Effects of rock ductility on the fracability of ductile reservoirs: An experimental evaluation

Abstract

Deep hydrocarbon resources below 4000 m are an important replacement for shallow-depth resources; however, as depth increases, the difficulty and cost of development increase sharply. One of the possible reasons is that deep reservoir rocks exhibit strong ductility, which causes difficulties to stimulate a well-developed fracture network in rocks. To date, the mechanism by which rock ductility affects reservoir fracturing remains unclear, and there is a lack of quantitative evaluation models for reservoir fracability. To overcome the problems, in this study, artificial rock specimens with different ductility were fabricated using similar materials based on the geological data and petrophysical properties of the target deep reservoir; the specimens were classified using the proposed ductility index BIf. The hydrofracturing and CT scanning experiments of the artificial specimens with different BIf values under various horizontal stress differences Kh were conducted. The spatial fractal index Df was proposed to characterize the three-dimensional morphology and development of the fracturing network. The fracability evaluation function of ductile rock correlating the indices BIf and Kh was established. It was shown that ductile rocks tended to form non-planar hydraulic fractures with obvious tortuosity and the degree of fracture network development was lower than that of brittle rocks. The maximum breakdown pressure was approximately twice that of the brittle specimen, while the corresponding injection energy was approximately 5–10 times and the energy dissipation of fracture propagation was approximately 4–6 times those of the brittle rock. When Kh ≥ 0.3 the maximum horizontal principal stress played a major role in controlling the hydraulic fracture propagation in ductile rocks. If Kh < 0.3, the fracture propagation in rocks was mainly governed by material ductility rather than the maximum horizontal principal stress. This study provides a new way to explore and quantify the effect of rock ductility on the fracability of deep reservoirs.