Sandstone field with waterflood, gas and miscible solvent injection
Backgound and geological information
The field is a structural stratigraphic trap that has been divided into several vertical zones. Complex, systematic depositional and diagenetic changes resulted in a dual pore system that was further impacted by structural and hydrocarbon histories, resulting in a highly variable vertical and areal distribution of net pay, porosity, and water saturation. Porosity values range from 10 to 30% with an average of 22%. Average well-zone permeabilities ranged from 100 to 1800 md with a field average of 500 md.
The primary depletion mechanisms were gas-cap drive along with a very weak waterdrive (rock properties deteriorate rapidly off structure) and potential solution-gas drive. Waterflooding was planned as part of the initial field development to prevent significant pressure depletion.
Waterflooding was started in conjunction with an infill-drilling program that reduced spacing to 80 acres per well. Inverted seven- and nine-spot injection patterns were applied to areas of the oil column not overlain by the gas cap. A small enriched-hydrocarbon miscible injection project was later expanded to additional areas of the field.
Horizontal wells have been used extensively in areas close to the gas/oil contact (GOC) to capture reserves from relatively thin oil columns that were previously uneconomic because of severe coning problems. A gas-cap cycling project has been expanded three times. A few 40-acre-spaced wells were drilled in areas of poor drainage, especially where locally large shale sections prevent good continuity. However, a more economical approach has been to drill 1,500- to 2,000-ft horizontal sidetracks, which can realize up to 85% of the 40-acre well recovery but at less than one-third the cost.
Estimated ultimate recovery is 60% original oil in place (OOIP) with a miscible contribution of 10% OOIP in affected areas. The contribution of reservoir management to these high recovery levels is difficult to quantify. Initial estimates of recovery were in the range of 40 to 45% of OOIP. It is clear that a continuing effort to discover improved depletion techniques has added significantly to ultimate recovery.
Field surveillance and management
The unit collects surveillance data to manage the reservoir effectively. This includes:
- Day-to-day production tests
- Production/injection profiles for wells
- Bottomhole-pressure surveys across the field
Additionally, repeat cased-hole neutron logs are taken to monitor gas movements across the reservoir. On new wells, a comprehensive suite of openhole logs is obtained to provide geologic information and to supplement production data and surveillance. The unit actively collected special reservoir surveillance data from:
- Pressure-pulse and pressure-transient well tests
- Repeat formation tests
- Specially obtained cores
Both single-well chemical-tracer tests and log-inject-log tests have been conducted, and specialized core data have been obtained in the waterflood areas to measure and improve the effectiveness of the waterflood/miscible enhanced oil recovery (EOR) project. Additionally, gas samples are collected routinely from flood area production wells to measure returned miscible injectant to further optimize and improve the miscible EOR project and from gravity-drainage area wells to monitor movement of injected cycle gas. Many different simulation models have been developed and updated over time.
There are frequent discussions, at both the technical and managerial levels, of:
- Surveillance techniques
- Operating strategies
- Reservoir and facility management programs
- Future capital projects
Committees, forums, and teams continue to evolve to facilitate and improve these exchanges. In addition, corporate resources bring technical expertise and a worldwide perspective to the development decisions.
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