Numerical Simulation of Gas Hydrate Production Using the Cyclic Depressurization Method in the Ulleung Basin of the Korea East Sea

Study of the Appropriate Well Types and Parameters for the Safe and Efficient Production of Marine Gas Hydrates in Unconsolidated Reservoirs

The majority of marine hydrates are buried in unconsolidated or poorly consolidated marine sediments with limited cementation and strength. As a result, hydrate decomposition during production may cause the significant subsidence of the formation, necessitating a halt in production. The numerical model of unconsolidated hydrate formation, based on geomechanics, was established in order to elucidate the depressurization production process. The sensitive factors of unconsolidated hydrate production were determined by analyzing the influence of formation parameters and production parameters on gas production. Then, a safety formation subsidence was proposed in this paper, and the appropriate well type and parameters for the safe and efficient production of hydrates in unconsolidated formations of various saturations were determined. The sensitivity of gas production to the formation parameters was in the order of formation porosity, hydrate saturation, and buried depth, while the effects of the production parameters were BHP (bottom hole pressure), horizontal length, and the heat injection, in descending order. For hydrate reservoirs in the South China Sea, when the hydrate saturation is 20%, a horizontal well is necessary and the appropriate horizontal length should be less than 80 m. However, when hydrate saturation is more than 30%, a vertical well should be selected, and the appropriate bottom hole pressure should be no less than 3800 kPa and 4800 kPa for 30% and 40% saturation, respectively. Based on the simulation results, hydrate saturation was the key factor by which to select an appropriate production technique in advance and adjust the production parameters. The study has elucidated the depressurization production of marine unconsolidated hydrate formations at depth, which has numerous implications for field production.

Modeling of a Double Gas Hydrate Particle Ignition

This paper presents the numerical research findings for the conditions and characteristics of methane-propane hydrate particle ignition. The curves of the ignition delay times of a hydrate particle versus its size and ambient temperature were obtained. The effect of the rates of phase transformations (evaporation and dissociation) on the hydrate particle ignition behavior was analyzed. Following the mathematical modeling of the processes under study using different heating schemes of gas hydrates, the patterns of processes developing in a particle during the induction period were identified. It was established that the ignition behavior of methane, propane, and other gases was significantly different from that of other gases produced from hydrate decomposition. The established differences form the basis for predicting the characteristics of gas hydrate ignition at different power plants.

Geomechanically Sustainable Gas Hydrate Production Using a 3D Geological Model in the Ulleung Basin of the Korean East Sea

Although various simulation studies on gas hydrate production have been conducted, a single vertical well in the cylindrical system has been adopted in most research. However, this system has a limited ability to predict commercial production in gas hydrate reservoirs. In order to facilitate commercial production, a field-scale reservoir model with a multi-well system must be constructed using geological data, such as seismic data, well logging data, core data, etc. The depressurization method is regarded as a practical production strategy because it has high levels of production efficiency and economical effectiveness. However, this method can lead to subsidence due to the increased effective stress. In this work, we studied a production simulation strategy for commercial gas hydrate production. A three-dimensional geological model with a realistic field scale is constructed using seismic and well logging data from the Ulleung Basin of the Korean East Sea. All of the grids are refined in the I and J direction, and the grids near the production well are very small to consider realistic hydrate dissociation. The cyclic depressurization method is adopted for the increase in the geomechanical stability, rather than the non-cyclic depressurization method. Various case studies are conducted with alternating bottomhole pressures for the primary and secondary depressurization stages over 100 days. Geomechanical stability is significantly enhanced, while cumulative gas production is relatively less reduced or nearly maintained. In particular, all cases of the cumulative gas production at 6 MPa during the secondary depressurization stage are similar to the non-cyclic case, while the geomechanical stabilities of those cases are restored. This study is thought to have contributed to the development of technology for commercial gas hydrate production with a geomechanical stability study using a reservoir-scale model with a multi-well system.