Seismic Monitoring of Microbial Plugging for Microbial Enhanced Oil Recovery

Microbial plugging by insoluble biopolymer production in porous media is considered as a promising technique to increase sweep efficiency during water flooding for microbial enhanced oil recovery (MEOR). For successful application of this technique, detecting the regions of active microbial growth and monitoring biopolymer accumulation will be critical. Thus, developing techniques for monitoring in-situ bacterial growth, biopolymer formation, and hydraulic conductivity reduction is essential for successful application of this MEOR approach. We examined the feasibility of using seismic signatures (P- and S-wave velocity and attenuation) for monitoring the accumulation of insoluble biopolymers in unconsolidated sediments. A model bacterium, Leuconostoc mesenteroides was selected, which produces a certain type of insoluble biopolymer, called dextran when metabolizing sucrose. We performed a column experiment, where the model bacteria were grown and stimulated to produce insoluble dextran in a fully saturated fine sand-pack. Over the course of the experiment, ultrasonic transducers and bender elements were used to monitor the changes in P- and S-wave responses at an ultrasonic frequency range. After 20 days of the experiment, the hydraulic conductivity significantly decreased by approximately one order of magnitude. Meanwhile, P-wave velocity remained consistent and S-wave velocity increased by more than 200%. Amplitudes of the P-wave signatures decreased by 80% during accumulation of the soft biopolymer, indicating an increase in P-wave attenuation; and the S-wave amplitudes increased. The results suggest possibility that reduction in hydraulic conductivity by insoluble soft biopolymer formation can be seismically monitored at this laboratory scale.