煤体水力压裂过程中孔壁应变及电阻率响应特征试验研究

Laboratory test research on borehole strain and electrical resistivity response characteristic of coal samples in hydraulic fracture process

为全面分析压裂过程煤体的损伤破坏过程,提出水力压裂过程孔壁应变与电阻率同步监测的试验方法,利用自制的水力压裂试验系统,开展原煤水力压裂试验,得到煤体水力压裂过程的孔壁压缩应变–电阻率–水压曲线,分析孔壁应变与电阻率的响应机制。结果表明,煤体水力压裂过程中孔壁应变和电阻率呈现较强的规律性;孔壁应变曲线呈现"V"型,电阻率曲线呈现"■"型,这种变化趋势与水压变化相对应,当水完全占据孔内空间时会导致煤体电阻率达到最小,一旦压裂孔出现破裂,电阻率及孔壁应变会出现突变并逐渐达到最大值;孔壁应变曲线反映出压裂孔承压变形的强度,电阻率曲线则反映出试块内部裂隙的演化特征。煤体孔壁应变–电阻率–水压曲线蕴含了压裂孔承压破裂的丰富信息,为研究钻孔起裂行为提供了一种新方法。

In order to analyze damage evolution state of coal sample in hydraulic fracture process, a joint test method combining borehole strain and electrical resistivity has been proposed. The borehole strain and electrical resistivity response characteristic of coal samples in hydraulic fracture process have been studied with self-developed hydraulic fracture experimental system. The curves of borehole strain, electrical resistivity and water pressure have been obtained while the response mechanism is analyzed. The results show that there is strong regularity in borehole strain and electrical resistivity curves. The borehole strain curves show ＂V＂ shape while the electrical resistivity curves show ＂4-＂ shape. When water takes up the space of borehole, the electrical resistivity of coal samples reaches the minimum value. When the borehole fractured, borehole strain and electrical resistivity increase to the maximum value gradually. Borehole strain reflects the deformation characteristic of borehole during hydraulic fracture process while the electrical resistivity reflects the initiation and development of fissure inner coal samples. Volume fracturing of coal samples causes the dilatation of fissure, resulting in sudden change of electrical resistivity. The curves of borehole strain, electrical resistivity and water pressure have richinformation for fracturing of coal sample in hydraulic fracture process. And it provides a new method to analyze initiation behavior of coal sample in hydraulic fracture process.