Numerical Investigation of the Depressurization Exploitation Scheme of Offshore Natural Gas Hydrate: Enlightenments for the Depressurization Amplitude and Horizontal Well Location

Abstract

According to the recent test mining of marine natural gas hydrate (NGH), extensive indoor experimental and numerical simulation studies, depressurization is regarded as a relatively economical and mature choice. For the characteristics of NGH-bearing sediments, seabed settlement during the NGH exploitation process is inevitable. It is crucial to work on designing the hydrate exploitation scheme according to the gas production efficiency and subsidence. NGH production involves intricate thermal–hydrodynamic–mechanical phenomena, necessitating the use of numerical simulation tools to evaluate associated geo-mechanical responses. The self-developed numerical simulator repeats the previous research of relevant scholars and verifies the accuracy of the model. Based on the logging data of the Shenhu Sea area in the South China Sea, the research model is established. We investigated the case simulation of long-term production using the horizontal well depressurization method by adjusting the target pressure and wellbore location. The results show that increasing the depressurization amplitude can not only significantly improve the production efficiency but also increase the settling effect of the seabed formation. Increasing the wellbore layout depth can fully use the temperature and pressure conditions of the hydrate reservoir itself, thereby improving production efficiency. However, it also expands the scope of the overall settlement. In addition, it should also pay attention to the boundary effect between the decomposition range of hydrate and the overburden and underburden. The initial findings showcased in this research could enhance our comprehension of production and geomechanical behavior in the process of hydrate mining in the sea area and have a particular guiding significance for future mining scheme design.

Well Location Evaluation of Depressurized Production from the Multilayer Gas Hydrate Reservoir

“…Some studies indicate that the vertical location of horizontal wells significantly influences production efficiency, impacted by reservoir temperature–pressure gradients and variations in gas and water densities. − Moreover, depressurization-induced pore pressure reduction can increase the effective stress within the NGH reservoir, potentially causing stratum deformation or even shear damage. − By adjusting the vertical location of the horizontal well, the gas production and reservoir subsidence can be significantly changed. − However, current assessments of horizontal well locations exhibit a weak integration of gas production and geomechanical response. Meanwhile, some NGH reservoirs have been shown to have a complex phase condition with multiple layers from top to bottom, including a hydrate-bearing layer (HBL), three-phase layer (TPL), and free-gas layer (FGL), ,, and this will further bring uncertainties.…”