Gas-Cap Effects in Pressure-Transient Response of Naturally Fractured Reservoirs

Abstract

Summary This paper presents a theoretical framework of gas-cap effects in naturally fractured reservoirs. General pressure solutions are derived for both pseudosteady-state and unsteady-state matrix-fracture interporosity flow. Deviation from the fracture or fracture-matrix response occurs as the gas-cap effect is felt. Anomalous slope changes during the transition period depend entirely on the contrast between the fracture anisotropy parameter,λ1, and matrix-fracture interporosity parameter, λ, and between the total gas-cap storage capacitance (1 − ω1) and oil-zone matrix storage (1 − ω). A composite double-porosity response is observed for ω1 ≤ ω1c and 1.0 ≤ λ1/λ ≤ 1000. A triple-porosity response is observed for ω1 ≥ ω1c and 140 < ωλ1/λ < 1.0E05.