Deviations from transient linear flow behavior: A systematic investigation of possible controls on abnormal reservoir signatures

Abstract

Observed deviations from classical diffusion (specifically transient linear flow, TLF) for Permian Basin Wolfcamp shale oil wells have been historically attributed to reservoir geometric heterogeneity and consequent anomalous diffusion (AD). Recently, we have suggested that the combination of multiphase flow and variable operating conditions could create similar reservoir signatures (as quantified using classic flow-regime identification techniques). However, there are additional factors, not previously investigated, that could be affecting flow-regime identification (ID). This work aims to systematically investigate all possible causes of deviations from classical diffusion behavior. A general, new semi-analytical oil flow solution is proposed that explicitly accounts for multi-phase flow, stress-sensitive reservoir properties, and variable operational conditions. This is achieved through using the combination of modified oil-phase pseudo-pressure and material balance pseudo-time. Additionally, reservoir geometric heterogeneity and AD are incorporated into the exponent of the power-law relationship between rate-normalized oil-phase pseudo-pressure and material balance pseudo-time. A new workflow is also developed to improve flow regime ID while various reservoir complex behaviors are occurring simultaneously. Using the new workflow, a comprehensive investigation into which parameters are primarily causing deviations from TLF is performed in use of simulated cases with inputs that are similar to those used to history-match a Wolfcamp shale well. Further, errors in the determination of reservoir and fracture properties caused by misdiagnosis of the flow regime are analyzed. Our extensive investigations demonstrate that: 1) multi-phase flow, and in particular the appearance of gas flow (producing gas-oil-ratio ramps up) as flowing bottomhole pressure (FBHP) drops below the bubble point pressure, has the biggest impact on the deviation from TLF; 2) even for cases with high producing water-oil-ratio, water production has relatively minor effects; 3) pressure-dependent permeability has relatively small effects, even for cases when well FBHP changes strongly; 4) deviations from TLF in a Wolfcamp shale oil example may be primarily caused by the combination of multiphase flow and variable FBHP, while the AD effect is likely secondary. Importantly, if log-log flow-regime identification plots are not corrected for complex reservoir behaviors, the reservoir flow regime may become distorted, leading to the selection of incorrect models for production analysis and errors in reservoir/fracture properties estimates.