Crack Propagation Hypothesis and a Model To Calculate the Optimum Water-Soaking Period in Shale Gas/Oil Wells for Maximizing Well Productivity

Abstract

Currently, researchers and the industry believe that water invasion into a shale matrix should dominate the process of water soaking before flowback of hydraulic fracturing fluids. Based on laboratory observations with Tuscaloosa marine shale (TMS) cores, we postulate a hypothesis that cracks are formed in shale formations during and after hydraulic fracture stimulation and that they later contribute to improved well productivity. The formation of cracks contributes to improving well inflow performance, while the cracks also draw fracturing fluid from the hydraulic fractures, reduce fracture width, and consequently lower well inflow performance. The trade-off between crack development and fracture closure allows for an optimum water-soaking time, which could potentially maximize well productivity. A mathematical model was developed to describe the dynamic propagation of cracks based on the capillary-viscous force balance. The effect of crack formation on the long-term well productivity was analyzed using a previously published mathematical model for well productivity. A combination of the crack propagation and the well productivity models for the first time provides a technique for predicting the optimum fluid soaking time before flowback of hydraulic fracturing fluids. Sensitivity analyses show that reducing the viscosity of fracturing fluid could potentially speed up the optimum water-soaking time, while lowering the water-shale interfacial tension (IFT) could potentially delay the optimum water-soaking time. Real-time shut-in pressure data can be used in the crack propagation model to “monitor” crack development and identify the optimum water-soaking time before the flowback of hydraulic fracturing fluids for maximizing well productivity and the gas/oil recovery factor.