Study on the Elasto Viscoplastic Constitutive Model of Shale Influence on Fracture Conductivity

ABSTRACT Shale oil and gas reservoir is a new hotspot of oil and gas exploration and development. It is very important for fracturing production to produce artificial fractures with sufficient conductivity. Shale clay and organic matter content is high, which is easy to creep, and it makes the proppant embedded in the formation and affects the fracture conductivity.In this paper, an elasto-viscoplastic constitutive model is proposed to study the influence of shale creep on the conductivity. Isotropic linear elastic constitutive relation is adopted for elastic model, and the Perzyna model considering isotropic hardening is adopted for the viscoplastic model. Under the confining pressure of 5 MPa and the deviator stress of 10 MPa, 15 MPa and 20 MPa respectively, the triaxial compression creep experiments were carried out on the long 7 samples, and the material parameters in the viscoplastic model were determined by fitting the experimental data.The constitutive model is programmed into the user material subroutine UMAT of Abaqus using the return mapping algorithm. Then a triaxial compression numerical creep experimental model was established to verify the model. This model is applied to fracture conductivity model to study the influence of reservoir creep on the conductivity of artificial fractures and the sensitivity analysis of the influencing factors of fracture width was carried out. INTRODUCTION In the current context of "carbon peak" and "carbon neutral", the demand for natural gas as a clean and low-carbon fossil fuel is gradually increasing(Zhong et al., 2023). As an important source of natural gas, shale has attracted more and more attention(Katende et al., 2023). Hydraulic fracturing is the main means for developing of low-permeability oil fields such as shale (Maslowski & Labus, 2021). Whether to produce artificial fractures with sufficient conductivity is an important criterion for hydraulic fracturing evaluation(Lin et al., 2020).With the increase of closing pressure, the proppants at the fracture surface embed into the reservoir, and the embedded area is easy to block pores, rock crushing and shale flakes migrate, which hinders the flow of oil and gas(Zhang et al., 2014). In the porous media space composed of proppants, with the development of shale oil and gas, the proppant's effective stress increases, leading to the proppant being further embedded in the fracture surface.