Midale CO2 Flood Pilot
Authors: Beliveau D.A. 
Published: March 2010

Abstract
The Midale oilfield is part of a Mississippian carbonate trend in southeastern Saskatchewan. The Midale Unit has been under waterflood since 1962, and is approaching an 80% water-cut. In an attempt to significantly increase recovery from this large resource base, Shell Canada Limited and partners have begun piloting a tertiary miscible CO2 process. This article focuses on the planning and design of the CO2 pilot, which began in 1984. The design integrates techniques from both observation and production-style pilots. The synergistic analysis of data from many sources is crucial for timely scale-up of results from the project, which will conclude in 1988.

Introduction
Oil was discovered in the Midale field in June 1953, by the drilling of Shell Midale A-7-18-6–10 W2M. Rapid development over the next five years resulted in the drilling of some 300 wells on 32-ha spacing. Production from the field was erratic during these early stages due to market and pipeline restrictions. It soon became apparent that primary depletion in the field would be due almost entirely to solution-gas drive. Analysis showed waterflooding to be the preferred secondary recovery method, so a pilot waterflood project was undertaken in 1960 in two field-scale patterns. Encouraging results from this pilot led to the formation of the Midale Unit, and a field-wide water flood on inverted nine-spot patterns commenced in December 1962.
Oil production from the Unit peaked at 2300 m3/d in 1964, and declined steadily to 950 m3/d by 1985. Water-cut is approaching 80% in the mature operation. Average fluid injection and production rates in this low productivity reservoir are some 60 and 12 m3/d per well, respectively.
A charge in investment climate in the early 1980s resulted in an explosion of activity for the Midale Unit, including the tertiary CO2 pilot and an aggressive infill drilling program. As well, upgrade of the existing waterflood facilities is under consideration. This surge in development activity lays the groundwork for continued Unit oil production well into the 21st century.

Geology
The Midale field(1) is located in southeastern Saskatchewan along the northern margin of the Williston Basin. The oil accumulation in the Midale Unit is in the Mississippian-aged Charles Formation. The Midale Beds pay section occurs as a basal member of the first (lowest) Charles cycle. The carbonate section of the Midale Beds is usually overlain by the Midale Evaporite of the Ratcliffe Beds and underlain by the Frobisher Evaporite.
Classicall, the pay section has been divided into two major units, the upper Marly and lower Vuggy zones. The Marly is a microcrystalline dolomite with porosities in the 20% to 30% range, and net pay approximately 3.5 m out of a 6.5 m gross section. The underlying Vuggy is a micro- to coarsely crystalline fragmental limestone, with net pay of approximately 5.0 m out of a 10.0 m gross section and porosities of 10% to 15%. The non-pay in each zone is tighter carbonate rock. Both zones have initial water saturations in the net pay of approximately 40%, and matrix (air, unstressed) permeabilities in the 15 md range.