• Energy Research

To deal with sand production problems during the process of producing natural gas from hydrate-bearing sediments (HBS) using reservoir-fluid extraction method, a new gravel sizing method for sand control packing named “Hold coarse while eliminate fine particle (HC & EF method)” was developed for the clayey hydrate-bearing formations. Site X, in Shenhu area, South China Sea was taken as an example to describe detailed gravel sizing procedure. On the basis of analyzing basic particle size distribution (PSD) characteristics of HBS at Site X, the formation sand was divided into two components, which are coarse component and fine component. The gravel sizes for retaining coarse component and eliminate fine component were calculated, respectively. Finally, intersection of these two gravel sizes was taken as the proper gravel size for Site X. The research results show that the formation at Site X is clayey sand with poor sorting and uniformity, proper gravel size for upper segment packing is 143−215 μm, while that for lower segment packing is 240−360 μm. In consideration of the difficulty of layered sand control operation on offshore platform, proper gravel packing size for Site X is recommended as 215−360 μm. Key words: gas hydrate, production test, sand management, gravel sizing, South China Sea, Shenhu area

To deal with sand production problems during the process of producing natural gas from hydrate-bearing sediments (HBS) using reservoir-fluid extraction method, a new gravel sizing method for sand control packing named “Hold coarse while eliminate fine particle (HC & EF method)” was developed for the clayey hydrate-bearing formations. Site X, in Shenhu area, South China Sea was taken as an example to describe detailed gravel sizing procedure. On the basis of analyzing basic particle size distribution (PSD) characteristics of HBS at Site X, the formation sand was divided into two components, which are coarse component and fine component. The gravel sizes for retaining coarse component and eliminate fine component were calculated, respectively. Finally, intersection of these two gravel sizes was taken as the proper gravel size for Site X. The research results show that the formation at Site X is clayey sand with poor sorting and uniformity, proper gravel size for upper segment packing is 143−215 μm, while that for lower segment packing is 240−360 μm. In consideration of the difficulty of layered sand control operation on offshore platform, proper gravel packing size for Site X is recommended as 215−360 μm. Key words: gas hydrate, production test, sand management, gravel sizing, South China Sea, Shenhu area

Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.

Experiments were carried out by reacting H(2) gas with N(2) hydrate at a temperature of 243 K and a pressure of 15 MPa. The characterizations of the reaction products indicated that multiple H(2) molecules can be loaded into both large and small cages of structure II clathrate hydrates. The realization of multiple H(2) occupancy of hydrate cages under moderate conditions not only brings new insights into hydrogen clathrates but also refreshes the perspective of clathrate hydrates as hydrogen storage media.

Abstract Blockage of sand-control media is one of the main obstacles that affect gas production efficiency from gas hydrate-bearing sediments. Hydrate reformation is a potential plugging inducer of sand-control media. In this study, we conduct a series of experiments using steel-wired screen mesh to examine the hydrate-induced clogging of sand-control screen. The screen mesh sample was installed into a closed-circuit circulating system under gas-water two-phase flow condition to simulate bottomhole multiphase production processes. The results indicate that hydrate formation within the screen would causes permeability loss of the screen up to 98%. The pseudo-permeability of the screen sample shows dual-gradient decreasing characteristics during hydrate clogging. Therefore, we speculate that hydrate accumulating and hydrate particle bridging are the two main mechanisms causing screen plugging. The hydrate accumulating sub-process is mainly controlled by the degree of subcooling, while the hydrate particle-bridging sub-process is affected by fluid flow rate. Furthermore, the “J-shape” coiling of pressure-temperature relationship can be used as an indicator in diagnosing possible bottomhole screen plug during hydrate exploitation. Artificial interference of downhole temperature is strongly recommended to mitigate screen plugging induced by hydrate reformation.

Abstract Blockage of sand-control media is one of the main obstacles that affect gas production efficiency from gas hydrate-bearing sediments. Hydrate reformation is a potential plugging inducer of sand-control media. In this study, we conduct a series of experiments using steel-wired screen mesh to examine the hydrate-induced clogging of sand-control screen. The screen mesh sample was installed into a closed-circuit circulating system under gas-water two-phase flow condition to simulate bottomhole multiphase production processes. The results indicate that hydrate formation within the screen would causes permeability loss of the screen up to 98%. The pseudo-permeability of the screen sample shows dual-gradient decreasing characteristics during hydrate clogging. Therefore, we speculate that hydrate accumulating and hydrate particle bridging are the two main mechanisms causing screen plugging. The hydrate accumulating sub-process is mainly controlled by the degree of subcooling, while the hydrate particle-bridging sub-process is affected by fluid flow rate. Furthermore, the “J-shape” coiling of pressure-temperature relationship can be used as an indicator in diagnosing possible bottomhole screen plug during hydrate exploitation. Artificial interference of downhole temperature is strongly recommended to mitigate screen plugging induced by hydrate reformation.

Optimization of the depressurization pathways plays a crucial role in avoiding potential geohazards while increasing hydrate production efficiency. In this study, methane hydrate was formed in a flexible plastic vessel and then gas production processes were conducted at constant confining pressure and constant confining temperature. The CMG-STARS simulator was applied to match the experimental gas production behavior and to derive the hydrate intrinsic dissociation constant. Secondly, fluid production behavior, pressure-temperature ( P ‐ T ) responses, and hydrate saturation evolution behaviors under different depressurization pathways were analyzed. The results show that integrated gas-water ratio (IGWR) decreases linearly with the increase in depressurizing magnitude in each step, while it rises logarithmically with the increase in the number of steps. Under the same initial average hydrate saturation and the same total pressure-drop magnitude, a slow and multistage depressurization strategy would help to increase the IGWR and avoid severe temperature drop. The pore pressure rebounds logarithmically once the gas production is suspended, and would decrease to the regular level instantaneously once the shut-in operation is ended. We speculate that the shut-in operation could barely affect the IGWR and formation P ‐ T response in the long-term level.

Optimization of the depressurization pathways plays a crucial role in avoiding potential geohazards while increasing hydrate production efficiency. In this study, methane hydrate was formed in a flexible plastic vessel and then gas production processes were conducted at constant confining pressure and constant confining temperature. The CMG-STARS simulator was applied to match the experimental gas production behavior and to derive the hydrate intrinsic dissociation constant. Secondly, fluid production behavior, pressure-temperature ( P ‐ T ) responses, and hydrate saturation evolution behaviors under different depressurization pathways were analyzed. The results show that integrated gas-water ratio (IGWR) decreases linearly with the increase in depressurizing magnitude in each step, while it rises logarithmically with the increase in the number of steps. Under the same initial average hydrate saturation and the same total pressure-drop magnitude, a slow and multistage depressurization strategy would help to increase the IGWR and avoid severe temperature drop. The pore pressure rebounds logarithmically once the gas production is suspended, and would decrease to the regular level instantaneously once the shut-in operation is ended. We speculate that the shut-in operation could barely affect the IGWR and formation P ‐ T response in the long-term level.