CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims benefit of U.S. provisional patent application Serial No. 62/207,244 filed August 19, 2015, and entitled "Riserless Drilling System with Concentric Drill Pipe," which is hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] Not applicable.

BACKGROUND

[0002] Well systems include a borehole or well extending into a subterranean, hydrocarbon bearing formation. The well of offshore well systems extends from a sea floor and may include a wellhead mounted at the surface of the subsea well for providing access to the well and for supporting equipment of the well system mounted thereto. Following the drilling and completion of the well (via a drilling and/or completion system) of the subsea well system, hydrocarbons may be produced from the well through a subsea tree mounted to the wellhead. Over time, the rate of hydrocarbons produced from the subsea well may diminish until it is no longer economically viable to maintain production from the well. In such an event, it may become necessary to cease producing from the well and abandon the well such that the well is sufficiently sealed from the surrounding environment. In some applications, the well is temporarily sealed or abandoned with a cement plug placed near the surface of the well to seal the well from the surrounding environment. In the case of temporarily abandoned wells, it may become necessary at a point in the future to permanently abandon the subsea well, a process sometimes referred to as converting a temporarily abandoned well to a permanently abandoned well, to ensure that the abandoned well does not begin to leak to the surrounding environment sometime in the future. In other applications, the active or producing well may be immediately permanently abandoned in lieu of temporarily abandoning the active well.

BRIEF SUMMARY OF THE DISCLOSURE

[0003] An embodiment of a riserless well abandonment system comprises a surface vessel, a first mast extending from the surface vessel, a second mast extending from the surface vessel, wherein the second mast is spaced from the first mast, a running tool extending from the first mast, wherein a subsea intervention device is coupled to the running tool, and a concentric drill string extending from the second mast, wherein the concentric drill string comprises an inner drill pipe disposed within an outer drill pipe. In some embodiments, the subsea intervention device comprises a ram blowout preventer configured to seal a bore of the subsea intervention device, an annular blowout preventer, and a rotating control device configured to seal against the concentric drill string when the drill string extends through the subsea intervention device and is rotating relative to the subsea intervention device. In some embodiments, the concentric drill string comprises an inner drill pipe disposed within an outer drill pipe. In certain embodiments, the subsea intervention device is coupled to a wellhead disposed on the sea floor. In certain embodiments, when the concentric drill pipe extends into a borehole extending from the wellhead, an inlet fluid flowpath is formed between an annulus disposed between the inner drill pipe and the outer drill pipe and the borehole, when the concentric drill pipe extends into the borehole, a return fluid flowpath is formed between the inner drill pipe and the borehole. In certain embodiments, the concentric drill string further comprises a crossover port to provide fluid communication between the borehole and the inner drill pipe. In some embodiments, the riserless well abandonment system further comprises a drill bit coupled to the concentric drill pipe, wherein fluid communication is provided between the inlet fluid flowpath and the borehole through the drill bit. In some embodiments, the riserless well abandonment system further comprises a top drive coupled to the second mast and configured to rotate the concentric drill string, and a swivel coupled to the second mast and configured to provide fluid communication between the inlet fluid flowpath and a surface system disposed on the surface vessel. In some embodiments, the riserless well abandonment system further comprises a casing string extending into the borehole from the wellhead, a perforating gun coupled to the concentric drill string, wherein the perforating gun is configured to form perforations in the casing string.

[0004] An embodiment of a method of permanently abandoning a well using a riserless well abandonment system comprises extending a subsea intervention device from a first mast disposed on a surface vessel, extending a drill string from a second mast disposed on the surface vessel and spaced from the first mast while the subsea intervention device is extended from the first mast, coupling the subsea intervention device to a wellhead disposed on the sea floor, extending the drill string through the subsea intervention device, and circulating cement through a borehole extending from the wellhead using the drill string. In some embodiments, the method further comprises sealing against the drill string with a rotating control device of the subsea intervention device as the drill string rotates relative to the subsea intervention device. In some embodiments, the method further comprises drilling through a cement plug disposed in the borehole using a drill bit coupled to the drill string. In certain embodiments, the method further comprises forming a first perforation in a casing disposed in the borehole using a first perforating gun coupled to the drill string, and forming a second perforation in the casing axially spaced from the first perforation using a second perforating gun coupled to the drill string. In certain embodiments, the method further comprises circulating fluid through an annulus formed around the casing using the drill string. In some embodiments, the method further comprises circulating fluid into the borehole through an inner drill pipe of the drill string, and circulating fluid from the borehole through an annulus formed between the inner drill pipe and an outer drill pipe of the drill string.

[0005] An embodiment of a method of permanently abandoning a well using a riserless well abandonment system comprises coupling a subsea intervention device to a wellhead disposed on the sea floor, extending a concentric drill string through the subsea intervention device, circulating fluid into a borehole extending from the wellhead through an inlet fluid flowpath extending through an inner drill pipe of the concentric drill string, circulating fluid from the borehole through a return fluid flowpath extending through an annulus formed between the inner drill pipe and an outer drill pipe of the concentric drill string, and circulating cement through the borehole using the concentric drill string. In some embodiments, the method further comprises extending the subsea intervention device from a first mast disposed on a surface vessel, and extending the concentric drill string from a second mast disposed on the surface vessel and spaced from the first mast while the subsea intervention device is being extended from the first mast. In some embodiments, the method further comprises forming a perforation in a casing disposed in the borehole using a perforating gun coupled to the concentric drill string, and circulating fluid into an annulus surrounding the casing through the perforation using the concentric drill string. In certain embodiments, the method further comprises pumping cement into the annulus using the concentric drill string. In certain embodiments, the method further comprises forming a first cement plug in a bore of the casing using the drill string, and forming a second cement plug in the bore of the casing using the drill string, wherein the second cement plug is axially spaced from the first cement plug.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] For a detailed description of the various exemplary embodiments disclosed herein, reference will now be made to the accompanying drawings in which: [0007] Figure 1 is a schematic view of an embodiment of a well system in accordance with principles disclosed herein;

[0008] Figure 2 is a schematic view of another embodiment of a well system shown in a first position in accordance with principles disclosed herein;

[0009] Figure 3 is a schematic view of the well system of Figure 2 shown in a second position;

[0010] Figure 4 is a schematic view of the well system of Figure 2 shown in a third position;

[0011] Figure 5 is a schematic view of the well system of Figure 2 shown in a fourth position;

[0012] Figure 6 is a schematic view of the well system of Figure 2 shown in a fifth position;

[0013] Figure 7 is a schematic view of the well system of Figure 2 shown in a sixth position;

[0014] Figure 8 is a schematic view of the well system of Figure 2 shown in a seventh position;

[0015] Figure 9 is a schematic view of the well system of Figure 2 shown in an eighth position;

[0016] Figure 10 is a schematic view of another embodiment of a well system shown in a first position in accordance with principles disclosed herein;

[0017] Figure 11 is a schematic view of the well system of Figure 10 shown in a second position;

[0018] Figure 12 is a schematic view of the well system of Figure 10 shown in a third position;

[0019] Figure 13 is a flow chart illustrating an embodiment of a method of permanently abandoning a well using a riserless well abandonment system in accordance with principles disclosed herein; and

[0020] Figure 14 is a flow chart illustrating another embodiment of a method of permanently abandoning a well using a riserless well abandonment system in accordance with principles disclosed herein.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0021] The drawing figures are not necessarily to scale. Certain features of the disclosure may be shown exaggerated in scale or in somewhat schematic form, and some details of conventional elements may not be shown, all in the interest of clarity and conciseness. 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 connection, or through an indirect connection via other devices and connections.

[0022] The following discussion is directed to various embodiments of the disclosure. One skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.

[0023] Referring to Figure 1, an embodiment of a well or drilling system 100 is shown schematically. Drilling system 100 comprises a riserless offshore drilling system, or in other words, an offshore drilling system configured to circulate drilling fluids to and from a borehole without needing a riser for conducting the drilling fluids. In the embodiment shown, drilling system 100 generally includes a surface system 102, a drill string 150, and a wellhead system 170. In some embodiments, the components of surface system 102 are disposed at a surface or waterline on a vessel, such as a semi-submersible drilling vessel or drill ship. In the embodiment shown in Figure 1, surface system 102 of drilling system 100 generally includes an inlet fluid conduit 104 for injecting or providing drilling fluids to a borehole 2 extending into a subterranean earthen formation 4 from a sea floor 6, and a return fluid conduit 130 for returning drilling fluids from the borehole 2. Return conduit 106 includes a choke manifold 108 for selectively applying a backpressure and controlling fluid flow through return conduit 106, a degasser 110 for removing gas from a fluid flow passing through conduit 106, and a flowmeter 112 for measuring the rate of fluid flow through conduit 106. Additionally, return conduit 106 includes a shale shaker 114 for removing cuttings and other debris from fluid flowing through return conduit 106 and a return fluid tank 116 for receiving recirculated drilling fluids from return conduit 106.

[0024] Surface system 102 of drilling system 100 additionally includes a drilling fluids or mud separator 118 configured to receive drilling fluids from return fluid tank 116 and separate the drilling fluids between a high density fluid outputted to a high density tank 120 and a low density fluid outputted to a low density tank 122. In some embodiments, the high density fluid received in high density tank 120 comprises high density drilling fluid or mud while the low density fluid received in low density tank 122 comprises sea water. A pair of high density pumps 124 are in fluid communication with high density tanks 120 and are configured to pump the high density fluid into drill string 150 via inlet conduit 104. Similarly, a pair of low density pumps 126 are in fluid communication with low density tanks 122 and are configured to pump the low density fluid into drill string 150 via inlet conduit 104. Further, surface system 102 includes a low density fluid conduit 128 and a low density valve 130 disposed between low density conduit 128 and inlet conduit 104 where low density valve 130 is configured to selectively adjust the density of fluid flowing through inlet conduit 104 by controlling the amount of low density fluid delivered to inlet conduit 104 from low density tank 122 via low density pumps 126. In this manner, the density of drilling fluids provided to drill string 150 may be adjusted as desired in light of drilling conditions.

[0025] Drill string 150 is configured to provide a conduit for the circulation of drilling fluids between the surface system 102 and the borehole 2. In the embodiment shown in Figure 1, drill string 150 comprises a plurality of threaded concentric drill pipe joints, and thus, includes an inner tubular member or drill pipe 152 and an outer tubular member or drill pipe 154, where inner drill pipe 152 extends within outer drill pipe 154. In some embodiments, drill string 150 comprises concentric drill pipe manufactured by ReelWell AS located at Dusavikveien 35, 4007 Stavanger, Norway. In this configuration, a return fluid flowpath 153 extends through the bore of inner drill pipe 152 for fluids returning to surface system 102 from borehole 2 and an inlet fluid flowpath 155 extends through an annul us formed between an inner surface of outer drill pipe 154 and an outer surface of inner drill pipe 152 for communicating drilling fluids from surface system 102 to borehole 2. In the embodiment shown in Figure 1, surface system 102 of drilling system 100 additionally includes a top drive 132 and a swivel 134. Top drive 132 provides for fluid communication between return flowpath 153 disposed in the inner drill pipe 152 of drill string 150 and the return conduit 106. Swivel 134 provides for fluid communication between inlet conduit 104 of surface system 102 and the inlet flowpath 155 disposed radially between the inner drill pipe 152 and outer drill pipe 154 of concentric drill string 150.

[0026] In the embodiment shown in Figure 1, a drill bit 136 is attached to a lower terminal end of drill string 150 for forming borehole 2. Drill bit 156 includes ports (not shown) disposed therein for providing fluid communication between inlet flowpath 155 and the borehole 2. Additionally, a crossover port 158 is disposed proximal drill bit 156 for providing fluid communication between borehole 2 and return flowpath 153. In this arrangement, drilling fluids are pumped into the inlet flowpath 155 of drill string 150 from inlet conduit 104 of surface system 102 via swivel 134, displaced along inlet flowpath 155 and expelled from drill string 150 and into the borehole 2 via the ports in drill bit 156, and then recirculated to return conduit 106 of surface system 102 via return flowpath 153 and top drive 132. Cuttings and other debris collected in borehole 2 may be removed from the recirculated drilling fluid via shale shaker 114 described above. Additionally, back pressure and fluid flow rate through return conduit 106 may be selectively managed via choke manifold 108. In this manner, drilling system 100 provides a closed loop drilling system encompassing conduits 104, 106, and flowpaths 153, 155, where fluid flow is provided by pumps 124 and/or 126.

[0027] Wellhead system 170 of drilling system 100 is configured to provide access to borehole 2 while sealing borehole 2 from the surrounding environment or sea 7. In the embodiment shown in Figure 1, wellhead system 170 generally includes a wellhead 172 and a well containment device 180. Wellhead 172 is disposed at the sea floor 6 and provides structural support for well containment device 180. Additionally, wellhead 172 supports a casing string 174 extending therefrom into borehole 2, which is cemented into place within borehole 2 via cement 176. Well containment device 180 is configured to seal borehole 2 from the sea 7. In this embodiment, well containment device 180 includes a rotating control device (RCD) 182 configured to seal against an outer surface of outer drill pipe 155 of drill string 150 as drill string 150 rotates in response to torque applied thereto by top drive 132 of surface system 102. However, in other embodiments, well containment device 180 may comprise a blowout preventer (BOP) stack including multiple BOPs, such as ram BOPs and annular BOPs. Additionally, in this embodiment, well containment device 180 includes a pair of manifolds or fluid lines 184 extending therefrom for selectively providing fluid communication to an inner bore 186 of well containment device 180. In some embodiments, manifolds 184 may be utilized for providing additional fluid flowpaths between surface system 102 and the borehole 2. For instance, manifolds 184 may be utilized as choke and/or kill lines for providing additional control in managing borehole 2.

[0028] Referring to Figures 2 and 3, another embodiment of a well system 200 is shown schematically. Well system 200 includes features in common with well system 100 shown in Figure 1, and shared features are numbered similarly. In the embodiment shown in Figures 2 and 3, well system 200 comprises a well abandonment system 200 generally configured for permanently abandoning a subsea well. Particularly, abandonment system 200 is configured to permanently abandon a subsea well without the need for running a marine riser, and thus, abandonment system 200 comprises a riserless well abandonment system in this embodiment. As will be explained further herein, the ability to abandon a subsea well without needing to run a marine riser may significantly decrease the amount of total time required for performing the abandonment operation, thereby significantly decreasing the overall costs incurred during the performance of the abandonment.

[0029] In the embodiment shown in Figures 2 and 3, well abandonment system 200 generally includes a surface vessel 202 comprising a first or main mast or rotary 204 and a second or auxiliary mast or rotary 206 laterally spaced from main mast 204. Although surface vessel 202 is shown in Figures 2 and 3 as comprising a drilling ship, in other embodiments, surface vessel 202 may comprise other types of surface vessels, such as a semi-submersible platform. The inclusion of primary and auxiliary masts 204 and 206 allows for the simultaneous running of multiple strings from surface vessel 202, thereby decreasing the amount of time required for performing the abandonment operation of borehole 2. Further, because well abandonment system 200 comprises a riserless well abandonment system, strings extending from masts 204 and 206 may be alternatingly inserted into borehole 2 without needing to decouple and retract a marine riser from wellhead 172, reducing the time required for performing the abandonment operation.

[0030] In the embodiment shown in Figures 2 and 3, primary mast 204 is configured to run subsea a well containment device or subsea intervention device (SID) 230, which is attached to a lower terminal end of a string or running tool 208 extending vertically from primary mast 204. Additionally, in the embodiment shown in Figures 2 and 3, auxiliary mast 206 is configured to run subsea the concentric drill string 150 discussed in relation to well system 100 shown in Figure 1, which includes drill bit 156 attached to a lower terminal end thereof. Surface system 102 is also provided on surface vessel 202 for assisting in the operation of concentric drill string 150, and thus, auxiliary mast 206 includes top drive 132 and swivel 134 of surface system 102. Although in this embodiment surface vessel 202 includes surface system 102, in other embodiments, surface vessel 202 may include other surface systems configured to operate with concentric drill string 150.

[0031] Wellhead 172 is positioned atop borehole 2 for providing access to borehole 2, similar to the configuration of wellhead system 170 of well system 100 shown in Figure 1. Additionally, in the embodiment shown in Figures 2 and 3, well abandonment system 200 includes a subsea connector 210 coupled to wellhead 172 for releasably coupling with SID 230. In this embodiment, subsea connector 210 comprises an H-4 subsea connector provided by GE Oil & Gas Company located at 7105 Business Park Dr. Houston, Texas 77041; however, in other embodiments, subsea connector 210 may comprise other subsea connectors known in the art. Additionally, in the embodiment shown in Figures 2 and 3, a plurality of casing strings extend into borehole 2 from wellhead 172, where each casing string is supported by a corresponding casing hanger secured within wellhead 172.

[0032] Particularly, borehole 2 includes a first or surface casing string 212, a second or first intermediate casing string 214, a third or second intermediate casing string 216, a fourth or third intermediate casing string 218 (shown in Figure 3), and a fifth or inner casing string 220. Unlike casing strings 212, 214, 216, and 220, third intermediate casing 218 includes a hanger that couples to an inner surface of second intermediate string 216 within borehole 2 and beneath wellhead 172. Further, casing strings 212, 214, 216, and 218 each terminate at a corresponding annular casing shoe 212s, 214s, 216s, and 218s at a lower terminal end thereof. Although in the embodiment shown in Figures 2 and 3 well abandonment system 200 includes casing strings 212-220, in other embodiments, well abandonment system 200 may include other configurations of casing strings. In still other embodiments, the borehole 2 of well abandonment system 200 may be uncased. In some embodiments, surface casing 212 has a diameter of approximately 36", first intermediate casing 214 has a diameter of approximately 28", second intermediate casing 216 has a diameter of approximately 22", third intermediate casing 218 has a diameter of approximately 16", and inner casing 220 has a diameter of approximately 13 5/8"; however, in other embodiments, the diameters of casing strings 212, 214, 216, 218, and 220 may vary.

[0033] In the arrangement shown in Figures 2 and 3, surface casing 212 is disposed radially outer casing strings 214, 216, and 220, and is disposed directly adjacent or physically engages an inner surface or wall 3 of borehole 2. First intermediate casing 214 is positioned radially within and adjacent to surface casing 212, forming a first annulus 213 between an outer surface of first intermediate casing 214 and both (moving axially) an inner surface of surface casing 212 and the inner surface 3 of borehole 2. Similarly, second intermediate casing 216 is positioned radially within and adjacent to first intermediate casing 214, forming a second annulus 215 between an outer surface of second intermediate casing 216 and both (moving axially) an inner surface of first intermediate casing 214 and the inner surface 3 of borehole 2. Further, inner casing 220 is positioned radially within and adjacent to second intermediate casing 216 and third intermediate casing string 218, forming a third annulus 217 between an outer surface of inner casing 218 and both (moving axially) an inner surface of second intermediate casing 216, an inner surface of third intermediate casing 218, and the inner surface 3 of borehole 2. Finally, third intermediate casing 218 is positioned radially within and adjacent to second intermediate casing 216, forming a fourth annulus 219 between an outer surface of third intermediate casing 218 and both (moving axially) the inner surface of second intermediate casing 216 and the inner surface 3 of borehole 2.

[0034] To secure casing strings 212, 214, 216, and 218 within borehole 2 and to restrict fluid communication between borehole 2 and the surrounding environment (e.g., the sea 7), cement 211 is positioned radially between the outer surface of each casing string 214, 216, 218, and 220 and the inner surface 3 of borehole 2. Additionally, cement 211 is also positioned within annuli 213, 215, 217, and 219 to secure intermediate casing strings 214, 216, 218, and 220. Further, in the embodiment shown in Figures 2 and 3, a surface cement plug or blockage 222 is formed within a bore 220B of inner casing 220 to restrict fluid flow between bore 220B and the surrounding environment. With cement 211 positioned between the inner surface 3 of borehole 2 and the outer surface of casing strings 214, 216, 218, and 220, and with surface plug 222 positioned within the bore 220B of inner casing 220 proximal wellhead 172, borehole 2 is disposed in a temporary abandoned conditioned. Thus, while borehole 2 is temporarily sealed from the surrounding environment, per applicable regulations, borehole 2 may need to be permanently abandoned to mitigate the possibility of the formation of a leak between borehole 2 and the surrounding environment.

[0035] SID 230 is configured to provide well control over borehole 2 during the performance of the abandonment operation, as will be described further herein. For instance, SID 230 is configured to seal borehole 2 from the surrounding environment and to prevent fluids within borehole 2 from escaping into the surrounding environment in the event of an uncontrolled influx of fluids from the formation 4 into the borehole 2 or from the release of trapped pressurized fluid within borehole 2. In some embodiments, SID 230 includes an 18 ¾" inner diameter and is configured to sustain 15,000 pounds per square inch (PSI) of internal pressure; however, in other embodiments, SID 230 may comprise varying sizes and pressure ratings. As described above, SID 230 is configured to land on and releasably couple with subsea connector 210. In the embodiment shown in Figures 2 and 3, SID 230 generally includes a pair of ram BOPs 232, a disconnect 234, an annular BOP 236, and an RCD 238. Additionally, SID 230 is coupled with an accumulator bank 240 configured to provide hydraulic power to ram BOPs 232 and annular BOP 236. In some embodiments, the components of SID 230 (e.g., ram BOPs 232, disconnect 234, and annular BOP 236, etc.) are actuatable via a remotely operated subsea vehicle (ROV). In other embodiments, the components of SID 230 are actuatable via an intervention workover control system (IWOCS). [0036] The ram BOPs 232 of SID 230 are configured to selectively actuate to seal a bore of SID 230. Particularly, ram BOPs 232 are configured to shear a tubular (e.g., drill string 150) disposed within SID 230 to thereby restrict any fluid communication therethrough. Annular BOP 236 of SID 230 is configured to selectively actuate and seal against an outer surface of a tubular disposed within the bore of SID 230. In this arrangement, fluid may still be communicated through a bore of the tubular while an annulus formed between an outer surface of the tubular and an inner surface of the SID 230 is sealed via the annular BOP 236. Disconnect 234 of SID 230 is generally configured to allow for the subsea disconnection of ram BOPs 232 from annular BOP 236 and RCD 238. In this manner, ram BOPs 232 may be left connected with wellhead 172 for management of borehole 2 while annular BOP 236 and RCD 238 may be retrieved to surface vessel 202, such as following the complement of the permanent abandonment of borehole 2. RCD 238 is configured to seal against an outer surface of a tubular extending through SID 230 as the tubular rotates and/or is displaced axially through SID 230. In this manner, borehole 2 may remain sealed from the surrounding environment even as a rotating tubular is extended into and/or out of the borehole 2.

[0037] Referring generally to Figures 2-8, well abandonment system 200 may be utilized to permanently abandon borehole 2. Particularly, in the embodiment shown in Figures 2-8, well system 200 may be utilized to "convert" borehole 2 from the temporarily abandoned or sealed condition shown in Figures 2 and 3 to a permanently abandoned condition illustrated in Figure 8. As shown particularly in Figure 2, to permanently abandon borehole 2 in this embodiment the SID 230 is run or extended towards wellhead 172 on running tool 208 using main mast 204 while simultaneously concentric drill string 150 and drill bit 156 coupled thereto are run or extended towards the sea floor 6 using auxiliary mast 206. By running SID 230 and concentric drill string 150 simultaneously via main mast 204 and auxiliary mast 206, the overall time to complete the permanent abandonment of borehole 2 is reduced as drill string 150 may be extended towards the sea floor 6 before SID 230 is landed on wellhead 172 and coupled thereon via subsea connector 210.

[0038] As shown particularly in Figure 3, once SID 230 is extended towards wellhead 172 with running tool 208 and using main mast 204, SID 230 is releasably connected or latched to subsea connector 210. In some embodiments, following the connection of SID 230 with subsea connector 210, the connection formed between SID 230 and subsea connector 210 is pressure tested. Additionally, in some embodiments, the individual ram BOPs 232 and annular BOP 236 are operated with either an ROV or IWOCS to confirm their functionality prior to proceeding further with the permanent abandonment of borehole 2. Once the connection between SID 230 and subsea connector 210 has been verified with a pressure test, and the individual BOPs of SID 230 have had their operability confirmed, then control over borehole 2 has been established via a successful connection of SID 230 with connector 210 and wellhead 172.

[0039] As shown particularly in Figure 4, following the connection of SID 230 with wellhead connector 210, concentric drill string 150 is positioned over and inserted into SID 230 until drill bit 156 coupled to the terminal end of drill string 150 contacts or "tags" surface cement plug 222. In some embodiments, once drill bit 156 has tagged surface cement plug 222, annular BOP 236 of SID 230 is actuated into a closed position sealing against an outer surface of the outer drill pipe 154 of concentric drill string 150, and SID 230 is pressure tested a second time with drill string 150 extending therethrough to ensure the seal provided by annular BOP 236 against concentric drill string 150. Following the pressure test of SID 230, drilling fluids begin to be circulated through concentric drill string 150 using surface system 102 in a manner similar to the circulation of drilling fluids through drill string 150 discussed above in the context of the well system 100 shown in Figure 1.

[0040] Particularly, after annular BOP 236 of SID 230 is actuated into an open position, drilling fluids are pumped from surface vessel 202 through the inlet fluid flowpath 155 extending through concentric drill string 150, and into bore 220B of inner casing string 220 via ports disposed in drill bit 156. The drilling fluids are then recirculated to surface vessel 202 from bore 220B of inner casing 220 via crossover port 158 and return flowpath 153 extending through drill string 150. Once drilling fluids begin circulating through drill string 150, drill string 150 is rotated via top drive 132 disposed on auxiliary mast 206 and extended or "stripped" further through SID 230 to allow drill bit 156 to drill through surface cement plug 222. Although in this embodiment drill string 150 is rotated via top drive 132, in other embodiments, drill string 150 may be coupled with a bottom hole assembly (BHA) including a mud motor for rotating drill bit 156 in response to the circulation of drilling fluids through concentric drill string 150.

[0041] As shown particularly in Figure 5, following the drilling out of surface cement plug 222, fluid pressure and gas content of bore 220B of inner casing 220 may be checked via the fluid communication established between bore 220B of inner casing 220 and surface system 102 with the circulation of drilling fluids therebetween. Following the performance of any measurement of fluid pressure or gas/fluid content within bore 220B of inner casing 220, concentric drill string 150 is further inserted into bore 220B of casing 220 while drilling fluids are circulated through bore 220B of inner casing 220 and concentric drill string 150. As discussed above, while drill string 150 is stripped through SID 230, RCD 238 acts to seal against the outer surface of drill string 150 to seal borehole 2 from the surrounding environment.

[0042] As drill string 150 is extended through bore 220B of inner casing 220, fluids previously trapped within inner casing 220 via the now-removed surface cement plug 222 are circulated out of inner casing 220 along a circulation flowpath 224 with the drilling fluids circulating through inner casing 220 and drill string 150. In this manner, any hydrocarbons or other fluids that have leaked into borehole 2 from formation 4 may be removed therefrom via the circulation flowpath 224 provided by concentric drill string 150. Moreover, due to the closed circulation fluid loop provided by concentric drill string 150, fluids disposed in borehole 2 may be circulated therefrom without needing to run a marine riser to wellhead 172 to provide for a recirculation flowpath, thereby reducing the overall time required for permanently abandoning borehole 2. In some embodiments, drill string 150 is extended substantially through the entire length of borehole 2 until drill bit 156 is positioned proximal a bottom or toe of the borehole 2, allowing for the circulation of borehole fluids along substantially the entire length of borehole 2. In other embodiments, drill string 150 is extended to a point where circulation flowpath 224 is positioned at or below a depth where inner casing 220 will be perforated, as will be discussed further herein. Additionally, in some embodiments, bore 220B of inner casing 220 is circulated with sea water from low density tank 122 (shown in Figure 1) of surface system 102 depending on the flow rate achievable while circulating low density fluids.

[0043] As shown particularly in Figure 6, once inner casing 220 and borehole 2 have been sufficiently circulated and washed with fluids from surface system 102 (sea water or high density fluids such as oil based muds, depending on the application), drill string 150 is pulled from borehole 2 and retracted to surface vessel 202. Following the retraction of concentric drill string 150 to surface vessel 202, a perforating tool or gun 252 is coupled to the lower terminal end of concentric drill string 150 and is run through SID 230 and into the bore 220B of inner casing 220 using auxiliary mast 206. Although in this embodiment drill string 150 is run into borehole 2 using auxiliary mast 206, in other embodiments, drill string 150 may be run or extended towards sea floor 6 using main mast 204 while concentric drill string 150 and corresponding drill bit 156 are simultaneously circulating or washing borehole 2, as described above. For instance, in applications where it is necessary to utilize main mast 204 for a purpose other than perforating casing strings of borehole 2, drill string 150 may be run into borehole 2 using auxiliary mast 206; however, in applications where main mast 204 is available for running drill string 150, main mast 204 may be used to run drill string 150 while borehole 2 is being washed with drill string 150.

[0044] In the embodiment shown in Figure 6, drill string 150 additionally includes an actuatable or settable packer 254 disposed proximal to but axially spaced from perforating gun 252. In this embodiment, following the insertion of drill string 150 (including gun 252 and packer 254) into the bore 220B of inner casing 220, perforating gun 252 is positioned above the shoe 218s of third intermediate casing string 218. In some embodiments, perforating gun 252 is positioned approximately 200' above shoe 218s; however, in other embodiments, the vertical distance or depth between perforating gun 252 and shoe 218s of third intermediate casing string 218 may vary. Once perforating gun 252 has been positioned above shoe 218s of third intermediate casing 218, packer 254 is "set" such that packer 254 sealingly engages the inner surface of inner casing string 220 and perforating gun 252 is actuated or fired to perforate inner casing 220 to provide for fluid communication between bore 220B of inner casing 220 and third annulus 217. Additionally, annular BOP 236 of SID 230 is closed to sealingly engage an outer surface of drill string 150.

[0045] As shown particularly in Figure 7, in some embodiments, following the perforation of inner casing 220 to form perforations 256, the fluid pressure within bore 220B of inner casing 220 below packer 254 is checked. Additionally, in some embodiments, an injectivity test is performed using drill string 150 to determine the treatment parameters and operating limits for performing a squeeze cementing operation within borehole 2. Particularly, following the performance of any pressure or injectivity testing, perforating gun 252 is released from drill string 150 and allowed to descend towards the bottom or toe of borehole 2. Additionally, as shown particularly in Figure 8, cement is pumped through drill string 150 from surface vessel 202 to squeeze at least a portion of third annulus 217 with pumped cement 211 via perforations 256. Particularly, in this embodiment, cement 211 is pumped to the portion of third annulus 217 formed between the outer surface of inner casing 220 and the inner surface of third intermediate casing 218. Additionally, cement is pumped into the portion of bore 220B of inner casing 220 below packer 254 such that a lower cement plug 260 is formed therein and positioned proximal the shoe 218s of third intermediate casing 218.

[0046] Following the squeezing of third annulus 217 with cement 211 and the formation of lower cement plug 260, the lower terminal end of concentric drill string 150 is released from packer 254 with packer 254 remaining in sealing engagement with the inner surface of inner casing string 220. Once the drill string 150 is released from packer 254, fluid pressure within the portion of bore 220B of inner string 220 above packer 254 may be measured. In some embodiments, a pressure test may be performed of the portion of bore 220B of inner casing 220 disposed above packer 254. Additionally, annular BOP 236 of SID 230 is opened to permit concentric drill string 150 to travel axially through SID 230. Following the performance of any pressure measurements or tests within bore 220B of inner casing string 220 and the opening of annular BOP 236, drill string 150 is retracted from the borehole 2 until the lower terminal end of drill string 150 is positioned at the appropriate depth for placing a surface cement plug, proximal an upper end of borehole 2. In some embodiments, the depth for placing the surface cement plug is approximately 200' from the sea floor 6; however, in other embodiments, the surface cement plug may be placed at varying depths from the sea floor 6. Once the lower terminal end of concentric drill string 150 is positioned at the selected surface cement plug setting depth, cement is circulated through drill string 150 and into the portion of bore 220B of inner casing string 220 positioned above packer 254. Cement is continuously circulated until the column height of cement positioned above packer 254 forms a balanced surface cement plug 262 extending vertically from packer 254.

[0047] Once the balanced surface cement plug 262 is set, the borehole 2 has been successfully permanently abandoned with third annulus 217 squeezed with cement 211, packer 254 sealingly engaging the inner surface of inner casing 220, lower cement plug 260 formed in bore 220B of inner casing 220 beneath packer 254, and balanced surface cement plug 262 formed in bore 220B of casing 220 above packer 254. As shown particularly in Figure 9, following the successful permanent abandonment of borehole 2, SID 230 may be retrieved from the wellhead 172. Particularly, concentric drill string 150 is removed from borehole 2 and SID 230 and running tool 208 is run into SID 230 from main mast 204. As running tool 208 is inserted into SID 230, concentric drill string 150 may be fully retracted to surface vessel 202 using auxiliary mast 206 to reduce the overall time required for retrieving SID 230 and retracting drill string 150. Once running tool 208 is inserted into SID 230, SID 230 may be unlatched from subsea connector 210 and retrieved from wellhead 172. In some embodiments, SID 230 is retried to surface vessel 202, while in other embodiments, SID 230 may be transported to another borehole for performing a permanent abandonment thereon.

[0048] Referring to Figures 10-12, another embodiment of a well system 300 is shown schematically. Well system 300 includes features in common with well system 200 shown in Figures 2-9, and shared features are numbered similarly. Similar to well system 200 described above, well system 300 comprises a well abandonment system 300 generally configured for permanently abandoning a subsea well. Particularly, well abandonment system 300 is configured to permanently abandon a subsea well without the need for running a marine riser, and thus, well abandonment system 300 comprises a riserless well abandonment system in this embodiment.

[0049] As shown particularly in Figure 10, in the embodiment shown in Figures 10-12, prior to perforating inner casing string 220, concentric drill string 150 is run into the borehole 2 with a first or lower perforating tool or gun 302, a second or upper perforating tool or gun 304, and settable packer 254 disposed axially between perforating guns 302 and 304. In this configuration, concentric drill string 150 is configured to form a plurality of axially spaced perforations in inner casing string 220. Particularly, concentric drill string 150 is run into borehole 2 until lower perforating gun 302 is positioned similarly within borehole 2 as perforating gun 252 of well abandonment system 200 shown in Figure 6. In this position, lower perforating gun 302 is axially aligned with the portion of third annulus 217 disposed between the outer surface of inner casing 200 and the inner surface of third intermediate casing 218. Additionally, in this position, upper perforating gun 304 is axially aligned with the portion of third annulus 217 disposed between the outer surface of inner casing 200 and the inner surface of second intermediate casing 216.

[0050] Following the axial positioning of concentric drill string 150 and perforating guns 302 and 304 described above, lower perforating gun 302 is actuated or fired to form lower perforations 306 which are positioned similarly in inner casing 220 as the perforations 256 formed in well abandonment system 200. Additionally, upper perforating gun 304 is actuated or fired to form upper perforations 308 in inner casing string 220, which are axially spaced from lower perforations 306. Once lower perforations 306 and upper perforations 308 are formed in inner casing string 220, and any pressure or injectivity tests of borehole 2 are performed, lower perforating gun 302 is released from the lower terminal end of concentric drill string 150 and drilling fluids, such as low density fluids including sea water, are circulated through drill string 150. As shown particularly in Figure 11, the formation of axially spaced perforations 306 and 308 allows for circulation of drilling fluids through third annulus 217 along an annulus circulation flowpath 310. The circulation of drilling fluids along annulus circulation flowpath 310 allows for the removal or circulation of hydrocarbons or other fluids previously trapped within third annulus 217 out of the borehole 2, which may be replaced with drilling fluids, such as sea water.

[0051] As shown particularly in Figures 11 and 12, once bore 220B of inner casing 220 and third annulus have been circulated with drilling fluids, cement is pumped through concentric drill string 150 to squeeze third annulus 217 with cement and to form lower cement plug 260 and surface cement plug 262 in the bore 220B of inner casing 220. Following the squeezing of third annulus 217 with cement 211 and the formation of cement plugs 260 and 262, the borehole 2 is successfully permanently abandoned, and SID 230 is retrieved from wellhead 172 using running tool 208 in a similar manner to the retrieval of SID 230 in wellhead abandonment system 200 shown in Figure 9.

[0052] Referring to Figure 13, a flowchart illustrating a method 400 of permanently abandoning a well using a riserless well abandonment system is shown. At block 402 of method 400, a subsea intervention device is extended from a first mast disposed on a surface vessel. In some embodiments, block 402 comprises extending SID 230 (shown in Figures 2- 9) from main mast 204 (shown in Figure 2) disposed on surface vessel 202 (shown in Figure 2). At block 404 of method 400, a drill string is extended from a second mast disposed on the surface vessel and spaced from the first mast while the subsea intervention device is extended from the first mast. In some embodiments, block 404 comprises extending concentric drill string 150 (shown in Figure 2) from auxiliary mast 206 (shown in Figure 2) disposed on surface vessel 202. At block 406 of method 400, the subsea intervention device is coupled to a wellhead disposed on the sea floor. In certain embodiments, block 406 comprises coupling SID 230 to wellhead 172 (shown in Figure 2) using subsea connector 210 (shown in Figure 2). At block 408 of method 400, the drill string is extended through the subsea intervention device. In some embodiments, block 408 comprises extending concentric drill string 150 through the SID 230. At block 410 of method 400, cement is circulated through a borehole extending from the wellhead using the drill string. In certain embodiments, block 410 comprises circulating cement 211 (shown in Figures 2-9) through borehole 2 (shown in Figures 2-9) extending from wellhead 172 using concentric drill string 150.

[0053] Referring to Figure 14, a flowchart illustrating a method 450 of permanently abandoning a well using a riserless well abandonment system is shown. At block 452 of method 450, a subsea intervention device is coupled to a wellhead disposed on the sea floor. In some embodiments, block 452 comprises coupling SID 230 (shown in Figure 2) to wellhead 172 (shown in Figure 2) disposed on sea floor 6 using subsea connector 210 (shown in Figure 2). At block 454 of method 400, a concentric drill string is extended through the subsea intervention device. In some embodiments, block 454 comprises extending concentric drill string 150 (shown in Figures 1 and 2) through SID 230. At block 456 of method 450, fluid is circulated into a borehole extending from the wellhead through an inlet fluid flowpath extending through an inner drill pipe of a concentric drill string. In some embodiments, block 456 comprises circulating fluid into borehole 2 (shown in Figures 2-9) extending from wellhead 172 through inlet fluid flowpath 155 (shown in Figure 1) extending through an annulus formed between inner drill pipe 152 and outer drill pipe 154 (shown in Figure 1) of concentric drill string 150. At block 458 of method 450, fluid is circulated from the borehole through a return fluid flowpath extending through an annulus formed between the inner drill pipe and an outer drill pipe of the concentric drill string. In certain embodiments, block 458 comprises circulating fluid from borehole 2 through return fluid flowpath 153 (shown in Figure 1) extending through inner drill pipe 152 (shown in Figures 1) of concentric drill string 150. At block 460 of method 450, cement is circulated through the borehole using the concentric drill string. In certain embodiments, block 460 comprises circulating cement 211 (shown in Figures 2-9) through borehole 2 using concentric drill string 150.

[0054] While exemplary embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teaching herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Furthermore, thought the openings in the plate carriers are shown as circles, they may include other shapes such as ovals or squares. Accordingly, it is intended that the following claims be interpreted to embrace all such variations and modifications.