Study On Sandstone Fracturing Test And Strata Fracturing Pressure Prediction Model Based On Thick-walled Cylinder Method

In underground fluid injection projects such as shale gas development,geothermal development,and oil and gas stimulation,how to avoid formation rock damage due to fluid injection is a key issue.In order to avoid safety accidents such as destruction and instability of formation rocks during fluid injection,it is necessary to obtain key data such as fracture pressure of formation rocks.In engineering,the methods of obtaining formation fracture pressure are mainly divided into field test method and model prediction method.The model prediction method is more widely used because of its rapidity and simplicity.Therefore,the accuracy of the model prediction method has become a main content of the study by scholars.In the existing prediction models of formation fracture pressure,the influence of the complex underground environment on rock damage characteristics is ignored,which reduces the accuracy of the prediction model.In the underground fluid injection project,the failure form of formation rocks is generally tensile failure.In order to explore the tensile failure characteristics of formation rocks,an indoor rock failure test is required.Among the existing rock tensile failure test methods,only the thick-walled cylinder method can better simulate the complex environment of formation rocks.In this paper,based on the modified thick-walled cylindrical tensile failure instrument,the stress environment and natural factors of the formation rock are simulated,and the effects of water and fluid loading rate and confining pressure on the tensile failure characteristics of the rock are mainly analyzed.The main work of this article is as follows:(1)An experimental study of crack initiation and propagation during water-induced sandstone tensile failure was carried out.The sandstone fracture pressures of multiple groups of dry and saturated conditions were compared and tested.The crack initiation pressure and unstability of each group of sandstones were analyzed,and the influence factors of water on rock crack growth were discussed.(2)Infiltration cracking tests under different pressurization rates were carried out.In theory,the fracturing pressure model of thick-walled cylinder under variable pressure rate seepage and constant pressure rate seepage is deduced.The experimental results are used to analyze the mechanism of variable pressure seepage on the fracturing process.The degree of influence were of the main influencing factors such as permeability and tensile strength on rock fracturing pressure was studied.(3)Sandstone cracking tests were carried out under different confining pressures.The sandstone cracking tests under different confining pressure conditions were carried out for two kinds of sandstones.Based on the test data,the tensile strength of the samples in each group of experiments was calculated,and the relationship between the confining pressure and the tensile strength of the rock was fitted.The law of the change of the tensile strength of the rock with the confining pressure is studied,and parameters that can characterize the law in the actual formation are proposed.Based on this parameter,the existing formation fracture pressure model is modified.Combined with the measured data,the reliability of the revised model is verified.