Development of a Mechanistic Model for Capillary Pressures and Relative Permeabilities to Estimate Oil and Gas Recoveries for Unconventional Reservoirs Under Depletion and Gas Injection
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A mechanistic model was created to calculate recovery factors and capillary pressure curves for unconventional reservoirs. A key aspect of the model is that it accounts for the effect of fluid confinement that occurs inside small diameter pores in unconventional reservoirs. Confinement effects are incorporated through the calculation of capillarity via the Young-Laplace equation. The model is intended to provide an estimate of recovery factors that is faster and simpler to use than a full reservoir simulation. The model simulates initial depletion of the reservoir and then simulates one or more gas injection steps as an enhanced oil recovery technique. The results from the model are unique to the specific reservoir fluid composition, pore size distribution, degree of depletion, injection gas composition, injection gas amount and number of injection steps specified for the run. The results from the model are compared against small-scale reservoir simulation runs under equivalent conditions. The simulator used for the comparison is a Matlab based, compositional research simulator that is capable of modeling pore confinement effects. For an idealized reservoir model, the mechanistic model and the reservoir simulation results match for both the initial depletion and gas injection portions of the run. For larger reservoir models containing a greater number of cells, the mechanistic model matches the simulation results closely for the initial depletion. However, the two methods do not match for the gas injection portion of the run. This mismatch occurs because the mechanistic model is not capable of modeling the complex mixing of the injection gas with the reservoir fluids as the gas moves through the reservoir volume. To isolate the impact of confinement on production, each reservoir simulation is run once with confinement effects considered and again using bulk fluid behavior (no confinement effects). All other run parameters are held constant between the runs. For a black oil reservoir fluid, the confinement effects increase the ratio of oil to gas production. For a volatile oil, confinement has a minimal impact on the ratio of oil to gas production.
Czernia, Bartosz Tadeusz (2018). Development of a Mechanistic Model for Capillary Pressures and Relative Permeabilities to Estimate Oil and Gas Recoveries for Unconventional Reservoirs Under Depletion and Gas Injection. Master's thesis, Texas A & M University. Available electronically from