Natural Gas Hydrate Production Research Articles

Numerical analysis of the geomechanical responses during natural gas hydrate production by multilateral wells

Multilateral wells are acknowledged as a promising well type to achieve the commercial development of natural gas hydrate. Previous studies have validated the efficacy of multilateral wells for hydrate mining without considering the geomechanical responses. However, effective stress increases during hydrate extraction by depressurization, and the reservoir skeleton degrades with hydrate dissociation, which might induce stress concentration, stratum subsidence, seafloor inclination, and other hazards. Understanding the geomechanical responses is thus crucial for the design of hydrate exploitation utilizing multilateral wells. As a continuation of our previous study, a coupled thermal-hydrologic-mechanical model was constructed to investigate the geomechanical responses during hydrate production by multilateral wells. Results reveal that multilateral wells can upgrade geomechanical risks and gas production capacity for hydrate exploitation. Stress concentration and stratum sinks are prevalent around the multilateral well. Increasing production pressure may alleviate stratum subsidence but has a noticeable impact on gas production. Since stratum inclination generated by hydrate mining with multilateral wells is minimal, geological catastrophes are most improbable. For utilizing multilateral wells in hydrate extraction, it is recommended to pay more attention to wellbore and wellhead safety.

Undrained Shear Properties of Shallow Clayey-Silty Sediments in the Shenhu Area of South China Sea

Suction piles are used to ensure wellhead stability during natural gas hydrate production in the Shenhu area of the South China Sea (SCS). Undrained shear properties of clayey-silty sediments play a critical role in the stability analysis of suction piles. However, it has not been fully studied. This study conducts a series of undrained triaxial shear tests on shallow clayey-silty sediments in the Shenhu area of SCS, and stress–strain curves under different overconsolidation ratio (OCR) conditions are obtained. OCR effects on undrained shear properties of clayey-silty sediments are discussed, and a model to predict the pore pressure coefficient at failure is proposed. Results show that the isotropic compression index is 0.175, and the isotropic swelling index is 0.029. The undrained shear strength is proportional to the effective confining pressure, and the proportionality coefficient is 0.42 for normally consolidated specimens, while the undrained shear strength of OC specimens nonlinearly increases with OCRs increasing. The proposed model aptly predicts the pore pressure coefficient at the failure of clayey-silty sediments of SCS with different OCRs.

Numerical simulation of gas extraction performance from hydrate reservoirs using double-well systems

Multi-well systems can increase the drainage area and hence enhance the productivity of natural gas hydrate (NGH) production. Fundamentally, the double-well system is the key and basis of understanding the multi-well system and formulating field strategies for gas extraction from hydrate-bearing sediments. However, production performance and multi-well production strategies for recovering NGH by using depressurization have not been treated in detail. Here we develop a three-dimensional model to describe double-well production characteristics from the low-permeability hydrate reservoir and analyze well-deployment strategies. The productivity of this double-well system is 1.5–2.5 higher than that of the single well under the same conditions, which is triggered by enlarging the drainage area. The well spacing alters flow characteristics and corresponding geophysical fields, which is the key variable for the double-well system in gas recovery. Its range can be determined based on the combination of productivity and hydrate dissociation front. Besides, due to the limited enhancement in productivity using the double-well system, the multi-well system assisted with reservoir reconstruction technologies and stimulation methods is of significant practical value in realizing commercial exploitation of NGH.

Research Development in the Traditional Methods and Water Flow Erosion for Natural Gas Hydrate Production: A Review

Natural gas hydrates are considered a possible sustainable energy source. Various methods have been proposed and developed in the past few decades to exploit hydrate. The purpose of this paper is to review the research advances, application potential, and limitations of hydrate production methods, including four traditional methods (depressurization, thermal stimulation, inhibitor injection, and CO2 replacement), and novel water flow erosion. For the four traditional methods, there still exist many bottlenecks to achieve commercial hydrate production owing to a series of problems such as vast sand production, ice generation, hydrate reformation, lower energy efficiency, and huge water production. For the water flow erosion method, it can be a novel strategy to promote hydrate decomposition by introducing the chemical potential difference and accelerating mass transfer. Significantly, if the traditional methods assist with water flow erosion, the problems such as insufficient decomposition driving force and ice generation are prevented. However, there is always a gap between the experimental results and the practical application of the water flow erosion method. Therefore, future work should be undertaken to investigate the selection criteria of suitable water flow rates for different reservoirs and the ecological security of the hydrate reservoir during the water flow erosion process.

Marine CSEM 3D modeling with a downhole dipole source for natural gas hydrate production monitoring

Marine CSEM 3D modeling with a downhole dipole source for natural gas hydrate production monitoring

An Interface Parametric Evaluation on Wellbore Integrity during Natural Gas Hydrate Production

Based on the whole life cycle process of the economic exploitation of natural gas hydrate, this paper proposes the basic problem of stabilizing the wellbore for the basic conditions that must be met to ensure the integrity of the wellbore for exploitation: revealing the complex mechanism of fluid–solid–heat coupling in the process of the physical exchange of equilibrium among gas, water, and multiphase sand flows in the wellbore, hydrate reservoir, and wellbore, defining the interface conditions to ensure wellbore stability during the entire life cycle of hydrate production and proposing a scientific evaluation system of interface parameters for wellbore integrity.

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.

Numerical investigation of natural gas hydrate production performance via a more realistic three-dimensional model

Numerical simulation plays a crucial role in the prediction of natural gas hydrate production performance. However, most existing models are two-dimensional or three-dimensional with idealized geometries and uniform parameter assignments, which cannot depict the effects of stratigraphic undulation and spatial variability of reservoir physical parameters on gas production. Therefore, a convenient method to convert the image information (more accessible) into parameter attribute values (required for model construction) was proposed in this study. Using the converted data of reservoir depth, thickness, and porosity, a more realistic three-dimensional model was innovatively constructed. Then, the influences of reservoir fluctuations and spatial variability of physical parameters on production performance were quantitatively analyzed. It was found that placing the production well in an elevated area can facilitate gas production. Specifically, Well 1 (located in the highland) had a 34.1% higher normalized gas production rate (i.e., production rate per unit well length) and a 14.9% lower normalized water production rate than Well 3 (located in the flat area) in the free gas layer. In addition to reservoir fluctuations, the exploitation efficiency of the gas hydrate-bearing layer was also affected by the thickness. The spatial variability of hydrate saturation and that of gas saturation in the study area were not very prominent, and the gas production rate obtained by the heterogeneous scheme was approximately 10% different from that of the homogeneous scheme. However, although the spatial variability of porosity was also not great (no more than 2%), when the cubic law was used to calculate the corresponding permeability, the gas production rate obtained by the heterogeneous scheme was nearly 20% different from that of the homogenous scheme. This study demonstrates the need to use a more realistic three-dimensional model for gas hydrate production performance prediction and is expected to provide an important reference for well location selection.

Natural gas hydrate accumulation mechanisms considering the multi-phase seepage and exploitation disturbance in porous media

Pressure gradient is established in reservoirs when the depressurization method is applied to decompose natural gas hydrates (NGHs). When the local reservoir pressure is in the thermodynamically stable area, hydrate reformation and blockage may occur and lead to incomplete hydrate exploitation. However, studies have so far not focused on the effect of hydrate saturation change and multiphase flow at the different exploitation stages on NGHs production behaviors, thus, it was the research priority in this study. The NGHs saturation at different exploitation stages is characterized by controlling the initial hydrate saturation, and the multiphase flow in reservoirs is simulated by controlling the water–gas flowrate. The results show that the onset time of blockage and amount of hydrate reformation were negatively associated with initial hydrate saturation (Shi) at a given water–gas flowrate. When the Shi elevated 1.8 times, the onset time of blockage and increment of hydrate saturation decreased by 0.7 times in the flow system. However, when the Shi < 12 %, the hydrate accumulation, and flow blockage were avoided with the water–gas flowrate of 10–1 mL/min. Other conditions being the same, the onset time of blockage was increased with the water phase fraction, while the increment of hydrate saturation was decreased with the water mass fraction. At a given gas flowrate of 1 mL/min and initial hydrate saturation of 40 %, when the water flowrate improved 7.5 times, a factor of 2.6 decreases in the increment of hydrate saturation was confirmed, from around 33.73 % to 12.95 %. Furthermore, the hydrate reformation and accumulation can prevent the gas overflow by forming the hydrate barrier, thus, a stable pressure was observed after flow blockage.

Sensitivity and Uncertainty Analysis for Natural Gas Hydrate Production Tests in Alaska

The Japan Research and Development Consortium for Pore Filling Hydrate in Sand (MH21-S), the U.S. Department of Energy National Energy Technology Laboratory (DOE NETL), and the U.S. Geological Survey (USGS) are planning a long-term production test from gas hydrate (methane hydrate) bearing reservoirs in the Prudhoe Bay Unit in northern Alaska. Prediction of gas and water production using 2D reservoir simulation models can be very useful for decision-making for long-term production testing. However, the input parameters applied to the simulation model, which are essentially based on the most likely values obtained from the well-log interpretation and core sample measurements of the Hydrate-01 stratigraphic test well (STW), have some degrees of uncertainty. In this study, to evaluate the impact of uncertainties in the input model on the prediction results of gas and water production as well as to analyze how each input parameter contributes to the uncertainties of prediction results, we conducted uncertainty analysis and parameter sensitivity analysis based on two different methods (the regression-based method and the Sobol’ method). The sensitivity analyses reveal that the parameters regarding relative permeability (exponential of the gas relative permeability (Ng) and residual gas saturation (Sgcr)) and the intrinsic/effective permeability of the lower B1 sand significantly contribute to the uncertainties of the gas and water production predictions. In addition, the uncertainty analyses reveal that the P50 value of cumulative gas production can result in less than the reference case with the most likely values for the input parameters due to the uncertainty of Ng. These results indicate that reducing the uncertainties of gas relative permeability and residual gas saturation is essential for more reliable numerical simulations both for prediction of test well performance and subsequent history matching of field test results.

A Novel Wellbore-Wall Heating Method without External Energy Injection for Natural Gas Hydrate Production—A Heat Transfer Device

Natural gas hydrate (NGH) dissociation is a heat-absorbing process, and the cooling around the wellhead is more pronounced during depressurization production. Low temperature will cause NGH regeneration or ice formation, blocking gas flow paths and reducing extraction efficiency. In this study, a novel heat transfer device (HTD) was innovatively proposed to alleviate this problem. Theoretical analysis and numerical simulations were used to research the methodological principles, applicable conditions, and expected benefits of the HTD. Results show that the HTD utilizes the characteristics of the geothermal gradient to rapidly transfer energy from the lower reservoir to the wellbore wall, which in turn raises the temperature and prevents the ice and NGH regeneration causing the blockage from adhering to the wellbore wall. The heat transfer radius, the length of the endothermic section, and the operating temperature difference make a tremendous difference in the heat transfer efficiency of the HTD. The HTD may be more suitable for Class 1 reservoir conditions and help to improve gas production under the depressurization method in the Shenhu sea of the South China Sea. The device can achieve continuous self-heat transfer without external energy injection to significantly reduce costs, which provides a new idea for marine NGH production.

Experimental study on hydraulic fracturing behavior of frozen clayey silt and hydrate-bearing clayey silt

There are critical problems in the field trials of natural gas hydrate (NGH) production, such as low production rate, limited recovery range and short period of stabilized production. Reservoir stimulation is a potential means to promote gas recovery from NGHs since previous studies have proved that gas production of other unconventional gas reservoir can be highly enhanced by hydraulic fracturing. However, few studies pay attention to the hydraulic fracturing behavior of clayey silt NGH reservoir. In this work, the effects of sediment properties and fracturing fluid parameters on fracture initiation, propagation and morphology were experimentally investigated by hydraulic fracturing of frozen clayey silt and methane hydrate-bearing clayey silt. The results indicated that the uneven distribution of ice in sediments led to non-uniform fracture propagation phenomenon, such as single-wing fracture and fracture deflection. The fracturing fluid with injection rate of 90 mL/min was conducive to the formation of complex fractures in sediments, which indicated that that increasing injection rate could greatly increase the effect of reservoir reconstruction. When the axial pressure (8 MPa) was 2 MPa higher than the confining pressure, the fracture propagated in the direction perpendicular to the minimum principal stress, which revealed the magnitude of minimum in-situ stress difference controlling the fracture propagation direction. In addition, this work showed the fracability of clayey silt hydrates even with low saturation, which revealed the feasibility of hydraulic fracturing of marine methane hydrate reservoir.

Source-Reservoir Characteristics and Accumulation of Gas Chimney-Type Gas Hydrates in Qiongdongnan Basin, Northern South China Sea

The Qiongdongnan Basin is an important gas hydrate exploration area in the South China Sea, but the gas hydrate accumulation process is poorly understood. By selecting an Lingshui (LS) target area and using first-hand geochemical data, three-dimensional seismic data, and an independent thermal insulation and pressure maintaining shipborne core analysis system, in this study, comprehensive geological evaluation was carefully conducted around the gas source and reservoir of the gas chimney hydrates, and the accumulation process was investigated. Geochemical data for a total of 47 sets of gas samples revealed that the gas source of the hydrates in the study area was mainly thermogenic gas supplemented by mixed gas. It was predicted that the contribution of the thermogenic gas to the hydrate accumulation was up to 70%. Using the independent shipborne core analysis system, the characteristics of the low-temperature combustible gas hydrates were determined, and the reservoir in the study area was characterized as non-diagenetic to weakly diagenetic, rich in clay and silt (D50 = 15.1–34.1 μm), weakly self-sustaining, and strongly heterogeneous. Based on the differentiated seismic response of the hydrate layer-gas bearing hydrate layer-shallow gas layer at the top of the gas chimney, the accumulation process in the target area was determined to be as follows: remote thermogenic gas transportation, local microbial genetic gas accumulation, episodic dynamic gas chimney reservoir formation, and source-reservoir control. In addition, the exploration ideas of an effective supply from a gas source and a shallow large-scale sand body are emphasized. The results of this study provide an important reference for the trial production of natural gas hydrates in the South China Sea.

Experimental study on the influence of brine concentration on the dissociation characteristics of methane hydrate

Brine stimulation is a common method for assisting depressurization to produce hydrate, which significantly improves the efficiency of hydrate production. In this work, to study the influence of brine concentration on the natural gas hydrate production process, a novel cylindrical hydrate production simulation experimental device was established, and investigations of hydrate dissociation experiments induced by depressurization in conjunction with brine injection were carried out. The effects of different brine concentrations on the gas production rate and heat transfer during production were studied. The distribution, flow, and concentration evolution of brine during hydrate production were analyzed. Brine injection can significantly enhance cumulative gas production during the depressurization stage, increase the sensible heat for hydrate dissociation, enhance heat transfer between the hydrate reservoir and the environment during the recovery process, and significantly improve the gas production rate of hydrate. The increase in the gas production rate caused by brine injection mainly occurs at the end of the depressurization stage and the beginning of the constant pressure stage. The injected brine is mainly located in the middle and upper areas of the reactor. In the bottom area of the reactor, the hydrate has higher saturation and longer dissociation time, which is slightly affected by the injected brine. A higher hydrate saturation is not beneficial to brine injection and does not promote hydrate dissociation. Additionally, in the experiments with brine injection, the resistance decreases more obviously during the depressurization stage due to more hydrate dissociation. The resistance also shows that in the bottom area of the reactor, there is a higher hydrate saturation and a lower brine effect on hydrate.

Numerical simulation on depressurization extraction of natural gas hydrate based on a coupled thermal–hydraulic–chemical model

To explain the dissociation law in the depressurization extraction of natural gas hydrate (NGH), a coupled thermal–hydrologic–chemical (THC) model was established. The coupled model was confirmed through the data of Masuda’s depressurization experimentation and the first NGH extraction in the Shenhu Area. After verifying the applicability of the numerical model, a long-term (1000 days) gas productivity of the NGH reservoir at the W17 test site was explored. This paper quantitatively analyzed the effect of reservoir permeability, temperature and porosity on NGH production capacity (cumulative methane production). The results revealed that the increase in reservoir permeability and temperature could significantly enhance methane production. The subsequent NGH extractions should raise the permeability and temperature of the target hydrate reservoir. Under the same NGH saturation, cumulative methane production did not always increase with increasing reservoir porosity. When the reservoir porosity increased to 0.40, the cumulative methane production decreased with the increase in porosity. The results showed that the combined extraction approach of depressurization and thermal flow injection is more efficient than only the depressurization method.

Study on Hydrate Risk in the Water Drainage Pipeline for Offshore Natural Gas Hydrate Pilot Production

Hydrate plug in the drainage line is a serious flow assurance problem for the pilot production of offshore natural gas hydrates. Current research focuses on hydrate deposition in the annular flow and the oil-dominated system. The multiphase flow system in the drainage line is a water-dominated system which is normally a bubbly flow. In this work, a new model is developed to study the temperature and pressure field in the drainage line considering that the flow pattern is bubbly flow. Combining with the methane hydrate phase equilibrium curve, the hydrate formation region in the drainage line can be established. The hydrate formation region is enlarged with the ESP pressure increasing and the water production rate decreasing, since the ESP can supply extra pressures in the drainage line and the heat transfer phenomenon is enhanced between the drainage line and environment under the low water production rate condition. The model pointed out that the risk of hydrate formation rises up as the hydrate concentration increases beyond 6%. This study can lay a theoretical foundation for the efficient prevention of gas hydrates in the drainage line during offshore natural gas hydrate pilot or long-term production.

Research progress and challenges of natural gas hydrate resource evaluation in the South China Sea

As an efficient clean energy, natural gas hydrate (NGH) has become a hot topic in recent researches. Since 1990s, China has made great achievements and progress in NGH exploration in the South China Sea (SCS), including determination of the favorable distribution areas and favorable strata thickness, identification of the dual source for accumulation, evaluation of the prospective gas contents, verification of the widespread existence, and confirmation of the technical recoverability of NGH resources. However, there are three major challenges in the NGH studies. First, all the 24 national key and major projects in the SCS focused on trial production engineering and geological engineering in the past 20 years, while 8 of the 10 international NGH research projects focused on resource potential. Second, resource evaluation methods are outdated and some parameter selection are subjective. Third, the existing resource evaluation results are low-level with a great uncertainty, and cannot be used to guide NGH exploration and production or strategic research. To improve the evaluation of NGH resources in the SCS, future researches should focus on four aspects: (1) improve the research on the criterion of the objective existence of NGH and the method of prediction and evaluation; (2) apply new theories and methods from the global NGH research; (3) boost the research on the difference and correlation of the conditions of hydrocarbon migration and accumulation in different basins; (4) innovate the theory and method of NGH resource potential evaluation.

Numerical Analysis of the Geomechanical Responses During Natural Gas Hydrate Production by Multilateral Wells

Numerical Analysis of the Geomechanical Responses During Natural Gas Hydrate Production by Multilateral Wells

Stability analysis of seabed strata and casing structure during the natural gas hydrates exploitation by depressurization in horizontal wells in South China Sea

Natural gas hydrates (NGHs) are a new type of clean energy with great development potential. However, it is urgent to achieve safe and economical NGHs development and utilization. This study established a physical model of the study area using the FLAC^3D software based on the key parameters of the NGHs production test area in the South China Sea, including the depressurization method, and mechanical parameters of strata, NGHs occurrence characteristics, and the technological characteristics of horizontal wells. Moreover, this study explored the law of influences of the NGHs dissociation range on the stability of the overburden strata and the casing structure of a horizontal well. The results are as follows. With the dissociation of NGHs, the overburden strata of the NGHs dissociation zone subsided and formed funnel-shaped zones and then gradually stabilized. However, the upper interface of the NGHs dissociation zone showed significant redistribution and discontinuity of stress. Specifically, distinct stress concentration and corresponding large deformation occurred in the build-up section of the horizontal well, which was thus prone to suffering shear failure. Moreover, apparent end effects occurred at the end of the horizontal well section and might cause the deformation and failure of the casing structure. Therefore, it is necessary to take measures in the build-up section and at the end of the horizontal section of the horizontal well to prevent damage and ensure the wellbore safety in the long-term NGHs exploitation.

Numerical and experimental investigation on a downhole gas-liquid separator for natural gas hydrate exploitation

Natural gas hydrate (NGH) is widely considered as an alternative energy source due to its tremendous reserves, cleanness and high energy density. During the exploitation process, discharging the gas-liquid phase to the surface for separation not only affects the pump performance but also increases the space requirements of the offshore platform. Taking NGH production test data in South China Sea for example, a downhole spiral gas-liquid separator is designed for high gas-liquid ratio (liquid content is less than 10%) in this paper. The forces of droplets in the separator are analyzed and the CFD model for gas-liquid two phase flow is established. The RNG k-ε turbulence model is used for analyzing the velocity field distribution and the pressure field distribution in the separator. The influence of structure parameters (pitch, cycle number of spiral piece, and the outer diameter of spiral pipe) and scale on the separation performance is investigated. The energy loss increases sharply after scaling in the separator and some necessary measures should be taken to avoid the generation of the sale and the secondary hydrate. Based on steepest ascent design, response surface methodology and particle swarm optimization, the optimal structure of the separator with high separation efficiency 87.26% and low pressure drop 2614.19Pa is obtained. The experimental platform is built for testing the gas-liquid separator. The comparison of numerical simulations and experiments show that the calculation model for the hydrate gas-liquid separation is correct and feasible. The separation device with the optimal structural parameters has good separation performance.