RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/154,787, filed April 30, 2015, and entitled Downhole Axial Coring Method and Apparatus, the entirety of which is incorporated herein by reference.

BACKGROUND

[0002] Axial coring operations are commonly employed in the evaluation of hydrocarbon reservoirs. Analysis of the obtained core may reveal valuable data concerning subsurface geological formations including parameters such as permeability, porosity, and fluid saturation that are useful in the exploration for petroleum, gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.

[0003] A conventional coring operation resembles a standard rotary drilling operation in that it makes use of a drill bit deployed at the distal end of a length of drill pipe. A coring bit includes cutting elements deployed about the periphery of the bit and surrounding a large central hole (referred to as a throat). The throat is intended to allow the formation core to pass unbroken into core catching barrels seated within coring collars deployed above the bit. Obtained cores generally range from about 30 to about 300 feet depending on the needs of the operation.

[0004] While such coring operations and equipment are conventional, certain difficulties can arise. For example, certain drilling conditions can lead to a significant reduction in the rate of penetration of drilling while coring, thereby increasing costs. Moreover core breakage and/or core jamming in the core barrel can lead to numerous complications including a virtual stoppage in drilling that may ultimately require the coring assembly to be tripped out the hole (which greatly increases the cost and time required to complete the operation).

SUMMARY

[0005] A method for coring a subterranean wellbore is disclosed. The method includes rotating a coring assembly in the wellbore to drill and core the well. The coring assembly includes a coring bit, a core barrel, and a measurement sub. A plurality of dynamics measurements are made while coring (drilling) using the measurement sub. The dynamics measurements are processed to identify the onset of an undesirable coring state. A drilling parameter used to drill and core the well is changed to mitigate the identified undesirable coring state. The undesirable coring state may include, for example, excessive axial vibrations or core jamming. The drilling parameter may include a drill string rotation rate and/or a weight on bit.

[0006] The disclosed embodiments may provide various technical advantages. For example, the disclosed dynamics measurements may enable certain deleterious coring states to be quickly identified and mitigated before damaging the core or the coring assembly. The disclosed methods may also enable the coring operation to continue at more efficient drilling parameters thereby potentially saving time and money.

[0007] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] For a more complete understanding of the disclosed embodiments, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 depicts a drilling rig on which the disclosed embodiments may be utilized.

[0010] FIG. 2 depicts one embodiment of the coring assembly shown on FIG. 1.

[0011] FIG. 3 depicts a flow chart of one disclosed method embodiment.

[0012] FIG. 4 depicts a flow chart of another disclosed method embodiment.

[0013] FIG. 5 depicts a plot of axial load versus time showing the mean, maximum, and minimum loads.

[0014] FIG. 6 depicts a plot of axial load versus time showing half the difference between the maximum and minimum loads and 2 times the standard deviation of the load for the data depicted on FIG. 5

[0015] FIG. 7 depicts a plot of full bandwidth load variance (standard deviation squared) and low frequency variance (less than 20 Hz) for the data depicted on FIG. 5.

[0016] FIG. 8 depicts a plot of axial acceleration versus time showing the mean, maximum, and minimum accelerations for the same time interval as FIG. 5.

[0017] FIG. 9 depicts a plot of torque versus time showing the mean, maximum, and maximum torques for the same time interval as FIG. 5.

[0018] FIG 10 depicts a plot of full bandwidth torque variance and low frequency torque variance for the data depicted on FIG. 9. [0019] FIG 11 depicts a plot of full bandwidth rotation rate variance and low frequency rotation rate variance for the time interval depicted on FIGS. 5-10.

[0020] FIG. 12 depicts a plot of axial load versus time showing the mean, maximum, and minimum loads at the back end of axial vibration event.

[0021] FIG. 13 depicts plots of the maximum axial load minus the minimum axial load and the drill string rotation rate versus time at the back end of axial vibration event shown on FIG. 10.

[0022] FIG. 14 depicts a flow chart of yet another disclosed method embodiment.

[0023] FIG. 15 depicts a plot of torque versus time during a period with core-jamming.

[0024] FIG. 16 depicts a plot of load versus time for the same time interval as FIG. 15.

[0025] FIG. 17 depicts a plot of torque to weight ratio for the same time interval as FIG. 15.

[0026] FIG. 18 depicts a plot of axial displacement versus time for the same time interval as FIG. 15.

[0027] FIG. 19 depicts a plot of axial acceleration versus time for the same time interval as FIG. 15.

[0028] FIG. 20 depicts a plot of torque and low-frequency acceleration for the same time interval as FIG. 15.

DETAILED DESCRIPTION

[0029] The disclosed embodiments relate generally to a downhole axial coring method and apparatus and in particular to a method and apparatus for employing mechanics and/or dynamics measurements while coring to improve coring operations. In some embodiments, a method and apparatus that may provide an early warning of potential problems during coring so that mitigating actions may be taken.

[0030] FIG. 1 depicts one example of an offshore drilling assembly, generally denoted 10, that may be suitable for employing the disclosed embodiments. In FIG. 1 a semisubmersible drilling platform 12 is positioned over an oil or gas formation (not shown) disposed below the sea floor 16. A subsea conduit 18 extends from deck 20 of platform 12 to a wellhead installation 22. The platform may include a derrick and a hoisting apparatus for raising and lowering the drill string 30, which, as shown, extends into borehole 40 and includes a coring assembly 50 which is described in more detail below with respect to FIG. 2.

[0031] It will be understood that the deployment illustrated on FIG. 1 is merely an example and that the disclosed embodiments are not limited to use with a semisubmersible platform 12 as depicted. The disclosed embodiments are equally well suited for use with any kind of subterranean drilling operation, either offshore or onshore. Nor are the disclosed embodiments limited to oilfield applications, as coring operations may be employed in a number of non-oilfield field applications, for example, including mining operations and geological research.

[0032] Those of ordinary skill will readily appreciate that a coring operation is similar to a rotary drilling operation in that a drill bit is rotated in a subterranean wellbore to drill ahead. The drilling speed (rate of penetration) is generally related to the rotary speed (rotation rate of the drill string) and the downforce (weight on bit). A coring operation differs from a conventional rotary drilling operation in that the coring bit includes an open hole (throat) for receiving the formation core. The core is received in an inner barrel in the drill collar above the bit. Drilling continues until a predetermined core length is achieved or until the core barrel jams and further coring becomes impossible. Upon the completion of drilling a core catcher may be deployed to break off and retain the core within the inner barrel while tripping out of the hole. Tripping is generally slow so that formation gases within the core may equalize and escape into the lower pressure atmosphere.

[0033] FIG. 2 depicts one example embodiment of a coring assembly 50. The depicted embodiment includes a coring bit 52 having a throat 53 for receiving a formation core. A core barrel assembly including inner and outer core barrels 62 and 64 is deployed above the bit 52. The inner and outer core barrels 62 and 64 are configured to rotate with respect to one another about a common longitudinal axis. A bearing assembly (not shown) is intended to isolate the inner core barrel 62 from the rotational energy of the drill string so that the inner core barrel 62 (and the core) remain rotationally stationary with respect to the wellbore while drilling. As depicted, the inner core barrel 62 defines an open cylinder 66 sized and shaped to receive the core. The outer core barrel 64 is rotationally coupled with the bit 52 (and the drill string 30) and may include one or more stabilizers 68, for example, as depicted. The outer core barrel 64 further includes a flow channel (not shown) for delivering drilling fluid to the bit 52. Coring assembly 50 may further include a core catcher 55 for breaking off and retaining the core within the inner core barrel 62 upon completion of the coring operation.

[0034] In the depicted embodiment, coring assembly 50 may further include a measurement while drilling (MWD) sub 70, a mechanics and/or dynamics measurement sub 75, and a bumper sub 80. The MWD sub 70 may include substantially any suitable surveying sensors, for example, including accelerometers, magnetometers, and/or gyroscopic sensors well known to those of ordinary skill in the art. The MWD sub 70 may further include a telemetry system, such as a mud pulse telemetry system or an electromagnetic telemetry system for communicating with the surface. Such telemetry systems are well known in the art. The MWD sub may also communication with the surface via a wired drill pipe communications channel. [0035] The mechanics and/or dynamics measurement sub 75 may include substantially any suitable sensors for making drilling mechanics and/or dynamics measurements while drilling. These measurements are referred to collectively herein as dynamics measurements. For example, measurement sub 75 may include a number of accelerometers configured to measure various vibrational modes (including axial, lateral, and torsional vibrations). Measurement sub 75 may further include a number of strain gauges for measuring various strain components from which system loads and/or stresses may be derived (e.g., including weight on bit, torque on bit, and bending moments). The measurement sub 75 may include, for example, the Drilling Mechanics Module or the OptiDrill sub available from Schlumberger Technology Corporation.

[0036] Measurement sub 75 may be deployed in close proximity to a bumper sub 80 (e.g., axially between the core barrel assembly and the bumper sub 80). As is known to those of ordinary skill in the art, the bumper sub 80 is a spring loaded telescoping sub that allows for independent axial motion above and below the sub while communicating full torsional stresses. During normal drilling the sub is closed such that the specified weight on bit is transferred to the bit. The stroke length of the sub is intended to enable the drill string to be put in the slips at the surface without lifting the bit off bottom.

[0037] With continued reference to FIG. 2, it will be understood that (while not explicitly depicted) coring assembly 50 may include one or more conventional drill collars, for example, deployed between the measurement sub 75 and the bumper sub 80. Such drill collars may also be deployed elsewhere in the assembly 50. Moreover, additional drill collars may also be deployed above the bumper sub 80 such that the bumper sub 80 remains closed at the weight on bit values employed during a coring operation.

[0038] Referring now to FIG. 3, a flow chart of one disclosed coring method 100 is depicted. A coring assembly (such as the example embodiment shown on FIG. 2) is deployed in and rotated in a subterranean wellbore at 102 to drill (and obtain an axial core from) the wellbore. Dynamics measurements are made at 104 while drilling in 102. The measurements may then be processed at 106 to identify the onset of an undesirable drilling/coring state. At 108, one or more drilling parameters (e.g., weight on bit or drill string rotation rate) may be adjusted to change the drilling state.

[0039] As described in more detail below, substantially any suitable dynamics measurements may be made at 104. For example, the measurements may include axial load, axial acceleration, torque, rotation speed, and the like. These measurements may be further processed to obtain maximum and minimum values, maximum minus minimum values, average values, a standard deviation (or other known deviations), energy spectra, and the like. The measurements may be obtained at substantially any frequency, e.g., in a range from about 10 to about 10,000 Hz and may further be processed using substantially any suitable high pass, low pass, and/or bandpass filters.

[0040] The measurements may be further processed at 106 to identify substantially any deleterious or potentially deleterious drilling state. For example, as also described in more detail below, the measurements may be processed to identify undesirable axial vibration modes, e.g., those having forces that exceed the compressional force across the bumper sub. Such force variations may cause the bumper sub to open and close while drilling which can lead to equipment damage and a reduced drilling speed. The measurements may also be processed to identify core sticking and/or jamming in the inner core barrel.

[0041] FIG. 4 depicts a flow chart of another disclosed method embodiment 120. As in method 100, a coring assembly is deployed in and rotated in a subterranean wellbore at 102. Dynamics measurements are made at 124 while drilling. The measurements include at least one of an axial load and an axial acceleration. The measurements are processed at 126 to identify the onset of undesirable (e.g., potentially deleterious) axial vibrational modes. The weight-on-bit and/or the drill string rotation rate may be adjusted at 128 to mitigate the undesirable axial vibration.

[0042] In certain coring operations, a zero-mean axial vibration with an amplitude (measured as a stress) greater than the compression force on the bumper sub may cause tension across the closed face of the sub possibly causing it to open. After opening, the sub generally closes with a sudden impact creating further axial vibrations. This impact at the face of the sub may (in certain drilling conditions) develop into a large non-linear resonance with a period approximately equal to the two-way time for axial waves between the bumper sub and the bit. While at its initiation, the amplitude of such axial vibrations may be relatively small (e.g., equal to the compression force on the bumper sub) it may amplify rapidly to levels greater than the overall weight-on-bit. In extreme cases, the drill bit can vibrate on and off the bottom (bit bounce), thereby potentially damaging the core and/or the coring assembly. Such vibrations may further cause a significant reduction in the drilling speed and initiate other harmful vibrational modes (e.g., torsional resonant modes). Axial vibrations may also cause core breakage and jamming and require premature tripping of the drill string.

[0043] The intent of method 120 is to identify axial vibrations at an early stage so that they can be suppressed (or possibly even eliminated). Such axial vibrations may be identified, for example, by monitoring the axial load while drilling. In one embodiment a measurement of the axial load deviation may be indicative of the onset of axial vibrations. For example, axial vibrations may be indicated when the difference between maximum and minimum loads exceeds an average (mean) load or when the standard deviation of the axial load exceeds half the mean load. [0044] Another identifying feature of the initiation of axial vibrations may be a negative axial load across the bumper sub (which is indicative of tension across the bumper sub causing it to open). This may be indicated, for example, when a difference between the maximum and minimum loads in a predetermined time interval is greater than twice the compressional load on the bumper sub. It may also be indicated when a standard deviation of the measured load in a predetermined time interval is greater than the compression load on the bumper sub divided by the square root of two. Severe axial loads may be indicated when the difference between maximum and minimum loads exceeds twice the mean load or when the minimum load is less than or equal to zero (which can be indicative of the drill bit lifting off bottom).

[0045] In some embodiments, axial vibrations may be identified by an energy (or variance) spectrum. For example, the axial energy (or variance) above a predetermined frequency threshold may increase significantly (e.g., by a factor of 3, or 10, or 20, or even 100 or more) when the bumper sub begins to oscillate (open and close). In one embodiment, the onset of axial vibrations may be indicated when the axial variance at the resonance frequency (and above) of the BHA is 3 (or 10 or 20 or more) times greater than the axial variance at frequencies less than the resonance frequency.

[0046] In some embodiments, axial vibrations may be identified by increased variance in axial accelerations (e.g., an increase in the maximum minus the minimum acceleration in a predetermined time period). Variations in torque and rotation speed may also be indicative of the above described axial vibration modes.

[0047] FIGS. 5-11 depict example plots of dynamics data that may be used to identify the onset of deleterious axial vibrational states. Such measurements may be obtained, for example, at 124 while drilling and processed at 126 of method 120 (FIG. 4). The following examples were obtained during a subterranean coring operation and are intended to further illustrate the disclosed embodiments but, of course, should not be construed in any way as limiting the scope thereof. The depicted measurements were made using a Drilling Mechanics Module (the DMM or OptiDrill available from Schlumberger Technology Corporation) deployed below the bumper sub (e.g., as depicted on FIG. 2).

[0048] FIG. 5 depicts a plot of axial compressive load versus time showing the mean, maximum, and minimum loads. The load values were obtained by converting strain gauge measurements to axial load using mathematical techniques known to those of ordinary skill in the art. The mean, maximum, and minimum loads represent mean, maximum, and minimum values obtained over 2 second intervals during the coring operation. As depicted on FIG. 5, the mean, maximum, and minimum load values were essentially constant (having values of about 20,000 pounds of force) between 0 and 600 seconds. Between 600 and 1,000 seconds the mean load value remains substantially constant while the maximum and minimum load values diverge greatly. The sharp oscillations in the maximum and minimum load values beginning at about 600 seconds are believed to be the result of the bumper sub banging (as indicated). Moreover, the occurrence of negative minimum load values beginning at about 850 seconds are indicative of axial tension and are believed to result from bit bounce (severe axial vibrations that lift the coring bit off bottom).

[0049] FIG. 6 depicts a plot of axial compressive load versus time showing half the difference between the maximum and minimum loads (Max-Min)/2 and the square root of two times the standard deviation (STD) of the measured load values in each 2 second interval for the data depicted on FIG. 5. As depicted, these two measured load parameters give a similar response. The values remain small and nearly constant (having values between about 1000 and 2000 pounds of force) between 0 and 600 seconds. Oscillations (e.g., significant oscillations) are observed beginning at about 600 seconds (which are believed to be related to bumper sub banging). Moreover, these parameters appear to reach a saturation beginning at about 800 seconds at values that are indicative of bit bounce (as described above with respect to FIG. 5).

[0050] As also described above, the data plotted on FIGS. 5 and 6 indicate that deviations in the axial load may be indicative of potential or actual axial vibrations. For example, when one or more of the axial load deviations (e.g., Max-Min/2, Max-Mean, Mean-Min, or V2 STD) is greater than half the load on the bumper sub a potential for vibrations may be indicated (and a warning signal may be given). In some embodiments, one or more of the axial load deviations exceeding the load on the bumper sub may further indicate the onset of axial vibrations. One or more of the axial load deviations exceeding the weight-on-bit may be taken to indicate severe (and potentially damaging) axial vibrations.

[0051] Moreover, the minimum load values may be evaluated as an indicator of axial vibrations. For example, if the minimum load value is less than 2000 pounds greater than the net weight of the coring assembly components below the bumper sub, the potential for axial vibrations may be imminent. If the minimum load value is less than the net weight of the coring assembly components below the bumper sub, the onset of axial vibrations may be indicated.

[0052] FIG. 7 depicts a plot of full bandwidth load variance (standard deviation squared) and low frequency load variance (less than 20 Hz) for the data depicted on FIG. 5. The high frequency load variance (greater than 20 Hz) is the difference between the two plotted lines. As depicted the full bandwidth load variance and the low frequency load variance are essentially equal to one another between 0 and 600 seconds indicating that all of the axial energy (which is proportional to the load variance) is below 20 Hz (i.e., that the high frequency load variance is approximately zero). The full bandwidth load variance was observed to increase significantly beginning at about 600 seconds with the onset of the axial vibrations. The vibrations appear to be fully developed by 800 seconds at which point the high frequency load variance had at least 200 times more energy than the low-frequency load variance. This is believed to be indicative of severe axial vibrations at the resonance frequency of the BHA (and its harmonics). The axial resonance frequency for the BHA configuration was 24.5 Hz.

[0053] As described above, the onset of axial vibrations may be indicated when the full bandwidth load variance is three times the low frequency load variance (or equivalently when the high frequency load variance is twice the low frequency load variance). High (and potentially damaging) axial vibration levels may be indicated when full bandwidth load variance is twenty times the low frequency load variance (or when the high frequency load variance is twenty times the low frequency load variance).

[0054] FIG. 8 depicts a plot of axial acceleration versus time showing the mean, maximum, and minimum acceleration values for the same time interval as FIG. 5. The acceleration values were obtained using an axial accelerometer deployed in the DMM. The mean, maximum, and minimum accelerations represent mean, maximum, and minimum values obtained over 2 second intervals during the coring operation. The mean axial acceleration remains constant at about 1G over the entire interval (as would be expected in a vertical wellbore). The maximum and minimum values were within about 0.1G of the mean between 0 and 350 seconds and within about 0.3G of the mean between 350 and 600 seconds. Beginning at about 600 seconds the maximum and minimum acceleration values diverged significantly from the mean to values exceeding 1G deviation by about 700 seconds. The axial accelerations may therefore be thought of as a confirming indicator of the load variations depicted on FIGS. 5 and 6 in which a deviation of the acceleration (e.g., Max-Min/2, Max-Mean, Mean-Min, or V2 STD) of greater than about 0.5G (or 1G) may be indicative of potential (or likely) axial vibrations.

[0055] FIG. 9 depicts a plot of torque versus time showing the mean, maximum, and maximum torques for the same time interval as FIG. 5. The torque values were obtained by converting strain gauge measurements to torque using mathematical techniques known to those of ordinary skill in the art. The mean, maximum, and minimum torques represent mean, maximum, and minimum values obtained over 2 second intervals during the coring operation. As depicted on FIG. 9, the mean, maximum, and minimum torque values increased gradually between 0 and 500 seconds at which point they decreased significantly. The observed decrease was believed to be due to a lithology change in the wellbore. Beginning at about 600 seconds the maximum and minimum torque values began to spread significantly as indicated. The measured torque may also be used as a confirming indicator of the above described axial vibrations, for example, when the maximum torque minus the minimum torque is greater than the mean torque.

[0056] FIG. 10 depicts a plot of full bandwidth torque variance (standard deviation squared) and low frequency torque variance (less than 20 Hz) for the data depicted on FIG. 9. The high frequency torque variance (greater than 20 Hz) is the difference between the two plotted lines. As depicted, the full bandwidth torque variance and the low frequency torque variance are essentially equal to one another between 0 and 600 seconds indicating that all of the torsional energy (which is proportional to the torque variance) was below 20 Hz. The full bandwidth torque variance was observed to increase significantly beginning at about 600 seconds with the onset of the axial vibrations. The vibrations appear to be fully developed by 800 seconds at which point the high frequency torque variance had at least 20 times more energy than the low frequency torque variance. In this example, the onset of axial vibrations was indicated when the full bandwidth torque variance is at least three times the low frequency torque variance (or when the high frequency torque variance was is twice that of the low frequency torque variance). Additionally, potentially damaging axial vibrations were indicated when the full bandwidth torque variance was at least 10 times the low frequency torque variance.

[0057] FIG. 11 depicts a plot of full bandwidth rotation rate variance (standard deviation squared) and low frequency rotate rate variance (less than 20 Hz) for the same time period as FIGS. 5-10. The high frequency rotation rate variance (greater than 20 Hz) is the difference between the two plotted lines. As depicted, the full bandwidth rotation rate variance and the low frequency rotation rate variance are essentially equal to one another between 0 and 600 seconds. The full bandwidth rotation rate variance was observed to increase significantly beginning at about 600 seconds with the onset of the axial vibrations. The vibrations appear to be fully developed by 800 seconds at which point the high frequency torque variance had at least 20 times more energy than the low-frequency torque variance. While high levels of rotation rate variance can occur for other reasons (e.g., stick slip oscillations), this example indicates that rotation rate variance may also be indicative of axial vibrations. In this example, the onset of axial vibrations were indicated when the full bandwidth rotation rate variance was at least five times the low frequency rotation rate variance.

[0058] FIG. 12 depicts a plot of axial load versus time showing the mean, maximum, and minimum loads at the back end of axial vibration event. The mean, maximum, and minimum loads were obtained using the same methodology as described above with respect to FIG. 5. Severe axial vibrations were observed between 2000 and 2080 seconds as indicated, for example, by the negative minimum load values. The axial load was observed to dissipate between about 2100 and about 2400 seconds such that the spread between the maximum and minimum loads was relatively small (e.g., in this example less than about 5000 pounds force).

[0059] FIG. 13 depicts plots of (i) the maximum axial load minus the minimum axial load and (ii) the mean drill string rotation rate versus time at the back end of axial vibration event shown on FIG. 12. The maximum minus the minimum loads were obtained using the same data shown on FIG. 12. The drill string rotation rates were measured downhole using magnetometer measurements and techniques known to those of ordinary skill in the art. The mean drill string rotation rates were obtained over 2 second intervals during the coring operation. As depicted, the axial vibrations were mitigated by reducing the rotation rate of the drill string (from about 70 to about 55 rpm).

[0060] It will be understood that various actions may be taken to suppress or mitigate the above described axial vibrations. For example, the rotation rate of the drill string may be reduced as described above with respect to FIG. 13. The excitation energy causing the vibrations is believed to ultimately come from the interaction of the drill bit with the rock. Lowering the drill string rotation rate reduces the power available and thus a sufficient reduction in rotation rate may allow the vibrations to dissipate.

[0061] The axial vibrations may also be suppressed or mitigated by changing the weight-on-bit. For example, the weight on bit may be increased so that the amplitude of the vibration is not sufficient to open and close the bumper sub. Such a response may be most effective during the early stages of vibration when the vibration amplitude remains small. The weight on bit may also be decreased such that the bumper sub remains open (thereby eliminating the banging as the sub closes). Such a decrease may require the traveling block to be raised since the axial vibrations tend to significantly reduce the rate of penetration of drilling.

[0062] FIG. 14 depicts a flow chart of another disclosed method embodiment 140. As in method 100, a coring assembly is deployed in and rotated in a subterranean wellbore at 102. Dynamics measurements are made at 144 while drilling. The measurements include at least one of a torque and an axial displacement or acceleration. The measurements are processed at 146 to identify the onset core jamming in the inner barrel. Mitigating actions may be taken at 148 in response to the measurements and processing in 144 and 146.

[0063] When the core sticks or jams in the core barrel, the weight of the drill string tends to be supported largely (or at least partially) by the core, rather than by the drill bit. This tends to further cause a rapid decrease in the rate of penetration (as the actual weight on bit can become essentially zero). Such jamming can also cause the core to fracture and may ultimately require the coring assembly to be tripped out of the wellbore. [0064] The weight on bit may be adjusted in response to episodes of core jamming or sticking. In non-severe cases, temporarily increasing the weight on bit may enable a sticking/jamming core to move more freely in the core barrel. In the event of a fully jammed core, the weight on bit may be increased to activate shear pins in an assembly such as the Jambuster® (available from Baker Hughes).

[0065] FIGS. 15-20 depict example plots of dynamics data that may be used to identify the onset of core jamming while drilling. Such measurements may be obtained, for example, at 144 while drilling and processed at 146 of method 140 (FIG. 14). The following examples were obtained during a subterranean coring operation and are intended to further illustrate the disclosed embodiments but, of course, should not be construed in any way as limiting the scope thereof. The depicted measurements were made using a Drilling Mechanics Module (the DMM or OptiDrill available from Schlumberger Technology Corporation) deployed below the bumper sub (e.g., as depicted on FIG. 2).

[0066] FIG. 15 depicts a plot of mean torque versus time during a coring operation. The torque values were obtained by converting strain gauge measurements to torque using mathematical techniques known to those of ordinary skill in the art. The plotted torque values represent the mean values obtained over 2 second intervals during the coring operation. The measured mean torque values showed noticeable oscillation, however, were generally in a range from about 3000 to about 7000 foot pounds (e.g., between 5500 and 6400 seconds and between 7200 and 8000 seconds). FIG. 13 further shows periods of core jamming (or sticking) at which the measured mean torque is significantly reduced (e.g., to about 1000 foot pounds).

[0067] FIG. 16 depicts a plot of mean load versus time for the same time interval as FIG. 15. The mean load values were obtained as described above with respect to FIG. 5. As depicted the mean load values oscillate within a range from about 22,000 to about 26,000 pounds of force. There is little change in the mean load values with time during the core jamming events shown on FIG. 15.

[0068] FIG. 17 depicts a plot of the torque to weight (load) ratio for the same time interval as FIG. 15. The behavior of the ratio is similar to that of the torque (since the load is nearly constant as shown on FIG. 16) with the ratio also being reduced during periods of core jamming. For the depicted example, the coring bit had a radius of 6.25 inches (0.52 feet). The ratio of torque to load is a measure of distance, and may thus be compared to the bit radius. This example shows that there may be a danger of the core sticking or jamming when the plotted ratio is below 1/5 of the bit radius (in this case 0.1ft), and a likelihood of jamming when the ratio is below 1/10 of the bit radius (here 0.05 ft). [0069] FIG. 18 depicts a plot of the root mean square axial displacement versus time for the same time interval as FIG. 15. The root mean square axial displacements were obtained via integrating axial accelerometer measurements via techniques known to those of ordinary skill in the art. The depicted measurements were also band pass filtered to remove low frequency displacements (those less than 1 Hz). The high frequency displacement values were small and essentially constant (having a value of about 0.2 mm) during normal core drilling. During the periods of core jamming the high frequency (greater than 1 Hz) axial displacements increased significantly (e.g., to values greater than 1 mm in the depicted embodiment). An increase in axial displacement may be seen during core jamming since as there tends to be little or no load on the bit and it is therefore free to translate axially even when there are minimal axial vibrations. This example shows that displacements exceeding 0.4 mm (or 0.8 mm) may be indicative of potential or actual core jamming. Moreover, when the displacement exceeds three times the mean displacement level, there may be a danger or likelihood of core jamming.

[0070] It will be understood that while not depicted, axial velocity measurements may also be obtained by integrating the accelerometer data and may be used to identify incidents of core sticking or jamming. Similar criteria may be utilized to identify potential core jamming incidents (e.g., an axial velocity exceeding three times the mean velocity).

[0071] FIG. 19 depicts a plot of root mean square low frequency axial acceleration versus time for the same time interval as FIG. 15. The measured axial acceleration values were band pass filtered to remove accelerations outside a 0.05 to 2 Hz frequency band. The plotted acceleration values were small and essentially constant (having a value of about 2 mG) during normal core drilling. During the periods of core jamming (or sticking) the band pass filtered acceleration increased significantly (e.g., to values greater than 10 mG in the depicted embodiment). This example shows that low frequency accelerations exceeding 4 mG (or 8 mG) may be indicative of potential or actual core jamming. Moreover, when the low frequency acceleration exceeds three times the mean value, there may be a danger or likelihood of core jamming.

[0072] FIG. 20 depicts a plot of torque and low-frequency acceleration for the same time interval as FIG. 15 and therefore compares the data plotted on FIGS. 17 and 19. The core jamming incidents are clearly indicated in both the torque data and the low-frequency acceleration data. Note that both data streams indicate the onset of core jamming at about 6500 seconds and about 8000 seconds. The combination of torque and acceleration, velocity, and/or displacement may be used to prediction incidents of core jamming with increased accuracy. Thus, for instance, while the ratio of torque to load in FIG. 17 indicates that there was a danger of jamming at 5500 seconds, and probable jamming at about 6500 and 8000 seconds, the acceleration measurement shown on FIG. 19 may indicate that no jamming occurred at 5500 seconds. Moreover, the precise onset and cessation of jamming on the other two occasions was pinpointed precisely.

[0073] It will be understood that the example data plotted on FIG. 20 indicates that a combination of the torque measurements (or the torque to weight ratio) and the displacement measurements data may advantageously provide a strong indicator of potential, likely, or actual core jamming. For example, when the plotted ratio in FIG. 17 is below 1/5 of the bit radius (or 1/10 of the bit radius) and the axial displacements are greater than 4 mm (or 8mm) there may be high likelihood of core jamming.

[0074] It will be further understood that the criteria used to identify core jamming may also be used to identify a full core barrel. In other words, when the core fill the barrel and abuts the upper end thereof, the dynamics response may be similar to that of core jamming.

[0075] In certain embodiments it may be desirable to transmit at least a portion of the dynamics measurements to the surface while drilling. Numerous downhole telemetry techniques are known to those of ordinary skill in the art. For example, a wired drill pipe connection may provide a high bandwidth communication channel between the surface and the BHA. Such a communication channel may enable a high volume of sampled data to be sent to the surface such that the processing in 106, 126, and 146 (FIGS. 3, 4, and 14) may be conducted uphole (e.g., at a surface computer).

[0076] Other known telemetry techniques (e.g., electromagnetic telemetry or mud pulse telemetry) are generally significantly slower than wired drill pipe. When such techniques are utilized, the processing at 106, 126, and 146 (FIGS. 3, 4, and 14) may be advantageously performed downhole (e.g., using a downhole controller or processor). A portion of the processed data may then be transmitted to the surface. For example, maximum, minimum, and mean loads may be transmitted to the surface in any predetermined sampling period (e.g., 30-60 seconds). In some embodiments, the standard deviation of the axial load may be transmitted to the surface.

[0077] In other embodiments (especially for use when the telemetry rate is very low), the processed data may be reduced to a warning value indicative of the degree of axial vibrations or core jamming. For example, the warning value may include a two bit axial vibration parameter and a two bit core jamming parameter (each having four levels such as none, low, medium, and high) indicating the level of vibrations or jamming present. These parameters may be computed using the indicators described above. For example, a four level axial vibration parameter may be obtained from a deviation of the axial load as follows: Level 0: ALD < 0.5· BSC; Level 1: 0.5· BSC < ALD < BSC; Level 2: BSC < ALD < WOB; and Level 3 WOB < ALD (where ALD is the axial load deviation, BSC is the bumper sub compression, and WOB is weight on bit). Other warning parameters may be readily derived based on the data presented above and the corresponding identifying criteria discussed. In such embodiments the warning value may be periodically transmitted to the surface (or may be transmitted only when mitigation is required).

[0078] It will be understood that while not shown on FIGURES 1 and 2, that the disclosed coring assembly embodiments include an electronic controller. A suitable controller may be deployed, for example, in the MWD tool 70, the measurement sub 75, and/or elsewhere in the coring assembly. The controller may include one or more electronic processors (e.g., microprocessors) in communication with sensors in the measurement sub 75 and configured to receive and process the dynamics measurements. The controller may further include various signal processing circuitry, an A/D converter, processor readable memory, and/or a data storage device. The controller may also include processor-readable or computer-readable program code embodying logic, including instructions for processing the dynamics measurements, for example, for computing maximum, minimum, and mean measurement values, standard deviations, variances, and the like. The controller may also include hardware and/or software low pass, high pass, and/or band pass filters.

[0079] A suitable controller may further include a timer including, for example, an incrementing counter, a decrementing time-out counter, or a real-time clock. The controller may further include multiple data storage devices, various sensors, other controllable components, a power supply, and the like. The controller may also include conventional receiving electronics, for receiving signals from the various sensors deployed in the measurement sub 75. The controller may also optionally communicate with other instruments in the drill string, such as telemetry systems that communicate with the surface. Moreover, one skilled in the art will readily recognize that the multiple functions described above may be distributed among a number of electronic devices (controllers).

[0080] A downhole axial coring method and apparatus and certain advantages thereof have been described in detail. The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the disclosure. Those skilled in the art should appreciate that they may readily use the disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Additionally, it should be understood that references to "one embodiment" or "an embodiment" of the present disclosure are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. For example, features shown in individual embodiments referred to above may be used together in combinations other than those which have been shown and described specifically. Accordingly, any such modification is intended to be included within the scope of this disclosure. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not just structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke means-plus-function for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for' together with an associated function.