Volumetric Balance Method To Monitor Field Performance Of Gas-Miscible Floods
Authors: Okazawa T. , Lai F.S.Y. 
Published: March 2010

Abstract
A methodology has been developed for quantitatively evaluating (1) the flow distribution of the injected solvent toward producer wells, and (2) the solvent-swept volume between each injector producer well pair from injection-production data. At a steady-stale hydrocarbon production, the production rate reflects the solvent flow- rate to the producer. At the same time, the volumetric balance between cumulative solvent injected and cumulative hydrocarbon production from the producer reflects the volume swept by the solvent. An application of the method to the hydrocarbon miscible flood in Judy Creek ‘A’ Pool yielded data on the solvent-swept volumes which are consistent with the gravity override theory.

Introduction
Since the hydrocarbon miscible flood was started in Judy Creek A Pool(1) in May 1985, production performance has indicated that the gravity override by the solvent is affecting solvent conformance. This conclusion is substantiated by a comparison between injection logs and production logs; i.e. although solvent injections are reasonably uniform down to the lowest perforated zones, solvent production occurs only at the top of the uppermost geological zones. Although such qualitative evidence or gravity override abounds, no adequate technique has been available for quantitative estimation of the solvent swept volume based on field data.
For the purpose of evaluating the performance of gas-miscible floods at sufficiently early stages, the "Volumetric Balance Method" was developed to determine the solvent distribution and the solvent-swept volume for each injector-producer pair from field injection-production data. This paper describes the basic principle of the "Volumetric Balance Method ", potential problems and their remedies in applying the method to field operating conditions, and actual field examples. Although Field application examples or the technique in the Judy Creek hydrocarbon miscible flood project are used, the technique is also applicable to CO2 miscible floods.

Volumetric Balance Method

Basic Principle
Let us assume that in a reservoir (schematically shown in Fig. 1) Injector I is supplying injected fluids toward Producer P as well as toward other directions. Solvent and water injection was started at Injector I upon completion or waterflood, i,e. the reservoir was assumed to retain only the residual oil saturation after waterflood (Sorw). It is assumed that the solvent flow rate (qP toward Producer P is a fraction (f) of the total solvent injection rate (qI) at Injector I and that the solvent-swept pore volume toward Producer P is VS. The swept volumes would be determined by factors such as reservoir size, continuity and gravity override. Figure 1 depicts the case of gravity override.
During the solvent flood, the swept pore volume (VS) has a non-aqueous phase saturation (Sso; solvent and oil combined) which depends on fractional flow conditions and is higher than the initial waterflood residual oil saturation (Sorw). If piston-like displacement were considered, the non-aqueous phase saturation (Sso) would reach the Producer P and production or tertiary oil would start when the volume of injected solvent toward Producer P (QP) became equivalent to VS (Sso - Sorw). In other words, the solvent-swept pore volume (VS) would become filled with the saturation Sso.