Hydrate Reservoir Research Articles

Heat utilization efficiency analysis of gas production from hydrate reservoir by depressurization in conjunction with heat stimulation

The heat utilization efficiency of heat injection is a key factor for the available heat stimulation method in natural gas hydrate, and the heat transfer characteristics during natural gas production from hydrate reservoir require further analysis. In this work, based on the laboratory tests by the Pilot Hydrate Stimulator (PHS), the heat transfer process of hydrate decomposition in the PHS was investigated, and the real-time decomposition rate and the energy utilization efficiency of the hydrate decomposition were quantitatively studied. The experimental results showed that the energy consumption of hydrate decomposition was mainly obtained from the sensible heat hydrate bearing sediments and the heat transfer from the environment in the depressurization process. In the initial stage of heat injection, the energy consumption of hydrate decomposition mainly came from the heat contained in the injected hot water and the heat transfer through the boundaries. In the later stage of heat injection, all of the energy consumption for hydrate decomposition was obtained from heat injection. In this work, the heat transfer coefficient between the reservoir and the environment was first proposed to quantify the heat transfer through the boundary. The results showed that the heat transfer coefficient presented large fluctuations during the initial stage of the depressurization and the period when the reservoir temperature reached the environment temperature in the heat injection process. Meanwhile, the coefficient remained stable at the later stage of heat injection. These results indicated that the change in gas-water hydrate inside the reservoir and the change in reservoir temperature affected the heat transfer coefficient between the reservoir and the environment. Furthermore, the energy utilization efficiency and gas production rate were optimized for hydrate production when the reservoir temperature was controlled within the range close to the environmental temperature. These results may be of great significance for heat injection control during field production from hydrate reservoirs.

A Review of Stimulation Technologies for Weakly-Consolidated Natural Gas Hydrate Reservoirs

As an unconventional clean resource with huge reserves and a wide distribution, natural gas hydrates (NGHs) have good application prospects. However, due to limited understanding and available production technology for NGHs, there is still a large gap between current production tests and commercial exploitation. A breakthrough in reservoir stimulation technologies is key to realizing the industrialization of NGHs in the future. Through a comprehensive summary of hydrate production test cases in Japan and China, this paper highlights the difficulties in the transformation of weakly-consolidated reservoirs. By systematically reviewing the theory and technology of hydrate reservoir transformation and engineering applications, this paper elucidates in detail the technical principles and mechanisms of several available stimulation technologies for weakly-consolidated reservoirs, and assesses the feasibility of their application to increase the production of NGHs. Existing problems and challenges are summarized and future prospects are discussed. Finally, suggestions are put forward for research and development of transformation technology for weakly-consolidated NGHs reservoirs in the future.

Characterization of the sediments in a gas hydrate reservoir in the northern South China Sea: Implications for gas hydrate accumulation

As a potential major energy resource, gas hydrates are widely found in sub-oceanic sediments and could play an important role in the future of fuels. Many studies have been applied on gas hydrates reservoir to understand its accumulation as well as potential industrial values, while the characteristics of sediments favoring gas hydrates accumulation are still unclear. Here, we investigate a gas hydrates reservoir in Shenhu area, Pearl River Mouth Basin, South China Sea (SCS), studying reservoir sediments features to understand controlling factors of gas hydrates accumulation. We sampled sediments of drilling site W19 in this area, which has thick, high saturation gas hydrate layers, for sediments' physical properties analysis (including the grain size, mineral composition, and specific surface area), as well as biological abundance in the gas hydrate-bearing layer and adjacent layers. The results show fine-grained turbidite sediments characterized by poor sorting, high foraminifera abundance, and low clay mineral content, are beneficial to gas hydrate accumulation, while suspended sediments characterized by fine sorting, high calcareous ultra-microfossil abundance, and high clay mineral content, are not conducive to gas hydrate accumulation. Based on these results, we propose that the gas hydrate accumulation in the SCS is controlled by multiple factors, including the sedimentary processes, sorting, biological abundance, mineral composition, and specific surface area. It could potentially reveal the regularity of gas hydrate accumulation on a global scale.

Imitating the effects of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments: An experimental study

The drilling fluid invasion into hydrate-bearing sediments will change the geomechanical properties of the reservoir and may lead to uncontrolled geological disasters in the worst case. Native sediments are replaced with artificial samples in this study to investigate the effect of drilling fluid invasion on the strength behaviors of hydrate-bearing sediments. The triaxial test is used primarily to assess the strength behaviors of hydrate-bearing sediments at varying temperatures, pressures, and hydrate saturation levels. The process of water-based drilling fluid penetrating into hydrate-bearing sediments is then experimentally imitated under various experimental conditions, with reservoir temperature of 4°C and pore pressure of 10 MPa. The possible repercussions of drilling fluid invasion and hydrate phase transition are identified by testing the mechanical properties of sediments under various invasion times and temperatures. The findings reveal that when hydrate saturation rises from 5% to 35%, the gas hydrate sediments shift from strain hardening to strain softening, with the critical hydrate saturation value of transition is between 15% and 25%. Peak strength increases with increasing hydrate saturation and pressure, and decreases with increasing temperature, whether drilling fluid invasion is present or not. The peak strength, Young’s modulus, shear modulus, and secant modulus of hydrate sediments all decreased significantly after drilling fluid invasion, although the Poisson’s ratio rose. These mechanical parameters are related to temperature and pressure under the action of drilling fluid. Finally, engineering and research recommendations for reducing the risk of drilling fluid invasion and hydrate dissociation are made based on experimental findings and theoretical analysis. This study innovatively examine the geomechanical mechanical properties of drilling fluid invading hydrate reservoir, which is critical for avoiding production concerns.

Multiphase flow and geomechanical responses of interbedded hydrate reservoirs during depressurization gas production for deepwater environment

The coupled multiphase flow and geomechanical responses are very important considerations in planning offshore hydrate production. In this work, a novel thermo-hydro-mechanical (THM) coupled simulator has been developed to investigate the THM responses of interbedded hydrate reservoirs using depressurization method in the offshore India. Based on the detailed geological data at the NGHP-02-16 site, a more realistic hydrate reservoir model is constructed to analyze the unique multiphase flow and geomechanical responses induced by depressurization. The results indicate that the average gas production rate of this work is 6370 ST m3/d, which is significantly lower than the previous numerical result of about 9000 ST m3/d using the same 3 MPa production pressure. This is mainly because the geomechanical response gives negative feedback to flow through the stress-dependent porosity and permeability. The reservoir heterogeneity results in the non-uniform hydrate dissociation and effective stress changes in the marine sediments. The depressurization results in significant increase of shear stress and vertical compaction in the hydrate reservoir. The maximum shear stress is no more than 2 MPa and is mainly concentrated in the top and bottom regions of the production interval. The maximum seafloor subsidence is 1.15 m during 100 days depressurization test. Sensitivity analysis results suggest that the production pressure and perforation length are two important parameters affect the gas productivity and mechanical stability. The low production pressure is recommended under the premise of mechanical stability for gas production from hydrate reservoirs. Perforation in the underlying water-saturated sand layer will lead to more serious water production and seafloor subsidence. The coupled multiphase flow and geomechanical results presented in this work are helpful to design the safe hydrate production scheme in the future.

Effects of a Multistage Fractured Horizontal Well on Stimulation Characteristics of a Clayey Silt Hydrate Reservoir.

The gas production from clayey silt natural gas hydrate (NGH) reservoir in the South China Sea faces the problem of low connectivity between the reservoir and the production well, which seriously reduces the gas production rate. Multistage fractured horizontal well (MFHW) is regarded as an effective technical means to improve gas production for an unconventional reservoir with low permeability. In this paper, a three-dimensional numerical simulation model was built to study the promotion effects of MFHW technology on gas production from a clayey silt NGH reservoir. The temporal and spatial evolution characteristics of the NGH reservoir with and without multiple fractures were compared and analyzed in detail. In addition, the influences of the fracture number, permeability, and morphology on the stimulation effect on gas production through MFHW technology were discussed. The results indicated that the fractures with high conductivity provided a fast channel for gas and water flow and increased the contact area between the horizontal well and the NGH reservoir, which had a positive effect on increasing gas production from the clayey silt NGH reservoir. Increasing fracture number, fracture permeability, and the area of fracture morphology effectively improved the gas production rate and total gas production, but the stage of the high gas production rate only lasted for a short time. This study demonstrated the production behavior of MFHW technology in the clayey silt NGH reservoir, which was helpful for understanding this technology’s stimulation effect.

A state-of-the-art review and prospect of gas hydrate reservoir drilling techniques

Securing energy means grasping the key link in the national development and security strategy. Under the goals of carbon peak and carbon neutrality, the overall tendency of energy development is to increase the proportion of natural gas while stabilizing oil consumption, and the global primary energy is entering the era of natural gas. Gas hydrate in deep seabed shallow strata and extremely cold permafrost regions has piqued the interest of researchers due to its abundant resources, widespread distribution, and high energy density. Although the drilling of hydrate wells is still fraught with unknowns and challenges due to the technological barriers between countries, complex on-site working conditions, and unique physical chemical properties, accumulation forms, and occurrence characteristics of gas hydrate, more than ten successful trial productions around the world have opened the door of hope for the development of this potentially new energy. The gas hydrate reservoir drilling technique is the frontier and hotspot of scientific and technological innovation and competitiveness around the globe today, reflecting the level of oil and gas technical advancement. At the national level, it possesses strategic and revolutionary features. Innovative drilling techniques, scientific well location layout, appropriate wellbore structure and well trajectory design, efficient drilling fluid, qualified drilling and completion equipment, and successful pressure-temperature preserved coring may all provide a strong guarantee for the successful completion of gas hydrate wells. This review comprehensively reviews the drilling techniques and engineering measures that can be used to develop gas hydrate. It focuses on the research advancement of important hydrate drilling technologies and the enlightening significance of these developments in the application of hydrate drilling. This work will deliver valuable experience as well as comprehensive scientific information for gas hydrate exploration and drilling.

Enhancing gas recovery from natural gas hydrate reservoirs in the eastern Nankai Trough: Deep depressurization and underburden sealing

Depressurization is recognized as the most effective way to recover natural gas hydrate. However, the bottomhole pressure is usually controlled above the quadruple point and few studies focus on reducing the pressure below that one. Even though some laboratory-scale studies have been performed, the application is not validated in the reservoir-scale production. Therefore, this study constructs a prediction model based on the field survey data in Japan to test the feasibility of deep depressurization (i.e., reducing the bottomhole pressure below the quadruple point) in the actual reservoir. The effect of ice formation on gas production is discussed in detail. Moreover, a method of deep depressurization combined with underburden sealing is innovatively proposed to further stimulate productivity and the different sealing effects are quantitatively analyzed. The results suggest that deep depressurization can effectively enhance gas production by forming ice and increasing driving force for hydrate dissociation, and the flowing channel blockage may be negligible. Underbuden sealing can further improve productivity by increasing pressure drop transfer under deep depressurization. A relatively higher sealing ratio corresponds to a higher gas recovery and a higher gas-to-water ratio, and the total gas production can increase by about 41.9% when the sealing ratio reaches 50.

Review of Sand Control and Sand Production in a Gas Hydrate Reservoir

Review of Sand Control and Sand Production in a Gas Hydrate Reservoir

Mathematical Model of the Decomposition of Unstable Gas Hydrate Accumulations in the Cryolithozone

We present a generalization of the mathematical model of gas discharge from frozen rocks containing gas-saturated ice and gas hydrates in a metastable state (due to the self-preservation effect) caused by the drop in external stress associated with various geodynamic factors. These factors can be attributed, for example, to a decrease in hydrostatic pressure on a gas-bearing formation due to glacier melting, causing an isostatic rise, or to the formation of linear depressions in the bottom topography on the shelf due to iceberg ploughing. A change in external pressure can also be associated with seismic and tectonic deformation waves propagating in the lithosphere as a result of ongoing strong earthquakes. Starting from the existing hydrate destruction model, operating at the scale of individual granules, we consider a low-permeable hydrate and ice-saturated horizontal reservoir. Generalization is associated with the introduction of a finite threshold for the external pressure drop, which causes the destruction of the gas hydrate and gas-saturated microcavities of supramolecular size. This makes it possible to take into account the effect of anomalously high pressures occurring in the released gas as a result of partial hydrate dissociation. Numerical and approximate analytical solutions to the problem were found in the self-similar formulation. A parametric study of the solution was carried out, and regularities of the hydrate decomposition process were revealed.

Geochemical characteristics of gases associated with natural gas hydrate

With more natural gas hydrate samples recovered and more research approaches applied to hydrate-associated gas studies, data concerning the geochemical characteristics of hydrate-associated gases have been increased significantly in the past decades. Although systematic reviews of hydrocarbons are available, fewer studies have focused on the systematic classification of gas hydrates, yet. In this study, the primary origins and secondary processes that affect the geochemical characteristics of the gases are discussed. The primary origins are affected mainly by the type and /or maturity of the organic matter, which determine the main signature of the gas is microbial gas or thermogenic gas in a broad scheme. Apart from primary origins, secondary processes after gas generation such as migration, mixing, biodegradation and oxidation occur during the migration and/or storage of gases can significantly alter their primary features. Traditional methods such as stable isotope and molecular ratios are basic proxies, which have been widely adopted to identify these primary origins and secondary processes. Isotopic compositions of C2+ gases have been employed to identify the precursor of the gases or source rocks in recent years. Data from novel techniques such as methane clumped isotope and noble gases bring additional insights into the gas origins and sources by providing information about the formation temperature of methane or proxies of mantle contribution. A combination of these multiple geochemical approaches can help to elucidate an accurate delineation of the generation and accumulation processes of gases in a gas hydrate reservoir.

Possible links with methane seepage and gas hydrate dynamics inferred from authigenic barite records in the northern south china sea

Numerous methane seepage events occurred in periods of low or falling sea level since 330 ka BP, which is attributed to decrease in hydrostatic pressure and subsequent gas hydrate dissociation in the northern South China Sea (SCS). The seepage intensity likely decrease due to gas hydrate stabilization once there was a relatively high-stand sea level. However, there are few geochemical records of decline in upward methane flux in the northern South China Sea. Here, combing porewater and solid-phase analyses, the geochemical cycling of barium was investigated in two piston cores from sites HD109 and HD319 within two areas with inferred gas hydrate occurrence in the Taixinan Basin of the northern SCS, in order to track the net decrease in the upward methane flux and to estimate the total duration time of these events in the studied sediments. The results indicate that there are four intervals with barium enrichments in the sediment section overlying the occurrent sulfate-methane transition zone (SMTZ) at both cores, suggesting the SMTZs have downward migrated through time. Based on the excess barium contents and the diffusive Ba2+ fluxes above the current SMTZ, we estimate the total time for barium accumulation at both cores is about ten thousand years. It is suggested that some methane seepage events temporarily enhance the upward flux of methane, inducing anaerobic oxidation of methane and associated SMTZ close to the sediment surface before the Holocene. After the most intensive seepage event ceased in the post-glacial period, the upward methane flux decreased and the SMTZ migrated downward gradually, preserving enrichments of diagenetic barite. Overall, these new data confirm the episodic decrease in upward methane flux recorded by authigenic barite after the last glacial maximum, which is likely related to the stabilization of underlying gas hydrate reservoir. This study may fill in the gap of the geochemical records of the variations in methane seepage and gas hydrate system during the post-glacial period in the northern SCS.

The double-edged characteristics of the soaking time during hydrate dissociation by periodic depressurization combined with hot water injection

The combination of depressurization and thermal stimulation is considered as a promising production method of gas hydrates. In this study, the experiments of methane hydrate dissociation by periodic depressurization combined with hot water injection (D&H) were conducted. Each cycle of D&H was consisted of the hot water injection stage, the soaking stage, and the depressurization stage (below the equilibrium pressure). The influences of the soaking time (4 min, 8 min, 12 min) on the hydrate dissociation behavior, the ratio of gas to water, and the energy efficiency were analyzed. The experimental results showed that, in the early period of depressurization combined with hot water injection stage (DHS), most of the consumed heat (70–90 %) during hydrate dissociation was provided by the heat transferred from surroundings. The proportion of the heat of the injected hot water to the consumed heat in the early period of DHS was only 10–30 %, and it quickly increased to 50–60 % in the later period of DHS. The calculation showed that the reduction of the soaking time improved the heat transfer rate from surroundings to the hydrate reservoirs at the soaking stage, thereby increasing the hydrate dissociation rate. However, the reduction of the soaking time decreased the thermal efficiency and the energy efficiency during hydrate production. For the soaking time of 4 min, 8 min, and 12 min, the range of the thermal efficiency were respectively 0.34–0.82, 0.50–1.06, and 0.54–1.20, and the range of the energy efficiency were respectively 6.24–14.89, 8.91–18.82, and 9.62–21.13. Meanwhile, the reduction of the soaking time also decreased the ratio of gas production to water production, which was adverse for hydrate production. This study makes up for the deficiency that the current understanding of soaking time in periodic depressurization combined with heat injection is insufficient. The research shows that the soaking time has a double-edged characteristic, which are helpful for choosing a suitable soaking time during gas hydrate production by periodic depressurization combined with hot water injection in actual field.

Extraction of Submarine Gas Plume Based on Multibeam Water Column Point Cloud Model

The gas plume is a direct manifestation of sea cold seep and one of the most significant symbol indicators of the presence of gas hydrate reservoirs. The multibeam water column (MWC) data can be used to extract and identify the gas plume efficiently and accurately. The current research methods mostly start from the perspective of image theory, which cannot identify the three-dimensional (3D) spatial structure features of gas plumes, reducing the efficiency and accuracy of detection. Therefore, this paper proposes a method for identifying and extracting the gas plume based on an MWC point cloud model, which calculates the spatially resolved homing of MWC data and constructs a 3D point cloud model of MWC containing acoustic reflection intensity information. It first performs noise suppression of the 3D point cloud of the MWC based on the symmetric subtraction and Otsu algorithm by leveraging the noise distribution of the MWC and the reflection intensity characteristics of the gas plume. Then, it extracts the point cloud clusters containing the gas plume based on Density-Based Spatial Clustering of Applications with Noise (DBSCAN) according to the density difference between the gas plume point cloud and the background MWC point cloud and next identifies the point cloud clusters by feature matching based on fast point feature histograms (FPFHs). Finally, it extracts the gas plume point cloud set in the MWC. As evidenced by the MWC data collected from gas hydrate enrichment zones in the Gulf of Mexico, the location of gas plume extracted by this method is highly consistent with that of gas leakage points measured during the cruise. Using this method, we obtained the point cloud data set of gas plume for the first time and accurately characterized the 3D spatial morphology of the subsea gas plume, providing technical support for gas hydrate exploration, subsea gas seepage area delineation, and subsea seepage gas flux estimation.

Analysis of factors affecting microwave heating of natural gas hydrate combined with numerical simulation method

As a clean and abundant unconventional natural gas resource, natural gas hydrate (NGH) holds the characteristics of safety, high efficiency and sustainable exploitation, which helps to alleviate the energy shortage of China, reduce the foreign-trade dependence of oil and gas, and ensure the national energy security. Microwave heating is a significant method that has been used in natural gas hydrate exploration. By using the microwave heating, the NGH in the reservoir formations would be heated, decomposed and stimulated thanks to taking advantage of microwave heating's unique characters: efficiency, high speed, clean and pollution-free. This paper established the temperature under microwave heating gas hydrate distribution theory model, and by using the finite element method for simulating temperature field of microwave heating gas hydrate, this paper analyzed the natural gas hydrate in the microwave field temperature distribution in the influencing factors. Microwave has a significant heating effect on the hydrate reservoir in the immediate vicinity of wellbore, and it is not affected by the initial conditions of reservoir. The temperature can rise to above 50°C within 1 h which is higher than the phase equilibrium temperature at the time of hydrate decomposition and is helpful to improve the decomposition rate of hydrate. The frequency is set at 915 MHz, and the feed port has a spiral arrangement with a length of 10 mm, which greatly expands the microwave heating range.

Source of the sand-rich gas hydrate reservoir in the northern South China Sea: Insights from detrital zircon U–Pb geochronology and seismic geomorphology

Sand-rich gas hydrate reservoirs were initially drilled in the Qiongdongnan Basin of the northern South China Sea in 2019. In this study, the source of the sands in the gas hydrate reservoirs in the Quaternary and their transport routes in the northwestern South China Sea were unraveled by detrital zircon U–Pb geochronology and seismic geomorphology to predict the distribution of the sand-rich gas hydrate reservoirs. In the Quaternary, gas hydrate reservoirs in the study area were mainly deposited as submarine fans which could be sourced from the southwest based on core-log-seismic data. The detrital zircon U–Pb ages indicate that the Quaternary sediments in the Qiongdongnan Basin could be sourced from the Red River, Truong Son Belt, and Hainan Island provenances with contributions of 40.9%, 39.0%, and 20.0%, respectively. Sediments from Hainan Island, mixed with the sediments from the Red River and Truong Son Belt by turbidity and oceanic currents, could be transported to the study area through the northwestern slope of the Qiongdongnan Basin which is the main sediment transport route of the sand bodies, and deposited as submarine fans for a favorable exploration area for gas hydrates.

Gas recovery enhancement from fine-grained hydrate reservoirs through positive inter-branch interference and optimized spiral multilateral well network

A spiral multilateral well network is a promising production method to enhance long-term gas recovery from prevalent fine-grained hydrate reservoirs. However, practical application is greatly restricted before the optimal well network parameters are determined and the mechanism behind a unique phenomenon in multilateral wells, namely inter-branch interference, is clear. In this study, we numerically optimized the well configuration and spacing when spiral multilateral wells were deployed in two typical fine-grained hydrate reservoirs, i.e., ultra-low permeability hydrate reservoirs (ULPHR, <1 mD) and low-permeability hydrate reservoirs (LPHR, >1 mD). The mechanism behind inter-branch interference was innovatively revealed. The results indicated that the number of spiral branches should be increased, and equidistant branches should be deployed uniformly in the lower ULPHR or throughout LPHR to enhance production efficiency. A wide spacing of spiral multilateral wells with long branches contributed to long-term productivity in fine-grained hydrate reservoirs with any permeability; however, narrow spacing was more favorable for short branches or short-term production. Our study found three inter-branch interference stages during gas production, namely, “no effect” stage, “positive” stage, and “negative” stage; all the three stages are controlled by reservoir permeability, production distance, and production time. Owing to the “positive” interference effect, longer equal-length branches resulted in superior long-term production enhancement in ULPHR, particularly for lengths greater than 30 m. Gas production from LPHR using only two optimal spiral multilateral wells exhibited high production performance similar to that of the sandy hydrate deposits in Japan, suggesting that the optimal spiral multilateral well network is promisingly suitable for commercial production in the future.

Investigating geological storage of carbon dioxide in a gas hydrate reservoir of the Shenhu area, South China Sea

Deep-water shallow-buried sand layers above the base of methane hydrate stability zone (BMHSZ) have been recognized as favorable reservoirs for gas hydrate accumulation. Carbon dioxide (CO2) injection into the sand layers provides significant advantages over onshore geological storage options because of CO2 hydrate formation and high CO2 density under conditions of pressure and temperature. However, reservoir characterization and geological conditions for storing CO2 in gas-charged sand layers below the BMHSZ have not been well studied.We investigate deep-water shallow-buried strata in the Shenhu area by analyzing and interpreting 3D seismic and well logs data. We present the W07–W23 gas hydrate reservoir that consists of two Lower Pleistocene sand zones: gas hydrate and free gas accumulation within levee facies in the upper zone and free gas accumulation within channel facies in the lower zone. Analysis of well data shows a 51 m thick total gas section interbedded with gas hydrate and inferred gas hydrate saturation as high as 20% of sediment pore space. Average porosity in the reservoir is estimated from log data at approximately 50%. Permeability in the hydrate-free sediments, as inferred from empirical equations retrieved from the offset wells, ranges 3–5 mD. We estimate that a total of 13–40 × 109 m3 of gas could be contained within the two zones and they may store roughly 5.7–11.4 × 107 ton of CO2. These results provide an improved understanding of gas hydrate reservoirs for occurrence of gas hydrate, free gas accumulations and CO2 storage capability below the traditional BMHSZ.

Microscopic molecular insights into methane hydrate growth on the surfaces of clay minerals: Experiments and molecular dynamics simulations

Clay is widely present in hydrate reservoirs and drilling and completion fluids. A quantitative understanding of hydrate growth characteristics on clay solid phase surfaces is vital for predicting hydrate formation and preventing secondary hydrate generation in solid-phase-containing drilling fluid systems. The experiments and nonequilibrium molecular dynamics simulations are performed to investigate the characteristics of methane hydrate growth on the surfaces of kaolinite and montmorillonite. The experiments and dynamics simulated results demonstrate that a large number of bound waters is absorbed on the clay surface. However, the bound water does not participate in hydrate formation. The mechanisms by which clay minerals influence the formation of hydrates are different. Kaolinite reduces the induction time of hydrate formation, but the massive amount of bound water on the kaolinite surface reduces the total amount of free water molecules, which hinders the formation of hydrates, and the inhibition effect on hydrates is more significant than that with montmorillonite. Significantly, the hydrolysis of interlayer cations on the montmorillonite surface inhibits the formation of hydrates. This study will help us to understand the mechanism for gas hydrates formation on reservoir clay minerals, and to predict the amount of hydrate reservoir reserves, which will further aid hydrate resource drilling and exploration.

Numerical analysis of gas recovery enhancement from natural gas hydrate reservoir by using a large-diameter casing

Multi-well patterns and complex structural wells can enlarge the contact area between the wellbore and natural gas hydrate (NGH) to improve productivity significantly, but the construction cost of drilling is still too high. The previous study has proposed a self-entry exploitation device (SEED) with an enlarged device diameter, allowing more NGHs to dissociate simultaneously. While for conventional wellbore, a large-diameter casing may be applied in NGH development to enlarge the wellbore diameter of the production section. This work continued to study the influences of enlarged device/wellbore diameter on gas production characteristics, temperature-pressure evolutions, and the dissociation front movement pattern. The sensitivity analysis was also carried out by various reservoir and depressurization parameters under different wellbore diameters. Results show that the enlarged wellbore diameter can enlarge the drainage area and increase the dissociation efficiency. The temperature and pressure around the wellbore remain relatively stable, meeting the conditions for long-term production. Also, the dissociation front moves farther away faster as the wellbore diameter enlarges, accelerating NGH dissociation at distant wells. Finally, the sensitivity analysis reveals that the enlarged wellbore diameter enhances gas production significantly in unfavorable production parameters: poor production pressure, low permeability, low NGH saturation, and narrow reservoir thickness.