Chemical interactions between the injected fluids and the minerals during the fracturing process can affect fluid flow and production. However, there is still a need to understand the impact of geochemistry on the interactions at the rock-fluid interface and how these interactions affect the wettability of the rock and fluid flow in organic-rich mudrocks. In this paper, we quantify the mineral-fluid affinity by performing adsorption calculations. Molecular dynamics simulations (MDS) are carried out to (i) quantify the adsorption of fracturing fluids on the surface of minerals, (ii) perform sensitivity analysis on the composition of fracturing fluid and reservoir temperature on adsorption and mobility, and (iii) analyze the spatial distribution of water and chemicals on mineral surfaces.
The minerals evaluated include illite and calcite, and the fracturing-fluid components are methanol, citric acid, sodium chloride, and water. We evaluate the effect of each chemical separately. First, systems composed of mineral surfaces in contact with brine are generated. Then, we analyze the mineral in contact with a solution containing brine and methanol and with a solution composed of brine and citric acid. MDS are carried out in the canonical (NVT) ensemble at the temperature of 330 K to evaluate the adsorption of the fracturing fluid. To quantify the impacts of reservoir temperature, we carry out MDS at a temperature of 360 K.
Results suggest that methanol does not have a strong effect on water adsorption and the ion spatial distribution on the mineral’s surface. We found that citric acid tends to form aggregates and that some cations present in the solution might participate in these aggregates. When methanol or citric acid is added to the brine solution, the mobility of both sodium and water on the illite surface decreases. The effects of each additive on the affinity between the mineral and fracturing fluids were also investigated. We found that the number of hydrogen bonds between illite and the fluid did not change when additives were added. However, the number of hydrogen bonds between calcite and the fluid was affected when methanol was added to the system.
The quantification of adsorption in the molecular scale provides a fundamental understanding of the electrochemical interactions between the rock surface and the fracturing/reservoir fluids at reservoir conditions, which enables the enhanced design of fracturing-fluid composition for different reservoir types. This information can also be used to quantify the impacts of injected and reservoir fluids on the wettability of the rocks.