Investigation Of Hydraulic Fracturing In Low Permeability Rock Based On Distinct Element Model

Hydraulic fracturing,as a physical and mechanical method to improve the porosity and permeability of rocks by fracturing underground dense rock formations with high-pressure fluid,has been widely used in oil and gas exploitation,geothermal resource development,radioactive nuclear waste disposal and geostress measurement.The application of the hydraulic fracturing has greatly increased the production of shale gas extraction and changed the world energy pattern.However,the engineering site is still faced with such problems as high breakdown pressure of the formation and unsatisfactory fracturing effect.It is an urgent problem for fracturing design engineers and researchers to solve how to reduce the high fracture pressure in the deep reservoir while ensuring the fracturing effect,so as to reduce the pressure injection system load and fracturing cost.This requires an in-depth discussion of the mechanical mechanism of rock hydraulic fracturing and many influencing factors,such as the physical and mechanical properties of rock,in-situ stress,fluid viscosity and injection method,borehole size and natural fracture in rock.In this paper,based on the advantages of Discrete Element Model in the analysis and calculation of geotechnical mechanics,using PFC^2D5.0 to calibrate the microscopic parameters of the model through Uniaxial Compression and Brazilian test,and establishes a discrete particles model with the same physical and mechanical properties as the low permeability rock.In Cundall fluid-coupling theory on the basis of considering the rock permeability and saturation condition to write algorithms to reproduce rock hydraulic fracturing process,through the numerical simulation of hydraulic fracturing under different in-situ stress to validate the reliability of the algorithm,and a series of numerical simulation of hydraulic fracturing to discuss the influence of borehole size,fluid viscosity,rock permeability and second fracturing on hydraulic fracturing.The numerical simulation results show that:borehole size has a significant effect on the hydraulic fracturing,the large borehole can effectively lower the breakdown pressure.Due to fluid seepage,fluid viscosity and rock permeability affect thydraulic fracturing.Breakdown pressure decreases with the decrease of injection fluid viscosity and increases with the decrease of rock permeability.It is found that breakdown pressure during the second fracturing is obviously lower than that during the first fracturing,and the initial fracture has a inducing effect on the second fracture initiation and expansion,and the hydraulic fracture still starts and expands along the direction of the initial fracture.