Effect of Gas Injection Rates on the Performance of a Thin Oil Rim: A Simulation Study

Abstract

Oil recovery from thin oil rims under a strong aquifer and large cap is challenging. Low oil recovery efficiency, water coning, gas cusping and the upward and downward shifts of the oil water contact (OWC) and gas oil contact (GOC) respectively are problems. However, studies have shown that injecting natural gas at the OWC can significantly improve oil recovery efficiency and reduce water production. But the effect of varying gas injection rates on the performance of such reservoirs needs to be studied, and that is the focus of the simulation conducted in this work. The model of a thin oil rim with a strong underlying aquifer and large gas cap in the Niger Delta was simulated under six different gas injection rates at the OWC to study the performance of the reservoir. Results show that as gas injection rate increased, the oil recovery efficiency and gas-oil ratio (GOR) increased almost linearly with the exception of gas injection rate of 2500Mscf/d which is not strong enough to push back water influx into the reservoir. The highest recovery efficiency was almost 62% at the highest gas injection rate of 15000Mscf/d which also gave the highest GOR and an insignificant volume of produced water. Every gas injection rate has its merits and demerits but the critical factors are oil recovery efficiency, volume of produced water and GOR. Hence, it is recommended that gas injection rates at the OWC be carefully selected based on goals the operating company wants to achieve.