The modeling of geochemical reactions is critical for managing production from reservoirs as they occur in all stages of production from an oil and gas field. Primary production from shale gas fields is modeled as adsorption reactions. Geochemical reactions also impact rock wettability that is making low salinity water flooding, an increasingly popular secondary production process. Tertiary recovery process of gas injection relies on accurate phase behavior analysis of hydrocarbon-injected gas mixture that is also impacted by geochemical reactions. A series of projects have been initiated to model the geochemical reactions for these different applications.
CO2 injection in oil reservoirs has the dual benefit of enhancing oil recovery as well as sequestering a greenhouse gas. CO2 injected in carbonate reservoirs, such as those found in the Middle East, can react with ions present in the brine and the solid calcite in the carbonate rocks. These geochemical reactions impact the phase behavior of in-situ hydrocarbon fluids and prediction of miscibility pressures thereby impacting oil recovery predictions from compositional simulations. Hence, it is important to model the impact of geochemical reactions on real oil during CO2 injection.
A practical method to use the Gibbs free energy function for integrating phase equilibrium computations and geochemical reactions has been developed. This method has the advantage of combining different thermodynamic models – the hydrocarbon phase components normally characterized using an Equation of State (EOS), while the aqueous phase components usually described using an activity coefficient model.
The first project seeks to quantify how geochemical reactions impact oil recovery predictions during CO2 injection in carbonate reservoirs. A real oil sample (Shell field courtesy * Birol Dindoruk) shall be used to show how a combination of Pitzer activity coefficient model and Peng Robinson (PR) Equation of State can result in changes in oil recovery predictions. In a separate project, the impact of geochemical reactions on the minimum miscibility pressure predictions shall be quantified using this approach. The change in minimum miscibility pressure in the presence of geochemical reactions depends on two factors: 1) the volume ratio (and hence molar ratio) of the aqueous phase to the hydrocarbon phase and 2) the salinity of the brine. The modified phase behavior, arising out of geochemical reactions, will be implemented in our in-house reservoir simulator IPARS (Integrated Parallel Accurate Reservoir Simulator).
In a third project, the convergence properties of different activity coefficient models shall be analyzed to identify the model most suited for compositional simulation. This shall help implement the most appropriate model that can integrate phase behavior computations as well as geochemical reactions.
In addition to above research projects pertaining to gas flooding process, we are initiating projects to help explain the low salinity water flooding project. The objective is to identify and isolate the geochemical reactions that are responsible for changing rock wettability during the low salinity water flooding process. Core flood experimental data for different combination of brine ions and rock types shall be analyzed to isolate the ions that are likely responsible. In case of many ions, principal component analysis shall be used to statistically determine the main ions responsible for the process. Having isolated primary ions, the geochemical reactions responsible for the process shall be determined. An appropriate activity coefficient model shall be used to explain the observation of fluid outlet concentration. This will help make predictions on ion concentrations that should be injected to change rock wettability to enhance oil recovery.
In addition to above projects that focus on oil and gas production, geochemical reactions also have important applications in remediation of aquifers and safe disposal of nuclear wastes. The resulting changes in IPARS and TRCHEM shall be used for applications in these fields.
*This is a joint work with Birol Dindoruk at Shell Oil E&P Company.