Abstract

The Midale Unit in southeastern Saskatchewan has been under waterflood operations for 40 years, and substantial quantities of remaining oil make the Unit a good target for implementation of an EOR process. Although naturally fractured, the Midale Unit proved to be a good candidate for a miscible CO2 flood. Encouraging results from a demonstration CO2 flood in a small part of the Midale Unit (?2,000 acres) prompted an extensive study to evaluate the potential of the fullfield CO2 flood. The study involved analytical modeling and several compositional simulation models, ranging from a small, element-of-symmetry model to the comprehensive, field-scale model.

This paper describes the compositional simulation conducted on a large 3D model containing 40,000 grid blocks. The 204-well model covered 20 inverted nine-spot waterflood patterns and the CO2 demonstration flood area including both vertical and horizontal wells; this comprises approximately 25% of the total Unit. The reservoir fluid was characterized using a 9-component Peng-Robinson equation-of-state model. Pattern-based permeability anisotropy was a key parameter used to accurately match individual well response to the waterflood and the CO2 flood. A very good history match was obtained on both the field and the well scale.

The tuned model was used to predict future reservoir performance under various operating and development scenarios. Various CO2 injection strategies were considered, including Continuous CO2 flood, Water-Alternating-Gas (WAG) and Hybrid WAG. Under each strategy a detailed sensitivity modeling was performed investigating the optimal CO2 volumes, injectivity, WAG ratio and the benefits of pressure support and limiting the gas production. The study showed that incremental recoveries of 11–15% OOIP can be expected by implementing the optimized CO2 flood scenario. Results of the compositional model were then used to tune the streamline simulator for the full field model. A full field development plan was delivered in 4 months.

Introduction

The Midale field, located in SE Saskatchewan (Figure 1), was discovered in 1953 and initially delineated on 80 acre spacing. The field produced under competitive drainage until unitization in late 1962, at which time an inverted nine-spot waterflood scheme was implemented. An extensive vertical infill program was undertaken during the mid 1980's to modify the waterflood patterns to increase recovery. In the late 1980's and mid 1990's, horizontal wells and multi-legged perpendicular horizontals were drilled to further improve waterflood conformance. To-date, the Unit has recovered over 20 E6m_ of oil (primarily from waterflood operations) representing about 24% of the Unit's OOIP.

A CO2 Flood Pilot project was initiated in 1984 to investigate the applicability of using CO2 to recover some of the large volume of remaining oil that would not be recovered by waterflood operations1–3. This project involved the drilling of 10 closely spaced wells in an area 4.4 acres in size, and it generated an enormous amount of reservoir and geological information. Based on results from the CO2 Pilot project, a larger-scale Midale CO2 Flood Demonstration Project4, consisting of a six-pattern CO2 flood located in the southwestern part of the Unit, began operations in 1992.

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