Abstract

The Rosebank olida Beds pool of southeast Saskatchewan, discovered in 1955, has produced more than 4.8 × 106 m3 of light oil to December, 1990. Oil occurs in the Mississippian Alida Beds in a subcrop trap. The oil reservoir consists of the Skeletal Wackestone unit, 25 to 30 m thick, overlain by the Upper Oolitic unit, up to 26 m thick. The two reservoir units are quite dissimilar in reservoir properties. The Upper Oolite has vuggy and coarse intergranular porosity with high horizontal permeability and low capillarity and water saturation. The Skeletal Wackestone has fine intercrystalline porosity with relatively low permeability and high capillarity and water saturation. The pool has performed well under natural water drive and it is expected that at least 32 percent of the oil will be recovered.

From late 1990 to September, 1993, eight horizontal wells were drilled in the northern part of the pool. The performance of a recent horizontal well (Saskoil Unit D9--I D3-10) and surrounding vertical wells was studied. The horizontal well has maintained rates of over 50 m3/dwilh a WOR of 0.3 m3/ m3/ since April, 1992. The horizontal well is expected to recover 190,000 m3/ of oil an incremental 24% of the original oil in place. To date there are no signs of detrimental effects on the offsetting vertical wells.

High initial production and high recoveries from horizontal wells at Rosebank are considered to result from two mechanisms.:

  1. In the Skeletal Wackestone. characterized by a network of fine intercrystalline porosity of low permeability, increased production and reserves result from the greater length of borehole opened;

  2. In the Upper Oolite, the porous streaks with high permeability are erratic and it is considered likely that the horizontal borehole connects to isolated or semi-isolated "compartments" that otherwise would be produced slowly if they were produced at all.

Introduction

The Rosebank pool occurs along the subcrop of the Mississippian Alida Beds on the sub-Watrous uncooformily approximately 225 km southeast of Regina, Saskatchewan (Fig. 1). The carllonate-hosled oil pool at a depth of 1070 m was discovered in 1955 and had produced 4.8 × 106 m3 (30.2 × 106 STB) of oil having a density of 828 to 841 kg/m3 (37 ° to 39 ° API) to December 31, 1990. Oil production was from about 90 vertical wells; about 50 of the vertical wells are in the unitized south portion of the pool (Rosebank Voluntary Unit No. I) and about 40 wells are in the non-unitized northern part of the pool.

This paper concerns the geology and the development through horizontal wells of the northern part of the Rosebank Alida Beds pool in Tp 5, Rge. 32 WIM. Horizontal development began in late 1990 with drilling of the CN et al. Rosebank HZ A9-14-AI5-14-5-32 well from 09-14 to 15-14-5-32. Results on the first well were reported by Jamieson in 1991 1 and additional results were given in June, 1993.2

This Rosebank study is part of a larger project that is examining the effects of geology. drilling practices and completion practices on horizontal well productivity and reserves.

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