A Compositional Material Balance for Combination Drive Reservoirs with Gas and Water Injection

Abstract

Abstract Compositional material balance methods have been extended to solution gas-gas cap-water drive reservoirs. Partial or complete pressure maintenance as well as reservoir repressuring by gas and/or water injection may be considered. Increments of reservoir withdrawals and injections equivalent to a given time increment are used as the basic iterative step to allow the extension. An example field study shows how the reservoir volumes, extent of the water drive, and relative permeability ratio function may be estimated. Knowing these, the methods can be used to predict reservoir behavior. Introduction The compositional material balance is similar to conventional material balances in many aspects. The assumption of a pressure-homogenous reservoir with no transients is common to all conventional material balances although numerical methods are different. The compositional material balance also contains this assumption.Inclusion of phase equilibria directly in reservoir calculations is the key difference and improvement in the compositional material balance. A continuous account of the compositions of the reservoir fluids and effluents is determined. Calculations of fluid phases and properties can be based on these compositions. Excepting fluid phases and properties determined in this manner, the compositional material balance uses procedures exactly analogous to conventional methods.Brinkman and Weinaug and Jacoby and Berry have given compositional material balances for simple solution gas drive reservoirs; subsequently Jacoby and Berry, extended their procedure to include pressure maintenance by gas injection at constant reservoir pressure.This paper extends compositional material balance techniques to a more general combination drive reservoir with various types of pressure maintenance operations. The procedure for this extended compositional material balance and an example of its field application are presented. PROCEDURE The numerical calculations of this compositional material balance are performed on a high-speed digital computer. The most comprehensive case the present compositional material balance program can handle is a solution gas-gas cap-water drive reservoir withwater production,water influx,separate or commingled production from the oil and gas zones,any surface processing arrangement including LPG and gasoline production, andgas injection into the gas and/or oil zones.* This description of the procedure applies to this most comprehensive case. Appropriate deletions in the procedure can be made for more simple mechanisms. ASSUMPTIONS in common with conventional material balances, the compositional material balance makes the following assumptions:the total reservoir is at a uniform pressure at a given time;the volumetric ratio of gas to liquid withdrawal at reservoir conditions is directly proportional to their mobilities; andinstead of assuming a pressure- dependent total gas-oil equilibrium, the compositional material balance makes a significantly different and more rigorous assumption as follows. Thermodynamic equilibrium exists for each component in the liquid-gas mixtures in the reservoir and in surface processing.These three assumptions are fundamental in the compositional material balance procedure. Other assumptions involve arbitrary simplifications of the reservoir mechanisms. This compositional material balance uses the following simplifying assumptions:no bulk mixing occurs between the gas and oil zone;total mixing occurs between any injected gas and the contents of the injection zone;no in situ hydrocarbon liquid is produced from the gas zone;no reservoir gas is retained behind any net bottom-water influx;the residual saturation of hydrocarbon liquid at reservoir conditions behind any net bottom-water influx is constant; andfluids retained behind any net bottom-water influx exert no further influence on the reservoir. These six assumptions are not fundamental. Modifications of the present procedure can be made for different assumptions of the reservoir mechanisms. THE TIME INCREMENT Previous compositional material balances have used a pressure increment for simple solution gas drive reservoirs and an increment of injected gas for pressure maintenance. These increments are suitable for these simple cases. They are, however, inadequate for reservoirs with time dependent drive mechanisms including water drive. JPT P. 1233^