Research progress on rock mechanical behavior under concept of reconstructed deep in-situ multi-physics conditions

LU Mingyuan; ZHOU Hongwei; WANG Jun; MAO Tingting; PENG Ruidong; XUE Dongjie

doi:10.13225/j.cnki.jccs.CQ25.0861

Abstract：With the increasing demand for underground resources in China,deep geomechanical issues have become a critical constraint on the advancement of mining,oil,gas,and geothermal engineering into the deeper crust.Deep rocks are typically subjected to coupled in-situ multi-physics conditions,characterized by high stress,high pore pressure,and high temperature.Their mechanical behavior is therefore governed by the in-situ conditions.However,the conventional concept of rock mechanical experiments lacks correspondence with the in-situ conditions,making it difficult to reveal their intrinsic mechanical behavior.To address this gap,the concept of deep in-situ multi-physics conditions is introduced,and a series of stress-,pore pressure-,and temperature-reconstructed experiments on 1 000-6 400 m cores from the Songliao Basin are summarized.Stress-strain responses under reconstructed stress and pore pressure conditions are further pre-dicted through numerical modeling.In addition,NMR experiments on 800 m coal samples from the Zhaolou Coal Mine in Shandong Province are used to validate the thermally induced response mechanisms of porous media under variable tem-perature conditions.The results show that:under in-situ stress-reconstructed experiments,applying time-dependent stress boundaries equivalent to the in-situ conditions can effectively suppress stress release effects in deep cores.This procedure promotes microcrack closure and intergranular contact,thereby enhancing structural compactness and load-bearing capa-city.Compared to conventional triaxial experiments,the elastic modulus and peak deviatoric stress increase by approxim-ately 10%,and the results further highlight the interplay between in-situ stress and diagenetic composition on the mechan-ical behavior of rocks at different depths.Under in-situ pore pressure-reconstructed experiments,the introduction of non-uniform pore pressure generally leads to a reduction in mechanical strength,with this weakening effect diminishing with increasing depth(confining stress).This suggests that in-situ pore pressure exerts a weakening influence on rock strength,though its magnitude is constrained by the confining stress.Under in-situ temperature-reconstructed experiments,coal and sandstone samples exhibit a non-monotonic variation in porosity,elastic modulus,and Poisson's ratio across the temperat-ure range from room to in-situ and over-in-situ conditions.These results confirm a thermoelastic recovery mechanism within the room-to-in-situ temperature range,with noticeable thermo-damage and thermo-plastic behavior occurring only beyond the in-situ temperature threshold.Overall,these findings demonstrate that in-situ condition reconstructions offer reliable methods for restoring in-situ boundaries at the laboratory scale,thereby effectively reducing the mechanical devi-ation caused by the mismatch of experimental boundaries.Based on this,a three-stage experimental approach,including"Reconstruction-Variable Loading-Response,"is proposed for deep solid resource development.In this framework,recon-structed in-situ conditions serve as the experimental baseline,while conditional variable loading acts as the intermediate process linking engineering disturbances to rock mechanical responses.This path-dependent approach enables the charac-terization of deep rock behavior across the full transition from stable in-situ states to disturbed and unstable failure re-gimes,providing an expandable experimental methodology for synchronized parameter testing,stability evaluation,and failure mechanism identification in the development of deep underground resources.