and a through passage 121 extending axially from first end 120a to second end 120b. In this embodiment, first end 120a comprises a planar surface disposed in a plane oriented perpendicular to axis 105. When portions 110, 120 are secured together with screws 130, the planar surfaces of ends 110b, 120a are seated and compressed flush against each other. As best shown in FIG. 2, second end 120b defines trailing end 100b of projectile 100 and through passage 121 defines the portion of cavity 101 extending through second portion 120. Passage 121 defines an opening 121 a at end 120a and an opening 121 b at end 120b. The annular radially outer surface of second portion is concave and has a maximum outer diameter that is less than or equal to the outer diameter of base 112.

[0054] As best shown in FIG. 2, when portions 110, 120 are secured together, passages 111 , 121 are coaxially aligned with axis 105. In addition, the diameter of opening 121 a at end 120a is less than the diameter of opening 111 b at end 110b. As a result, first end 120a defines a planar annular shoulder 102 that extends radially inward along passage 101 when portions 110, 120 are secured together. Shoulder 102 lies in a plane oriented perpendicular to axis 105 and faces opposing shoulder 115. The portion of passage 101 , 111 extending axially from shoulder 115 to shoulder 102 has a uniform and constant radius, and further, defines a cylindrical cavity sized to siidingly receive sleeve 140.

[0055] Referring again to FIGS. 2 and 7, second portion 120 includes a plurality of circumferentially-spaced drainage ports 124 and a plurality of circumferentially-spaced retrieval holes 125 In this embodiment, two drainage ports 124 are angularly spaced 180° apart and two retrieval holes 125 are angularly spaced 180° apart. Ports 124 and holes 125 extend radially from the radially outer surface of second portion 120 to passage 121. Ports 124 allow downhole fluids that may access passages 101 , 111 , 121 to drain therefrom. Holes 125 provide attachment points for a cable or wire that is used to pull bullet 100 from the borehole sidewall after being propelled therein. In other words, after bullet 100 has been shot into the subterranean formation, bullet 100 is pulled from the subterranean formation via a cable that is attached to holes 125 Second portion 120 also includes a plurality of circumferentially-spaced elongate, through slots 126 that extend axially from end 120b and extend radially from the radially outer surface of second portion 120 to passage 121. Slots 126 are used to position and load bullet 100 into a discharging mechanism of the downhole tool that propels bullet 100 into the borehole sidewall.

[0056] Referring still to FIGS. 2 and 7, a plurality of circumferentially-spaced holes 122 extend axially from end 120a to drainage ports 124 and a plurality of circumferentially-spaced counterbores 123 extend axially from end 120a into portion 120. In this embodiment, two holes 122 are angularly spaced 180° apart and two counterbores 123 are angularly spaced 180° apart. In addition, holes 122 and counterbores 123 are uniformly spaced apart with one hole 122 circumferentially disposed between each pair of circumferentially adjacent counterbores 123. Holes 122 are internally threaded and configured to mate with and threadably receive screws 130, whereas counterbores 123 have smooth cylindrical inner surfaces that slidably receive mating pins. In particular, counterbores 117, 123 and holes 116, 122 are positioned such that portions 110, 120 can be rotated relative to each other about axis 105 to align each hole 116 with a corresponding hole 122, and to align each counterbore 117 with a corresponding counterbore 123 to secure portions 110, 120 together As will be described in more detail below, screws 130 extend through aligned holes 116, 122 and pins extend through aligned counterbores 117, 123.

[0057] Referring now to FIGS. 2, 9, and 10, sleeve 140 has a first end 140a, a second end 140b axially opposite end 140a, and a cylindrical through passage 141 extending axially from end 140a to end 140b. Passage 141 defines an opening 141 a at end 140a and an opening 141 b at end 140b. When bullet 100 is shot into the borehole sidewall, a sample of the formation material (i.e., the core sample) is received in passage 141 of sleeve 140. Thus, passage 141 may also be referred to as a“core sample cavity.” An annular lip or flange 142 extends radially outward from sleeve 140 at lower end 140b. The radially inner cylindrical surface of sleeve 140 defining passage 141 and openings 141 a, 141 b is disposed at a uniform and constant diameter. The radially outer cylindricai surface of sieeve 140 Is disposed at a uniform and constant diameter between end 140a and flange 142.

[0058] As best shown in FIG. 2, sleeve 140 is coaxially and removably positioned in passage 111 of first portion 110 with end 140a axially abutting and seated against shoulder 115, with end 140b axially abutting and seated against end 120b and shoulder 102, and flange 142 axially positioned and compressed between ends 110b, 120a, thereby securing and retaining sieeve 140 within passage 101 and bullet 100. The inner diameter of sleeve 140 is equal to the diameter of opening 111a, thereby forming a smooth contiguous cylindrical surface along passage 101 across the transition between opening 111a and sleeve 140. The outer diameter of sleeve 140 (between end 140a and flange 142) is substantially the same as the inner diameter of first portion 110 between shoulder 115 and end 110, and thus, sleeve 140 slidingly engages first portion 110. Although sieeve 140 can be advanced axially into and out of passage 101 , 111 via sliding engagement of cylindricai surfaces, in other embodiments, the sleeve (e.g., sleeve 140) may be removably disposed within a passage of the bullet by other means such as mating threads.

[0059] As previously described, when bullet 100 travels into the subterranean formation, ends 100a, 110a initially contact the formation. It should be appreciated that shoulder 115 extends radially inward across end 140a of sleeve 140, and thus, protects leading end 140a of sieeve 140 from direct impact forces when bullet 100 is shot into the formation. As bullet 100 penetrates the borehole sidewall, formation material is received into passage 140 of sleeve 140 via openings 101a, 111 a.

[0060] Although the embodiment of bullet 100 shown in Figures 1 and 2 includes X-ray transparent sleeve 140, in other embodiments, a sleeve (e.g., sleeve 140) is not included in the bullet (e.g., bullet 100). In such embodiments lacking a sleeve, the remaining components of the bullet (e.g., first portion 110 and second portion 120) are X-ray transparent, thereby allowing the bullet to be subjected to direct imaging with X-ray tomography without removing the core sample therefrom. For example, in sleeveless embodiments, the core sample may be directly captured within the passage of the bullet (e.g., passage 101 ). Still further, although the embodiment of bullet 100 shown in Figures 1 and 2 includes multiple components or parts that are coupled together (e.g., first portion 110, second portion 120, and sieeve 140), in other embodiment, the bullet (e.g., bullet 100) is a single, unitary structure (e.g., a monolithic structure) that is X-ray transparent. Such embodiments reduce the number of components of the bullet while still enabling the core sample captured within the bullet to be imaged with X-ray tomography without removal of the core sample therefrom. For example, Figure 11 illustrates an embodiment of a percussion sidewall core (PSWC) projectile or bullet 100’ that is similar to bullet 100 previously described except that bullet 100’ is formed as a single, unitary structure that is X-ray transparent, and thus, the core sample captured within bullet 100’ can be imaged with X-ray tomography without removing the core sample from therefrom.

[0061] Referring now to FIG. 12, an embodiment of a method 160 for assembling bullet 100 previously described is shown. For purposes of clarity and further explanation, method 160 will be described with reference to FIGS. 1-9 described above

[0062] Starting in block 162 and with portions 110, 120 decoupled and separated, method 160 begins by positioning sleeve 140 in passage 111 of first portion 110. In particular, first end 140a is inserted into opening 111 b and sleeve 140 is axially advanced through passage 111 until first end 140a axially abuts shoulder 115 of first portion 110 and flange 142 contacts base 112 of first portion 110. Next, in block 164, and with sleeve 140 seated in passage 114 and portions 110, 120 axially spaced apart, first portion 110 is coaxially aligned with second portion 120 and rotationaliy oriented to: (1 ) align holes 116 of first portion 110 with holes 122 of second portion 120; and (2) align counterbores 117 of first portion 110 with counterbores 123 of second portion 120. Moving now to block 166, while maintaining the rotational alignment and axial spacing of portions 110, 120, a pin is placed in and extends axially from each counterbore 123, and then portions 110, 120 are axially pushed together to bring ends 120a, 140b into contact according to block 166. Due to the alignment of counterbores 117, 123, the portions of the pins extending from counterbores 123 advance axially into mating counterbores 117 as portions 110, 120 are moved axially together. Seating of the pins in aligned counterbores 117, 123 maintains the alignment of holes 116, 122 while screws 130 are placed in holes 116 and screwed into corresponding aligned holes 122 in block 168. The screws 130 are tightened to compress sleeve 140 between portions 110, 120, thereby forming bullet 100. Once assembled, bullet 100 is loaded into a downhole tool that can be used to propel bullet 100 into a borehole sidewall to obtain a core sample.

[0063] Referring now to FIG. 13, an embodiment of a method 170 for obtaining, extracting, and imaging a core sample with bullet 100 previously described is shown. It should be appreciated that method 170 is performed after buliet 100 has been assembled via method 160 described above, loaded into a downhole tool configured to fire bullet 100 into the formation, lowered downhole in the downhole tool, and positioned at the desired location of the borehole.

[0064] Starting in block 171 buliet 100 is shot or propelled into the borehole sidewall in block 171. Buliet 100 is oriented such that leading end 100a leads bullet 100 into the formation. As bullet 100 penetrates the formation, a portion of the formation (i.e., the core sample) passes through openings 101 a, 111 a and into core sample cavity 141 of sleeve 140. Next, in block 172, buliet 100 (with the core sample in cavity 141 ) is pulled or otherwise removed from the formation and retrieved to the surface. Holes 125 can be used to puli bullet 100 from the sidewall, or other means known in the art can be used to remove bullet 100 from the sidewall. It should be appreciated that the generally cylindrical core sample captured within cavity 141 breaks away from the formation as bullet 100 is pulled from the borehole sidewall. Once bullet 100 is removed from the sidewall, bullet 100 with the core sample therein is retrieved to the surface.

[0065] Once bullet 100 is at the surface, bullet 100 is partially disassembled by decoupling and separating portions 110, 120 in block 173. In particular, screws 130 are unthreaded from holes 122 of second portion 120, and then first portion 110 and second portion 120 are pulled axially apart. The pins disposed within counterbores 117, 123 sliding!y engage portions 110, 120 and resist relative rotation of portions 110, 120 about axis 105, but do not resist portions 110, 120 from being pulled axially apart. Next, in block 174, sleeve 140 (with the core sample disposed therein) is removed from first portion 110 by pushing and/or pulling sleeve 140 axially from passage 111 via opening 111 b in end 110b.

[0066] Moving now to block 175, sleeve 140, with the core sample disposed therein, is directly imaged via X-ray tomography to produce digital images. Since sleeve 140 is X-ray transparent, sleeve 140 does not attenuate or otherwise substantially inhibit the passage of X-rays therethrough and imaging of the core sample therein. Thus, in this embodiment, the core sample is not removed from sleeve 140. Next, in block 176, the digital images are processed using techniques know in the art (e.g , numerical methods and simulations) to determine various physical, mechanical, and petrophyscial properties of the core sample including, without limitation, porosity, absolute permeability, relative permeability, formation factor, elastic moduli, and the like.

[0087] It is to be understood that certain aspects of the disclosure such as block 176 of method 170 may be implemented by a computer system. For purposes of this disclosure, a computer system may include any instrumentality or aggregate of instrumentalities operable to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, detect, record, reproduce, handle, or utilize any form of information, intelligence, or data for business, scientific, control, or other purposes. For example, a computer system may be a personal computer or tablet device, a cellular telephone, a network storage device, or any other suitable device and may vary in size, shape, performance, functionality, and price. The computer system may include random access memory (“RAM”), one or more processing resources such as a central processing unit (“CPU”) or hardware or software control logic, read-only memory (“ROM”), and/or other types of nonvolatile memory. Additional components of the computer system may include one or more disk drives, one or more network ports for communication with external devices as well as various input and output (I/O) devices, such as a keyboard, a mouse, and a video display. The computer system also may include one or more buses operable to transmit communications between the various hardware components.

[0068] The computer system may also include computer-readable media. Computer-readable media may include any instrumentality or aggregation of instrumentalities that may retain data and/or instructions for a period of time. Computer-readable media may include, for example, without limitation, storage media such as a direct access storage device (e.g., a hard disk drive or floppy disk drive), a sequential access storage device (e.g., a tape disk drive), compact disk, CD-ROM, DVD, RAM, ROM, electrically erasable programmable read-only memory (“EEPROM”), and/or flash memory; as well as communications media such wires, optical fibers, microwaves, radio waves, and other electromagnetic and/or optical carriers; and/or any combination of the foregoing. Without being limited by this or any particular theory, the topology of the leading end or nose portion of a percussion sidewall core (PSWC) projectile or bullet (e.g., bullet 100) can influence the penetration of the bullet into the formation, the momentum lost at primary contact with the formation, the mechanism by which the bullet moves through the formation (e.g., cut or plough), the depth of penetration, and the stresses experienced by the core sample captured by the bullet. In the embodiment of bullet 100 previously described, leading end 100a is generally rounded and convex, and nose portion 114 extending axially (relative to axis 105) from leading end 100a of bullet 100 is tapered. However, in other embodiments, the leading end of the bullet (e.g., end 100a) and the nose portion extending from the leading end (e.g., nose portion 114) can have different topologies. For example, FIGS. 14-16 illustrate alternative geometries for the leading ends and nose portions of bullets in accordance with principles described herein. In FIGS. 14-16, the alternative geometries are shown in connection with the first portions of multi-part bullets similar to bullet 100 previously, however, it should be appreciated that such alternative geometries can be used with other bullet designs and configurations (single-piece, multi-piece, etc.).

[0089] Referring now to Figure 14, an embodiment of a first or leading portion 210 of a percussion sidewall core (PSWC) projectile or bullet is shown. First portion 210 is substantially the same as first portion 110 previously described and can be used in place of first portion 110. In particular, first portion 210 includes a central axis 205, a first end 210a defining the leading end of the corresponding bullet, and a nose portion 214 extending axially from first end 210a. Similar to nose portion 114 previously described, nose portion 214 tapers radially inward moving axially toward end 210a. As shown in Figure 13, nose portion 214 is defined a radially outer surface 214a of first portion 210 and a radially inner surface 214b of first portion 210. Outer surface 214a is disposed at an outer diameter that increases moving axially from end 210a. In particular, in this embodiment, outer surface 214a comprises a plurality of axially adjacent frustoconlcal surfaces moving axially from end 210a, and inner surface 214b is a cylindrical surface extending axially from end 210a. In particular, outer surface 214a may be described as being oriented at an acute taper angle a relative to inner surface 214b - taper angle a is measured between surfaces 214a, 214b in cross-sectional side view. Taper angle a is preferably between 0° and 90° along each section of outer surface 214a (e.g., along each frustoconlcal surface), and more preferably between 10° and 80° along each section of outer surface 214a (e.g., along each frustoconlcal surface). In general, the smaller the taper angle a, the sharper and more aggressive the corresponding portion of outer surface 214a. In this embodiment, taper angle a decreases (is less aggressive) moving axially along outer surface 214a toward end 210a to reduce stress concentrations and enhance the toughness of annular cutting edge 216 at end 210a. Due to the plurality of axially adjacent frustoconica! surfaces along outer surface 214a of nose portion 214, end 210a is not rounded and convex, and thus, annular cutting edge 216 is defined by an annular line disposed in a plane oriented perpendicular to axis 205 at end 210a.

[0070] Referring now to Figure 15, another embodiment of a first or ieading portion 310 of a percussion sidewall core (PSWC) projectile or bullet is shown. First portion 310 is substantially the same as first portion 110 previously described and can be used in place of first portion 110. In particular, first portion 310 includes a central axis 305, a first end 310a defining the Ieading end of the corresponding bullet, and a nose portion 314 extending axially from first end 310a. As shown in Figure 15, nose portion 314 is defined by a radially outer surface 214a and a radially inner surface 214b as previously described. Namely, outer surface 214a comprises a plurality of axially adjacent frustoconica! surfaces moving axially from end 310a, and inner surface 214b is a cylindrica! surface. However, in this embodiment, end 310a does not include an annular cutting edge defined by an annular line disposed in a plane oriented perpendicular to axis 205. Rather, in this embodiment, end 310a includes an annular serrated cutting edge 316 comprising a plurality of circumferential!y-adjacenf serrations 317 in side view.

[0071] Referring now to Figure 16, yet another embodiment of a first or Ieading portion 410 of a percussion sidewall core (PSWC) projectile or bullet is shown. First portion 410 is substantially the same as first portion 110 previously described and can be used in place of first portion 110. In particular, first portion 410 includes a central axis 405, a first end 410a defining the leading end of the corresponding bullet, and a nose portion 414 extending axially from first end 410a. As shown in Figure 16, nose portion 414 is defined by a radially outer surface 214a and a radially inner surface 214b as previously described. Namely, outer surface 214a comprises a plurality of axially adjacent frustoconicai surfaces moving axially from end 410a, and inner surface 214b is a cylindrical surface. However, unlike first portion 110, in this embodiment, end 410a is defined by a sinusoidal or wavy profile 416 in side view.

[0072] As previously described, when embodiments of percussion side wail core (PSWC) projectiles or bullets described herein are advanced into the formation, the core sample is captured therein. In embodiments described herein including a sleeve (e.g., sleeve 140) removably disposed within the bullet (e.g., bullet 100), the core sample is captured within the sleeve, and in embodiments described herein comprising a single-piece bullet, the core sample is captured within a receptacle of the single-piece bullet. In either case, compressive stresses may be experienced by the captured core sample as the bullet advances into the formation and receives the core sample. To reduce the compressive stresses experienced by the core sample, which may undesirably alter physical properties of the core sample If sufficiently large, one or more relaxation or stress relief slots may be provided in the bullet to allow excess core sample material to escape the capture receptacle. For example, referring now to FIGS 17 and 18, embodiments of a first portion 110’ and a sleeve 140’ are shown. First portion 110’ and sleeve 140’ are the same as first portion 110 and sleeve 140, respectively, as previously described with the sole exception that a plurality of uniformly circumferentially-spaced relaxation slots 183 are provided in first portion 110’ and a plurality of uniformly circumferentially-spaced relaxation slots 184 are provided in sleeve 140’.

[0073] Referring briefly to FIG. 17, each slot 183 extends radially through first portion 110 from its radially outer surface to passage 111 , and further, each slot 183 extends axially from end 110a to base 112. Thus, slots 183 are oriented parallel to axis 105. In this embodiment, two slots 183 angularly spaced 180° apart are provided, however, in other embodiments, one, three or more slots 183 may be provided.

[0074] Referring briefly to FIG. 18, each slot 184 extends radially through sleeve 140 from its radially outer surface to passage 141 , and further, each slot 184 extends axially from end 140a. Thus, slots 184 are oriented parallel to axis 105. In this embodiment, four slots 183 angularly spaced 90° apart are provided, however, in other embodiments, any suitable number of slots 184 may be provided. Slots 184 may be circumferentially aligned or staggered with respect to slots 183.

[0075] The width and length of slots 183, 184 can be adjusted to achieve the desired stress relief of the core sample, while ensuring sufficient rigidity of first portion 110 and sleeve 140 they advance into the formation. In addition, the number, shapes, and positions of slots 183, 184 may vary, depending on a hardness of the subterranean formation, and a material and shape of corresponding bullet. Although slots 183, 184 are linear in this embodiment, in other embodiments, the slots (e.g., slots 183, 184) may include curves and/or bends.

[0076] Referring now to Figures 19 and 20, an embodiment of a first or leading portion 510 of a percussion sidewall core (PSWC) projectile or bullet is shown. First portion 510 can be used in place of first portion 110 previously described. In this embodiment, first portion 510 is similar to first portion 110 previously described but has a relatively smaller volume of material in the penetrating portion, and thus, presents a reduced frontal area to the formation, thereby decreasing the impact area and volume. These characteristics offer the potential to advantageously reduce the stress and strain applied to the core sample, as well as reduce the explosive force needed to advance the corresponding bullet into the borehole sidewall. Portion 510 is made of a rigid, durable materiai(s) suitable for use in a downhole environment and capable of maintaining their shape and integrity while penetrating the borehole sidewall.

[0077] In this embodiment, first portion 510 has a central axis 515, a first end 510a defining the leading end of the corresponding bullet, a second end 510b opposite end 510a, a base portion 512 at end 510b, a penetrating portion 514 extending axially from end 510a to base 512, and a through passage 511 extending axially from end 510a to end 510b. The tip of first portion 510 at end 510a is generally convex and radiused. Unlike nose portion 114 and intermediate portion 113, which define the penetrating portion of first portion 110 previously described, in this embodiment, penetrating portion 514 has a radially outer surface 514a that comprises a cylindrical surface extending axially from end 510a to base 512 and a radially inner surface 514b that comprises a cylindrical surface extending axially between ends 510a, 510b. Thus, penetrating portion 514 has a uniform radial thickness Tsi ₄ measured radially from inner surface 514b to outer surface 514a. The radial thickness T514 is relatively small to reduce the volume of penetrating portion 514 and the impact area of end 510a. Inner surface 514 defines a portion of passage 511 extending from end 510a that receives the core sample

[0078] In this embodiment, penetrating portion 514 also includes a plurality of uniformly circumferentially-spaced relaxation or stress relief slots 516. Each slot 516 extends radially through penetrating portion 514 from outer surface 514a to passage 511 , and further, each slot 516 extends axially from end 510a and terminates proximal base 512. Thus, slots 516 are oriented parallel to axis 505 In this embodiment, four slots 516 angularly spaced 90° apart are provided, however, in other embodiments, one, two, three, five, or more slots 516 may be provided.

[0079] The width and length of slots 516 can be adjusted to achieve the desired stress relief of the core sample, while ensuring sufficient rigidity of penetrating portion 514 as it advances into the formation. In addition, the number, shapes, and positions of slots 516 may vary, depending on a hardness of the subterranean formation, and a material and shape of corresponding bullet. Although slots 516 are linear in this embodiment, in other embodiments, the slots (e.g., slots 516) may include curves and/or bends. It should be appreciated that slots 516 also reduce the volume of penetrating portion 514.

[0080] Referring stiii to Figures 19 and 20, a plurality of uniformly circumferentially-spaced support webs or ribs 517 extend between penetrating portion 514 and base 512. In particular, ribs 517 extend axially from end 510a along outer surface 514a to base 512, and then radially outward along the surface of base 512. Ribs 517 support and add rigidity to the relatively thin penetrating portion 514.

[0081] As compared to a similarly sized penetrating portion (e.g., penetrating portion of first portion 110), penetrating portion 514 offers the potential for a reduced impact area and a reduced penetrating volume. Without being limited by this or any particular theory, the lower the impact area and the lower the penetrating volume of the penetrating portion of a bullet, the lower the force needed for the penetrating portion to penetrate the formation to the desired depth and the lower the stress applied to the core sample. As used herein, the term“impact area” refers to the frontal area (e.g., cm ² ) of the penetrating portion of a bullet during a core sample acquisition operation. The volume of the penetrating portion of a bullet can be quantified and characterized by the“barrel volume,” the“chamber volume,” and the “total volume” (or ratios thereof), where the “barrel volume” is the volume of the material (e.g., metal) that forms the penetrating portion of the bullet, the“chamber volume” is the volume of the penetrating portion of the bullet that receives the core sample, and the“total volume” is the sum of the“barrel volume” and the“chamber volume.”

[0082] In general, the forces needed for different bullets to penetrate the formation to the desired depth and the stresses applied to the core sample by different bullets can be quantified and compared via the impact area, barrei volume, chamber volume, and total volume (or ratios thereof). For example, for one exemplary set of similarly sized first portions 110, 510 (each having a chamber volume of about 4,000 mm ³ ), first portion 110 has an impact area of about 100 mm ² , whereas first portion 510 has an impact area of about 50 mm ² . Thus, for obtaining a similarly sized core sample (about 4,000 mm ³ core sample), first portion 510 has about 50% of the impact area as compared to first portion 110. Stated differently, for first portion 110, the ratio of the impact area to core sample size is about 0.025 (100 mm ² /4,000 mm ³ ), whereas for first portion 510, the ratio of the impact area to core sample size is about 0.0125 (50 mm ² /4,0Q0 mm ³ }. In embodiments described herein that provide penetrating portions with reduced impact areas (e.g., penetrating portion 510), the impact area is preferably less than 200 mm ² , more preferably less than 100 mm ² , and even more preferably less than 75 mm ² ; and the ratio of the impact area to the core sample volume is preferably less than 0.020.

[0083] As another example, portions 113, 114 of first portion 110 described above define a penetrating portion having a chamber volume of about 4,000 mm ³ , a barrel volume of about 8,000 mm ³ , and a total volume of 12,000 mm ³ (sum of the barrel volume and the chamber volume). In contrast, penetrating portion 514 described above has a similarly sized chamber volume of about 4,000 mm ³ , but a smaller barrel volume of about 1 ,000 mm ³ and smaller total volume of 5,000 mm ³ . Thus, for obtaining a similarly sized core sample (about 4,000 mm ³ core sample), penetrating portion 514 has a barrel volume that is about 12% of the barrel volume of the penetrating portion of first portion 110 (1 ,000 mm ³ /8,000 mm ³ }. Stated differently, for first portion 110, the ratio of barrel volume to chamber volume (i.e., core sample size} is about 2.00 (8,000 mm ³ /4,0GG nm ³ ) (i.e., the barrel volume is about 200% larger than the chamber volume) and the ratio of the chamber volume to the total volume is about 0.33 (4,000 mm ³ /12,000 mm ³ ) (i.e., the chamber volume is about 33% of the total volume), whereas for first portion 510, the ratio of barrel volume to chamber volume is about 0.25 (1 ,000 mm ³ /4,000 mm ³ ) (i.e., the barrel volume Is about 25% of the chamber volume) and the ratio of the chamber volume to the total volume is about 0.80 (4,000 mm ³ / 5,000 mm ³ ) (i.e., the chamber volume is about 80% of the total volume). In embodiments described herein that provide reduced volume of the penetrating portion, the ratio of the barrel volume to the chamber volume is preferably less than 1 .00 (i.e., the barrel volume is less than the chamber volume), more preferably less than 0.50, and even more preferably less than 0.30; and the ratio of the chamber volume to the total volume is preferably greater than 0.50 ) (i.e., the chamber volume is greater than 50% of the total volume), and more preferably between 0.50 and 0.85 (i.e., the chamber volume is 50% to 85% of the total volume).

[0084] As previously described, cylindrical inner surface 514b extends from end 510a to end 510b. Thus, inner surface 514b does not include an annular shoulder (e.g., shoulder 115). Accordingly, this embodiment of first portion 510 is not designed for use with a distinct and separate inner sleeve (e.g., sleeve 140). Rather, the core sample is received in passage 511 , and can be removed from passage 511 and imaged or imaged through penetrating portion 514 in embodiments where penetrating portion 514 is X-ray transparent.

[0085] Referring now to FIG. 21 , an embodiment of a percussion sidewall core (PSWC) projectile or bullet 600 is shown in particular, bullet 600 has a central or longitudinal axis 605, a first or leading end 600a, a second or trailing end 600b axially opposite end 600a, and an inner cavity 601 extending axially from end 600a to end 600b. Similar to bullet 100 previously described, in this embodiment, builet 600 is an assembly of separate and distinct components. In particular, bullet 600 includes a first portion 610, a second portion 120 removably coupled to first portion 610 with screws (not shown), and a cylindrical sleeve (not shown) (e.g., sleeve 140) removably disposed between portions 610, 120. Portions 610, 120 and the sleeve are coaxially aligned such that each shares the same central axis 605. Portions 610, 120 and the sleeve are made of a rigid, durable material(s) suitable for use in a downhole environment and capable of maintaining their shape and integrity while penetrating the borehole sidewall.

[0086] Second portion 120 is as previously described with respect to bullet 100. First portion 610 is similar to first portion 110 previously described but has a relatively smaller volume of material in the penetrating portion and presents a reduced frontal area to the formation, thereby decreasing the impact area and penetrating volume. These characteristics offer the potential to advantageously reduce the stress and strain applied to the core sample, as well as reduce the explosive force needed to advance the corresponding bullet into the borehole sidewall.

[0087] Referring now to FIGS. 21 and 22, in this embodiment, first portion 610 includes a central axis 615, a first end 610a defining the leading end 600a of bullet 600, a second end 610b opposite end 610a, a base portion 612 at end 610b, a penetrating portion 614 extending axially from end 610a to base 612, and a through passage 611 extending axially from end 610a to end 610b. Passage 611 defines a portion of cavity 601 . The tip of first portion 610 at end 610a is generally convex and radiused. in this embodiment, penetrating portion 614 has a radially outer surface 614a comprising a cylindrical surface extending axially from end 610a to base 612 and a radially inner surface 614b comprising an annular planar shoulder 616 proximal end 610a and a cylindrical surface extending axially from shoulder 616 to end 610b. Shoulder 616 lies in a plane oriented perpendicular to axis 605 and faces end 610b. Shoulder 616 functions in the same manner as shoulder 115 previously described to hold a sleeve (e.g., sleeve 140) within bullet 600.

[0088] Axially below shoulder 616, penetrating portion 614 has a uniform radial thickness Tei ₄ measured radially from inner surface 614b to outer surface 614a. The radial thickness Tei ₄ is relatively small to reduce the volume of penetrating portion 614.

[0089] Referring still to Figures 21 and 22, a plurality of uniformly circumferentialiy-spaced support webs or ribs 517 as previously described extend between penetrating portion 614 and base 612. In particular, ribs 517 extend axially from end 610a along outer surface 614a to base 612, and then radially outward along the surface of base 612. As previously described, ribs 517 support and add rigidity to the relatively thin penetrating portion 614.

[0090] As compared to a similarly sized penetrating portion (e.g., penetrating portion of first portion 110), penetrating portion 614 has a reduced impact area at end 610a and a reduced total penetrating volume. In embodiments described herein that provide reduced impact area at the penetrating end (e.g., end 610a) and reduced volume of the penetrating portion, the impact area is preferably less than 200 mm ² , more preferably less than 100 mm ² , and even more preferably less than 75 mm ² ; and the ratio of the impact area to the core sample volume is preferably less than 0.020; and further, the ratio of the barrei volume to the chamber volume is preferably less than 1 00 (i.e., the barrel volume is less than the chamber volume), more preferably less than 0.50, and even more preferably less than 0.30; and the ratio of the chamber volume to the total volume is preferably greater than 0.50 ) (i.e., the chamber volume is greater than 50% of the total volume), and more preferably between 0.50 and 0.85 (i.e., the chamber volume is 50% to 85% of the total volume)

[0091] As previously described, inner surface 614b includes shoulder 616 for removably securing a sleeve within bullet 600. Accordingly, this embodiment of first portion 610 is designed for use with a distinct and separate inner sleeve (e.g., sleeve 140). The core sample passes axially through end 610a and into the sleeve disposed in passage 611. The sleeve with the core sample disposed therein is removed from the remainder of bullet 600 by separating portions 610, 120 as previously described with respect to bullet 100, and imaged through the X-ray transparent sleeve. [0092] Referring now to FIG. 23, an embodiment of a percussion sidewai! core (PSWC) projectile or bullet 700 is shown in this embodiment, bullet 700 has a central or longitudinal axis 705, a first or leading end 700a, a second or trailing end 700b axially opposite end 700a, and an inner cavity 701 extending axially from end 700a to end 700b. Similar to bullets 100, 600 previously described, in this embodiment, bullet 700 is an assembly of separate and distinct components. In particular, bullet 700 includes a first portion 710, a second portion 120 removably coupled to first portion 710 with screws (not shown), and a cylindrical sleeve (not shown) (e.g., sleeve 140) removably disposed between portions 710, 120. Portions 710, 120 and the sleeve are coaxially aligned such that each shares the same central axis 705. Portions 710, 120 and the sleeve are made of a rigid, durable materia!(s) suitable for use in a downhole environment and capable of maintaining their shape and integrity while penetrating the borehole sidewall.

[0093] Second portion 120 is as previously described with respect to bullet 100. First portion 710 is similar to first portion 110 previously described but has a relatively smaller volume of material in the penetrating portion and presents a reduced frontal area to the formation, thereby decreasing the impact area and penetrating volume. These characteristics offer the potential to advantageously reduce the stress and strain applied to the core sample, as well as reduce the explosive force needed to advance the corresponding bullet into the borehole sidewall.

[0094] Referring now to FIGS 23 to 25, in this embodiment, first portion 710 includes a central axis 715, a first end 710a defining the leading end 700a of bullet 700, a second end 710b opposite end 710a, a base portion 712 at end 710b, a penetrating portion 714 extending axially from end 710a to base 712, and a through passage 711 extending axially from end 710a to end 710b. Passage 711 defines a portion of cavity 701. In this embodiment, penetrating portion 714 includes an annular tip 716 at end 710a and a plurality of uniformly circumferentially spaced support ribs 717 extending axially from tip 716 to base 712. Annular tip 716 of first portion 710 at end 710a is generally convex and radlused. The radially inner surface of each rib 717 is disposed at a uniform radial distance from axis 715 moving axially from tip 716 to base 712. Annular tip 716 extends radially inward relative to the radially inner surfaces of ribs 717, thereby defining an annular shoulder 718 along passage 711. Shoulder 718 lies in a plane oriented perpendicular to axis 715 and faces end 710b. Shoulder 718 functions in the same manner as shoulder 115 previously described to hold a sleeve (e.g., sleeve 140) within bullet 700. In this embodiment, no wall or other structure is circumferentially disposed between ribs 717, and thus, ribs 717 support annular tip 716 and provide rigidity to penetrating portion 714. As a result, penetrating portion 714 may be described as including a plurality of circumferentially-spaced slots 719, with each slot 719 being circumferentially positioned between each pair of circumferentially adjacent ribs 717. More specifically, each slot 719 extends axially from annular tip 716 to base 712, and extends circumferentially between each pair of corresponding circumferentially adjacent ribs 717.

[0095] The circumferential width and length of ribs 717 and slots 719 can be adjusted to achieve the desired stress relief of the core sample, while ensuring sufficient rigidity of penetrating portion 714 as it advances into the formation. In addition, the number, shapes, and positions of ribs 717 and slots 719 may vary, depending on a hardness of the subterranean formation, and a material and shape of corresponding bullet. Although ribs 717 and slots 719 are linear in this embodiment, in other embodiments, the ribs (e.g., ribs 717) and/or slots (e.g., slots 516) may include curves and/or bends. It should be appreciated that slots 719 reduce the volume of penetrating portion 714.

[0096] As compared to a similarly sized penetrating portion (e.g., penetrating portion of first portion 110), penetrating portion 714 has a reduced impact area at end 710a and a reduced total penetrating volume. In embodiments described herein that provide reduced impact area at the penetrating end (e.g., end 610a) and reduced volume of the penetrating portion, the impact area is preferably less than 200 mm ² , more preferably less than 100 ² , and even more preferably less than 75 mm ² ; and the ratio of the impact area to the core sample volume is preferably less than 0.020; and further, the ratio of the barrel volume to the chamber volume is preferably less than 1.00 (i.e., the barrel volume is less than the chamber volume), more preferably less than 0.50, and even more preferably less than 0.30; and the ratio of the chamber volume to the total volume is preferably greater than 0.50 ) (i.e., the chamber volume is greater than 50% of the total volume), and more preferably between 0.50 and 0.85 (i.e., the chamber volume is 50% to 85% of the total volume).

[0097] The radially inner surfaces of ribs 717 and shoulder 718 define an annular cavity within passage 711 for removably securing a sleeve within bullet 700. Accordingly, this embodiment of first portion 710 is designed for use with a distinct and separate inner sleeve (e.g., sleeve 140). The core sample passes axially through end 710a and into the sleeve disposed in passage 711. The sleeve with the core sample disposed therein is removed from the remainder of bullet 700 by separating portions 710, 120 as previously described with respect to bullet 100, and imaged through the X-ray transparent sleeve.

[0098] While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the disclosure. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1 ), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps.