Unconventional reservoirs such as the Eagle Ford have had tremendous success over the last decade, and while wells come on at high rates, they drop quickly and the recovery factors are low, which suggests the need for enhanced oil recovery. One method that has become popular is cyclic (huff-n-puff) gas injection. In this method, gas is injected down a well for some time, and then the injection well is turned back into a producer until the production drops, and the process is repeated. A few companies have successfully tested this in the field, 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 this paper, the relative significance of four proposed recovery mechanisms is examined: (1) oil swelling, (2) viscosity reduction, (3) vaporization, and (4) pressure support. A numerical flow simulation model is used to study these effects. A model of an unconventional reservoir is constructed where all these mechanisms are present and contributing to the recovery. To validate the model, it is history matched to a pilot gas injection project in the Eagle Ford. A primary case and a full gas injection case are completed. A run is also completed where all of the recovery mechanisms for gas injection are turned off, which provided a result that is similar to primary production. Then each mechanism is turned on and off (one at a time), and the model is re-ran to determine the relative contribution of each mechanism. This process is carried out for different reservoir fluids from low gas-oil-ratio (GOR) black oils to liquid rich gas condensate.
By evaluating the recovery mechanisms for fluids at various GORs, charts are created that show how important each mechanism is as a function of different reservoir fluids. All mechanisms provide some contribution, but their significance varies as a function of GOR. Pressure support provides similar small response for all fluid types. Vaporization is most important for gas condensate reservoirs (high GORs), but it plays a role for all fluid types. Oil swelling has a large impact for low GOR oils, but diminishes for higher GOR fluids, and viscosity reduction plays a minor role only for low GOR cases.
Most importantly, the impacts of the mechanisms on recovery are better understood for these processes. Currently huff-n-puff gas injection has been applied successfully in parts of the Eagle Ford, but this play has a wide range of in-situ fluid types; and likewise, other unconventional basins have a large variety of reservoir fluids. By better defining the recovery mechanisms for cyclic natural gas injection, EOR can be improved in existing unconventional plays and better designed for new areas.