Evaluation of the impact of multi-phase flow on reservoir signatures in the Wolfcamp shale

Abstract

The dominant transient flow regime for multi-fractured horizontal wells (MFHWs) producing from low-permeability and shale (unconventional) reservoirs has historically been interpreted to be transient linear flow (TLF) with analysis performed in the framework of classical diffusion (CD). Recently, observed deviations away from this classical behavior for Permian Basin Wolfcamp shale (oil) wells have been attributed to reservoir heterogeneity and anomalous diffusion (AD). However, some wells in the basin also produce below bubble point pressure and exhibit characteristics of multi-phase flow. The objective of the current study is therefore to investigate if multi-phase flow may also be contributing to observed deviations from classic TLF behavior.The conventional log-log diagnostics used to identify flow regimes do not account for reservoir complexities such as multi-phase flow and reservoir heterogeneity. Failure to correct for these effects when they are occurring may result in misdiagnosis of flow regimes. A new workflow is therefore introduced herein to improve flow regime identification when reservoir complexities are exhibited, with a specific focus on multi-phase flow. The workflow involves the correction of log-log diagnostics for multi-phase flow through the use of modified pseudo-variables (pseudo-pressure and pseudo-time) once characteristics of multi-phase flow are identified (e.g. rapid increase of gas-oil ratio after bubble point pressure is reached). Although reservoir heterogeneity and AD are accepted causes of deviations from TLF, the impact of multi-phase flow has not been investigated in detail. Therefore, corrections to pseudo-variables for multi-phase flow, a known reservoir complexity exhibited by Wolfcamp shale wells, are presented. Pressure-dependent permeability is also accounted for in pseudo-variable calculations, although its impact is demonstrated to be relatively minor in this study.Application of the new workflow to a simulated case and a Wolfcamp shale field case demonstrates the following: 1) multi-phase flow, and in particular the appearance of a mobile gas phase after two-phase oil and water production, results in deviations from classical TLF behavior when data is analyzed using conventional (uncorrected) diagnostics; 2) application of the modified diagnostics to a simulated case that includes multi-phase flow results in the “true” flow regime signature of TLF being observed; 3) application of the modified diagnostics to a field case exhibiting evidence of multi-phase flow reduces the deviation from TLF.This study suggests that multi-phase flow may be impairing our ability to identify flow regimes using conventional flow regime identification methods.