| With the rapid growth of our country's economy and industry,the demand for coal is also increasing.At the same time,the mining conditions of coal resources are also deteriorating.Problems such as gas outburst and roof falling threaten the safe production of mines.Hydraulic fracturing is the effective way to solve the above problems.Since the rock is a porous medium,there is often a radial pore pressure gradient in the borehole formed by fluid seepage during the fracturing process.However,the existing research about the influence of pore pressure gradient on hydraulic fracturing is extremely limited.This article uses physical model experiments to explore the law and numerical simulations to verify the method,study the effect of radial pore pressure gradient on rock hydraulic fracturing in detail.Prepare standard coarse sandstone samples and test the mechanical parameters,then simulate the seepage process through Comsol Multiphysics,calculate the seepage time required to form a stable radial pore pressure gradient and the distribution law of pore pressure and its gradient inside the sample.The simulation results show: with the seepage pressure increases,the seepage time to form a stable radial pore pressure gradient decreases;after the seepage is completed,the pore pressure inside the sample gradually increases from the inside to the outside.The pore pressure gradient decreases from the bare hole section to both sides in the radial direction,and is same in the axial direction.At the same time,the simulation results are verified by the pre-seepage experiment,and the final seepage time in the formal experiment was determined to be 24 h.In view of the influence of the radial pore pressure gradient of the borehole on the hydraulic fracturing process,with the help of the conventional triaxial fluid-solid coupling hydraulic fracturing experiment system of the research group,hydraulic fracturing experiments were carried out on coarse sandstone samples under different radial pore pressure gradients,compare and analyze the changes on water pressure?acoustic emission data and osmotic pressure during the experiment,and perform3D-XRM experiments on some samples after hydraulic fracturing,then observe the crack morphology through visualized 3D reconstruction software.The results show:under the influence of radial pore pressure gradient,the water pressure would rise during the crack propagation stage after the initial rupture of the sample,and the rising pressure peak is greater than the initial rupture pressure,This is because when the crack propagates,the pore pressure around the crack increases.Due to the existence of the critical pore pressure gradient,when the pore pressure is higher,the corresponding fracture pressure will also increase;with the radial pore pressure gradient increases,the uneven force inside the sample becomes more and more obvious,and the force transmission between the pores and the rock matrix particles also increases.The pressure drop when rupture occurs in the sample and the AE energy at the time of rupture increase,and the change in osmotic pressure is also more obvious;when the radial pore pressure gradient increases,the pore pressure at the same position in the sample also increases.Therefore,under the same pump injection flow and time,as the pore pressure gradient increases,the overall fracture in the sample The extended range and the degree of opening are reduced.Use the Comsol Multiphysics to simulate the radial pore pressure gradient and conventional hydraulic fracturing,The simulation results show: the fracture pressure of the sample increases when the radial pore pressure gradient exists,which is consistent with the experimental results,which proves the law of the radial pore pressure gradient on rock fracturing.There are 51 figures,11 tables and 84 references in this thesis... |
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