Analysis of the Waterflood Response of a Naturally Fractured Reservoir

The 500 million barrel Midale oilfield is part of a trend of large Mississippian oil accumulations located in southeastern Saskatchewan, along the northern margin of the Williston Basin. The field was discovered in 1953 and developed on 80-acre spacing. In 1962 Midale was unitized for waterflooding, with 83 320-acre inverted ninespot patterns (Figure 1).

Waterflood performance in Midale is dominated by a system of oriented natural vertical fractures typically spaced 1–4 feet apart. Production wells located "ontrend" from injectors (aligned with the fractures) showed sharp early response to waterflood. In contrast, response of "offtrend" producers was smooth and delayed. Oil produced to-date represents recovery of 20% of the original oil in place (OOLP); ultimate waterflood recovery is predicted to be only 24% OOLP. Current watercut in the mature operation is about 80% (Figure 2). Even with the natural fractures, the Midale Unit is a low-productivity reservoir, with average production rates of 75–100 STB/D/well.

A major reservoir and process mechanism characterization effort began in the mid-1980s to guide ongoing Midale field development opportunities. A detailed study team consisting of a geologist, a petrophysicist, and production, reservoir, and research engineers spent three years analyzing waterflood performance. The large-scale effort was dictated by the complexity of predicting performance in a heavily fractured reservoir. Obviously, good communication and cross-discipline integration of expertise was a key to the project’s success. The resulting reservoir and process model honors all available data and can match performance of the field-scale waterflood as well as a tertiary CO₂ flood pilot.