Numerical simulation of hydraulic fracture propagation in conglomerate reservoirs

Abstract

Hydraulic fracturing is a necessary production enhancement procedure for low permeable conglomerate reservoirs. To study the hydraulic fracture (HF) propagation mechanism in strong heterogeneous conglomerate reservoirs, a numerical simulation based on the modified global cohesive zone method (CZM) was proposed. Two types of geological conglomerate models were built, and natural fractures were also considered in each model. A comprehensive discussion of the treatment parameters for the fracture geometry and width was also conducted. Numerical simulations suggest that a zig-zig fracture can be formed in all cases, while micro fractures emerge near the main fracture, and the number of micro fractures is larger for the composite gravel model than for the single gravel model. The presence of stiff gravel can affect the fracture propagation path and the associated conductivity and can sometimes result in an asymmetric fracture propagation path. The contribution effect of micro fracture on production might be debatable because of the relatively small fracture width. Four different mechanical behaviors, namely, penetration, deflection, attraction and termination, can be observed. In addition, fracture penetration tends to occur near the wellbore zone, while fracture branching generally occurs at the HF tip. The number of fracture deflection modes dominates the fracture behavior regardless of the distance between the HF and the gravel. The in situ stress condition determines the overall fracture propagation path, but the fracture starts to deviate under a low principal stress difference. An increase in the pump rate controls the fracture length due to a sufficiently large fluid injection volume. The increase in fracturing fluid viscosity does not affect the fracture geometry to a large extent but can heavily arrest fracture closure, thus improving fracture width, which can mitigate the large fracture width reduction along the gravel edge. Compared to the homogeneous rock model, the numerical simulation based on the adopted CZM method can be from a more complex fracture type with the conglomerate model, while numerical results can be validated by the triaxial fracturing simulation in the whole and local areas, which demonstrates the ability to perform a numerical simulation of HF propagation in reservoirs with strong heterogeneity.