Initial Results of Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Logging-While-Drilling Operations
Published:December 01, 2009
- PDF LinkChapter PDF
Boswell Ray, Timothy S. Collett, McConnell Dan, Frye Matthew, Shedd William, Godfriaux Paul, Dufrene Rebecca, Mrozewski Stefan, Guerin Gilles, Cook Ann, Shelander Dianna, Dai Jianchun, Jones Emrys, 2009. "Initial Results of Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Logging-While-Drilling Operations", Unconventional Energy Resources: Making the Unconventional Conventional, Tim Carr, Tony D’Agostino, William Ambrose, Jack Pashin, Norman C. Rosen
Download citation file:
The Gulf of Mexico gas hydrates Joint Industry Project (the JIP), a cooperative research program between the US Department of Energy and an international industrial consortium under the leadership of Chevron, conducted its “Leg II” logging-while-drilling operations in April and May of 2009. JIP Leg II was intended to expand the existing knowledge base on gas hydrates in the Gulf of Mexico to include the evaluation of gas hydrate occurrence in sand reservoirs. The selection of the locations for the JIP Leg II drilling was the result of a geological and geophysical prospecting approach that integrated direct geophysical evidence of gas hydrate-bearing strata with evidence of gas sourcing, gas migration, and occurrence of sand reservoirs within the gas hydrate stability zone. Logging-while-drilling operations for JIP Leg II included the drilling of seven wells at three sites. Despite drilling the deepest and most technically challenging well yet attempted in a marine gas hydrate program, the expedition was on time, under budget, and met all its scientific objectives. Minimal operational problems were encountered with the advanced LWD tool string, and the continual refinement of drilling parameters enabled the successful management of a range of shallow drilling issues, including borehole breakouts and shallow gas and water flows. Two wells drilled in Walker Ridge Block 313 (WR 313) confirmed the pre-drill predictions by discovering gas hydrates at high saturations in multiple sand horizons having reservoir thicknesses up to 50 ft. In addition, drilling in WR 313 discovered an unpredicted, thick, strata-bound interval of shallow finegrained sediments having abundant gas-hydrate-filled fractures. Two of three wells drilled in Green Canyon Block 955 (GC 955) confirmed the pre-drill prediction of extensive sand occurrence having gas hydrate fill along the crest of a structure associated with positive indications of gas source and migration. Well GC955-H discovered ~100 ft of gas hydrate in sand at high saturations. Two wells drilled in Alaminos Canyon Block 21 (AC 21) confirmed the pre-drill prediction of potential extensive occurrence of gas hydrates in shallow sand reservoirs at low saturations.
Further data collection and analyses at AC 21 will be needed to better understand the nature of the pore filling material. The JIP plans to use the results of Leg II to plan Leg III drilling and coring operations anticipated to occur in 2010.
The Gulf of Mexico Gas Hydrate Joint Industry Project (“The JIP”) is a cooperative research program between the U.S. Department of Energy and a consortium of U.S. and international energy industry companies under the management of Chevron. Partners in the JIP include ConocoPhillips; Japan Oil, Gas, and Metals National Corporation; StatoilHydro; Schlumberger; Halliburton; Total; Korea National Oil Company; Reliance Industries; and the Minerals Management Service (MMS). The project was initiated in 2001 to investigate the occurrence, nature, and implications of gas hydrate in the Gulf of Mexico. In 2005, the JIP completed Leg I drilling, logging, and coring operations designed primarily to assess gas hydrate-related hazards associated with drilling through the clay-dominated sediments that typify the shallow sub-seafloor in the deep water Gulf of Mexico (Ruppel et al., 2008).
In order to properly characterize the nature and implications of gas hydrates in the Gulf of Mexico, the JIP has supported a number of critical fundamental science and technology development efforts, including development of improved tools and techniques for remote sensing (Xu et al., 2004; Dai et al., 2008), well-bore stability modeling (Birchwood et al., 2007), and field sample analysis (Yun et al., 2006). The JIP has provided to the science community the first public data on gas hydrate-bearing sand reservoirs in the Gulf of Mexico (Smith et al., 2006; Latham et al., 2008; Boswell et al., 2009) and has contributed extensive experimental datasets related to the impact of gas hydrate on the physical properties of sediments of various grain sizes (see Santamarina and Ruppel, 2008).
Upon analysis of Leg I results, the JIP determined that it had adequately addressed its concerns regarding drilling safety issues related to gas hydrate at the moderate to low concentrations that typify occurrences in fine-grained sediments (Birchwood et al., 2008). Therefore, the JIP and the DOE decided to expand its effort to assess a range of issues related to the occurrence of gas hydrate within sand reservoirs (Jones et al., 2008). To enable this work, geoscientists from the U.S. Geological Survey (USGS), the Department of Energy’s National Energy Technology Laboratory (NETL), the MMS, AOA Geophysics, the Naval Research Lab, and Rice University collaborated to evaluate and prioritize various prospects with respect to the potential of encountering high concentrations of gas hydrate in sand reservoirs (Hutchinson et al., 2008). The group evaluated these sites through integrated geological and geophysical analyses and ultimately developed the site descriptions and prioritizations that were implemented in JIP Leg II (Hutchinson et al., 2009a, 2009b, 2009c; Shedd et al., 2009b).
JIP Leg II Site Selection
In 2006, the JIP and its collaborators began detailed geologic and geophysical evaluations of numerous potential drill sites in the Gulf of Mexico, seeking evidence for active petroleum systems (gas sources and migration pathways) co-located with sand-prone lithofacies. An initial primary target was provided by Chevron through public release of well and seismic data around the “Tigershark” well in Alaminos Canyon (AC) Block 818 (Smith et al., 2006). Subsequently, a review of gas hydrate indicators within existing industry well log data throughout the Gulf of Mexico conducted by the MMS yielded evidence of extensive sand occurrence within the shallow sediments but provided limited additional compelling evidence of gas hydrate-bearing sands in the available downhole log datasets. Therefore, in 2007, the JIP conducted an open workshop to identify additional drilling opportunities, in which locations in Walker Ridge (WR) Block 313 and Green Canyon (GC) Block 955 were brought forward by JIP contributor AOA Geophysics (Fig. 1).
By 2008, the JIP and its collaborators had compiled for the AC 818, GC 955, and WR 313 sites the following: (1) geologic interpretations and prioritized drilling targets from the site selection group coordinated by the USGS (Hutchinson et al., 2008); (2) pre-stack, full-waveform 3-D inversions for gas hydrate saturation from WesternGeco; (3) drilling hazards assessments from AOA Geophysics; and (4) borehole stability models and operational recommendations from Schlumberger Geomechanics. Analysis of “Tigershark” well drilling data revealed evidence of formation overpressures at AC 818, and the site was dropped from further consideration for the Leg II drilling program. Leg II logging-while-drilling (LWD) operations were slated for the spring of 2008, but the JIP and the DOE elected to postpone operations when delivery of the contracted drill rig was delayed until after the start of the 2008 hurricane season. Plans were then made to conduct Leg II in the spring of 2009.
The JIP used the additional time to continue site evaluation activities through the rest of 2008 and early 2009. This effort benefitted greatly from continuing work within the MMS, which revealed additional drilling opportunities at sites in East Breaks (EB) 922, GC 781/825, and AC 21/65 (Fig. 1). Hazards analysis and permitting activities were then begun for these sites late in 2008. However, as the date of the expedition approached, it was clear that permissions to occupy the GC 781/825 site would not be gained from companies operating in nearby facilities. As a result, the site was dropped from the drilling plans early in 2009. Ultimately, the JIP, with the support of AOA Geophysics, conducted hazard analyses and obtained permits for more than 20 locations in the WR 313, GC 955, AC 21/65, and EB 992 sites.
JIP Leg II Operations
JIP Leg II was conducted from April 16 to May 7, 2009 aboard the Dynamically-Positioned (DP) Modular Drilling Unit (MODU) Q4000 owned and operated by Helix, Inc. On-board science operations were managed and conducted by project participants from the USGS, NETL, MMS, AOA Geophysics, and Lamont-Doherty Earth Observatory (LDEO) of Columbia University. Baker-Hughes mud engineering expertise also played a critical role in project operations. The selection and sequence of drilling locations from among the 20 permitted locations was determined during the course of the expedition through discussion by the science team and Chevron management onshore, as where drilling depth and drilling/logging parameters (see Collett et al., 2009). Selected drill locations were modified up to 500 ft from permitted locations per MMS regulations based on insights from previous drilling during the expedition. Figure 2 is a schematic illustrating the nature of JIP sites, locations, and targets.
JIP Leg II focused exclusively on the collection of a comprehensive suite of LWD data. The bottom-hole-assembly featured a state-of-the-art tool configuration that provided the most detailed log data yet acquired in a marine gas hydrate project. Tools employed included Schlumberger’s GeoVision (resistivity and gamma-ray), EcoScope (resistivity and neutron and density porosity), TeleScope (Measurement-while-drilling [MWD] data), SonicVision (P-wave acoustic velocity) as well as the more recently-developed PeriScope (full azimuthal resistivity) and MP3 (azimuthal P- and S-wave acoustic velocity). These tools provided full 3-D information on both acoustic and electrical properties of the sediment enabling the improved evaluation of gas hydrate in both pore-filling and fracture-filling modes. The LWD and associated MWD tools also provided “real-time” drilling performance data that allowed us to optimize the drilling plan as we advanced through the program. LWD logging methods, operations, and results are summarized by Mrozewski (2009), Guerin et al. (2009a, 2009b), and Cook et al. (2009). The expedition acquired LWD data from 15,380 ft of sedimentary section in seven holes drilled at the WR 313, GC 95, and AC 21 sites. The performance of the Q4000 crew in safely and efficiently drilling the wells was outstanding. Similarly, the complex, state-of-the-art Schlumberger LWD tool string functioned extremely well, having only minimal operational issues and no lost-time incidents.
JIP Leg II began at 15:30 on April 16, 2009, when the Q4000 completed work for another client at Green Canyon Block 195. The vessel sailed to its first location in Walker Ridge Block 313, and the WR 313-G well was spudded at 17:30 April 18. The well reached a total depth of 10,200 ft (RKB) at 19:00 April 20. Initial plans to then move in “dynamic positioning (DP) mode” (a short intra-site move taken between locations with the drill string lifted approximately 500 above the sea-floor) to a second Walker Ridge site were changed in order to assess the condition of the LWD string and to address several potential tool data quality issues (Mrozewski et al., 2009; Guerin et al., 2009a; 2009b; Cook et al., 2009). The ship sailed to Green Canyon H Block 955 and drilled three holes (GC 955-I, GC 955-H, and GC 955-Q) from 11:30 April 22 to 20:00 April 28. The ship then returned to Walker Ridge and drilled the WR 313-H well, completing operations at 11:00 May 1. A 175 nautical mile transit was then made to Alaminos Canyon Block 21, where the AC 21-A and AC2 1-B wells were drilled. Planned operations in EB 992, where the JIP had permitted four sites, were complicated by the arrival of the rig Ocean Valiant in northeastern AC 24 on May 2 to conduct development operations on behalf of ExxonMobil. ExxonMobil representatives were extremely supportive of the JIP project, and gave permission for two locations (EB 992-A and EB 992-C) to be drilled. However, based on drilling results from AC 21-A and AC 21-B, the science team determined that further drilling was not cost-effective, and at midnight on May 4/5 drilling operations were ended. The Q4000 was then demobilized at sea with the assistance of the MV Mia, and Leg 2 ended at approximately noon on May 6, 2009.
Drilling operations within JIP Leg II were marked by the constant challenge of optimizing data quality while maintaining borehole stability. In addition, several of the targets were exceptionally deep: the two wells drilled in Walker Ridge 313 (at more than 3,000 feet below the seafloor) exceed by more than 1,000 ft the previous record for the deepest gas hydrate research wells (NGHP Expedition-01 Site 17, Andaman Islands; see Collett et al., 2008). The process of drilling the JIP Leg II wells provided new insights into the optimal drilling strategies for marine open-hole drilling programs (Collett et al., 2009). Most notably, original plans to drill these deep holes using minimal mud were revised due to difficulties with borehole stability issues observed in the first well drilled (WR 313-G). Despite the large volumes of gas hydrate that the expedition encountered, it was apparent that the primary drilling hazards that needed to be managed during the JIP Leg II program were not gas hydrate related, but were instead the common problems that face all drilling programs: borehole stability, drill cutting removal, gas releases into the borehole, and water flows. These issues are particularly acute in a shallow scientific drilling and coring data in which the wells are drilled “open-hole” without surface conductors or drilling mud returns. Additional experience was also gained relative to the expected response of thick gas hydrate bearing units to drilling, providing further opportunities to improve future gas hydrate drilling protocols (Collett, et al., 2009).
Walker Ridge 313
The WR 313 drill site lies in ∼6,500 ft of water within the “Terrebonne” mini-basin in the northern Gulf of Mexico. The basin is elongated from north to south and has a central salt-cored ridge that splits the basin into eastern and western halves. The western basin has been intermittently bounded by structural highs on the west, south, and east, resulting in a reversal of gradient for channelized/turbidity flows entering from the north. During periods of relative uplift of these bounding ridges, coarser sediment delivery systems experience reduced-to-reversed gradients, resulting in diminishment of channelized facies and the deposition of “ponded” sheet sands within the mini-basin. The ongoing uplift of the margins has continued to deform the sedimentary section through recent time, and much of the strata exhibit steep dips from the basin flanks into the basin center. An existing industry well (the WR 313 #1), located high on the eastern margin of the western Terrebonne mini-basin, shows signs of elevated resistivity throughout the well, but log data was often poor, and no sands of seismically resolvable thickness were apparent.
The primary attribute of the WR 313 site suggesting that the site was prospective for gas hydrate in sands were a series of anomalous seismic responses that aligned with the inferred base of gas hydrate stability (see McConnell and Kendall, 2000; Shedd et al., 2009a; Hutchinson et al., 2009a). These seismic events, when traced downdip to the west, switch seismic “polarity” from a strong positive response to a strong negative response at a common horizon that crosscuts stratigraphy (McConnell and Zhang, 2005).
This configuration of seismic responses was interpreted to indicate free-gas accumulations (the negative anomalies) being trapped within porous and permeable sand horizons by significant accumulations of overlying gas hydrate within the sediment pore space. Detailed mapping of the area conducted by the MMS revealed several such prospective seismic horizons, providing numerous potential drilling opportunities at multiple stratigraphic levels. Pre-drill estimates of gas hydrate saturation conducted by WesternGeco indicated high saturations within several of these units (Fig. 3). In addition to testing the hypothesis linking these phase reversals to gas-hydrate-filled sands, the site offered the opportunity to drill multiple targets at a single location, as well as to drill a single horizons at multiple locations to test the lateral heterogeneity of gas hydrate and to determine the controls on degree of reservoir fill above the base of gas hydrate stability zone.
Two wells were drilled at the WR 313 Site (see McConnell et al., 2009a: Cook et al., 2009). The first (WR 313-G) targeted the most areally-extensive amplitude anomaly in a location near the base of gas hydrate stability zone. The second (WR313-H) was drilled approximately roughly 1 nautical mile updip to the east and tested the primary G-well target horizon near the updip termination of anomalous amplitudes. The H-well also provided a test of a second amplitude anomaly near the base of gas hydrate stability zone, as well as an opportunity to test a third stratigraphic unit in a location below the stability zone (Fig. 4).
WR 313-G well
The WR 313-G well was drilled in 6562 feet of water. While drilling the primarily muddy sediments above the primary target at the WR 313-G well, a zone of elevated resistivity (from 4 to 10 ohm-m) was encountered through a thick interval from 7,458 ft to 7,850 ft below rig floor. Initial interpretation is that this zone marks a stratal-bound interval of clay-dominated sediments having fracture-filling gas hydrate. The sedimentary section logged from 7,840 to 9,400 ft below the rig floor included numerous porous and resistive sands and silts up to 10 feet thick within a predominantly fine-grained section. As drilling proceeded, the lack of use of heavy drilling fluids and slow penetration rates (both designed intentionally to maximize the quality of the data recorded by the logging tools) made it difficult to remove cuttings or well-bore cavings from around the drill string. At a depth of about 9,600 ft below the rig floor, a decision was made to continuously pump a 10.5 ppg drill mud which improved the hole cleaning capabilities and the well could be safely advanced with minimum hole stability problems. The main target was encountered as expected at 9,412 ft below rig floor and included a net of ∼30 ft of sand containing gas hydrate at high saturations within a total 70-ft gross interval (Fig. 5).
WR 313-H well
The WR 313-H well was located ∼1 nautical mile to the east of the -G well in 6450 feet of water. The shallow, fracture-filling gas hydrate occurrence was again observed at ∼7050 to 7400 ft below rig floor. As with the G-well, the underlying sediments contained interbedded muds and thin sands, and virtually every unit indicated gas-hydrate fill. Sediments interpreted to be correlative with the G-well primary target were reached at 8,755 ft below rig floor and had graded into a sand-poor interval having reduced porosity but still saturated with gas hydrate. The top of the primary H-well target was logged at 9,096 ft below rig floor (Fig. 6). This unit consisted of two lobes of very clean sand, each with sharp basal and upper contacts. Resistivity in the upper (15 ft-thick) lobe was very high (∼30 to 300 ohm-m). The lower lobe (21 ft-thick) was less resistive (∼3 to 30 ohm-m). Drilling continued below the inferred base of gas hydrate stability zone, penetrating the third target at 9,624 ft below rig floor, which consisted of a series of 10-15 foot thick, highly-conductive sands.
Green Canyon 955
The gas hydrates JIP site selection team has identified numerous potential targets in Green Canyon Block 955. The site is located in over 6,500 ft of water just seaward of a major embayment in the Sigsbee Escarpment (“Green Canyon”) which appears to have served as a persistent focal point for sediment delivery into the deep Gulf of Mexico. The area is traversed by a prominent and long-lived channel/levee complex that has transported and deposited large volumes of sandy sediment from Green Canyon to the deep Gulf of Mexico abyssal plain (McConnell, 2009b). The southwest corner of Block 955 includes a recently formed structural high caused by deeper mobilization of salt. The crest of the structural high is cut by complex network of faults that can provide potential pathways for migrating fluids and gases (McConnell, 2000; Heggeland, 2004). Geophysical data reviewed during assessment of the site indicates a complex set of geophysical responses near the inferred base of gas hydrate stability (Fig. 7). Some of these responses are suggestive of free gas and some indicative of gas hydrate, but all are limited to depths that are near or below the inferred base of the gas hydrate stability zone. The pre-drill predictions of gas hydrate saturations developed by WesternGeco are shown in Figure 8.
The Green Canyon site combined many of the features required for the formation of significant gas hydrate accumulations, including sources of gas, pathways for gas to migrate into the gas hydrate stability zone, and porous sands within the stability zone in which gas hydrate can accumulate. A motivation behind the JIP’s selection of this site was to test the hypothesis that gas hydrate accumulations within sands at the base of gas hydrate stability restrict the vertical migration of gas into shallower units within the structure. In addition, the drilling was designed to test the hypothesis that gas hydrate could exist in the seismically-muted section just above the inferred base of gas hydrate stability zone. One possibility was that gas hydrates concentrated within sands occur in close association with faults or fractures in orientations not readily recorded in the seismic data Another hypothesis was that the gas hydrate saturation at the top of the seismically inferred sand package was broadly gradational, serving to mute the seismic response of the in-situ gas hydrate. Three wells, the GC 955-I, -H, and -Q wells, were drilled in Green Canyon from April 22 to April 28. For more detail on these wells, see McConnell et al. (2009b) and Guerin et al. (2009a).
GC 955-I well
GC 955-I was drilled in 6770 feet of water in a position very close to a prominent, late-stage channel axis (to maximize the occurrence of sand reservoirs) in a location with relatively muted geophysical indications (i.e., seismic amplitudes) of gas hydrate. The well encountered more than 300 ft of porous sands as predicted; however the sands contained primarily water and only modest indications of gas hydrate. The well also flowed water (presumably from within these thick, porous sand zones), requiring roughly a day of effort to control the flow. A summary log display of the GC 955-I well is provided in Figure 9. For more detail, see McConnell et al. (2009b) and Guerin et al. (2009a).
GC 955-H well
GC 955-H was drilled in 6670 feet of water about 1 mile southwest of the I-location in a structurally higher position on the domal structure. The well targeted strong geophysical anomalies associated with features suggestive of gas hydrate at a projected depth of 8,025 ft below rig floor. While drilling the shallow section, a thick zone of gas hydrate-filled fractures in mud-rich sediments was observed from ∼7250 to 7670 ft below rig floor. At 7,795 ft below rig floor, the well encountered the top of a thick gas-hydrate-bearing sand interval. Three gas-hydrate-bearing zones of 88 ft, 13 ft, and 3 ft thick, separately and underlain by water-saturated intervals, were logged within a single contiguous sand body (Fig. 10). Log quality within the sand is highly erratic, with resistive zones displaying almost perfectly in-gage holes, and water-bearing zones being significantly washed out.
GC 955-Q well
Based on the results of the GC955-H well, the science team and the JIP elected to drill the GC 955-Q well (water depth of 6,516 feet) in a separate fault block in a structurally higher position, potentially placing the sand reservoir higher into the gas hydrate stability zone. On seismic data, this location exhibited a thick sequence of high-amplitude geophysical responses that had been assessed a “high” risk of free gas in pre-expedition hazards analysis (McConnell et al., 2009b); however, the science team determined that this risk had been sufficiently mitigated by the lack of significant free gas observed below hydrate in the -H well and the mud handling capabilities of the Q4000. No indications of shallow, fracture-filling gas hydrate were observed. At a depth of 7,921 ft below rig floor, the well encountered gas hydrate-bearing sand, which continued to a depth of at least 7,974 ft below rig floor (the deepest data point provided by the MP-3 acoustic tool, which was located 35 ft above the bit [Fig. 11]). At a depth of 8,014 ft below rig floor, drilling was halted when a single gas release from the well was visually observed by the Q4000’s remotely operated vehicle (ROV). The LWD assembly was removed, and the well re-entered and cemented.
Alaminos Canyon 21/65
Proposed JIP Leg II drilling sites in AC 21/65 lie within the Diana sub-basin and target anomalous seismic reflections that occur approximately 600 feet below the seafloor and 800 feet above the inferred base of gas hydrate stability. A related site, East Breaks 955, located about 5 miles to the east, provided additional opportunities to test slightly different settings within a genetically related prospect. Two primary features drive the prospectivity of these sites. First, an existing (1995) industry well (the EB 992 #001 “Rockefeller” well) logged a thick and slightly resistive sand at shallow depths (Shedd et al., 2009b; Frye et al., 2009). Log analysis of the LWD resistivity indicated potential gas hydrate saturation ranging from 20 to 40%, but with some uncertainty due to the potential poor quality of the log data. Second, seismic data showed strong seismic reflectors at both the top and base of this seismic inferred sand and again, polarity and amplitude consistent with low to moderate gas hydrate saturation. However, this interpretation was uncertain due largely to the limited data on the acoustic nature of shallow, unconsolidated, high-porosity sediments.
No pre-drill seismic analyses for gas hydrate saturation has been conducted for these sites; however, preliminary seismic mapping of this sand body conducted by the MMS clearly delineates its extent, and shows wide occurrence through a large portion of the south Diana sub-basin Throughout the area, the geophysical expression of the target interval is consistent, but there is only limited evidence of gas sources or only minor variation in seismic amplitudes that might reflect local variations in gas hydrate saturation. The sediments are also very young, being no more than 440,000 years in age (Frye et al., 2009).
The primary hypotheses to be tested at the site included: (1) the opportunity to test log and seismic interpretations of low- to moderate-saturation gas hydrates in reservoir-quality sands – an occurrence that has rarely been observed; and (2) the potential that the 1995-vintage LWD resistivity data were degraded, due to poor logging conditions (typical of the shallow portions of large-diameter deep boreholes) or perhaps complex reservoir architecture (such as very-thinly interbedded sands and shales), resulting in a composite low-resistivity “pay”. Two wells were drilled through the prospective shallow sand facies in AC 21 (Fig. 12). A third permitted well in AC65 was not drilled, and none of the permitted wells in EB 992, were drilled. For additional detail on the scientific results of the JIP Leg II LWD operations at the AC 21 site, see Frye et al. (2009) and Guerin et al. (2009b).
AC 21-A well
The AC 21-A well was drilled in 4,889 feet of water and featured a complex geophysical response (including a series of four seismic events of higher magnitude) not commonly seen elsewhere in AC 21/65 and EB 992 sites. As expected, the well encountered two sands (at 5,429 and 5,459 ft below rig floor) separated by a 15-ft shale. The resistivity in these sands was consistently ∼2 ohm-m (Fig. 13), only slightly more resistive than the bounding shales (1.5 ohm-m). No clearly water saturated sands were observed in the well. A zone of slightly increased resistivity (up to 2.5 ohm-m) within fine-grained sediments was logged from 6,193 to 6,452 ft below rig floor, and may be associated with the base of gas hydrate stability zone in the area.
AC 21-B well
The AC 21-B well was drilled in 4,883 feet of water approximately 1.2 nm to the north of the B location and targeted a relatively thick interpreted sand interval having seismic response typical of the unit of a large region throughout the AC 21, AC 65, and surrounding blocks. This well logged a single sand body 125 ft thick at 5,403 ft below rig floor (Fig. 14). As with the A well, the resistivity of the sand was remarkably consistent at 1.8 to 2.5 ohm-m.
This report provides an initial review of JIP Leg II operations and results. Further work, including detailed examination of the full LWD datasets and recalibration of pre-drill seismic interpretations, will be conducted and will provide improved interpretations of the nature of the gas-hydrate bearing units encountered.
The results of drilling in WR313 clearly confirmed the geological/geophysical model that links phase reversals of strong amplitude and appropriate polarity to substantial accumulations of gas hydrate in deeply buried sand reservoirs. As with WR 313, drilling in GC955 found gas hydrate to be closely associated with strong seismic amplitude events of positive (“peak”) polarity; however, no significant sand reservoirs were found in the overlying, seismically muted section. In addition, both the WR 313 and GC 955 sites featured unanticipated thick zones of fracture-dominated gas hydrate occurrence in muds. At Walker Ridge, this occurrence appeared to be strata-bound and sub-regional, whereas at GC 955, the occurrence was more heterogeneous and perhaps related to local faulting. Drilling results at AC 21 were consistent with pre-drill interpretations of low- to moderate-gas hydrate saturation over a potentially large area. However, lack of data on reservoir mineralogy and pore water geochemistry made it difficult to assess nature or degree of pore fill with great confidence.
JIP Leg II set out to confirm the existence of gas hydrate in sand reservoirs in the Gulf of Mexico, and to test existing approaches and technologies for pre-drill appraisal of gas hydrate concentration. The results of the expedition indicate that both objectives were fully achieved. From a management standpoint, Leg II was extremely successful, being completed on time and under budget and with zero injuries. Operationally, the expedition provided significant new information on the optimal drilling and well control protocols for deep gas hydrate research projects. Technically, the operation of the LWD tools was outstanding. Scientifically, the expedition was a clear success, yielding extremely valuable and advanced datasets on gas hydrate occurrences ranging from low to high saturation in sands as well as thick sections of fracture-filling gas hydrate in muds. Perhaps most importantly, the expedition validated the integrated geological and geophysical approach used in the pre-drill site selection process, and provided increased confidence in the assessment of gas hydrate volumes in the Gulf of Mexico.
It is expected that further evaluation of the complex geology of these sites, including both conventional and pressure coring, will add significantly to the understanding of the nature and occurrence of gas-hydrate-bearing sands in the marine environment.