A Composite Transient Model for Multi-Fractured Horizontal Well in Tight Gas Reservoirs Based on Multi-Factor Effect

Abstract

In low-permeability tight reservoirs, the natural gas transient flow is affected by multi-factor including stress-sensitivity, threshold pressure gradient, or both. This paper aims to synthetically study transient pressure response as well as transient rate behavior of a multi-fractured horizontal well in tight gas reservoirs. With the new mathematics model, the flow characteristics coupled with such multi-factor can be described. The new composite seepage model of a line-sink incorporating both stress-sensitive permeability and threshold pressure gradient was established. Employing fractures discretization and superposition principle yielded the final model of a multi-fractured horizontal well. The model was semi-analytically solved by Pedrosa's transformation, perturbation theory, and integration transformation method. Not only pressure transient analysis, but also rate transient analysis are separately performed with relevant parameters. A total of six flow periods can be divided according to the same standard of time stages for pressure type curves and rate type curves. It is found that stress-sensitivity results in upward tendencies in both of pressure and rate derivative curves reflecting larger pressure drop during intermediate and late flow regimes; threshold pressure gradient leads to more difficult fluid flow acting a steeper upwarping on pressure curve and downwarping on production rate curve. Finally the variation trend of the corresponding curve will be more apparent under the combined impact of stress-sensitivity and threshold pressure gradient. The research of this paper can provide some insights into well dynamic forecasting during exploiting such reservoirs, and contribute to establish the theoretical basic for transient analysis efficiently.