A Performance Evaluation of Canada's Snipe Lake/Beaverhill Lake A Pool
- G.L.V. Springate (Esso Resources Canada Ltd.) | I.D. Muir (Esso Resources Canada Ltd.) | T.R. Caldwell (Esso Resources Canada Ltd.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1992
- Document Type
- Journal Paper
- 390 - 396
- 1992. Society of Petroleum Engineers
- 6.5.2 Water use, produced water discharge and disposal, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.6 Drilling Operations, 5.1 Reservoir Characterisation, 2.4.3 Sand/Solids Control, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 4.1.5 Processing Equipment, 5.3.4 Integration of geomechanics in models, 4.6 Natural Gas, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 4.1.9 Tanks and storage systems, 5.8.7 Carbonate Reservoir, 5.4.1 Waterflooding, 5.1.5 Geologic Modeling, 5.5 Reservoir Simulation
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This paper describes the evaluation of the production performance of the Snipe Lake/Beaverhill Lake A (BHL-A) pool. The purpose of this investigation was to determine the reasons behind the pool's low projected recovery (32%) relative to similar pools (40%) and to recommend steps to remove the causes. This work was initiated because the pool's production is declining under the existing depletion scheme (essentially a downdip waterflood). The evaluation involved simultaneous geological and engineering investigations that improved the quality of data interpretation and the timeliness of the effort and reduced the overlap of work that otherwise might have occurred. The detailed reservoir model developed indicated that the production performance is controlled by the pool's geology. Fluid movement is restricted vertically between reservoir cycles and horizontally within the major reservoir cycle. An analysis of previous infill drilling and producer-to-water-injector conversions demonstrated that incremental recovery of oil was achieved; 4.5 % (422 × 103 m3) front infill wells on 32-ha well spacing and 6.0% (740 × 103 m3) from increased water injection. Converting the entire pool to a 65-ha pattern waterflood will increase the recovery by 1.9% (900 × 103 m3). A 32-ha pattern waterflood has the potential of 5.6% (1740 × 103 m3) additional recovery.
The Snipe Lake BHL-A pool is located 260 km northwest of Edmonton, Alta. The pool is one of a number of large Swan Hills atoll-reef complexes of Middle-Upper Devonian Age in west-central Alberta. These reef complexes are situated on the areally extensive Swan Hills carbonate platform (Fig. 1). They intertongue with and are overlain by deeper marine basinal limestones of the Waterways formation. Topographically high areas of the platform formed the sites for subsequent reef growth. The Snipe Lake reef complex occurs on the extreme northwestern portion of this platform. The Snipe Lake complex is up to 110 m thick and covers an area of about 750 × 103 m2. The present burial depth is 2620 m, with a structural dip to the southwest of 9 m/km. Only the eastern margin of the complex contains oil. The major portion is below the oil/water contact, and as a result, very little information is available for areas other than the eastern margin. This oil-prone portion of the reef covers 70 × 103 m2.
The pool Was discovered by the drilling of Well 10-21-71-18W5 in 1962. In a short time, almost the entire pool was drilled at 65-ha well spacing. Since discovery, 145 wells have been drilled to delineate the pool. Currently, 46 are oil producers, 10 are produced-water injectors, 6 are freshwater injectors, 50 are suspended, and 35 are abandoned. The original depletion mechanisms of the pool were oil expansion and aquifer influx. The aquifer influx was of limited energy, and as a result, a water-injection scheme began in Oct. 1964. Unitization of the pool was completed in March 1965. In April 1986, Esso Resources Canada Ltd. was named the unit operator. Current oil production is 550 m3/d with a WOR of 2.0 and a GOR of 75.
Reservoir quality in the Snipe Lake platform-reef complex can be related directly to depositional attributes. More than 95% of the porosity development is of primary interparticle and intraparticle porosity types. Reservoir facies consist of sediments deposited under high-energy, current-winnowing conditions. Hence, reservoir quality is related to depositional facies. Major changes in the distribution and type of facies occurred during the evolution of the platform-reef complex. To predict where reservoir facies shift in a temporal sense, a sequence stratigraphic framework was constructed by correlating major (8- to 12-m-thick) shallowing-upward cycles.
Fig. 2 schematically displays depositional facies distribution along a major cycle boundary. The paleobathymetric profile (foreslope dip averaging 2 to 3°) is semiquantitative. Water depths are probably minimum values owing to sediment compaction. Furthermore, the width of these facies belts and the amount arid distribution of carbonate foreslope sand facies varies according to reef margin configuration with respect to the predominant northeast paleowind direction and foreslope dip angle.
Margin facies are composed of varying amounts of abraded skeletal rubble and in-situ, thick encrusting tabular stromatoporoids. The margin occupies a narrow discontinuous belt that was probably never wider than tens to hundreds of meters at anyone time for each cycle (Fig. 3). Reef flat, margin/upper foreslope, and segments of the middle foreslope are important reservoir facies at Snipe Lake. The other major reservoir facies, allochthonous carbonate foreslope debris, can occur anywhere on the foreslope depositional profile (Fig. 2). Typically, carbonate foreslope sand is best developed in the strongly progradational B0 cycle and within platform-reef reentrants (Fig. 4). Reservoir facies show 10% to 15% average porosity, with permeability ranging from 30 to 100 md. Foreslope facies become more micritic in a downslope direction because currents become less effectIve winnowing agents with depth. Lower foreslope facies are composed of tight wafer stromatoporoid-crinoidatrypid brachiopod lime floatstones and boundstones. Nonreservoir facies are also encountered within the lagoon because of shelter from prevailing wave energy provided by the margin. Tight, muddy, restricted subtidal lagoon and tidal flat facies are more prevalent toward the platform-reef interior. Grainy, subtidal facies show better porosity development (average 3% to 5%) and occur in better circulated portions of the lagoon, such as the area fringing the reef flat debris apron (Fig. 3). Figs. 3 and 5 illustrate good agreement between depositional facies and reservoir facies and the importance of recognizing facies distribution to determine reservoir continuity within a given major cycle.
Reservoir vertical continuity can be understood best through detailed mapping of each major cycle in the entire pool. Fig. 6 displays the cycle-stacking pattern observed for the platform-reef complex. The platform (53 m thick) consists of five major cycles (A1 through A5) arranged in a backstepping manner (i.e., each successive platform cycle is areally smaller and thinner than the preceding cycle). The Snipe Lake reef began on the Cycle A5 accretionary topographic high and comprised four major cycles (B0 through B2 and C) within the unit boundary.
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