Experimental investigation on hydraulic fracturing of granite specimens with double flaws based on DIC

Abstract

Hydraulic fracturing is a crucial technology for shale gas exploitation and enhanced geothermal systems. However, the fundamental mechanisms of hydraulic fracturing, such as the mechanism (tensile or shear) of cracks, physical process of crack initiation, propagation, and coalescence, are not comprehensively understood. Therefore, further direct observations and quantitative investigations are required. Accordingly, an experiment of hydraulic fracturing on double-flaw granite specimens was performed. The fracturing processes were captured using a high-speed camera, and a quantitatively study based on digital image correlation (DIC) was achieved. DIC was proved to be a powerful analysis tool, especially for visualising the crack evolution process. Results show that the crack types in the hydraulic fracturing tests were primarily Type I or Type Ib tensile cracks. The process of strain localisation zones (SLZs) revealed the physical process of the hydraulic fracturing: SLZs initiated at or near pre-existing flaw tips before fracturing, and then cracks initiated and propagated along the SLZs. Some cracks initiated previously may close with the decrease in fluid pressure after fracturing and evolved into closure cracks. The bridging angle was the main factor determining the coalescence patterns, and the specimens exhibited direct coalescence (Category 6 or 7) when β ≥ 90°. SLZs were initiated at both inner tips in Category 6, while SLZs could only be observed at one inner tip in Category 7. The ultimate fracturing pressure first decreased then increased as the bridging angle increased.