Geochemical reactions and their influence on petrophysical properties of ultra-low permeability oil reservoirs during water and CO2 flooding

Abstract

Complex geochemical reactions can occur during the water and CO2 flooding for improved oil recovery, influencing the reservoir porosity and permeability, especially for low-permeability reservoirs. Core flooding experiments can be adopted to assess this effect, but it usually takes months or even years to accomplish. In this study, scaling and geochemical reaction experiments were firstly conducted to investigate the precipitation and dissolution of the main minerals induced by water and CO2 injection, based on a typical low-permeability oil reservoir in Xinjiang Oilfield China. Secondly, reactive flow simulation models were then established, based on the experimental data, to evaluate the influence of the geochemical reactions on the porosity and permeability in the reservoir. The experimental results show that the injection water can result in severe mineral precipitation after it mixes with the original formation water, while the injected CO2 can acidize the formation water and result in the dissolution of some minerals (e.g. calcite and silicate) and precipitation of dolomite, which can change the composition of the formation water. The simulation results show that severe carbonate precipitation can occur near the injection well, resulting in a decrease in porosity and permeability after 10 years of water injection. However, the change of rock minerals induced by CO2 injection can cause a slight increase in porosity and permeability near the injector, which benefits the injection of ultra-low permeability reservoirs. Therefore, CO2 flooding is recommended for improved oil recovery.