Experimental Study on Characteristics of Crack End Face in Hydraulic Fracturing of Glauberite Salt Rock

In order to study the mechanism of crack propagation dissolution under glauberite hydraulic fracturing and provide a reference for the fluid mining of glauberite salt deposit, fill the gap of only physical effect but no chemical effect in hydraulic fracturing, and enrich the hydraulic fracturing theory. The hydraulic fracturing experiments of glauberite salt samples under axial/confining pressures of 5/4 MPa, 7/4 MPa and 9/4 MPa were carried out by using laboratory test method. The crack morphology of the failure section was scanned by using 3D morphology scanner, and the test data were calculated and analyzed combined with the characterization formula of crack roughness. It is found that the glauberite hydraulic fracturing is the joint action of stress and dissolution, and the fracturing process can be roughly divided into three stages: water pressure rising stage, crack propagation stage and water pressure falling stage. The division of the three stages is the same as the previous research results, but the pressure fluctuation in the crack propagation stage is violent, which is quite different from the hydraulic fracturing experimental results of other rocks (such as coal and shale), which is caused by the dissolution of Glauber’s salt. Under different stress, the crack morphology of glauberite after hydraulic fracturing is different. When the vertical stress difference coefficient (k) ≤ 0.75, the glauberite salt rock will produce transverse cracks, and when k ≥ 1.25, the glauberite salt rock will produce longitudinal cracks. Under the same stress state, the roughness of Glauber’s salt water crack is greater than that of saturated salt water crack. Under different stress states, the roughness of Glauber’s salt water crack increases with the increase of axial pressure. The research results have important engineering value for in-situ leaching and fluid mining of glauberite deposit.