多直径岩芯液压致裂岩石抗拉强度快速试验机研发

Development of multiple-diameter core hydraulic fracturing machine to test tensile strength of rocks

为了更为准确地测定岩石的抗拉强度以降低钻杆式水压致裂原地应力系统柔性对最大水平主应力计算的影响,研发了多直径岩芯液压致裂岩石抗拉强度快速试验机。试验机参照水压致裂测试力学过程,基于岩石厚壁筒原理研发,主要核心部件包括加载系统、压力监测和数据采集系统、样品夹持系统3个部分。为了方便推广该套试验设备和此类试验方法,针对试验影响因素加压速率、中心盲孔深度和直径、空心岩柱内外径之比等因素开展了57个岩石样品的试验工作。试验结果显示,空心岩柱盲孔深度对测试结果影响不明显,但是会影响测试成功率,岩样盲孔深度可为岩柱高度的0.7~0.8倍。空心岩柱中心孔直径会显著影响测试结果,当中心孔直径大于20 mm时,中心孔的尺寸效应不再显著。如果考虑空心岩柱内外径之比,根据理论分析和试验数据可知,取0.3~0.5可获得更为稳定的测试数据。试验采用的加压速率为0.02 MPa/s,获得试验数据稳定可靠。此次测试样品福建长汀下古生界奥陶~志留系板岩的岩石抗拉强度平均值为8.4 MPa。这些认识为后续规范地开展此类测试奠定了初步基础。

In order to determine rock tensile strength in a precise way to minimize the negative impacts of the compliance of the drilling-rod hydraulic fracturing system on the determination of the maximum horizontal principal stress, the multiple-diameter core hydraulic fracturing machine to test tensile strengths of rock cores was developed and manufactured. Based on the hydraulic fracturing process and the thick rock cylinder model in rock mechanics, the test machine includes three major parts： load applying system, pressure monitoring and data acquisition system, specimen holding system. In order to make the test machine and the test method widely accepted and used, 57 rock sample tests were conducted with respect to the major influential factors, such as pressurization rate, depth and diameter of central blind hole, ratio of internal to external diameters of hollow rock cylinder. The test results show the depths of internal blind holes do not affect the test results significantly, but can lower the success rate of tests, therefore, it is suggested that the depth of intemal blind hole should be 0.7 - 0.8 times of the total height of rock specimen. The test results also indicate that the diameters of internal holes can affect the test results significantly, and the scale effect does not work effectively when the diameter of internal borehole is greater than 20 mm. Considering the ratio between the internal diameter of internal blind hole and the external diameter of rock specimen, it is suggested to adopt 0.3 - 0.5 to obtain more stable test results according to the theoretical analysis and test results. The adopted test pressurization rate was about 0.02 MPa/s in this research, which can help get reliable data. The tested tensile strength of slate specimens, extracted in the ordovician-silurian formation of the Paleozoic period, in Changting, Fujian Province, is about 8.4 MPa. These research findings make preliminary bases for standardizing such kind of test method and test machine.