Summary
Considerable research has been focused on the development of rate-transient-analysis (RTA) models to estimate the reserves of gas/condensate reservoirs. Currently, broadly deployed RTA tools rely on multiphase pseudopressure concepts to enable multiphase production-data analysis. In any multiphase pseudopressure calculation, the determination of the saturation/pressure (So/p) relationship plays a vital role because it directly influences the ability of multiphase RTA methods to reliably forecast original gas in place (OGIP). In this work, we present a thermodynamics-based So/p model that provides a better understanding of the phase behavior for the boundary-dominated gas/condensate reservoirs. The proposed So/p model is derived from the thermodynamic nature of extended black-oil formulations. A noniterative flash-calculation protocol is used to establish the So/p path in the condensate-buildup region. The developed method can be coupled with RTA tools and services for the calculation of multiphase pseudopressure. In this work, we present case studies of three gas/condensate reservoirs with different types of fluids. Two RTA multiphase analysis models are used to scrutinize the production data using the newly proposed So/p relationship, and results are compared with the use of a traditional steady-state method coupled with constant-volume-depletion (CVD) data. Results of the case studies show that RTA models that use the proposed So/p consistently yield more accurate OGIP estimation. Thus, this work presents a practical approach to remove commonly used yet potentially faulty assumptions in multiphase RTA applications for liquid-rich gas/condensate reservoirs.
