Experimental and numerical investigation of pore-scale mechanisms of microbial enhanced oil recovery (MEOR) using a microfluidics approach

Abstract

The utilization of microorganisms as an enhanced oil recovery (EOR) method has attracted much attention in recent years because it is a low-cost, easy to apply and environmentally friendly technology. However, the pore-scale mechanisms involved in microbial enhanced oil recovery (MEOR) that contribute to an additional oil recovery are not fully understood so far. This work aims to investigate the MEOR mechanisms using microfluidic technology, among others, bioplugging and changes in fluid viscosity as well as wettability alteration. Further, the contribution of these mechanisms to additional oil recovery was quantified. A novel experimental setup that enables investigation of MEOR in micromodels under elevated pressure, reservoir temperature and anaerobic and sterile conditions was developed. Micromodels designed based on real rocks structures were constructed with two different permeability zones for the investigation of bioplugging effects and conformance improvement during the MEOR process. An image processing algorithm was developed to estimate the bacteria growth and transport as well as fluid phase saturation in micromodels. To investigate the role of microorganisms in MEOR processes, the single and two-phase experiments were performed with fluids from a German high-salinity oil field selected for a potential MEOR application. Several parameters that govern the MEOR performance were investigated, including (1) bacteria growth, which is controlled by the nutrient concentration and incubation conditions as well as the flooding operation management, (2) bacteria community, (3) properties of porous media such as pore size and wettability. Furthermore, a numerical approach was applied to understand the significance of contributing mechanisms to oil displacement efficiency. As a result, in-situ bacteria growth was observed in the micromodel for both single and two-phase flooding experiments. During the injection, microbial cells were partly transported through the micromodel but also remained attached to the micromodel surface. These attached bacteria and biofilm formation cause the permeability reduction in micromodels and possibly the wettability alteration. Two-phase flow experiments in a customized heterogeneous micromodel showed a significant effect of bioplugging and improved the macroscopic conformance of the oil displacement process. The increase in differential pressure after bacteria incubation and microscopic visualization confirmed bioplugging in micromodels. The flow diversion of the tracer particles and the differences in the velocity field also confirmed that bioplugging might lead to improved conformance control. Additionally, during the metabolism, microorganisms were observed produced gas that could dissolve in the liquid phase, thus reducing the viscosity. This oil viscosity reduction was identified to contribute to incremental oil in micromodel experiments. The simulation results showed that the bacteria growth and transport in micromodels could be reproduced through the chemical reactions and the kinetic model. Furthermore, several oil displacement mechanisms during MEOR were evaluated in the simulation model, including bioplugging effect due to the attachment of bacterial cells, oil viscosity reduction due to the dissolution of CO2, and relative permeability change due to bacteria adsorption and biofilms onto the surface of the rock. Based on the simulation result, it can be concluded that these three mechanisms contribute to oil displacement efficiency during the MEOR process in micromodels.