Evaluation of Over-Pressured Gas Reservoirs: Reserves Estimation to Production Performance - A Case Study

Abstract

Geo-pressured reservoirs are challenging in terms of in-place estimation and maintaining sustained production. Estimation of Gas Initially In-place (GIIP) using P/z from early data shows incorrect estimation and resulted in over-predicting GIIP. Typically, over-pressure gas reservoirs show downward curvature in P/z plot early or slightly delayed depending upon the pool limit and the degree of overpressure. A rapid decline in production also observed in the over-pressured gas reservoirs resulted in low hydrocarbon recoveries. This paper aims for correct GIIP estimation in over pressured gas reservoirs and to better manage and maximize recoveries from the Field. Based on available pressure and production data, two approaches were used in order to estimate the GIIP of the Field. (1) Conventional P/z by using Hammerlindl method (2) Developed analytical model by incorporating rock compressibility value from the Geomechanical SCAL study. The calculation of GIIP from both the methods are somewhat in close agreement as the vital factor of rock compressibility was introduced in GIIP estimation, to cater for the effect of reducing porosity as the pressure drops, resulting in changing storage capacity of the rock. Additionally, consecutive pressure Build-up data history showed reduction in permeability with the time from 9 mD to 0.4 mD. This change in permeability is caused by near wellbore overburden stress increase resulting in rock expansion which in turn reduces/closes pore throats. This manifest as an apparently damaged region causing lower rates and poor recoveries. In this scenario the well would be shut-in at higher abandonment conditions. Basic geo-mechanical parameter of rock compressibility plays an important role in reserves estimation for over-pressured reservoirs. Previous literature and our evaluation suggest that optimum production strategies for such reservoirs are: (1) To produce at lower rates at which drawdown is less than half of prevailing pore pressures. (2) Drill more wells to sustained production while producing at lower rates. Both strategies under optimum conditions would result in uniform depletion and better recoveries. Production performance with time, rapidly decreases as permeabilities reduce due to near wellbore stress sensitive region caused by larger drawdown so a controlled drawdown is recommended.