Evaluation and optimization of gas assisted gravity drainage process

Abstract

Gas Assisted Gravity Drainage (GAGD) process involves injecting gas at the top of the pay zone through vertical wells and producing oil from a horizontal wells placed near the bottom of reservoirs. Injected gas accumulates at the top and displaces oil to the production well. The objective of this study is to determine the optimal oil production rate, gas injection rate and well pattern on the performance of GAGD applied in dipping reservoirs. Sensitivity analysis of relative permeability correlations, permeability anisotropy ratio and residual oil saturation is also performed. The results from reservoir simulation in dipping reservoirs indicate that oil recovery is significantly increased when performing GAGD. The oil recovery is in the range of 69% to 74%. At the end of production time, high oil recovery is obtained when very low injection and production rates are used. However, when considering at 30 years, higher oil production rate and gas injection rate results in higher oil recovery for the study reservoirs. In addition, when the production rate is fixed, increasing injection rate provides higher oil recovery. For well pattern, using one horizontal producer located at the deepest depth together with a vertical gas injector at the most updip location yields the highest oil recovery. For sensitivity analysis, relative permeability correlations provide insignificantly different oil recovery except for the production time, and increasing vertical to horizontal permeability ratio gives higher cumulative oil production. Furthermore, a decrease in residual oil saturation results in higher oil recovery and the extended production time. The result shows that decreasing oil saturation from 0.05 to 0.15 leads to an increase in oil recovery up to 13% at the end of production time.