Conductivity Evaluation of Cross Propped Fractures in Shale Reservoirs

Abstract

ABSTRACT: Volume fracturing is a key technology for shale gas development. It connects hydraulic fractures and natural fractures to form a large-scale cross propped fracture network. The conductivity of cross propped fractures is the key to the effect of shale volume fracturing. Based on the Kozeny model, the conductivity of a single fracture is calculated, and the conductivity of the single fracture is corrected by experimental data, taking into account the effect of formation closure stress. Based on the similarity principle of hydropower, an evaluation model for the conductivity of cross propped fractures is proposed. On the basis of the numerical simulation results of proppant transportation, the factors affecting the conductivity of cross propped fractures are analyzed. The results show that the proppant falls at a low velocity into the narrow natural fracture, and then moves slowly forward in the form of a creep in the fracture, where it is transported for a short distance. For wide natural fractures, the proppant is suspended into the fracture at a greater velocity and transported over a longer distance, forming a "tall and long" proppant dune morphology. When the natural fracture width ratio increases, the proppant is placed more unevenly in the cross fractures, the seepage resistance of proppant dunes in hydraulic fractures gradually increases, the seepage resistance in natural fractures has no obvious change, the total seepage resistance of cross fracture is greater, and the conductivity of the fracture is smaller. The numerical calculation model is used to evaluate the conductivity of cross propped fractures, which provides theoretical guidance for the optimization of shale gas volume fracturing schemes. 1. INTRODUCTION As a kind of clean energy, Chinese shale gas will break through 2 × 1012 m3 and 200 × 108 m3 respectively in proven geological reserves and total annual production in 2020. Increasing shale gas exploration and development under the "dual carbon" target is an important guarantee to ensure energy security and alleviate environmental problems (Long et al., 2021; Zhang et al., 2021). At present, the development of shale gas reservoirs at home and abroad usually adopts volume fracturing technology. Through the construction method of "fast injection velocity + large liquid volume + large sand volume", hydraulic fractures and natural fractures are connected, so as to realize the effective placement of proppant in the fractures, form large-scale cross propped fractures, and improves fluid seepage channels, thereby greatly increasing gas well productivity (Lu et al, 2019a; Lu et al., 2019b; Zhang, 2022).