| Unconventional oil and gas reservoirs in the oil and gas energy engineering are generally characterized by low permeability,which makes energy extraction difficult.As hydraulic fracturing has been widely used as an oil and gas production enhancement technology in this field,it is significantly important to fully understand the complex stress conditions on rock hydraulic fracture initiation,fracture expansion pattern and permeability change pattern before and after fracturing to develop a reasonable hydraulic fracturing scheme.Therefore,this paper conducts conventional triaxial compression,triaxial compression full permeability testing and permeability-hydraulic fracturing tests on low-permeability sandstone in turn.Meanwhile,the finite volume method(FVM)and ABAQUS numerical simulation software are used for numerical simulation of fracture initiation pressure and fracture expansion in sandstone.(1)Triaxial compression and full permeability testing of low permeability sandstone:specimens show shear damage and the damage surface is an irregular surface through the upper and lower surfaces.During triaxial compression and deformation,there are four stages:compression-density,elasticity,plastic yielding and post-peak damage,showing the deformation characteristics of plastic hardening,compression expansion and brittleness.Permeability of specimens in the process of triaxial compression and deformation generally shows a pattern of slow decrease and subsequent slow increase and final surge.(2)Low permeability sandstone permeability-hydraulic fracturing test: a whole process of hydraulic fracturing consists of five stages: slow pressure increase,rapid pressure increase,steady pressure increase,fracture development and sudden drop.Both fracture initiation pressure and fracture pressure in low permeability sandstone increase with increasing surrounding pressure and decrease with increasing bias pressure.There are three stages of hydraulic fracture evolution,which include initiation,development and closure,and the closure rate of hydraulic fracture width shows a certain increasing trend with the increase of surrounding pressure.Hydraulic fracture expansion pattern is significantly impacted by the effect of surrounding pressure.Under lower envelope pressure conditions,hydraulic fracture does not extend through the specimen to form a partially fractured specimen,whose fracture surface is approximately in one plane.Under higher envelope pressure conditions,hydraulic fractures extend through the entire specimen to form a fully fractured specimen with a distorted fracture surface,resulting in a larger surface area.Permeability of the partially fractured specimens is much more significantly impacted by the rock matrix than by hydraulic fractures,and the permeability of the fully fractured specimens remains essentially constant with change in stress.(3)Numerical simulations: firstly,based on the finite volume method,numerical simulations of the fracture initiation pressure in sandstone are conducted and the simulation results reflect more precisely the fracture initiation pressure variation pattern found in the tests.Secondly,ABAQUS software is used to create an extended finite element two-dimensional model to study the distribution of pore pressure,stress and displacement within the rock during the fracture extension process. |
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