Geochemical effects in two-phase flow



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Commercial reservoir simulators usually neglect the effect of the permeable media on phase equilibrium, and the thermodynamic interaction between the aqueous phase and the other phases present in the reservoir. These assumptions are not satisfied when gas is injected or produced, because water will be vaporized from the reservoir. This dissertation derives analytical and semianalytical solutions to predict water vaporization and mineral precipitation for gas injection and production in hydrocarbon reservoirs. It also develops and incorporates into the models phase equilibrium relationships that include the effect of capillary pressure and salinity in the predictions. A new equilibrium constant for mineral precipitation has been derived that takes into account capillary pressure and interfacial forces. For gas injection, a traveling wave solution has been adopted with a correction proposed to honor the material balance. The traveling wave solutions were tested against results from the compositional simulator Generalized Equation of State Model (GEM) and against experimental data. The results from the traveling wave solution matched the experimental data closely while GEM failed to make good predictions. A semi-analytical model has been developed for gas production. This semi-analytical model was also tested against results from GEM. The results closely matched the GEM solutions in the absence of capillary pressure, interfacial forces and salinity effects. The results of this research show that the reservoir could be completely depleted from water near the wellbore when gas is produced or under-saturated gas is injected. Therefore, the existence of a residual water saturation value has to be re-evaluated in reservoir modeling. It is also shown that the wetting phase is mobile even at extremely low values of wetting phase saturation: the paradigm of an immobile residual phase is mostly imposed by the limitations of measuring devices in the laboratory and by the length of time of the experiments used to measure relative permeability. A new method to obtain relative permeability at very low wetting phase saturation has been derived, based on fitting the relative permeability exponent to vaporization data in the falling rate period. This research has also quantified the reductions in porosity and absolute permeability caused by mineral precipitation.