Abstract

Gravel packs are commonly used to minimize sand production in unconsolidated sandstone reservoirs. The migration of particulates and fines into the gravel packs often has a severely reducing effect on the permeability of the packs and the subsequent productivity of the well. Detailed laboratory studies were undertaken using 12/20 and 20/40 mesh silica gravel to ascertain the optimum gravel size to minimize formation damage to gravel packs in the Battrum field.

The Battrum field is a shallow unconsolidated sandstone reservoir located in Saskatchewan, Canada, producing a low API gravity crude oil over an area of approximately 3900 hectares with 180 active injection and production wells. Currently a large portion of the field is under enhanced recovery using a commercial scale in-situ combustion technique. The results of detailed laboratory studies, including detailed size and sieve analysis of the formation, classification of the different lithologies and detailed laboratory tests to investigate at reservoir conditions the flow of gravel and fines into different sizes of gravel packs are documented. Tests include detailed petrography and computerized petrographic image analysis on actual sections of gravel and formation sand interfaces and illustrate the mechanism of fines entrainment in the gravel packs and provide associated permeability reduction data. Details of field gravel pack treatments are also provided in the paper. Test results indicated an increased propensity for gravel pack plugging in the coarser (12/20) mesh gravel in comparison to the 20/40 mesh gravel. Simultaneous multiphase flow of both oil and water was also found to have an increasing effect on rapidity and severity of apparent plugging. The results presented provide insight into optimum gravel to sand sizing ratios for this field and have potential application to other similar reservoirs.

THE BATTRUM FIELD

The Battrum field was discovered by Mobil Oil Canada (MOCAN) in 1955 and is located in southeast Saskatchewan, Canada (Figure 1). Mobil currently operates units 1, 2 and 3 of the field. The majority of the reserves in the reservoir are produced from four district stratigraphic layers from sandstone fades of the Jurassic Roseray Formation.

The four layers in the Roseray formation at Battrum define an off lapping parasequence set and have been numbered according to depositional sequence as R1, R2, R3 and R4. These layers were deposited in response to a west to east progradation of a wave dominated clastic shoreline. The layers are defined by marine flooding surfaces which document periods of relative sea level rise. The architecture of the Roseray units is determined by this origin and two episodes of post depositional erosion. Sedimentary structures indicate that the majority of the Roseray reservoir facies were deposited in a mechanical fashion by wave and/or wind generated currents. Layers R1 and R2 are the major producing units in the western portion of the field while layers P3 and R4 contain the majority of the reserves in the eastern section.

The reservoir sands are, in general, very fine to fine grained, moderately to well sorted and poorly consolidated. Reservoir quality is very much controlled by grain size with layer R2 containing the largest amount of high quality sand. Over 42% of the 39.2 × 10+6 m3 (247 million barrels) of moveable oil in place is present in this layer.

In general the better quality laminated Roseray sands (which were the subject of this study) represent moderately sorted quartzose sublitharentites with good modified primary intergranular porosity supplemented by occasional grain moldic porosity. The sandstone framework generally consists of subangular monocrystalline quartz grains with much lesser amounts of chert grains, rock fragments and detrital feldspars. There are only trace amounts of authigenic quartz cement and generally no carbonate cements.

Detrital, recrystallized and authigenic clays characterize the rock. Kaolinite clay dominates (about 4% of the bulk rock fraction) with lesser amounts of illite (2%) and smectite (1%). Quartz is the major residual constituent with trace amounts of potassic feldspar and pyrite. The high kaolinite concentration and trace smectite indicate that the Battrum matrix is potentially sensitive to fines and sand mobilization and some potential for clay swelling and sloughing.

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