Research On Effect Of Silica On Porosity And Permeability During Steam Injection

Steam injection has been found to be an efficient way to recover heavy oil. The viscosity of oil decreases as the temperature increases, which makes it possible to recover heavy oil or enhance the recovery rate by steam injection. But the injected steam may react with the silica in rock, the silica in rock may dissolve into the aqueous phase near the injection well at high temperature and be carried forward by the moving condensate, as the condensate travels away from the injector, it cools and the dissolved silica precipitates. Thus, steam injection may cause silica redistribution in reservoir, which causes the porosity and permeability of the reservoir to change. This paper will focus on the theoretic understanding about the silica redistribution effects on the reservoir porosity and permeability.In this study, we examined the kinetics of silica dissolution or precipitation. The reactive transport equation for silica was deduced and solved analytically using Laplace transforms. The results show that the silica concentration in the aqueous phase is the equilibrium silica solubility corresponding to the temperature of that location, which is called "local chemical equilibrium". The flow of condensate in reservoir is a non-isothermal process, and the temperature profile was gained through the conservation equation for energy, and the silica concentrations in aqueous phase were given in different temperature regions. On the basis of silica conservation equation in rock and silica concentration equations in aqueous phase, the reservoir porosity change during steam injection was examined through the relation of porosity and silica mass fraction. Network model was used to study the absolute permeability versus porosity relationships for silica redistribution in rock, and a power-law form was gained for porosity and permeability in a range of porosity change through the network model.The results drawn from the theory were validated by the experimental data of others. With the theory, the effect of steam injection on reservoir porosity and permeability can be evaluated, which is helpful to the heavy oil recovery by steam injection.