Swan Hills Unit #1: Adding Value with Seismic Through Reservoir Delineation and Characterization

History

The Swan Hills Unit #1 lies approximately 200 kilometres northwest of Edmonton, Alberta, Canada along the trend of several long-standing producing fields from the Swan Hills formation. Home Oil is the operator of Swan Hills Unit #1 with approximately 18% working interest. The field was discovered in 1957, two months after the discovery of Virginia Hills. The field was unitized in the Beaverhill Lake zone in 1963 and subsequently revised in 1983. It comprises 44,160 acres. The unitized lands have estimated 1,413 Mmbbls oil in place and produced cumulatively 540 million barrels to date with daily production rates currently at 27,000 bbls/day, 37mmcf/day gas and 130,000 bbls/day of water. Watercuts vary by geologic zone and geographic area. The average field watercut is 86 percent. Water flooding started as early as 1965, miscible flooding with an ethane rich solvent commenced in two phases in 1985 and 1988.

Geology

In 1990, Home Oil initiated a computer based geologic reservoir modelling study consisting of information from 485 wells to understand more fully the reservoir architecture of the field. The reef was subdivided into different layers and mapped accordingly. The resultant geologic perspective enhanced the knowledge of the reservoir and identified further opportunities. The geologic study identified potential for attic oil and possibly clean reef edge oil.

Seismic Program

In January of 1992, Home Oil acquired a 23.64 square kilometre 3D seismic program to delineate the reef edge so as to mitigate drilling risk for potentially trapped updip oil. Minimizing environmental impact by utilizing existing access and avoiding Forestry sample plots were concerns for the design and acquisition of the program. Data quality exceeded expectations. Vertical and lateral resolution limits for the reef edge were exceeded. Seismic interpretation delineated potentially undrained reserved which were identified as drilling targets. Subsequent vertical and horizontal drilling results substantiated the concept of clean reef edge oil. Reserve additions and increased daily production resulted.

Lithology Based Inversion

A lithology based seismic inversion was conducted on the 115,000 trace 3D data volume to enhance confidence in the position of the interpreted reef edge. On conventional seismic, the reef top could only be interpreted by inference due to tuning from other seismic events. The detailed geological modelling study was the source of corrections required for varying vintages of sonic and density log information, core porosity information and structural information. A log balancing technique was developed to compensate for logging errors due to tool calibrations, washouts and fluid saturations as defined by the geological work. A wavelet was extracted using a least squares simultaneous extraction from all of the calibrated logs. Although this ensured the accuracy of the wavelet, the reflectivity of the seismic data was also corrected using a simultaneous reflectivity colour correction. This ensured that the reflectivity of the seismic exhibited the correct periodicity prior to inversion.

The resulting inversion allowed the reef to be interpreted directly instead of by inference. Inversion pattern recognition was developed and employed to pick the base of the reef. An isovelocity map of the reef derived from the inversion data was then constructed using the enhanced data to interpret reef porosity. Porosity prediction estimates agree generally with well data at vertical resolution limits stretched to λ/6. The isovelocity map has been used to predict more porous regions of the reef and subsequently the best trajectory for subsequent horizontal drilling and vertical whips.

Results, Conclusions and Summary

Modem seismic technology associated with detecting increased oil reserves in aging productive reservoirs opens a host of opportunity for enhanced oil recovery schemes for many oil companies. The 3D seismic data has proven to be valuable through reservoir delineation applications by defining the reef edge, assisting with drilling program design, and mitigating drilling risk. The increased resolution realized by lithologic based seismic inversion has enhanced interpretor confidence. Enhanced deliverability rates are anticipated from the porosity prediction estimate derived from the inversion. This project aided appreciably to offset the production decline of the unit. Additional seismic data was acquired the following year at Swan Hills Unit # 1 to add yet further value. These drilling results are still pending.