Microbial enhanced oil recovery (MEOR) utilizes microbes for enhancing the recovery by two main mechanisms: 1) reduction of oil-water interfacial tension (IFT) by produced biosurfactant, and 2) selective plugging by microbes and metabolic products. A mathematical model for MEOR has been built. Our model organism is a surfactant-producing spore-forming bacterium that shows a great potential for MEOR. Application of spore-forming bacteria is an advantageous novelty of the present approach.
A spore is a dormant, very resistant version of the cell. Sporulation (turning bacteria into spores) occurs when bacteria are exposed to stresses, such as starvation. The remains of a sporulated cell serve as a substrate for the rest bacteria. An inverse process, reactivation, occurs when spores sense favorable conditions.
The model accounts for growth, substrate consumption, surfactant production, attachment/filtering out, sporulation and reactivation. The mathematical setup is a set of 1D transport equations involving reactions and attachment. Characteristic sigmoidal curves are used to describe sporulation and reactivation in response to substrate concentrations. Surfactant decreases IFT, modifying the relative permeabilities and thus decreasing the fraction of water in the flow. Attachment of bacteria reduces the pore space available for flow, i.e. the effective porosity and permeability. Clogging of specific areas may occur.
In order to obtain sufficient local concentrations of surfactant, substantial amounts of substrate should be supplied; however, massive growth of bacteria increases the risk for clogging at the well inlet areas, causing injectivity loss. In such areas starvation may cause sporulation, reducing the risk of clogging. Substrate released during sporulation can be utilized by attached vegetative bacteria and they will continue growing and producing surfactant, which prolongs the effect of the injected substrate. The simulation scenarios show that application of the spore-forming bacteria gives a higher total production of surfactant and the reduced risk of clogging, leading to an increased period of production and a higher oil recovery.