ABSTRACT

In early 1992, PanCanadian Petroleum Ltd. drilled the first exploratory horizontal well in southeast Saskatchewan at Ceylon, T6 R19 W2. A second horizontal well was also drilled into the Bakken Formation at 6500 ft (2000 m) depth on the large Ceylon structure in late 1992. The Bakken Formation occurs at the Mississippian to Devonian transition and consists of three informal units. Lower and upper organic rich shale units encase a middle sandstone and siltstone unit. The Bakken shales are pervasive throughout the Williston Basin. The middle Bakken sandstone development occurs primarily within the Saskatchewan section of the basin and is the reservoir target. The shales, which are the source rock within the thermally mature portions of the basin, are immature at Ceylon. Light 40° API oil migrated updip from the thermally mature areas and is trapped within the middle Bakken sandstone.

The Ceylon area was selected to evaluate the productive potential of the naturally fractured, thin pay section of the Bakken Formation with horizontal drilling technology. Although there are pools in Saskatchewan with significant Bakken production from vertical wells, most attempted completions have been uneconomic. All Bakken production in Saskatchewan is from the middle sandstone unit although numerous good horizontal wells were producing from the shale units in North Dakota by 1992. Horizontal drilling was attempted with the goal of increasing productivity and drainage area within a 6 ft (2 m) net pay section by intersecting a maximum number of open fractures. Reservoir heterogeneities, prediction of the oil/water contact, open fracture orientation and spacing, and drilling induced formation damage were all considerations addressed in the design and implementation of the drilling programs. A sophisticated program was designed to minimize formation damage by drilling with invert mud at slightly underbalanced conditions. An oriented core in the vertical pilot hole and a horizontal core were cut in the second well for fracture information and to determine in-situ water saturations within the transitional oil/water contact. Capillary pressure data from the vertical core was used to predict the transitional oil/water contact in the fine grained sandstone reservoir.

A large structural high at Ceylon, defined by 2-D and 3-D seismic, projects into the Hummingbird trough. Salt dissolution of the underlying Prairie Evaporite Formation and orthogonal sets of reverse faults coincide with the margins of the Ceylon structure. The 3-D seismic over the Ceylon structure was used to plan the well trajectories. Intersection of the oil/water contact resulted in high initial water cuts in both wells. Produced water chemistry and inert gases suggest the Bakken fractures are connected to deep seated strike-slip faults and act as conduits to deeper formation waters.

Drilling confirms a productive fractured middle Bakken sandstone reservoir exists along the structural crest of the Ceylon structure. Horizontal technology can be successfully utilized to target a 6-9 ft (2-3 m) net pay interval at 6500 ft (2000 m) true vertical depth. The horizontal well was maintained in the thin pay section over 3250 ft (1000 m) laterally with minimal formation damage. Compared to vertical wells, substantially more reservoir information, including fracture contribution to productivity, was obtained by horizontal drilling. However, high water cuts and declining oil production precluded a large Ceylon development program.