Optimization of the Blueberry Debolt Oil Pools: Significant Production Increases For a Mature Field
Authors: Carlson M.R. , Bora R. , Chugh S. , Loczy P. , Byl M. 
Published: March 2010

The Blueberry Debolt Oil Field of Northeastern British Columbia, discovered in 1956, has over 36 years of production history. It is a complex carbonate reservoir with the oil trapped against the updip termination of a thrust fault. Reservoir quality is extremely variable and controlled by dolomization and fracturing. There is a downdip water leg and early testing indicated the presence of an updip gas cap.

The production performance of the field was difficult to evaluate, in particular, the source of water production. Numerical modelling (simulation) has revealed significant insights into the production mechanisms in the reservoir. The source of water production is better understood and the location of the gas cap better defined. With this information a successful infill drilling program was implemented which has resulted in significant production increases for this very mature property.

The approach utilized and results obtained are described. Successful optimization of this field required integration of geological description, production performance, material balance, well test analysis, numerical simulation, and infill drilling results.

Location

The Blueberry field is located approximately 60 miles northwest of Fort St. John in Northeastern British Columbia. Figure 1(a) shows the general location. There are two pools, which are called the North and South Pools, respectively, as shown in Figure 1(b). The discovery well was drilled in 1956 at d-82-L/94-A-12 (North Pool). Commercial production was obtained from a depth of approximately 2,075 m (6,800 ft.). Production commenced in 1957, following further development drilling. The oil is mediumlight with a 36 ° API gravity.

Based on interpretations of DSTs and production tests, early in the life of the reservoir, a gas cap had been interpreted in both the North and South Pools.

Basic Geology

Production is obtained from the Mississippian Debolt formation. Oil is trapped against the updip extreme of a thrust fault related to the Laramide orogeny. The general structure is shown in Figure 2. There is thought to be only a minor amount of roll-over. The leading edge of the thrust fault trends northwest-southeast. Towards the south part of the pool there is a break in the trend of the main thrust. It was thought that there may be a lateral tear, with displacement taken up in two smaller thrusts. It was not clear if this lateral tear was sealing or not.

Lateral faults and/or low permeability are thought to separate the two main South and North Pools. The exact location of the division has not been determined.

Development

South Pool

At the beginning of the study, a total of 15 producers were drilled in the South Pool, including one horizontal well. Overall GOR levels have increased, from 113 m3/m3 (635 scf/bbl) initially to 278 m3/m3 (1,575 scf/bbl). Such a modest increase indicates some pressure maintenance. Watercuts have increased steadily to 50% at the start of the study. More detailed analysis, on a well by well basis, indicated that the updip wells in the southernmost part had the highest watercuts.