Multidisciplinary Reservoir Characterization and Simulation Study of the Weyburn Unit

Abstract

Summary Reservoir characterization is important to EOR and numerical simulation studies, especially for complex naturally fractured reservoirs with low-permeability matrix rock. Solvent movement and volumetric sweep are controlled by fracture distribution and connectivity and by matrix permeability variation. All these factors were considered in our reservoir characterization and simulation study. The reservoir characterization was so effective that, in a 63-well simulation study, 70% of the wells were history matched on the first simulation runs without modification to the original geologic/petrophysical data. This can be attributed to the team approach of using geological, reservoir engineering, and petrophysical input. Introduction The Weyburn Unit, in the southeastern corner of Saskatchewan, has produced medium-gravity crude oil from the fractured, low-permeability Midale beds of the Mississippian Charles formation since its discovery in 1954. Even though the Weyburn Unit has been on waterflood for more than 26 years, a significant target for EOR, horizontal wells, and waterflood optimization remains. The Weyburn field is located ~130 km southeast of Regina, Sask. (Fig. 1). The productive portion of the field covers ~180 km2. The field currently has 627 producing wells and 162 water injection wells on ~24-ha spacing. The Midale beds of the Weyburn field were produced by primary depletion from discovery until 1964, when an inverted nine spot waterflood scheme was implemented. Since 1985, both horizontal and vertical infill drilling programs have been undertaken to optimize waterflood performance. The Midale beds of the Mississippian Charles formation were deposited on a shallow carbonate shelf in the Williston basin. The reservoir is informally subdivided into the upper Marly and the lower Vuggy zones (Fig. 2). The Marly is a chalky intertidal dolostone with occasional limy or limestone interbeds. Porosity in the dolostones ranges from 16% to 38%, with an average of about 26%. Matrix permeability within this reservoir horizon ranges from 1 to >100 md, with an average of <10 md.