Analysis on the mechanical properties of granite rock near the wellbore after percussive drilling and AWJ perforation

Rock properties near the wellbore are particularly important for the development of geothermal wells, but few people have studied them. The mechanical properties of granite rock near the wellbore after percussive drilling and abrasive water jet (AWJ) perforation are analyzed and studied in this paper. The variation of P-wave velocity, stress-strain curve, tensile strength, uniaxial compressive strength, peak strain, elastic modulus, microscopic appearances of fractures with the distance from the wellbore is studied. The purpose is to provide theoretical and technical support for fracturing and perforation in geothermal resource development. The experimental results indicate that percussive drilling can generate a large number of impact fractures near the wellbore, which can reduce the mechanical properties of granite rock near the wellbore. Compared with the granite after conventional rotary drilling, the p-wave, tensile strength, compressive strength and elastic modulus of granite after percussive drilling can be reduced by 87.3%, 55.10%, 38.32%, 66.67%, respectively. In addition, we find that AWJ perforation can only produce a few fractures, that is to say, its effect on the mechanical properties of rock is not significant. The percussive drilling effect on the mechanical properties is much more intense than AWJ perforation. Percussive drilling can affect the wellbore in a range of 1.2-1.5 times the diameter of the wellbore, while AWJ perforation can affect the wellbore in a range of 0.4-0.6 times the diameter of the wellbore. On the other hand, although AWJ perforation cannot change the mechanical properties of granite rock near the wellbore significantly, the secondary fractures produced by AWJ perforation can connect the impact fractures produced by percussive drilling, forming crushed zones. It demonstrates that the assistant technique of percussive drilling can be a potential solution for the stimulation enhancement of geothermal resources.