Investigation on cracking propagation patterns in hydraulic fracturing under different vug distributions: Experimental and numerical case

Abstract

Natural vugs are developed in geothermal reservoirs. Communication of natural vugs can effectively improve reservoir stimulation, but the vug distribution affects its propagation pattern. In this work, large-size samples (500 × 500 × 500 mm) made of carbonate-like materials were prepared, and a series of true triaxial hydraulic fracturing tests were carried out to investigate the influence of the vug orientation and relative orientation between vugs on the propagation pattern. Moreover, the discrete element method (DEM) was used to further elucidate the effect of vug distribution on the cracking propagation mechanism. The results obtained that the increase in the vug orientation leads to an increase in peak pressure, and the relative orientation has less effect on peak pressure. In terms of propagation pattern, at low vug orientation and relative orientation (i.e. 0°, 30°), the induced crack will penetrate or deflect through the vugs, while at 45°, the induced cracks deflect but do not interpenetrate the vugs. Accordingly, numerical simulations demonstrate that the crack deflection induced by the vug is mainly due to microcracks generated by tensile stress concentration around the vugs under the pore pressure, but the high vug orientation suppresses the tensile stress domains, accompanied with an increase in the percentage of shear cracks with vug orientation. These results may improve the understanding of vuggy carbonate reservoir stimulation.