Modification of fluid-rock interfacial properties and wettability by surfactant-producing bacteria

Abstract

In a porous medium consists of two or more immiscible fluids, surface energy of fluids/rock interfaces affect the saturation, distributions and displacement of fluids. Modification of the interfacial properties of fluids in minerals by using surfactants has widely been proposed aiming on increasing the mobility of fluids through porous media. Surfactants are proven to effectively modify the interfacial tension and wettability in multi-phase fluids and rock systems, improving the displacement and sweep efficiency of fluids in porous media. In the meantime, biosurfactants have been drawing much attention as an alternative to the chemical surfactants for their biodegradability, ecological suitability and low toxicity. However, the question as the extent of microbial alterations in fluid wettability and interfacial tension under reservoir pressure and temperature conditions still warrants further investigation. In this Ph. D. dissertation, in aspect of biosurfactant as an enhancer for geologic carbon storage (GCS) and microbial enhanced oil recovery (MEOR), the potential and feasibility of biosurfactant producing bacteria for the modification of fluids wettability in brine and rock mineral systems have been experimentally investigated in reservoir conditions.

In this dissertation, Bacillus subtilis was selected as a model bacterium, and cultivated at reservoir conditions such as high pressure, high temperature, high salinity and extreme pH environments. And lipopeptide biosurfactant, surfactin, produced by B. subtilis was evaluated for modifying interfacial properties of immiscible fluids (i.e., $CO_2$-brine and oil-brine) and rock wettability. Surfactin produced by B. subtilis lowered $CO_2$-brine interfacial tension (IFT) and modified quartz and calcite minerals more $CO_2$-wet which further increased $CO_2$ sweeping efficiency in porous media. The functionality of biosurfactants varied to the changes of $CO_2$ phase and the rock minerals types. The wettability of $CO_2$ in calcite was comparably unpredictable because it involved calcite dissolution, re-precipitation reactions which roughened the calcite surface, where the wetting of fluids in rough surface is hard to define due to the contact angle hysteresis occurrence.

The functionality of surfactin in oil-brine-quartz systems was further investigated at high pressure and various temperature conditions. Despite the observation that capillary factor of oil-brine-quartz reduced by surfactin, the biosurfactant production yield and rate were significantly affected at elevated temperature. Additionally, the pore network model (PNM) and microfluidics experiments were conducted to evaluate the effect of biosurfactant on displacement efficiency in porous media. Further, the synergistic effect of biosurfactant with low amount of chemical surfactants were experimentally investigated. The effect of surface charge of calcite minerals on effectiveness of biosurfactant/chemical surfactant mixtures were identified.