Horizontal Wells In The Rosebank Alida Beds Pool, Saskatchewan
Authors: Vigrass L.W. , Vigrass R.G. 
Published: March 2010

The Rosebank Alida Beds (Mississippian) pool of southeastern Saskatchewan is a mature oil field that has produced close to 5 million m3 of light oil since discovery in 1955. Oil trapped at the subcrop is produced from two carbonate reservoirs: a lower unit with fine intercrystalline porosity and an upper unit with coarse vuggy porosity. The pool was originally developed on 32 ha (80 acre) spacing. Original pressure has been largely maintained by natural water influx.

Vertical as well as horizontal infill wells have not resulted in significant interference with production at offsetting original vertical wells. Nonetheless, several vertical infills have productivity that is considerably lower and water-oil ratio that is significantly higher than offsetting original wells. In contrast, seven of eight horizontal wells drilled in the northern part of the pool have been remarkably successful in increasing productivity and reserves.

The seven successful horizontal wells had first-year oil productivity averaging 40 m3/d, close to three times that of surrounding vertical wells drilled 25 to 40 years earlier, and initial water-oil ratios of 0.3, comparable to that of the original wells. Five of the horizontal wells have average ultimate reserves of 80,000 m3/well compared to an average 67,500 m3/well for 26 surrounding vertical wells. In the area studied, the vertical wells are expected to recover 37.5% of the original oil-in-place. The five horizontal wells are expected to recover an additional 8.6% of OOIP for a total recovery of at least 46.1%.

Geology and reservoir properties are important considerations in the location of successful horizontal wells. At Rosebank, among other considerations, horizontal wells oriented parallel to the paleotopographic ridge, presumably parallel to the main fracture trend, will recover more reserves than those oriented' transverse to the ridge.

Introduction

The Rosebank pool occurs along the subcrop of the Mississippian Alida Beds on the sub-Watrous unconformity approximately 225 km southeast of Regina, Saskatchewan (Figure 1). The carbonate-hosted oil pool at a depth of 1,070 m was discovered in 1955 and had produced 4.8 × 106 m3 (30.2 × 106 STB) of light oil to December 31, 1990. The pool has a strong water drive.

The southern part of the pool, developed on 32 ha (80 acre) spacing in 1955 and 1956, was unitized in 1971. The northern part of the pool was drilled on 32 ha (80 acre) spacing in 1966 and 1967. Several vertical infill wells on 16 ha (40 acre) spacing were drilled over the years with the greatest activity from 1987 to 1991. Horizontal development began in late 1990 and ten horizontal wells had been drilled to December 31, 1993. Results of the first horizontal well were reported by Jamieson(l,2).

Horizontal drilling of the carbonate reservoir at Rosebank has resulted in a remarkable increase of oil production in a mature oil field that has been on production for almost 40 years. There has been considerable controversy as to whether ultimate reserves are increased through horizontal infill drilling or whether there is simply an acceleration in the production of oil that would be produced by vertical wells over a longer period of time.