L41009 Material Balance Equation for Partly Depleted Oil Reservoirs Repressured with Gas

This project researched the use of a model oil reservoir and new concepts of reservoir engineering to develop a simple method of predicting oil recovery as a byproduct of gas storage operations. This paper is the fourth in a series reporting the progress of a study of reservoir performance during complete gas re-pressuring and gas cycling. It presents a new material balance equation (MBE) for calculating initial oil volume and relationship of average reservoir pressure to net gas in storage. The new MBE is particularly helpful in making field case history studies of oil reservoirs that have been completely re-pressured with gas. These studies must reveal the percentage of oil recovery, which requires, in turn, that the initial oil volume be determined accurately. A material balance equation was derived for calculating (1) the initial oil volume and (2) the relationship of average reservoir pressure to net gas in storage in reservoirs used for natural gas storage operations. The equation is also applicable to secondary-recovery operations where reservoirs are completely re-pressured and gas cycled. Using this equation minimized two probable sources of appreciable error in conventional material balance calculations: inaccurate measuring of and accounting for produced gas, and differences between true reservoir pressure and average reservoir pressures as derived from bottom-hole pressure measurements. A new procedure is presented for calculating water encroachment. By solving the conventional material balance equation and the derived equation for gas storage simultaneously, the values for initial oil in place and net water encroachment can both be determined.