Non-isothermal, multi-phase, multi-component flows through deformable methane hydrate reservoirs

We present a hydro-geomechanical model for subsurface methane hydrate systems. Our model considers kinetic hydrate phase change and non-isothermal, multi-phase, multi-component flow in elastically deforming soils. The model accounts for the effects of hydrate phase change and pore pressure changes on the mechanical properties of the soil. It also accounts for the effect of soil deformation on the fluid-solid interaction properties relevant to reaction and transport processes (e.g., permeability, capillary pressure, and reaction surface area). We discuss a ’cause-effect’ based decoupling strategy for the model and present our numerical discretization and solution scheme. We then proceed to identify the important model components and couplings which are most vital for a hydro-geomechanical hydrate simulator, namely, (1) dissociation kinetics, (2) hydrate phase change coupled with non-isothermal two phase two component flow, (3) two phase flow coupled with linear elasticity (poroelasticity coupling), and finally (4) hydrate phase change coupled with poroelasticity (kinetics-poroelasticity coupling). To show the versatility of our hydrate model, we numerically simulate test problems where, for each problem, we methodically isolate one out of the four aforementioned model components or couplings. A special emphasis is laid on the kinetics-poroelasticity coupling for which we present a test problem where an axially loaded hydrate bearing sand sample experiences a spontaneous shift in the hydrate stability curve causing the hydrate to melt. For this problem, we present an analytical solution for pore-pressure, which we subsequently use to test the accuracy of the numerical scheme. Finally, we present a more complex 3D example where all the major model components are put together to give an idea of the model capabilities. The setting is based on a subsurface hydrate reservoir which is destabilized through depressurization using a low pressure gas well. In this example, we simulate the melting of hydrate, methane gas generation, and the resulting ground subsidence and stress build-up in the vicinity of the well.