Predictions of Horizontal Well Performance In a Mature Waterflood, Weyburn Unit, Southeastern Saskatchewan
Authors: Galas C.M.F. , Churcher P.L. , Tottrup P. 
Published: March 2010

Abstract
The results of a parametric numerical simulation study led to the development of selection and screening criteria which were used to define horizontal well candidates within the Weyburn Unit of southeastern Saskatchewan. The relative impact of several reservoir and wellbore properties on horizontal well productivity was assessed.
This study shows that Weyburn horizontal well performance is improved if reservoir permeability is high, void age replacement is maintained, good communication exists between the two main producing zones, and effective wellbore length is maximized. The well spacing, location and position in the more highly oil-saturated zone were also found to contribute to the productivity of a horizontal well.
As a result of this and other engineering and geological work, three successful horizontal wells were designed and drilled in the Weyburn Unit in late 1992. These wells have produced over 19,000 m3 of oil in the first three months of production. Many of the findings of the parametric study, such as well orientation, expected watercut and optimum positioning within the pay, were confirmed by this drilling program.

Introduction
The Weyburn Unit is located approximately 130 km southeast of Regina, Saskatchewan (Figure 1). The field covers approximately 180 km2 and consists of 627 producing wells and 162 water injection wells.
Oil is produced from the Midale Beds of the Mississippian Charles Formation which were deposited on a shallow carbonate shelf in the Williston Basin. The reservoir is subdivided into an upper Marly and a lower Vuggy zone (Figure 2).
The Weyburn field was discovered in 1955 and was completely delineated by vertical drilling by 1960. In 1964, the field was unitized and a waterflood was implemented using inverted nine spot patterns oriented in the NE-SW direction. Between 1985 and 1992 over 150 vertical infill wells were drilled within the unit resulting in substantial incremental oil recovery. The infill drilling program concentrated on improving the walerflood performance and vertical wells were drilled on both 24 ha and 16 ha spacing's within some areas of the field.
To date, only 25.8% of the 178 million stock tank m3 of medium gravity crude oil have been produced. The remaining oil in place represents a significant target for improved recovery schemes such as horizontal drilling.
FIGURE 1: Location of Weyburn Unit. (Available in full paper)
In 1991, a horizontal drilling program was conducted in the Weyburn Unit. Three wells were drilled in different parts of the unit to evaluate the technical feasibility of applying horizontal drilling technology. With the number of economically attractive vertical infill targets declining, it was concluded that the future development of the unit would be dependent on the successful application of this technology. Although promising, the results of this drilling program fell far short of what had been anticipated with only one well (10Hz-II) producing any appreciable amount of oil. The low overall productivity of these wells and the success of other wells in the area drilled by other operators led to the development of the parametric numerical simulation study, which is the subject of this paper.