Unconventional reservoirs such as the Permian, Bakken and Eagle Ford have produced significant oil; however, the recovery factors for these kinds of reservoirs are still low. Some type of enhanced oil recovery (EOR) could improve the situation, and one EOR method with recent success in unconventional reservoirs is huff-n-puff (cyclic) gas injection. It has been implemented in a number of field trials, and the production data clearly indicates that incremental oil is being produced, but the physical mechanisms for the additional recovery are not well understood.
In a recent previous paper, we examined the relative importance of different recovery mechanisms including oil swelling, viscosity reduction, vaporization, and pressure support as a function of the reservoir fluid’s initial gas-oil ratio (GOR). A numerical flow model is used to study these effects, where all of these mechanisms are present and contributing to the recovery. In one run, all of the recovery mechanisms for gas injection were turned off. Then each mechanism was individually turned on, and the model run again to determine the relative contribution of each mechanism.
To understand if different conditions alter the relative importance of the proposed mechanisms, this paper evaluates the sensitivity of different reservoir conditions (e.g. reservoir pressures, reservoir fluid compositions, permeability, etc.) and of different operating conditions (e.g. injection pressure, injection rate, gas composition, cycle time, producing bottom hole flowing pressure, fracture surface area, etc.).
Changes that increase the maturity of the injection project cause more free gas to be present, and for these cases, the vaporization mechanism becomes more important. These include longer injection time, higher injection rates and smaller fracture areas. Interestingly, as matrix permeability and fracture surface area increase, the relative importance of gas injection decreases because so much more oil is produced on primary for these cases.
Most importantly, the impacts of the mechanisms on recovery are now better understood for these processes. Currently cyclic gas injection has been applied successfully in unconventional basins, but there are significantly different fracture parameters, rock properties and fluid types. By better defining the recovery mechanisms for cyclic natural gas injection, it can be adapted to the varying conditions found within most unconventional plays. This will result in better EOR designs for new areas and in improved recovery for existing unconventional plays.
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