Numerical investigation on hydrate reservoir deformation induced by depressurization production and analysis of CO2 reinjection potential

Abstract

Depressurization production causes reservoir deformation to change the physical and mechanical properties, thus affecting the fluid flow and production performance. The mechanical deformation and gas production characteristics of multilayer hydrate reservoir at the first depressurization production site in the Shenhu area need to be further simulated and investigated. In this paper, a multilayer hydrate reservoir model is established based on the real logging data of SHSC-4 well, and the simulation results are compared with the test production results to verify the model validity. The production performance and reservoir stability are evaluated by considering reservoir deformation and gas production behavior, and the CO₂ reinjection potential of the multilayer reservoir after production is analyzed by numerical methods. Low production pressure can serve to increase cumulative gas production, but reservoir deformation can also be an unfavorable factor hindering gas production. The negative effects of reservoir deformation caused by depressurization on gas production results need to be considered when numerical methods are used to evaluate reservoir production performance or optimize production design. Percentage contribution of free gas layer (FGL) decreases with the reduction of production pressure, and the gas production from the reservoir is mainly from hydrate-bearing layer (HBL) and three phase layer (TPL). There is a turning point in the production performance of HBL and TPL around 3 MPa. The gas production performance of HBL is better than TPL when the production pressure is lower than 3 MPa, and the percentage contribution of HBL and TPL are about 40% under different initial inherent permeability conditions. Permeability enhancement measures promote the propagation of low pore pressure in the reservoir, which is prone to cause large reservoir deformation. CO₂ reinjection leads to reservoir uplift around production well, and stress concentration distribution induced by depressurization production are mitigated. TPL has better CO₂ reinjection potential than FGL and HBL, and it accounts for about 50% of the total reinjected gas.