Pore pressure built-up in hydrate-bearing sediments during phase transition: A poromechanical approach

Due to the density contrast between the hydrate and methane gas,the pore pressure is accumulated in the sediment during the decomposition process of methane hydrate.This accumulation of pore pressure decreases the magnitude of effective stress,further triggering potential geological disasters such as landslide.This paper establishes a theoretical framework to investigate the evolution of fluid pressure in the hydrate-bearing sediments during the decomposition process.This model consists of two parts:an unsaturated thermo-poromechanical constitutive law as well as a phase equilibrium equation.Compared with the existing studies,the present work incorporates the effect of pore volume change into the pressure built-up model.In addition,the capillary effect is considered,which plays a nontrivial role in fine-grained sediments.Based on this model,the evolution of fluid pressure is investigated in undrained conditions.It is shown that four mechanisms mainly contribute to the pressure built-up:the density contrast between decomposing hydrate and producing fluid,the variation of pore volume,the compaction of hydrate due to variation of capillary pressure,and the thermal deformation of pore constituents induced by temperature change.Among these mechanisms,the density contrast dominates the pore pressure accumulation.Under the combined effect of these contributions,the evolution of fluid pressure exhibits a strong nonlinearity during the decomposition process and can reach up to dozens of mega Pascal.Nevertheless,this high-level pressure built-up results in a significant tensile strain,yielding potential fracturing of the sediment.

Elastic behavior of saturated porous materials under undrained freezing

The elastic behavior of saturated porous materi- als under undrained freezing is investigated by using a poro- mechanical approach. Thermodynamic equilibria are used to describe the crystallization process of the partially frozen solution in bulk state and confined state in pores. By phase transition at freezing, fusion energy, thermal contraction of solid, solution and ice crystals, volume changes of crystallization build up remarkable pore pressure that induces expansion or shrinkage of solid matrix. Owing to the lower chemical potential when pore water mixes with salts, fewer ice forms in pores. Penetration of ice into the porous materials increases the capillary pressure, but limits effect on the pore liquid pressure and the strain of solid matrix. On the contrary, the pore pressure induced by solution density rises as salt concentration increases and causes significant shrinkage of solid matrix.

A fully coupled finite element framework for thermal fracturing simulation in subsurface cold CO2 injection

Thermal fracturing could occur during cold CO2 injection into subsurface warm rock formations.It can be seen in a variety of fields such as carbon geo-sequestration,unconventional gas development,enhanced oil recovery,geothermal energy extraction,and energy geological storage systems.In CO2 geosequestion,limited degree of thermal fracturing due to the cooling effects of cold CO2 injection will enhance well injectivity,especially for those storage formations of low permeability.Thermal fracturing can therefore potentially enhance the injection efficiency and make positive impact on commercialization of CO2 geological storage.However,excessively developed fractures could break down the caprock and cause potential CO2 leakage into overlying rock formations.Risk analysis has to be done based on thermal fracturing simulation in order to maintain caprock integrity.Simulation of thermal fracturing during cold CO2 injection involves the coupled processes of heat transfer,mass transport,rock deforming as well as fracture propagation.To model such a complex coupled system,a fully coupled finite element framework for thermal fracturing simulation is presented.This framework is based on the theory of non-isothermal multiphase flow in fracturing porous media.It takes advantage of recent advances in stabilized finite element and extended finite element methods.The stabilized finite element method overcomes the numerical instability encountered when the traditional finite element method is used to solve the convection dominated heat transfer equation,while the extended finite element method overcomes the limitation with traditional finite element method that a model has to be remeshed when a fracture is initiated or propagating and fracturing paths have to be aligned with element boundaries.

Numerical Analysis for an Energy-stable Total Discretization of a Poromechanics Model with Inf-sup Stability

We consider a previously proposed general nonlinear poromechanical formulation, and we derive a linearized version of this model. For this linearized model, we obtain an existence result and we propose a complete discretization strategy – in time and space – with a special concern for issues associated with incompressible or nearly-incompressible behavior. We provide a detailed mathematical analysis of this strategy,the main result being an error estimate uniform with respect to the compressibility parameter. We then illustrate our approach with detailed simulation results and we numerically investigate the importance of the assumptions made in the analysis, including the fulfillment of specific inf-sup conditions.