Stress And Time Effects Of Fractured Sandstone Permeability

Natural uranium is the "cornerstone of nuclear military industry" and "nuclear power granary".Sandstone-type uranium mines are the most important type of uranium resources in China.Increasing the development of sandstone-type uranium resources can improve the guarantee capability of China’s uranium resources.In situ leaching is the main development method of sandstone-type uranium resources,which has high requirements on the permeability of uranium resource reservoirs,while the permeability of sandstone-type uranium mines in China is relatively low.Reservoir for renovation.Existing studies have confirmed that the blasting and permeabilityenhancing method is suitable for the stimulation of low-permeability sandstone uranium resource reservoirs,but the sustainability of the permeability-enhancing effect of reservoir stimulation still needs further research.The key scientific issue involved is to reveal the permeability of fractured rock mass after stimulation.Evolution law with stress and time.Therefore,this thesis adopts the comprehensive research methods of theoretical analysis,model construction and laboratory test to study the permeability evolution law of intact sandstone,single-fracture sandstone and multi-fracture sandstone under the action of stress and time,and establish a permeability characterization model.The main results are as follows:(1)The permeability of intact coarse sandstone decreases with the increase of axial pressure and confining pressure,and increases with the increase of pore pressure when the net confining pressure is less than 4 MPa,all of which conform to the exponential function relationship.However,when the net confining pressure is greater than 4 MPa,the permeability is hardly affected by the pore pressure.The permeability stress sensitivity coefficient decreases with the increase of axial pressure and confining pressure;it increases with the increase of pore pressure,but when the net confining pressure is lower than 2.5 MPa,the permeability stress sensitivity coefficient increases rapidly with the increase of pore pressure.In the same sample,the permeability stress sensitivity coefficient follows the following rules: pore pressure > confining pressure >axial pressure.(2)The concepts of fracture direction stiffness and fracture surface normal stress are introduced,and a rock seepage-stress model with a single fracture is established.The permeability of sandstone with single fracture is 1.5-17 times higher than that of intact coarse sandstone samples;the permeability of sandstone with single fracture decreases with the increase of axial pressure and confining pressure,showing an exponential function relationship whose exponential term is a quadratic function.When the net confining pressure is lower than 5 MPa,the permeability of sandstone with single fracture increases with the increase of pore pressure.When the net confining pressure is greater than 5 MPa,the increase of pore pressure has no obvious effect on the permeability.The fitting correlation coefficients between the single fracture seepage-stress model and the test data are all above 0.9,which verifies the validity of the model.(3)The generalized Kelvin model was embedded in the single-crack seepagestress model,and the single-crack seepage-creep model was established.In the singlecrack seepage-creep test,the permeability decreased sharply 48 h before the test,decreased slowly during 48-108 h,and gradually stabilized after 108 h.The maximum error between the experimental value and the model value is 8%,and the correlation coefficient is greater than 0.95,which confirms the reliability of the model and predicts that the permeability will eventually increase to 1.90-2.44 times that of the complete coarse sandstone.(4)The permeability-volume strain equation derived based on the KozenyCarmen equation is modified by introducing the volumetric strain correction coefficient,and a fracture network seepage-stress model is established.In the seepage-stress test of coarse sandstone with multiple fractures,the permeability is positively correlated with the change of volumetric strain,which decreases with the increase of axial pressure and confining pressure,and increases with the increase of pore pressure.The correlation coefficient between the experimental value and the model value is greater than 0.94,which verifies the validity of the model.(5)The volumetric strain and permeability curves under sandstone creep are treated as catenary lines,and the seepage-creep model of sandstone with fracture network is established.The evolution law of permeability of multi-fractured sandstone with time is similar to that of single-fracture,showing three stages of rapid,slow decrease and stable.The correlation coefficient between the test data and the model fitting is greater than 0.98,and the model predicts that the long-term permeability of the sample will increase to 2.30 times that of the complete coarse sandstone sample.