Design and history matching of a waterflood/miscible CO₂ flood model of a mature field: the Wellman Unit, West Texas

Abstract

The Wellman Unit is located in Terry County, West Texas. It is a Wolfcamp massive limestone reef. The reservoir is considered geologically unique due to the double cone-shape structure with a maximum closure of 825 feet above the OWOC. Three stages of depletion occurred over 50 years of production: 1) Primary depletion under a combination of solution gas drive and bottom water drive 2) Secondary recovery was initiated in 1979 via water injection into the flank of the reservoir and 3) CO₂ miscible injection was implemented in three wells on top of the structure in 1983. In this research, production and reservoir data was revised and integrated to develop a full field, 3-D, black oil simulation model. The primary objective was to reproduce via history matching, the historical performance of the reservoir under primary, secondary and tertiary stages of depletion. A secondary objective was developing a calibrated model that can be used to evaluate, design and plan future reservoir management decisions. To accomplish this task, a reservoir model was built that honors the structural configuration of the reservoir. A flexible grid system comprised of 16,767 grid blocks was built based on corner point - non orthogonal geometry. During the matching process, parameters were ranked according to uncertainty. The most uncertain parameter was the transmissibility and physical size of the under-lying aquifer system. Finally, the pseudo-miscible option based on the Todd and Longstaff theory was implemented as a modification to model behavior of the CO₂ miscible flood. The results can be summarized by the following: 1) History matching the primary depletion stage was satisfactory. 2) Extensive aquifer tuning was required to maintain sufficient reservoir pressure for CO₂ injection and match the field data during the waterflooding phase. 3) A complete pressure match was achieved through primary depletion, waterflooding and CO₂ injection, however the match on liquid production rate was compromised in order to tune the final pressure match from 1986-1995. The results of this work provide a foundation for future research into this hydraulically complicated reservoir. Once the complete calibrated model has been built, the simulation can be used to forecast future operations like blow-down or gas storage.