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The coalbed methane (CBM) desorption area is highly important in the placement of wells to produce an adequate working system. This paper develops a homogeneous seepage model of CBM that incorporates desorption and uses a finite-element method to obtain the pressure field in complex boundary cases influenced by adjacent wells. The pressure field is presented in a 2D and pseudo-3D form that visually demonstrates the shape of the pressure profile and the pressure expansion. A method of combined numerical computation and contour drawing technology is used to determine the CBM desorption area and pressure drop region. By analyzing the main factors that affect the CBM desorption region, we demonstrate that a higher value of critical desorption pressure yields a larger desorption area of the seam. The effect of the permeability is different from the value of the critical desorption. This effect is determined by the critical desorption pressure level and the intersection of pressure profiles with different permeabilities. Permeability anisotropy leads to a desorption area with an elliptical shape. We also demonstrate that a greater flux of methane will result in a greater desorption region, and the effects of the distance between wells and the desorption area are closely related to the permeability. The developed model is applied in a field case to predict the desorption area. [ABSTRACT FROM AUTHOR]

Based on the characteristics of fractures in naturally fractured reservoir and the discrete-fracture model, a fracture network numerical well test model was developed, which incorporated the fractures explicitly in the spatial domain. In two-dimensional problems, fractures were represented as one-dimensional line element and matrix space was discretized into linear triangular elements. Bottom hole pressure response curves and pressure field were obtained by solving the model equations with finite-element method. Through analyses of the bottom hole pressure curves and the fluid flow in pressure field, seven flow stages can be recognized on the curves, i.e. wellbore storage, transition, fracture linear flow, fracture network flow, matrix to fractures flow, system radial flow and boundary-dominated flow. Effects of reservoirs parameters, such as fracture conductivity, matrix permeability, fracture density and permeability anisotropy were studied. The analysis results demonstrated that fracture conductivity played a leading role in the fluid flow. Matrix permeability influenced the beginning time of flow from matrix to fractures. Fractures density was another important parameter controlling the flow. The fracture linear flow was covered under large fracture density. The pressure propagation was slower in the direction of larger fracture density. The same situation happened in the permeability anisotropy cases. [ABSTRACT FROM AUTHOR]

Gas production from hydrates induced by depressurization is a complex thermal-hydrodynamic-mechanical–chemical (THMC) coupled process. In this paper, we present a THMC coupled model to simulate the fluid flow in hydrate-bearing sediments (HBS) and the geomechanical behavior of HBS. The model is made of two subsystems, which are the fluid part of non-isothermal multi-phase flow with hydrate kinetic and solid part of geomechanical deformation. It accounts for two-way coupling effects between these two subsystems, i.e. the effect of pore pressure and hydrate dissociation on the solid mechanical behavior and the effect of stress on the hydraulic behavior. A new numerical method based on the hybrid control volume finite element method (CVFEM)-finite element method (FEM) is developed to solve the mathematical models. The local conservative CVFEM is used for the fluid part, and the standard FEM for the solid part. In the framework of hybrid CVFEM-FEM, the local conservation is reserved and the primary variables for the two subsystem are co-located. A multi-point flux approximation (MPFA) is adopted without orthogonal meshes so that it is very flexible to build complex geometrical models. The accuracy and reliability of the newly developed simulator QIMGHyd-THMC are tested by comparing with two experimental examples and a large-scale benchmark problem of other popular simulators. • A fluid-solid coupled mathematical framework is proposed for gas hydrate production. • A hybrid control volume finite element method and finite element method is presented. • Numerical method preserves local conservation for solving of fluid subsystem. • Co-located variable for simulation of fluid and solid is achieved. • Method and software are validated by experiments and comparing with other simulators. [ABSTRACT FROM AUTHOR]