Gas Hydrate Prospects Research Articles

First Successful Wireline Stress Testing in a Gas Hydrate Reservoir in the Hyuganada Sea, Japan

This study presents a stress testing operation conducted using a wireline formation tester in a newly discovered gas hydrate prospect located offshore in Japan. The campaign, which spanned from December 2021 to January 2022, involved drilling a well using logging-while-drilling technology. Subsequently, wireline formation testing and stress testing were successfully conducted at three different depths within a gas hydrate-concentrated zone. The testing was accomplished in a single riserless descent, with the primary goal of obtaining crucial data such as mobility, formation pressure, and fracture gradient for one of the prospects. This operation marked the first stress testing job performed with dual packers in an open water and deepwater environment specifically for gas hydrate reservoirs. The study also provides a comprehensive interpretation of the data gathered during the operation. Moreover, it evaluates various properties such as formation mobility, formation pressure, initial breakdown pressure, closure pressure, fracture propagation pressure, and instantaneous shut-in pressure.

Evaluation of Natural Gas Hydrate Fault System: A Case from a Sag in Deep-Water Slope Area of the Northern South China Sea

AbstractThe fault system is one of the structural carrier systems of gas hydrate accumulation, which plays a vital role in controlling the distribution of natural gas hydrate (NGH) accumulation. The previous studies mainly focus on summarizing the vertical migration mode of high flux fluid along the fault with obvious geophysical response characteristics on the seismic profile, such as “fault with gas chimney,” “fault with mud diapir,” and “fault with submarine collapse”, but lack of evaluation methods for the fault carrier system. We use the X sag in the deep-water continental margin slope area of the northern South China Sea as an example to study the fault systems closely related to NGH. This paper puts to use attribute technologies, such as coherence, curvature, and fusion, to analyze the characteristics and combination of the fault systems. We discussed migration patterns and evaluation methods of dominant fault carrier systems. This research proves that the strike-slip fault system in the platform area can directly connect the gas source bed with high-quality hydrocarbon generation to the gas hydrate stability zone (GHSZ). The activity of this fault system is more conducive to the accumulation of hydrocarbon in the GHSZ. This area has a good site for pore-filling gas hydrate prospecting and a preferential favorable fault carrier system. The composite fault system, consisting of a normal dip-slip fault system and a polygonal fault system, in the slope area can jointly communicate the biogenic gas-rich reservoir. Its activity and well-migration performance are the main reasons for the submarine gas leakage and collapse. It is a secondary favorable fault carrier system in the study area. There may be massive and vein natural gas hydrate formation in fractures in the leakage passage, and pore-filled gas hydrate may exist in the submarine nonleakage area. In this work, a three-factor evaluation method of the fault carrier system is proposed for the first time. This method is of great significance for the evaluation and exploration of NGH reservoirs in the continental margin slope area of the northern South China Sea.

Geological Reservoir Characterization of a Gas Hydrate Prospect Associated with the Hydrate-01 Stratigraphic Test Well, Alaska North Slope

Geological reservoir characterization is essential for accurate evaluation of gas production performance from gas hydrate reservoirs. Particularly, the understanding of reservoir architecture and heterogeneity is of great importance since these are considered as major controls on fluid hydrodynamic and thermodynamic conditions. This study deals with well log and three-dimensional (3-D) vertical seismic profile (VSP) data acquired from the Hydrate-01 Stratigraphic Test Well within the 7-11-12 prospect, Prudhoe Bay Unit, Alaska North Slope and reports on the results of geological/geophysical evaluation related to the geological structure and reservoir properties of the 7-11-12 prospect. The structural trends of the target reservoirs, based on well correlations, are mostly consistent with the predrill prediction using the surface seismic data, and infer the existence of subseismic faults cutting through the Hydrate-01 well. The 3-D VSP data confirm a down-to-the-east normal fault that offsets the reservoir units across the Hydrate-01 well, which is concordant with the well identification of the same fault, and indicate a northeast-dipping relay structure associated with the overstepping normal faults. The edge enhancement attribute associated with discontinuity generated from the 3-D VSP data shows small faults/fractures, possibly as part of a complex fault network within the imaged normal fault system. These results reveal that the 3-D VSP data provide detailed structural information that is not present from the surface seismic data. The Hydrate-01 well log data confirm the occurrence of gas hydrate at high saturation in the two targeted sand units (B1 and D1 sands), and the comparison to a nearby pre-existing well (7-11-12 well) shows the same general trend in gas hydrate saturation as a map of seismic impedance generated from surface seismic data. The well log data also suggest that the base of gas hydrate occurrence in the Hydrate-01 and 7-11-12 wells is almost aligned at the same depth in both of the targeted B1 and D1 sand reservoirs. Especially for the D1 sand in the Hydrate-01 well, the resistivity logs show a sharp transition from high gas hydrate saturation to fully water-saturated within the D1 sand, suggesting a common gas hydrate/water contact. The results of this study will be used to construct the geological models needed for reservoir simulation studies and they can provide important insights into the geological factors that control the occurrence of gas hydrate on the Alaska North Slope.

Research of Using Geophysical method to detect Natural Gas Hydrate-Application in Hala Lake area of Qinghai Province

Hara Lake is located in the south of Qilian Mountains. According to the previous geological survey in this area, the study area has a good prospect of gas hydrate prospecting. As a new type of clean energy, natural gas hydrate is seldom explored in our country. In order to find gas hydrate, two-dimensional reflection seismic exploration is carried out in this area, and the props are selected. Multiple coverage observation system and low frequency vibroseis. High quality 2-D seismic data are obtained by reflection seismic exploration, and the characteristics of velocity inversion anomaly and low frequency amplitude of gas hydrate are obtained, which provides a theoretical basis for gas hydrate acquisition.

Numerical simulation of AVO response characteristics from pore-filling gas hydrate in Qilian mountain permafrost, China

The seismic reflection wave amplitude variation with offset (AVO) technique can effectively extract the lithology and fluid parameters that are hidden in the seismic record, and thus can be applied to gas hydrate prospecting and assessments in the permafrost region. Based on the geological conditions of pore-filling gas hydrate reservoir in Qilian mountain permafrost, the forward simulation velocity model of gas hydrate, by the rock physics model of the equivalent medium theory, was established. The AVO response characteristics of the gas hydrate reservoir with different properties by the forward modeling velocity model and the ray tracing theory were studied. The sensitivity of elastic parameters, for gas hydrate reservoir identification and formation lithology, was also studied. The AVO attribute partition changing with the porosity is quite well; the AVO attribute partition changing with the saturation presents continuous transition and could not be distinguished easily, and the AVO attribute partition changing with the thickness of the gas hydrate reservoir is linear. The μ and E are the most sensitive parameters to the properties of the gas hydrate reservoir, the crossplot of μ and E can effectively identify the gas hydrate reservoir, and the crossplot of μ and λ/μ could identify the lithology of the gas hydrate reservoir.

Seismic analysis of the gas hydrate system at Pointer Ridge offshore SW Taiwan

Pointer Ridge is a gas hydrate prospect on the South China Sea continental slope offshore SW Taiwan. It is characterized by densely distributed bottom simulating reflections (BSRs), active gas seepage, and potential sandy gas hydrate reservoirs. To understand how the fluids have migrated toward the seafloor, and the role of geological processes in the gas hydrate system, we have collected and analyzed high-quality 2D and 3D reflection seismic data. We first mapped the spatial distribution of the BSRs, and interpreted a major normal fault, Pointer Ridge Fault (PR Fault). The NE-SW trending fault dips to the east, and separates the erosional regime to the west from the depositional regime to the east. One active vent site was identified directly above the PR Fault, while another is located on a topographic high to the west of the fault. On the hanging block of the fault we found at least one major unconformity. The seismic data indicate refilled channels with coarser-grained sediments in the hanging wall of the normal fault. Seismic attribute analysis shows subsurface fluid conduits and potential gas hydrate reservoirs. We propose two types of gas chimneys, which are separated by the fault. Gas plumes derived from hydroacoustic data are mostly from the footwall block of the fault. We infer that fluid flow is more active in the erosional environment compared to the depositional one, and that this is the result of reduced overburden. The methane-bearing fluids migrate upward along the PR Fault and chimneys and form hydrates above the base of the gas hydrate stability zone. Based on seismic interpretation and seismic attribute analysis, we postulate that the channel infill constitutes the most promising hydrate reservoirs in this geological setting. In the surveyed area of Pointer Ridge these channels occur mainly below the gas hydrate stability zone.

National Gas Hydrate Program expedition 02: Identification of gas hydrate prospects in the Krishna-Godavari Basin, offshore India

After completing the first expedition of India's National Gas Hydrate Program (NGHP-01) in 2006, it was concluded that for the next expedition (National Gas Hydrate Program 02; NGHP-02), a new drill site review effort should focus on identifying potential deep-water offshore gas hydrate accumulations in sand dominated depositional environments. Therefore, geological and geophysical data analysis and 3D seismic data interpretation along with associated seismic modeling were carried out in three areas of the Krishna-Godavari Basin: Areas B, C, and E. Conventional petroleum exploration approaches of seismic amplitude evaluation were adapted to prospect for potentially sand-rich depositional systems within the gas hydrate stability zone. Subsequently, these prospective areas were further assessed through the geological and geophysical evaluation of depositional setting, gas sources, and gas migration pathways. In Area B, prospecting focused on a large anticlinal structure with a prominent bottom-simulating reflector and several key horizons that indicated evidence for potential sand-hosted hydrate occurrences. In Area C, the prospects were distributed throughout various settings within a very large deep-water channel-levee-fan system with complex indications of potential gas hydrate occurrence in sand-prone seismic facies. In Area E, prospects were associated with high amplitude events within inferred channel-levee sequences. Based on the pre-expedition/onboard drill-site evaluation, the 22 most promising sites in the Krishna-Godavari Basin were identified and prioritized to investigate and delineate a total of 17 identified gas hydrate prospects. This paper describes the geo-scientific studies carried out prior to NGHP-02 for site identification, evaluation and prioritization. An important outcome of this study is the identification of two potentially producible gas hydrate systems inferred to host significant quantities of gas hydrate in stratigraphic-structural traps.

Gas hydrate reservoir identification, delineation, and characterization in the Krishna-Godavari basin using subsurface geologic and geophysical data from the national gas hydrate program 02 expedition, offshore India

Abstract During the National Gas Hydrate Program Expedition-02 (NGHP-02), logging while drilling, conventional coring and pressure coring, wire-line logging, vertical seismic profiling and formation pressure measurements were carried out in deep water portions of the Krishna-Godavari (K-G) and Mahanadi Basins along the eastern continental margin of India. A total of 42 research holes were completed for comprehensive data collection and measurements related to gas hydrate reservoir and host sediments properties with an objective to identify producible gas hydrate in sand-rich reservoirs that would be suitable for a future pilot production test. Shipboard and post cruise evaluation of physical properties measurements, pressure core dissociation studies and gas analysis of recovered core samples have confirmed the occurrence of sand hosted gas hydrate deposits in three operational areas named during NGHP-02 as Area B, Area C and Area E in the deep-water portion of the K-G Basin. The energy resource potential of the discovered gas hydrate deposits in the K-G Basin depends on many factors, including the quantity of available gas trapped as gas hydrate in the basin. Therefore, this study was undertaken to identify, characterize and delineate the discovered and confirmed gas hydrate prospects in the K-G Basin. Based on the integrated analysis of regional seismic data and NGHP-02 acquired borehole data, a total of 8 sand-rich reservoir constrained gas hydrate prospects have been identified and mapped in the K-G Basin (Areas B, C, and E) as described in this report.

Reservoir volume of gas hydrate stability zones in permafrost regions of China

Gas hydrate is a promising energy resource. Permafrost regions in China are natural gas hydrate prospects. In this study, we summarized the conditions for gas hydrate formation, and analyzed the probable thickness and area of gas hydrate stability zones in the permafrost regions in China. Then the probability of porosity within the gas hydrate stability zones was estimated from lithology, burial history, and compaction curves. Finally, the total reservoir volume for gas hydrate stability zones in permafrost regions was estimated from the joint probability distribution of the effective thickness, area, and porosity. The probable total reservoir volume of the gas hydrate stability zones in the permafrosts in China is in the range of 2.995 × 1012 m3 to 5.081 × 1012 m3, with the best estimate at 4.002 × 1012 m3. The Qinghai-Tibetan Plateau including Qilian (where the occurrence of natural gas hydrate has been proven by coring) has about 55% of the total reservoir volume of the gas hydrate stability zones in China, indicating it has a great resource potential. In the Mohe Basin the reservoir volume of the gas hydrate stability zone ranges from 0.49 × 1012 m3 to 0.79 × 1012 m3, suggesting it is a hopeful future resource exploration area. The best estimate of the total reservoir volume of the gas hydrate stability zones in Northwest China is 1.2 × 1012 m3 which is considerable. However, occurrence of gas hydrate is yet to be confirmed by future exploration. These estimates provide much needed data for further gas hydrate resource exploration and evaluation, as well as environmental protection.

Diapiric activities and intraslope basin development offshore of SW Taiwan: A case study of the Lower Fangliao Basin gas hydrate prospect

The architecture and distribution of mud diapirs are shaped by tectonic activity, sediment filling and unbalanced loading characteristics. Mud diapir development also controls spatial variations of intraslope basins in slope areas and can spur upward fluid migration with diapiric intrusion. The offshore area of southwestern Taiwan is an incipient collision zone in which thick sequences of deep marine sediments filled a rapidly subsided foredeep basin during the Pliocene. Large volumes of deposited sediment serve as source materials of diapiric ridges that extend NNE-SSW, and some mud diapirs even extend to on land SW Taiwan with subsurface signatures of gas. This study examines relationships between mud diapir and intraslope basin development in convergent tectonics through seismic and bathymetry data analyses. Four types of mud diapirs are identified: (1) buried symmetrical diapirs; (2) symmetrical diapirs extruded above the seafloor; (3) asymmetric and irregular diapirs; and (4) small individual diapirs manifested as mud intrusions found in local areas. A 3-stage model is proposed as a tool for describing the development and distribution of these types of diapirs. We further examine the relationship between mud diapirs and intraslope basin development patterns by analyzing 2D and 3D seismic images to reveal structural and sedimentary processes occurring in the Lower Fangliao Basin. This basin is characterized by BSR and amplitude anomalies of seismic profiles and is a prospect of the Taiwanese gas hydrate investigation project. An 8-stage development model with six depositional units is proposed as a means to explain the evolution of diapirs, submarine canyons, and fold and fault activities in the Lower Fangliao Basin, in turn revealing the relationship between mud diapir formation and intraslope basin development offshore of southwestern Taiwan.

Thermophysical method for quantitative estimation of hydrate content in samples imitating bottom sediments

Gas hydrates in sediments can dissociate on heating, which causes a characteristic change in temperature over time. This can be used to detect hydrates and estimate their content. The geothermal method for gas hydrate prospecting consists in obtaining thermograms and defining regularities related to the presence of gas hydrates in the medium. The aim of this study was a quantitative estimation of hydrate content in sediments, based on data from repeated measurements of the temperature of a linear heat source (needle probe). The power of the source is chosen so that hydrate does not decompose in the first measurement but decomposes in the second. Afterward, we solve the optimization inverse problem of determination of the model parameters, one of which is a hydrate content. Experimental data on heating of the laboratory samples are processed, and hydrate contents are obtained with an accuracy of 30%. These values agree with independent estimates. The analytical solution of the axially symmetric problem of gas hydrate dissociation, based on the solution of the Stefan problem, is applied as a mathematical model for hydrate heating and decomposition in the sample.

Numerical Simulation of Response Characteristics of Audio-magnetotelluric for Gas Hydrate in the Qilian Mountain Permafrost, China

Audio-magnetotelluric (AMT) method is a kind of frequency-domain sounding technique, which can be applied to gas hydrate prospecting and assessments in the permafrost region due to its high frequency band. Based on the geological conditions of gas hydrate reservoir in the Qilian Mountain permafrost, by establishing high-resistance abnormal model for gas hydrate and carrying out numerical simulation using finite element method (FEM) and nonlinear conjugate gradient (NLCG) method, this paper analyzed the application range of AMT method and the best acquisition parameters setting scheme. When porosity of gas hydrate reservoir is less than 5%, gas hydrate saturation is greater than 70%, occurrence scale is less than 50 m, or bury depth is greater than 500 m, AMT technique cannot identify and delineate the favorable gas hydrate reservoir. Survey line should be more than twice the length of probable occurrence scale, while tripling the length will make the best result. The number of stations should be no less than 6, and 11 stations are optimal. At the high frequency section (10∼1000 Hz), there should be no less than 3 frequency points, 4 being the best number.

Seismic studies of gas hydrate in the KG Basin, East Sea, offshore India

India has developed an ambitious research program after successfully unearthing gas hydrate layer at site 10 in Krishna Godavari (KG) Offshore, during first National Gas Hydrate Program (NGHP-1). After studying the result from NGHP-1, second NGHP has been firmed up with an objective to identify the Gas Hydrate prospect for test drilling using seismic data and well analysis in KG offshore areas. First time a 3D seismic data volume has been studied with objective to identify Gas Hydrate prone areas in Indian water. To achieve this objective seismic data has been calibrated with well log to identify the Top and bottom of Gas Hydrate Stability Zone. The extent of Gas hydrate is inferred on seismic section by large amplitude (opposite polarity of with respect to Sea bottom) bottom simulating reflector (BSR), which is following the sea bottom and also at places cutting across dipping srata. In this study, the BSR mapped and then RMS amplitude extracted within window of 100ms above of it. RMS amplitude distribution suggests the variance in Gas hydrate concentration spatially primarily guided by the porosity variation in shape of channel and scroll bar. Further RMS amplitude extraction 20 ms below BSR suggest the presence of free gas areas associated with Gas hydrate in the down dip towards the regional lows. The study concludes that there are three to four areas where positive acoustic impedance within GHSZ is available, probably indicative of high Gas Hydrate concentration due to better reservoir condition and may be tested.

Scale-dependent gas hydrate saturation estimates in sand reservoirs in the Ulleung Basin, East Sea of Korea

Through the use of 2-D and 3-D seismic data, several gas hydrate prospects were identified in the Ulleung Basin, East Sea of Korea and thirteen drill sites were established and logging-while-drilling (LWD) data were acquired from each site in 2010. Sites UBGH2–6 and UBGH2–10 were selected to test a series of high amplitude seismic reflections, possibly from sand reservoirs. LWD logs from the UBGH2–6 well indicate that there are three significant sand reservoirs with varying thickness. Two upper sand reservoirs are water saturated and the lower thinly bedded sand reservoir contains gas hydrate with an average saturation of 13%, as estimated from the P-wave velocity. The well logs at the UBGH2–6 well clearly demonstrated the effect of scale-dependency on gas hydrate saturation estimates. Gas hydrate saturations estimated from the high resolution LWD acquired ring resistivity (vertical resolution of about 5–8 cm) reaches about 90% with an average saturation of 28%, whereas gas hydrate saturations estimated from the low resolution A40L resistivity (vertical resolution of about 120 cm) reaches about 25% with an average saturation of 11%. However, in the UBGH2–10 well, gas hydrate occupies a 5-m thick sand reservoir near 135 mbsf with a maximum saturation of about 60%. In the UBGH2–10 well, the average and a maximum saturation estimated from various well logging tools are comparable, because the bed thickness is larger than the vertical resolution of the various logging tools. High resolution wireline log data further document the role of scale-dependency on gas hydrate calculations.

Geological and geophysical conditions for potential natural gas hydrate resources in southern South China Sea waters

The water depth in Nansha (南沙) waters, which is located in the southern South China Sea, varies from 200 to 2 500 m, with a deep-water (>500 m) area of 500 000 km2. In this region, there are many depositional basins with various structural features, prone to the accumulation of organic material. The temperature and pressure conditions in the deep-water environment are suitable for the preservation of gas hydrate. At several sites, we have recognized bottom-simulating reflectors (BSRs) from seismic data. Regional geology analyses show that the Nansha waters may have abundant gas hydrate prospects, especially in localities such as the Nansha trough and other deep-water basins of the central Nansha waters.

Anisotropic models to account for large borehole washouts to estimate gas hydrate saturations in the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II Alaminos Canyon 21 B well

Through the use of 3-D seismic amplitude mapping, several gas hydrate prospects were identified in the Alaminos Canyon (AC) area of the Gulf of Mexico. Two locations were drilled as part of the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II (JIP Leg II) in May of 2009 and a comprehensive set of logging-while-drilling (LWD) logs were acquired at each well site. LWD logs indicated that resistivity in the range of ∼2 ohm-m and P-wave velocity in the range of ∼1.9 km/s were measured in the target sand interval between 515 and 645 feet below sea floor. These values were slightly elevated relative to those measured in the sediment above and below the target sand. However, the initial well log analysis was inconclusive regarding the presence of gas hydrate in the logged sand interval, mainly because large washouts caused by drilling in the target interval degraded confidence in the well log measurements. To assess gas hydrate saturations in the sedimentary section drilled in the Alaminos Canyon 21 B (AC21-B) well, a method of compensating for the effect of washouts on the resistivity and acoustic velocities was developed. The proposed method models the washed-out portion of the borehole as a vertical layer filled with sea water (drilling fluid) and the apparent anisotropic resistivity and velocities caused by a vertical layer are used to correct the measured log values. By incorporating the conventional marine seismic data into the well log analysis, the average gas hydrate saturation in the target sand section in the AC21-B well can be constrained to the range of 8–28%, with 20% being our best estimate.

First-Order Estimation of In-Place Gas Resources at the Nyegga Gas Hydrate Prospect, Norwegian Sea

Gas hydrates have lately received increased attention as a potential future energy source, which is not surprising given their global and widespread occurrence. This article presents an integrated study of the Nyegga site offshore mid-Norway, where a gas hydrate prospect is defined on the basis of a multitude of geophysical models and one shallow geotechnical borehole. This prospect appears to hold around 625GSm3 (GSm3 = 109 standard cubic metres) of gas. The uncertainty related to the input parameters is dealt with through a stochastic calculation, giving a spread of in-place volumes of 183GSm3 (P90) to 1431GSm3 (P10). The resource density for Nyegga is found to be comparable to published resource assessments of other global hydrate provinces.

Pre- and post-drill comparison of the Mount Elbert gas hydrate prospect, Alaska North Slope

In 2006, the United States Geological Survey (USGS) completed a detailed analysis and interpretation of available 2-D and 3-D seismic data, along with seismic modeling and correlation with specially processed downhole well log data for identifying potential gas hydrate accumulations on the North Slope of Alaska. A methodology was developed for identifying sub-permafrost gas hydrate prospects within the gas hydrate stability zone in the Milne Point area. The study revealed a total of 14 gas hydrate prospects in this area. In order to validate the gas hydrate prospecting protocol of the USGS and to acquire critical reservoir data needed to develop a longer-term production testing program, a stratigraphic test well was drilled at the Mount Elbert prospect in the Milne Point area in early 2007. The drilling confirmed the presence of two prominent gas-hydrate-bearing units in the Mount Elbert prospect, and high quality well logs and core data were acquired. The post-drill results indicate pre-drill predictions of the reservoir thickness and the gas-hydrate saturations based on seismic and existing well data were 90% accurate for the upper unit (hydrate unit D) and 70% accurate for the lower unit (hydrate unit C), confirming the validity of the USGS approach to gas hydrate prospecting. The Mount Elbert prospect is the first gas hydrate accumulation on the North Slope of Alaska identified primarily on the basis of seismic attribute analysis and specially processed downhole log data. Post-drill well log data enabled a better constraint of the elastic model and the development of an improved approach to the gas hydrate prospecting using seismic attributes.