Abstract

Gas cycling enhanced oil recovery (GCEOR) is under intense investigation. A novel experimental procedure was developed to quantify the parameters that affect GCEOR performance. Porous media, exhibiting in-situ liquid permeability from 200 nD to 2 µD, were evaluated for GCEOR upside. The influence of cycling pressure, injection gas composition, soak time and level of primary depletion before initiation of GCEOR were measured.

The innovative experimental design for core-flow testing permitted the quantification of GCEOR using large lab-scale hydrocarbon pore volumes (HCPV). The unique experimental design allowed nano-darcy media to be tested using a time line comparable to conventional millidarcy media. Two dominant flow regimes were incorporated: matrix mass transfer into the fracture and flow within the fracture. Three mechanisms for EOR are described: extraction, swelling, and reduction of interfacial tension. Full reservoir conditions were reproduced and primary depletion followed by huff and puff GCEOR were evaluated, while changing the design parameters listed above. This work was performed on diverse oil and rock properties.

More than 30 primary depletion tests followed by GCEOR have been conducted. The effects of cycling pressure, injection gas composition, soak time, level of primary depletion before GCEOR, and other parameters were investigated. Due to large HCPV, good mass balance was maintained and sufficient fluids were produced, as a function of cycle number (huff and puff), in order to be able to measure effluent gas and liquid compositions and densities from each cycle along with the recovery of original oil in place (OOIP). All testing was done in order to quantify the relative benefit of huff and puff GCEOR compared to primary depletion recovery. Results indicate that recovery of OOIP can be more than doubled by implementing GCEOR: cycling pressure should be optimized (highest pressure does not necessarily perform the best); soak time/ huff time may compensate for non-optimal pressure operation; injection-gas composition can impact performance; gas utilization values are low compared to conventional continuous gas injection projects; less depletion before GCEOR initiation can accelerate recovery and can access residual oil that was not produced at higher levels of primary depletion.

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