Hydraulic path dependence of shear strength for compacted loess

Shear strength is an essential geotechnical parameter for assessing the landslide potential of loess slopes under rainfall infiltration and farm irrigation conditions on the loess plateau.However,the hydraulic path dependence of shear strength for compacted loess under varying rainfall infiltration conditions has not been thoroughly investigated yet.To this end,a series of direct shear tests and nuclear magnetic resonance(NMR)measurements are carried out on compacted loess.The shear strength tests were continuously implemented on loess specimens under scanning wetting paths besides initial drying paths.The experimental data quantitatively verify the significant effect of hydraulic paths applied to specimens on shear strength of compacted loess.The unique failure envelope of shear strength of loess is identified under the effective stress framework based on intergranular stress,which verifies that the effective stress framework can consider the effect of hydraulic paths on shear strength.Based on the effective stress,a shear strength formula is proposed to characterize shear strengths under varying hydraulic paths,in which the parameters from the soil-water retention curve and shear strength at saturated state are simply required.The proposed shear strength formula can properly predict the measured shear strength data of compacted loess experiencing three hydraulic paths.Furthermore,the distribution curves of transverse relaxation time for pore water in soil under varying hydraulic paths are simultaneously measured using the NMR method.The physical mechanism for the difference in shear strength of loess subjected to different hydraulic paths can be uncovered based on soil-water evolutions in pores in microscale.

Effect of hydraulic fracture deformation hysteresis on CO_(2)huff-n-puff performance in shale gas reservoirs

As a promising enhanced gas recovery technique,CO_(2)huff-n-puff has attracted great attention recently.However,hydraulic fracture deformation hysteresis is rarely considered,and its effect on CO_(2)huff-n-puff performance is not well understood.In this study,we present a fully coupled multi-component flow and geomechanics model for simulating CO_(2)huff-n-puff in shale gas reservoirs considering hydraulic fracture deformation hysteresis.Specifically,a shale gas reservoir after hydraulic fracturing is modeled using an efficient hybrid model incorporating an embedded discrete fracture model(EDFM),multiple porosity model,and single porosity model.In flow equations,Fick’s law,extended Langmuir isotherms,and the Peng-Robinson equation of state are used to describe the molecular diffusion,multi-component adsorption,and gas properties,respectively.In relation to geomechanics,a path-dependent constitutive law is applied for the hydraulic fracture deformation hysteresis.The finite volume method(FVM)and the stabilized extended finite element method(XFEM)are applied to discretize the flow and geomechanics equations,respectively.We then solve the coupled model using the fixed-stress split iterative method.Finally,we verify the presented method using several numerical examples,and apply it to investigate the effect of hydraulic fracture deformation hysteresis on CO_(2)huff-n-puff performance in a 3D shale gas reservoir.Numerical results show that hydraulic fracture deformation hysteresis has some negative effects on CO_(2)huff-n-puff performance.The effects are sensitive to the initial conductivity of hydraulic fracture,production pressure,starting time of huff-n-puff,injection pressure,and huff-n-puff cycle number.