Geomechanical Simulation of Partially Propped Fracture Closure and Its Implication for Water Flowback and Gas Production
Authors: Liu Yongzan , Leung Juliana Y. , Chalaturnyk Rick 
Published: May 2018

Summary
Uneven proppant distribution is often encountered in hydraulic fracturing. Despite propped and unpropped fractures exhibiting different closure behavior during shut-in and early-flowback periods as a result of changes in effective stress, modeling of the geometry and closure behavior of a partially propped fracture is rarely performed. Numerical simulation is used in this study to simulate the closure behavior of a partially propped fracture and to examine its effects on water flowback and gas production.
Explicit finite-difference geomechanical simulation is used to simulate the change in effective stress and the corresponding closure geometry of a partially propped fracture. Parameters including rock strength, in-situ stress condition, proppant compaction, and propped-fracture height and aperture are considered to understand their effects on the fracture-closure geometry. This partially propped fracture is then represented explicitly in the computational domain in a flow simulation to model fluid flow during the shut-in and flowback periods. Fracture volume and fracture conductivity are adjusted as a function of effective stress to represent the fracture-closure process. The effect of coupling of partially propped fracture closure and multiphase flow on water recovery and gas production is investigated. Implications of ignoring the physical process of fracture closure and complex partially propped fracture geometry are examined.
Geomechanical-simulation results reveal the potential formation of a residual opening above the proppant pack in a partially propped fracture. Three distinct parts are identified within a partially propped fracture: a propped region, an unpropped region, and a residual opening (arch). The size of the residual opening is most sensitive to the initial fracture aperture. Stress concentrations occur at the top of the proppant pack and lead to potential proppant crushing or embedment. In addition to water uptake into the matrix because of forced (large pressure differential across the matrix/fracture interface) and spontaneous (high capillary pressure in the matrix) imbibition, fracture closure during the shut-in and flowback periods could displace more water into the nearby matrix and reduce the final water recovery. The residual opening would exaggerate the effects of gravity segregation and hamper water recovery by providing a highly conductive flow path to gas flow, such that the water would accumulate near the bottom of the fracture. The implication is that more-aggressive drawdown should be implemented to recover the fracturing fluid.
This study offers a quantitative analysis of the geometry of a partially propped fracture and highlights its effects on water flowback and gas production. The existence of the residual opening, which is commonly ignored in most analysis, would indeed play a crucial role in the subsequent well performance and in-situ fluid distribution. A number of insights pertinent to practical fracture design and operational strategy are discussed.