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.

当前，研究人员和业内人士认为，水侵入页岩基质应主导水力压裂液回流之前的浸水过程。基于对塔斯卡卢萨（Tuscaloosa）海洋页岩（TMS）岩心的实验室观察，我们提出了一个假说，即在水力压裂增产期间和之后，在页岩地层中形成了裂缝，这些裂缝后来有助于提高油井生产率。裂缝的形成有助于改善井流性能，而裂缝也从水力压裂中抽出压裂液，减小裂缝宽度，从而降低井流性能。裂缝发展和裂缝闭合之间的权衡考虑到了最佳的浸水时间，这有可能使油井生产率最大化。建立了数学模型来描述基于毛细管-粘滞力平衡的裂纹的动态扩展。使用先前发布的井产能数学模型分析了裂缝形成对长期井产能的影响。裂纹扩展和井产能模型的首次结合为预测水力压裂液回流之前的最佳流体浸泡时间提供了一种技术。敏感性分析表明，降低压裂液的粘度可能会加快最佳的浸水时间，而降低水页岩界面张力（IFT）则可能会延迟最佳的浸水时间。