Abstract

For over three years, a multi-disciplinary project team has been evaluating thebusiness opportunity of injecting carbon dioxide into a portion of the Weyburn Unit to improve recovery. As with any project of this magnitude, having fullcycle expenditures in excess of 2 billion dollars, a comprehensive analysis ofthe opportunities and risks in the project had to be clearly defined andevaluated.

A risk assessment using a Monte Carlo simulation approach was undertaken tocombine the technical with the non-technical issues associated with the projectin order to define the full cycle opportunities and risks. The risk assessmentprocess was used to optimize the project configuration, to focus the team onkey issues, and to find ways to mitigate the risks inherent in theproject.

Uncertainties impacting the range in project returns appeared in the areas ofcommodity prices, reservoir performance, costs and fiscal terms. The riskassessment process, as it was applied to the Weyburn CO2 projectevaluation, is the subject of this paper.

Introduction

The Weyburn Unit, covering an area approximately 180 km2, isapproximately 130 km southeast of Regina, Saskatchewan (Figure 1). The Unit isoperated by PanCanadian Petroleum Limited on behalf of 40 working interestowners. The field was discovered in 1955 and produced under primary depletionuntil an inverted nine spot waterflood was implemented in 1964. Currentproduction averages 3150 m3/d of medium-sour crude from 650 oilproducers, of which 90 are horizontal wells. After waterflood, more than 116E6M3 of oil will remain in the reservoir, or 65 % of theoriginal 178 E6M3in place, representing a very attractivetarget for an enhanced recovery process.

The Midale beds of the Weyburn field were deposited on a shallow carbonateshelf in the Williston basin. The reservoir is unifonnly divided into the upper Marly, a chalky inter tidal dolostone with limestone interbeds, and a lowerVuggy zone, a heterogeneous and highly fractured subtidal limestone. A morecomplete reservoir characterization was discussed by Elsayed etal1.

An analogy to the Weyburn field can be drawn from the Midale Unit, is situatedjust east of the Weybum field. Shell Canada Ltd., the operator of the Unit, implemented a CO2 injection pilot in 1984 which was completed in1988. Due to encouraging results from the pilot2, Shell implementeda "CO2 demonstration project", an eight pattern flood in 1991 whichwas designed to improve oil recovery from the area and provide technical andeconomic data to further assess full scale development potential. This flood isstill in progress.

Input data forming the basis of this assessment incorporated some of the Midaleinformation as well as opinions from internal experts, industry consultants andother analogous CO2 floods in the Permian Basin3

Traditional evaluation process

Traditional engineering, design, and estimating methodology sets out a logicalstep-by-step process which leads through the project life cycle.

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