This paper presents an analytical model for full-flow-regime rate-transient analysis in unconventional volatile oil reservoirs. The model allows us to forecast long-term production based on transient production data and will be applicable in wells with substantial water production. We formulated and solved governing non-linear partial differential equations (PDEs) with an inner boundary condition of constant BHP. By defining pseudo-variables to transform the governing non-linear PDEs to linear forms, we were able to find solutions of pseudopressure-normalized rate for oil, gas and water phases that describe all flow regimes over the life of a multi-fractured horizontal well. For one-dimensional flow in closed reservoirs, our analytical solutions that show the relationship between pseudopressure-normalized rate and dimensionless time indicate a complicated decline with an exponential relation inside an infinite series. Our study also highlighted that simply using uncorrected PVT data without removing separator effects can be in error. More importantly, to provide an accurate pseudopressure calculation, we conducted numerous simulation studies and proposed saturation-pressure (S-P) relations to enhance the accuracy of calculated pseudopressures specifically for various volatile oils. Our analytical methods yielded reasonable interpretations of not only simulated data but also actual field data. The solutions were validated through comparisons with results from compositional simulation; the good agreements for both ordinary and near-critical volatile oils verified the accuracy of our analytical method; notably, the validations were almost exact during boundary-dominated flow. We also applied our methods to analyze production data from two wells in shale oil reservoirs in the Midland Basin. These cases illustrated that our model can handle with wells not only in transient (infinite-acting) flow but also in boundary-dominated flow.

Introduction
Background

Tight/shale oil development has increased significantly since 2010, driven by technological improvements that have reduced drilling costs and improved hydraulic fracturing technology in major tight/shale plays such as the Bakken, Eagle Ford, Permian Basin, and other resources through the world (EIA 2017). The U.S. Energy Information Administration also reported that the total reserves of shale oils worldwide are over three hundred billion barrels. In the next twenty years, successful development of these reservoirs will still be crucial to maintain current oil supplies and further to achieve even greater production and reserves levels. Because of the way hydrocarbons originated and accumulated in tight/shale rock formations, light oils have been found to be common fluids in most of these reservoirs. In production of these less-dense crude oils, we observe substantial gas flow, resulting in significant reduction of oil production once reservoir pressure drops below bubblepoint pressure. The industry urgently needs an accurate and practical method to forecast production, especially an analytical approach for field application to unconventional volatile oil reservoirs.

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