Studying the Potential of Calcite Dissolution on Oil Liberation from Rock Surfaces during Single-Well-Chemical-Tracer Tests by Coupling a Multiphase Flow Simulator to the Geochemical Package

Abstract

Calcite mineral dissolution has been considered to be an important mechanism for dampening the considerable pH-variation during Single-Well-Chemical-Tracer (SWCT) tests by improving the buffer capacity of the aqueous solution. Other parameters that also could have a great effect on the geochemistry of the reservoir during the SWCT tests are water buffer capacity, soluble hydrocarbon components, and temperature. Additionally, calcite mineral dissolution has been also presented over the last decade as an underlying mechanism for liberation of the adsorbed oil from the surface by modified salinity water injection (MSWI) in carbonate reservoirs. This contradiction of effects of calcite dissolution might pose a challenge for the accuracy of the SWCT tests in carbonates. This concern motivated us to highlight the potential of the calcite dissolution on the oil liberation from the carbonate rock surfaces during the SWCT tests by coupling a multiphase flow simulator to the geochemistry package PHREEQC. The results show that although the calcite dissolution is marginal during injection time, it might be substantial during shut-in time which is much longer. During shut-in time, the results show that the potential of calcite dissolution on the oil liberation from the rock surfaces could be more significant at higher reservoir temperatures although initial solid calcite concentration and buffer capacity also could have an effect. It is also clear that the pH of the system reaches the lowest level when the shut-in time reaches the transient time (i.e., injection and production times) and it is not changed significantly afterwards. At longer shut-in times, the additional ester hydrolysis and acid product is neutralized by the calcite dissolution and the buffer capacity of water. Therefore, the probability of the liberation of the adsorbed oil from the rock surface is higher at larger shut-in times so that test designs with shorter shut-in times and even as short as the transient time for the carbonate reservoirs is highly recommended. We hope that this study can be used to minimize the uncertainties of the SWCT tests and improve the reliability of the Sor measurements.