Depressurization Production Research Articles

Experimental observation of methane hydrate dissociation via different depressurization modes under water phase flow

As one of the most potential unconventional fossil fuels, the safe and efficient production of natural gas hydrates (NGHs) has become a prominent research area worldwide. Water flow erosion, which has been proven to be a powerful methane hydrate dissociation method recently, may be an effective way to mitigate the challenging ice generation problems urgently needed to be solved for depressurization production. In this study, we visually studied the effects of the three depressurization modes combined with water flow erosion on MH decomposition using in-situ magnetic resonance imaging. The time and spatial characteristics of the three-phase water–gas-hydrate during the MH production process are indirectly determined in sediment. Three stages of the energy recovery process are directly observed: the MH stable existing stage, the free gas production stage, and the MH decomposition stage. Our work indicates that all three combination modes effectively eliminate ice generated during the MH production process. Compared with the sudden depressurization method, radial extension appears on one side or two sides of the decomposition trends. In addition, the results denote that the higher water flow rate can efficiently enhance the effect of depressurization on hydrate dissociation. Meanwhile, the MH decomposition rate and formation stability benefit more from a faster water flow rate and depressurization rate.

Creeping microstructure and fractal permeability model of natural gas hydrate reservoir

The successful pilot testing of depressurization production for natural gas hydrate reservoir was conducted at Shenhu Area in the South China Sea. However, pressure changes due to depressurization production cause shrinkage of creeping pore and throat space for fluid flow, ultimately resulting in permeability damage in this reservoir. To optimize gas recovery for natural gas hydrate reservoir, it is important to understand the substantial variations of the pore structure and physical properties under different pressure conditions. In this study, by integrating computed tomography imaging with water flow experiments on Clay-silt sediment sample, we analyze how pore scale structural parameters (such as average pore and throat radius, pore and throat median radius, maximum pore and throat radius, and fractal dimension) change under different axial stresses by using fractal geometry approach. It is found that there is a negative relationship between axial stress and structural parameters. Meanwhile, with the axial stress increases, the range of pore and throat radius distribution located on the right of center distribution decreases. On the basis of fractal geometry theory, a fractal model is then proposed to explain the effects of axial stress and creeping microstructure on permeability for natural gas hydrate reservoir. Results show that the model provides good match with experimental data when the axial stress is larger, while poor agreements with experimental results at low pressure. This study helps us understand fundamental mechanism for permeability changes during the depressurization of gas hydrate reservoir.

Investigation of the hydrate reservoir production under different depressurization modes

Large reserves of natural gas hydrates exist, and the depressurization method has the greatest potential for gas hydrate reservoir recovery. Currently, the most commonly adopted depressurization simulation method is a constant bottom-hole pressure production scheme. This study proposes a new depressurization mode with decreasing bottom-hole pressure. The production characteristic was numerically investigated using this method. The results show the following: (1) As the depressurization exponent (n) decreases, the development effect improves, and production indexes including cumulative gas production/dissociation and gas-water ratio increase. However, the reservoir energy consumption is higher and the hydrate reformation is more severe. (2) Compared to the proposed depressurization mode, the hydrate production index of the constant bottom-hole pressure production (n = 0) is better. However, the hydrate reservoir energy consumption is higher and the hydrate reformation is more severe using constant bottom-hole pressure production. (3) To achieve a balance between production and reservoir energy consumption during depressurization production, the bottom-hole pressure should be controlled by selecting a suitable depressurization exponent between nmin and nmax, which can be determined through numerical simulations.

Influence of Intrinsic Permeability on the Production Performance of Shenhu Hydrate Sediment through Depressurization

Natural gas hydrate(NGH) is a clean resource with huge reserves. The depressurization method is an economical and effective exploitation method. In the process of depressurization, reservoir absolute permeability has an important influence on production results. Based on the data of Shenhu hydrate reservoirs, this paper established a depressurization production numerical simulation model. Then, the production performances such as pressure, temperature, gas production rate, cumulative gas production, and hydrate dissociation effect are all studied under different permeability conditions.study the change of reservoir pressure, gas production rate, cumulative gas production, reservoir temperature change and hydrate dissociation effect under different permeability conditions. Results show that higher permeability is conducive to the depressurization of hydrate reservoirs.

Numerical simulation of dissociation front of shenhu hydrate reservoirs by depressurization

Natural gas hydrates (NGH), widely distributed in the deep ocean and the permafrost, are considered as a potential energy resource. Depressurization is a promising method to exploit NGH reservoirs. There is a dissociation front between dissociated zone and dissociating hydrate zone during the production of NGH reservoirs by depressurization. The study of the movement of the dissociation front is very essential to understand the formation stability and predict the gas production performance of hydrate reservoirs. Based on the geological conditions of hydrate reservoirs in the Shenhu area, numerical simulation methods are used to investigate the shape and movement of the dissociation front during the depressurization production, and then the sensitive factors affecting the movement of dissociation front are analyzed. The results show that the dissociation front is not a definitely interface between the dissociated and dissociating zone but a transition zone, and the shape of the dissociation front is deemed to a non-piston model when depressurization is conducted. The movement rate of dissociation front 1 (the position where the hydrate starts to dissociate) is almost approximately linear, but the movement rate of dissociation front 2 (the hydrate saturation is reduced to 0) increases as approximately an exponential pattern. The greater movement rate of dissociation front could be observed clearly under the condition of the lower production pressure, the greater intrinsic permeability, the smaller initial hydrate saturation and the higher initial temperature condition, respectively. Compared to the one-step depressurization, the dissociation front 1 and dissociation front 2 are both delayed when the multi-steps depressurization is conducted. The formation temperature will decrease because of the endothermic effect of hydrate dissociation. If the ice point is reached, the extra latent heat generated from ice will contribute to the increase of the hydrate dissociation rate. Therefore, when the depressurization method is used to exploit NGH reservoirs, the pressure can be reduced to the vicinity of the phase equilibrium pressure corresponding to the freezing point.

Does the Mediterranean Sea have potential for producing gas hydrates?

Gas hydrate reservoirs are considered as future global energy resource, however, minimal studies have been conducted on one of the most important historical seas to investigate the potential - the Mediterranean Sea. In this study, it is aimed to investigate this potential. According to the analyses in this study, there are many indicators of gas hydrates in the Mediterranean Sea. These indicators are mainly appropriate pressure-temperature conditions of gas hydrate formation, source gas potential, and coarse sands potential. In this study, by using the literature marine survey data and drilling data of the Mediterranean Sea such as pressure, temperature, salinity, porosity, geothermal gradient, sand content of sediments, etc., it was estimated that up to 98.2 (median) standard trillion cubic meters of methane may be available in the potential producible gas hydrates of the Mediterranean Sea. Due to the paucity of bottom simulating reflectance in the Mediterranean Sea, three hypothetical Class 3 gas hydrate reservoirs in the Mediterranean Sea conditions were simulated by using depressurization production method with/without wellbore heating with HydrateResSim numerical simulator. When the depressurization pressure is lower, much more gas is produced but until certain value. The main reason of gas production stop in these three reservoirs is hydrate reformation along the wellbore. Wellbore heating at 50 °C extended gas production for a while but could not avoid hydrate reformation along the wellbore. Ice formation due to the endothermic gas hydrate dissociation is not the cause of gas production stop because the temperature of the Mediterranean Sea sediments is high.

Numerical analysis of depressurization production of natural gas hydrate from different lithology oceanic reservoirs with isotropic and anisotropic permeability

Natural gas hydrate (NGH) is a promising alternative energy and mainly distributes in deep oceanic sediments. The intrinsic permeability of hydrate-bearing layers can potentially influence heat and pressure transfer during depressurization-induced NGH dissociation. By taking siltstone, sand and clay reservoirs in Shenhu area of South China Sea as examples, this study numerically investigates the effects of the magnitude and anisotropy of reservoir permeability on NGH production process and main physical field evolution. Results reveal that permeability anisotropy could impede advective interaction of fluids in vertical direction, significantly changing temperature and pressure evolution during NGH dissociation. Consequently, NGH dissociation slows down, delaying the coming of peak gas production rate. Some degrees of permeability anisotropy even lead to much earlier termination of NGH dissociation. In this case, the more permeable sand reservoir could possibly have a lower gas production potential than the less permeable siltstone and clay reservoirs. The permeable overburden limits the fluid pressure reduction and weakens the efforts of depressurization production. The re-production by further lowering well pressure only lasts for a short time. Comparatively, the depressurization of constant production mass rate is more effective but only provides a small contribution for the total gas yield.

Numerical simulations for short-term depressurization production test of two gas hydrate sections in the Black Sea

Gas hydrates are considered as a promising energy source and the Black Sea has a high potential of gas hydrates. The Danube Delta of the Black Sea is the most well-known prospect in the Black Sea after many geological and geophysical studies such as bottom-simulation reflectors (BSR) and electromagnetic surveys. In this study, gas production simulations from two gas hydrate layers (6 m thick hydrate layer at 60 mbsf and 30 m-thick hydrate layer at 140 mbsf above BSR at 350 mbsf) at the same locations with approximately 50% hydrate saturation in the Danube Fan of the Black Sea were run with depressurization method separately at 2 MPa, 3 MPa, 4 MPa, 5 MPa, and 6 MPa by using HydrateResSim numerical simulators. Moreover, different production tests strategies were suggested in this region.

Numerical study of CO2 enhanced natural gas recovery and sequestration in shale gas reservoirs

Due to the ultra-fine pore sizes, Darcy's law is not applicable in shale gas reservoir. On the basis of the multiple binary gas transport mechanisms including viscous flow, Knudsen diffusion and ordinary diffusion, a new dual-porosity mathematical model is established to investigate the CO2 sequestration with enhanced natural gas recovery (CSEGR) in shale gas reservoirs. Meanwhile, the transfer function and well flow model considering the multiple mechanisms are developed. The mathematical model is implemented by the finite element simulation software COMSOL, and the accuracy of the numerical solution is verified by comparison with the classic analytical solution. Three sets of simulations were conducted to investigate the influence of CO2 injection on the development of shale gas reservoirs. The results show that CSEGR is feasible to achieve CO2 sequestration and enhance CH4 recovery in shale gas reservoirs, the CO2 storage and natural gas production rate can be improved highly with the increase of injection pressure. The CSEGR process can be divided into three stages including the early depressurization production period, the intermediate CO2 adsorption and CH4 replacement period and the late period of CH4 and CO2 produced simultaneously. In addition, the concentration variation and storage pattern of CH4 and CO2 gas are analyzed during the CSEGR process in shale gas reservoirs.

Numerical studies of methane production from Class 1 gas hydrate accumulations enhanced with carbon dioxide injection

Class 1 gas hydrate accumulations are characterized by a permeable hydrate-bearing interval overlying a permeable interval with mobile gas, sandwiched between two impermeable intervals. Depressurization-induced dissociation is currently the favored technology for producing gas from Class 1 gas hydrate accumulations. The depressurization production technology requires heat transfer from the surrounding environment to sustain dissociation as the temperature drops toward the hydrate equilibrium point and leaves the reservoir void of gas hydrate. Production of gas hydrate accumulations by exchanging carbon dioxide with methane in the clathrate structure has been demonstrated in laboratory experiments and proposed as a field-scale technology. The carbon dioxide exchange technology has the potential for yielding higher production rates and mechanically stabilizing the reservoir by maintaining hydrate saturations. We used numerical simulation to investigate the advantages and disadvantages of using carbon dioxide injection to enhance the production of methane from Class 1 gas hydrate accumulations. Numerical simulations in this study were primarily concerned with the mechanisms and approaches of carbon dioxide injection to investigate whether methane production could be enhanced through this approach. To avoid excessive simulation execution times, a five-spot well pattern with a 500-m well spacing was approximated using a two-dimensional domain having well boundaries on the vertical sides and impermeable boundaries on the horizontal sides. Impermeable over- and under burden were included to account for heat transfer into the production interval. Simulation results indicate that low injection pressures can be used to reduce secondary hydrate formation and that direct contact of injected carbon dioxide with the methane hydrate present in the formation is limited due to bypass through the higher permeability gas zone.

Geomechanical response of permafrost-associated hydrate deposits to depressurization-induced gas production

In this simulation study, we analyzed the geomechanical response during depressurization production from two known hydrate-bearing permafrost deposits: the Mallik (Northwest Territories, Canada) deposit and Mount Elbert (Alaska, USA) deposit. Gas was produced from these deposits at constant pressure using horizontal wells placed at the top of a hydrate layer (HL), located at a depth of about 900 m at the Mallik site and 600 m at the Mount Elbert site. The simulation results show that general thermodynamic and geomechanical responses are similar for the two sites, but with substantially higher production and more intensive geomechanical responses at the deeper Mallik deposit. The depressurization-induced dissociation begins at the well bore and then spreads laterally, mainly along the top of the HL. The depressurization results in an increased shear stress within the body of the receding hydrate and causes a vertical compaction of the reservoir. However, its effects are partially mitigated by the relatively stiff permafrost overburden, and compaction of the HL is limited to less than 0.4%. The increased shear stress may lead to shear failure in the hydrate-free zone bounded by the HL overburden and the downward-receding upper dissociation interface. This zone undergoes complete hydrate dissociation, and the cohesive strength of the sediment is low. We determined that the likelihood of shear failure depends on the initial stress state as well as on the geomechanical properties of the reservoir. The Poisson's ratio of the hydrate-bearing formation is a particularly important parameter that determines whether the evolution of the reservoir stresses will increase or decrease the likelihood of shear failure.