Consistent EUR Forecast in Permian Multiphase Unconventional Reservoirs with Pressure Normalized Rate Method

Abstract

Abstract Consistent Estimated Ultimate Recovery (EUR) forecasting has been a much-studied topic for the development and reserves estimation of unconventional resources. It is widely known that unconventional well EUR forecasts using Arp's Decline Curve Analysis (DCA) are not very accurate in early time. This is because Arp's empirical equations were originally defined for constant operating pressure and unconventional wells have significant early-time variations in operating pressure. Consequently, EURs based on early DCA are often overestimated. This paper develops the Pressure Normalized Rate Decline Curve Analysis (PNR DCA) method and demonstrates its ability to provide consistent EUR forecasts for Permian multiphase unconventional wells through multiple case studies and dynamic reservoir simulation. The authors developed PNR DCA method by normalizing actual oil rate to a rate at constant flowing pressure (PNR) and then applying decline curve analysis using the PNR to forecast EUR of Permian unconventional wells with limited production history. This method was first developed for a single-phase gas reservoir, the Haynesville shale (Xie et al., 2012). Lacayo and Lee (2014) introduced this method to forecast wells in four different shale plays (Eagle Ford, Woodford, Marcellus, and Bakken) for more accurate early forecasts, mostly for single-phase oil or gas reservoirs. This paper describes how the method was modified for more complicated Permian reservoirs, in which the resource contains mobile water and many wells feature early gas/oil ratio (GOR) increases, resulting in multiphase flow in the reservoirs. The method was validated with the data from over 100 wells with significant history (1-5 years) in the Bone Spring, Spraberry, and Wolfcamp formations in both the Midland and Delaware basins. The EUR estimated using the PNR DCA method was also found consistent with forecasts using history-matched dynamic reservoir simulation. The validation results show the PNR DCA method can provide relatively consistent EUR forecasts with limited production data, whereas Arp's DCA method can only forecast consistent EUR after flowing pressures stabilize, which can take a year to achieve. Results indicate that the PNR DCA method works for several sub-regional areas of the Permian Basin, including areas where water cut is as high as 80-90%.