Fracture-Face-Skin Evolution During Cleanup

Abstract

Summary There remains considerable debate about the impact of fracture-face-matrix damage on cleanup and productivity of gas wells. The impact of the Cinco-Ley fracture-face-skin factor has been studied extensively, normally with single-phase-flow examples. Two-phase-flow examples are usually limited to the impact on long-term production. This paper uses a new two-phase-flow simulator to demonstrate the impact of fracture-face-matrix damage on both fracture-treatment cleanup and fracture-face-skin evolution during cleanup and subsequent production. The simulator was validated by comparison of calculated skins with pressure-buildup simulations evaluated by use of a high-end, third-party, pressure-transient-analysis package. The study has demonstrated that fracture-face skin relative to gas flow can be calculated continually throughout a simulation of fracture-treatment cleanup and production. It was found that at lower matrix permeabilities and subsequent higher capillary pressure curves, the impact of water saturation in the damage zone becomes much more important. Specifically, the effective fracture-face skin relative to gas can be several times higher than expected on the basis of single-phase flow. Furthermore, the simulation results show that at lower matrix permeabilities, the time required to achieve a reasonable fracture-face skin relative to gas flow can require considerable production time, on the order of several weeks, even for moderate damage factors. The results demonstrate that in tight gas reservoirs (≈0.01 md and less), even a moderate amount of matrix damage in a fracture face can result in high fracture-face skins and exceedingly long times for treatment cleanup. As such, it becomes very important to minimize fracture-face-matrix damage during tight gas fracturing treatments.