Abstract

It is not trivial to identify and to quantitatively monitor production of injectants such as solvent and chase gas during a commercial gas-miscible flood. A new methodology was developed for evaluating produced amounts of those injectants and reservoir oil using readily available field data. This methodology follows two major logical steps. The first step is to determine overall compositions of the produced hydrocarbons. The new method can uniquely determine the total produced hydrocarbon composition from available field data, namely a separator gas composition and a produced gas-oil ratio, using an equation of state adequately tuned for specific crude oil. The second step is to determine the most plausible proportions of injected solvent, chase gas and reservoir oil in the produced hydrocarbon composition with the knowledge of the respective compositions. The entire process was packaged in computer software which had been tested for its reliability. The software has been in use for the past two years in Imperial Oil's gas-miscible projects for reporting solvent and chase-gas production. In these field applications, the software has proved to be capable of calculating solvent and chase-gas contributions even in situations with gas-lift wells.

Introduction

The first attempt of utilizing flash calculations based on the equation of state for determining injectant content in produced hydrocarbon during a hydrocarbon gas miscible flood was reported by Wu et al.1 in 1989. Their method was as follows; first create a hypothetical mixture of known reservoir oil and solvent compositions, then flash the mixture using a tuned equation of state at a first separator condition. This trial-and-error process was repeated with a change of the mixing ratio until the calculated separator-gas composition agreed with the measured composition. Their method could determine correctly the produced fluid compositions only if the produced fluid from the reservoir were a straight mixture of the assigned oil and solvent compositions. Unfortunately, injected solvent compositions vary with time and the displacement in the reservoir is not as ideal as straight mixing. As a result, they reported marked disagreements in GOR values between measured and predicted values.

The new methodology discussed here does not require a priori knowledge of injectant compositions for the determination of a produced fluid composition. Instead, a produced fluid composition is determined solely from measured separator gas composition and a GOR value. An equation of state tuned to a specific reservoir oil can determine a separator liquid composition which is in equilibrium with the measured gas composition. The produced fluid composition should be on the tie line connecting these gas and liquid compositions, and the GOR value determines the location of the produced fluid composition on the tie line. Once the produced fluid composition is determined, contributions of reservoir oil and injectants to form this composition are determined in a mathematically rigorous manner. The important feature of this methodology is that it can determine mole fractions of multiple injectants in the produced fluid.

The new methodology was packaged in software called PFAN (Produced Fluid ANalysis).

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