Transient pressure analysis for partially-penetrating wells in naturally-fractured reservoirs

Abstract

The major oil fields in Vietnam such as White Tiger and Rang Dong consist of fractured and weathered granite basement formation which are several thousand feet thick and are difficult to drill through. Consequently, these fields have been developed with wells that are completed only in the top few hundred feet of the formation. Fluid flow into the partially-penetrating wells is therefore non-radial, rendering inappropriate conventional transient pressure analysis based on radial flow of the fully-penetrating wells. An analytical solution has been developed that describes transient pressure behavior of the partially-penetrating wells in naturally-fractured reservoirs. The solution is obtained by combining the pseudo steady state model for naturally-fractured reservoirs with the partially-penetrating well model in homogeneous reservoirs. Verification of the analytical solution is made by comparison of pressures derived analytically with those obtained from numerical simulation. The new analytical solution is presented in the form of type curves graphing dimensionless pressure and pressure derivative versus dimensionless time for various reservoir thickness, completion interval fraction, fracture-matrix storativity ratio, and interporosity flow coefficient. The new type curves indicate that partial penetration and double porosity effects cause a characteristic shape of the curves at early and transition time. Transient well test analysis based on a fully-penetrating well yields erroneous results when applied to partially-penetrating wells in naturally-fractured reservoirs. The new type curves are particularly useful in estimating the matrix-fracture parameters and other reservoir engineering parameters for partially-penetrated naturally fractured reservoirs.