A coupled thermal–hydraulic–mechanical–chemical (THMC) model for methane hydrate bearing sediments using COMSOL Multiphysics

Abstract

Methane gas extraction by a deep well installed in methane hydrate bearing sediments (MHBS) found in deep subsea and permafrost regions is a coupled thermal–hydraulic–mechanical–chemical (THMC) process. The key processes include heat convection between layers, local deformation due to compaction, and stress relaxation caused by damage of the bonded structure. As improper production may induce formation compaction, sand production, and wellbore failures, a numerical code is needed to simulate the THMC processes during methane gas production so that geomechanics and production risks can be quantified. In this study, a nonlinear THMC model was implemented in the partial differential equations (PDE) and structural mechanics module of the COMSOL Multiphysics^® finite element code. This paper describes the non-linear coupled governing equations of the mechanical behavior during hydrate dissociation. In particular, it introduces a new thermodynamics-based constitutive model to simulate the mechanical behavior of hydrate bearing sediments. The performance of the newly developed code was examined by comparing the computed results with test data and other simulation results. The differences between fully coupled and semi-coupled models were analyzed. For example, heterogeneous turbidite layers observed in the Nankai Trough were modeled, and behaviors such as heat convection between different layers, shear stress and strain concentration were examined.

摘要

目的

水合物沉积物开采过程是一个热-水-力-化多场耦合过程,该过程包含了不同土层间的热对流、压缩引起的局部变形以及胶结结构破坏引起的应力松弛。不适当的开采会引起出砂、塌孔等破坏问题。本文旨在建立天然气水合物沉积物多场耦 合计算模型,以量化由开采引起的地质灾害风 险。

创新点

1. 通过COMSOL Multiphysics 实现水合物开采过程多场耦合有限元控制方程的计算;2. 建立的模型考虑变形-渗流双向全耦合过程。

方法

1. 通过理论推导,给出开采天然气水合物过程模拟的控制方程;采用偏微分方程模块实现除力学之外其他物理场的耦合计算;采用结构力学模块实现变形计算。2. 通过与试验数据进行比较验证模型的可靠性。3. 通过对比全耦合模型与半耦合 模型,分析双向耦合对水合物开采过程中沉积物 物理力学行为的影响。

结论

1. 所建立模型能够精确模拟水合物开采过程中沉积物的物理力学行为。2. 当考虑压缩对渗流的影响时,由于孔隙率的降低,计算得到的水合物分解速度要小于不考虑该影响时的速度。3. 由于存在层间对流效应,非均质模型计算得到的水合物 分解速度要快于均质模型。