Numerical analysis of thermal impact on hydro-mechanical properties of clay

As is known, high-level radioactive waste （HLW） is commonly heat-emitting. Heat output from HLWwilldissipate through the surrounding rocks and induce complex thermo-hydro-mechanical-chemical（THMC） processes. In highly consolidated clayey rocks, thermal effects are particularly significantbecause of their very low permeability and water-saturated state. Thermal impact on the integrity of thegeological barriers is of most importance with regard to the long-term safety of repositories. This studyfocuses on numerical analysis of thermal effects on hydro-mechanical properties of clayey rock using acoupled thermo-mechanical multiphase flow （TH2M） model which is implemented in the finite elementprogramme OpenGeoSys （OGS）. The material properties of the numerical model are characterised by atransversal isotropic elastic model based on Hooke＇s law, a non-isothermal multiphase flow model basedon van Genuchten function and Darcy＇s law, and a transversal isotropic heat transport model based onFourier＇s law. In the numerical approaches, special attention has been paid to the thermal expansion ofthree different phases： gas, fluid and solid, which could induce changes in pore pressure and porosity.Furthermore, the strong swelling and shrinkage behaviours of clayey material are also considered in thepresent model. The model has been applied to simulate a laboratory heating experiment on claystone.The numerical model gives a satisfactory representation of the observed material behaviour in thelaboratory experiment. The comparison of the calculated results with the laboratory findings verifies thatthe simulation with the present numerical model could provide a deeper understanding of the observedeffects. 2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

Desorption hysteresis of coalbed methane and its controlling factors: a brief review

Most coal reservoirs show high gas content with relatively low desorption efficiency,which restricts the efficiency of coalbed methane(CBM)extraction and single-well productivity.This review highlights the desorption hysteresis mechanism and its controlling factors as well as methods and models to reveal desorption hysteresis and potential solutions.Methane adsorption and desorption can be recorded by both gravimetric and volumetric experiments.Although different adsorption models are used,desorption is generally considered with the Langmuir model.Desorption hysteresis is influenced by the petrophysical composition,thermal maturity,pore structure distribution of the coal,reservoir temperature,and moisture and water content.Methods for calculating desorption hysteresis include the area index,hysteresis index and introduction of a hysteresis factor and a hysteresis coefficient.Molecular dynamics simulations of methane desorption are mainly based on theories of kinetics,thermodynamics,and potential energy.The interaction forces operating among coal,water,and methane molecules can be calculated from microscopic intermolecular forces(van der Waals forces).The desorption hysteresis mechanism and desorption process still lack quantitative probe methodologies,and future research should focus on coal wettability under the constraints of liquid content,potential energy adjustment mechanism,and quantitative analysis of methane desorption rates.Further research is expected to reveal the desorption kinetics of methane through the use of the solid–liquid–gas three-phase coupling theory associated with the quantitative analysis of methane desorption hysteresis,thereby enhancing the recovery rate and efficiency of CBM wells.