煤岩水力压裂过程中钻孔应变发展特征的试验研究

Experimental study of the features of the holeboring strain development in the hydraulic fracture process

为研究水力压裂过程中钻孔应变发展的特征,在不同应力条件下开展了钻孔水力压裂试验,测试分析了钻孔压裂段孔壁变形的规律。结果表明:在水压作用下,压裂段孔壁应变较为明显,存在拉伸和压缩2种类型,根据与水压的对应关系,2种应变曲线可分为4个发展阶段,即压裂管路排气阶段(应变小幅波动)、水压上升阶段(应变快速增加)、起裂延伸阶段(应变缓慢变化)和闭合阶段(应变急剧降低),其中压裂管路排气阶段对压缩应变的影响较小;拉伸应变的残余变形明显大于压缩变形,但压缩应变曲线与水压变化存在较好的一致性;孔壁应变是孔壁在水压作用下裂缝形成和扩展过程的表现。

The given article is aimed at introducing the results of its experimental study of the features of the hole-boring strain development in the hydraulic fracture process. For the said research purpose,we have conducted a series of experiments under the different stress conditions and made systematic analyses. The results of our study show that the deformation of the borehole proves to be remarkable under the water pressure whereas the surface of borehole displays two kinds of strain,that is,the tensile strain and the compressive strain. Based on the correlation between the strain curve and the water pressure curve,it is possible to divide the strain curve of the whole hydraulic fracturing process into 4 stages,that is,the gas emission stage,the water pressure increasing stage,the crack-formulation stage and the crack-gap disappearing stage. Different kinds of phenomena tend to appear during the aforementioned stages. For example,the gas emission stage reveals minor effect on the development of the compression strain while it makes the obvious effect on that of the tensile strain. Nevertheless,little fluctuations can be found in this stage. On the other hand,the borehole strain tends to increase quickly in the water pressure increasing stage. On the contrary,the borehole strain varies very little in the crack development stage. As to the last stage,the crack-gap disappearing stage,during which the borehole strain tends to decrease very quickly. Furthermore,when the samples tend to get broken under the high water pressure,there remains the residual strain that is going on with its work. Besides,the residual tensile strain of the borehole tends to be much higher than the compression strain is. However,the compression strain curves indicate obvious consistence with the water pressure curves,though while they may display opposite trend to the water pressure curves. Therefore,it is possible to recognize the borehole strain as a reflection to the development and propagation of the cracks of coal samples in hydraulic fracture process,with the effect of water pressure on the borehole surface. Thus,it can be concluded that the present research on the borehole strain can be of great interest for identifying the hydraulic fracture mechanism and the energy transfer of hydraulic fracture process,which can be made to provide a new method for experimental studies on the hydraulic fracture.