Understanding Anomalous Phase Behavior in Unconventional Oil Reservoirs

Abstract

Abstract Normal pressure/volume/temperature (PVT) analysis is based on a fundamental assumption that there is no potential difference between the gas a d liquid phases. The nanometer pore throats in unconventional reservoirs could impose a phase capillary pressure up to 1,000 psi. Therefore, the observed oil viscosity and bubble point pressure (Pb) in an unconventional oil reservoir are substantially lower than those reported by the PVT lab. Such reductions are further aggravated by the compaction effect and then become variable with pressure depletion. In this work, a set of non-linear fugacity equations was constructed by bringing capillary pressure in with the phase equilibrium equations. The corresponding PVT reports for 14 Bakken fluid samples were recalibrated by combining the newly developed non- linear fugacity equations and the pore throat reduction imposed by compaction effects. The resultant PVT tables help clarify all the inconsistent PVT observations. Examples are presented to demonstrate the significance of such PVT corrections. The corrected PVT tables were used in the Bakken reservoir simulation model and successfully resolved not only the inconsistent gas/oil ratio (GOR) issue between the model and real production data, but also substantially facilitated the history match and widened the optimal operational pressure window. A series of studies were completed to address the combined impact of the capillary pressure and compaction on Bakken reservoir fluid properties. The Pb could be reduced in a range from hundreds to over 1,000 psi, compared to the corresponding lab PVT report. Such reduction is more severe in the lower formation permeability and solution GOR scenarios. Furthermore, oil density and viscosity could also be reduced by up to 10% due to the lighter components migrating to the oil phase. This study reveals the cause of the delayed GOR increase and the strange stepwise GOR increases observed in unconventional oil reservoirs. This new understanding of the PVT property variation in an unconventional oil reservoir optimizes the operations procedure and will permit more reliable performance forecasts.