Effects of Non-Darcy Flow on Pressure Transient Analysis of Hydraulically Fractured Gas Wells

Abstract

Abstract Conventional well-testing techniques are commonly used to evaluate pressure transient tests of hydraulically fractured gas wells to estimate values such as formation permeability, fracture half-length, and fracture conductivity. When non-Darcy flow occurs along the fracture, analysis of the pressure transient test data using conventional analyses methods will produce incorrect values of fracture conductivity and fracture half-length. This work emphasizes the importance of non-Darcy flow in the hydraulic fracture and its effects on pressure buildup analyses of hydraulically fractured gas wells. A reservoir simulator was used to generate pressure drawdown and buildup data both with and without the effects of non-Darcy flow. These synthetic buildup tests were then analyzed using conventional well-testing techniques. We found that when non-Darcy flow occurs along the fracture, the estimated fracture conductivity and fracture half-length represent only a small fraction of the actual values. Also, the degree to which the non-Darcy flow affects the well-test results depends upon the values of fracture permeability, water saturation inside the fracture, and the production rate during the drawdown period. If the incorrect fracture properties obtained from conventional analysis are used in reservoir simulation forecasting, critical values such as production rate and total recovery will be miscalculated. Since conventional well-test analysis cannot be used to determine fracture parameters reliably, simulation history matching is the only appropriate method to correctly analyze buildup pressure response from hydraulically fractured wells in gas reservoirs. Introduction The flow of a viscous fluid through a porous medium normally is adequately described by Darcy's law.However, Darcy's law is inadequate for describing high-velocity flow. In 1901, Forchheimer1 proposed a generalized equation that takes into account the additional pressure drop resulting from high flow velocities:Equation 1 In hydraulically fractured gas wells, where large gas flow rates create high velocities, the pressure drop in the hydraulic fracture is dominated by non-Darcy flow. The resulting pressure distribution in the fracture affects the pressure distribution in the entire reservoir. For many years, large hydraulic fracture treatments have been used successfully to increase the flow rate from low to moderate permeability gas reservoirs. Numerous papers have been published to explain how to best compute the fracture half-length and fracture conductivity.2–10 Normally, pressure transient tests are run to evaluate these properties. Unfortunately, most analysts use conventional well-testing techniques that do not account for the effects of non-Darcy flow on the pressure distribution. Unless the effects of non-Darcy flow are considered in the analyses, the resulting values of fracture half-length and fracture conductivity will be incorrect.