钙芒硝盐岩水力压裂裂纹端面特征试验研究

为研究钙芒硝水力压裂作用下裂纹扩展-溶解机理,对钙芒硝矿层的流体化开采提供参考,同时填补水力压裂中只有物理作用而无化学作用的空白,丰富水力压裂理论,作者采用室内试验的方法,分别进行了钙芒硝试件在轴压/围压为5/4 MPa、7/4 MPa、9/4 MPa三种应力条件下的水力压裂实验,并采用3D形貌扫描仪对破坏断面的裂纹形态进行扫描,结合裂纹粗糙度表征公式对试验数据进行计算分析。研究发现:钙芒硝水力压裂是应力和溶解的共同作用,压裂过程大致可以分为水压升高阶段、裂纹扩展阶段、水压下降阶段3个阶段;3个阶段的划分与前人研究成果相同,但裂纹扩展阶段的压力波动剧烈,与其他岩石(例如,煤,页岩)水力压裂试验结果相差较大,其为钙芒硝溶解现象影响所致;不同应力下,钙芒硝水压致裂后裂纹形态不同;当垂向应力差异系数(k)≤0.75时,钙芒硝盐岩产生横向裂纹,k≥1.25时,钙芒硝盐岩将产生纵向裂纹;同一应力状态下钙芒硝清水致裂后裂纹端面粗糙度大于饱和盐水致裂后的粗糙度,不同应力状态下,水压致裂后粗糙度随轴压的增大而增大。研究成果对钙芒硝矿层的原位溶浸流体化开采具有重要的工程价值。

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 / 4MPa, 7 / 4MPa and 9 / 4MPa 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.