Weyburn CO2 Miscible Flood Conceptual Design and Risk Assessment

Abstract

Abstract For the three years leading up to the end of 1995, a multidisciplinary project team evaluated the business opportunity of injecting carbon dioxide into a portion of the Weyburn Unit to improve recovery. As with any project of this magnitude, a comprehensive analysis of the opportunities and risks in the project had to be clearly defined and evaluated. A risk assessment using a Monte Carlo simulation approach was undertaken to combine the technical with the non-technical issues associated with the project in order to define the full cycle opportunities and risks. The risk assessment process was used to optimize the project configuration, to focus the team on key issues, and to find ways to mitigate the risks inherent in the project. Key uncertainties impacting the range in expected project returns appeared in the areas of commodity prices, reservoir performance, costs and fiscal terms. The risk assessment process and conceptual development plan, incorporating studies and analysis completed up to the end of 1995, is the subject of this paper. Introduction The Weyburn Unit, covering an area approximately 180 km2, is approximately 130 km southeast of Regina, Saskatchewan (Figure 1). The Unit is operated by PanCanadian Petroleum Limited on behalf of the working interest ownership. The field was discovered in 1955 and produced under primary depletion until an inverted nine spot waterflood was implemented in 1964. Production at the end of 1995 averaged 3,150 m3/d of medium sour crude from 650 oil producers, of which 90 were horizontal wells. At that time, it was postulated that more than 116 ? 106m3 of oil was expected to remain in the reservoir after the completion of waterflood operations, or 65% of the original 178 ? 106m3 in place. This large remaining oil in place represented a very attractive target for an enhanced recovery process. The Midale beds of the Weyburn field were deposited on a shallow carbonate shelf in the Williston basin. The reservoir is uniformly divided into the upper Marly, a chalky inter-tidal dolo-stone with limestone interbeds, and a lower Vuggy zone, a heterogeneous and highly fractured sub-tidal limestone. A more complete reservoir characterization was discussed by Elsayed et al.(1) FIGURE 1: Weyburn unit outline. (Available in full paper) FIGURE 2: Risk analysis is the centerpiece of a risk management process. (Available in full paper) An analogy to the Weyburn field can be drawn from the Midale Unit, situated just east of the Weyburn field. Shell Canada Ltd., the operator of the Unit, implemented a CO2 injection pilot in 1984 which was completed in 1988. Due to encouraging results from the pilot(2), Shell implemented a "CO2 demonstration project, " an eight pattern flood in 1991 which was designed to improve oil recovery from the area and provide technical and economic data to further assess full scale development potential. This flood is still in progress. Input data forming the basis of this assessment incorporated some of the Midale information as well as opinions from internal experts, industry consultants and other analogous CO2 floods in the Permian Basin(3).