Abstract
Plugging high-permeability zones within oil reservoirs is a straightforward approach to enhance oil recovery by diverting waterflooding fluids through the lower-permeability oil-saturated zones and thereby increase hydrocarbon displacement by improvements in sweep efficiency. Sporosarcina pasteurii (ATCC 11859) is a nitrogen-circulating bacterium capable of precipitating calcium carbonate given a calcium ion source and urea. This microbially induced carbonate precipitation (MICP) is able to infill the pore spaces of the porous medium and thus can act as a potential microbial plugging agent for enhancing sweep efficiency. The following explores the microscopic characteristics of MICP-plugging and its effectiveness in permeability reduction. We fabricate artificial rock cores composed of Ottawa sand with three separate grain-size fractions which represent large (40/60 mesh sand), intermediate (60/80 mesh sand), and small (80/120 mesh sand) pore sizes. The results indicate a significant reduction in permeability after only short periods of MICP treatment. Specifically, after eight cycles of microbial treatment (about four days), the permeability for the artificial cores representing large, intermediate, and small pore size maximally drop to 47%, 32%, and 16% of individual initial permeabilities. X-ray diffraction (XRD) indicates that most of the generated calcium carbonate crystals occur as vaterite with only a small amount of calcite. Imaging by SEM indicates that the pore wall is coated by a calcium carbonate film with crystals of vaterite and calcite scattered on the pore wall and acting to effectively plug the pore space. The distribution pattern and morphology of microbially mediated CaCO3 indicate that MICP has a higher efficiency in plugging pores compared with extracellular polymeric substances (EPSs) which are currently the primary microbial plugging agent used to enhance sweep efficiency.
Highlights
Oil has and continues to play a dominant role in global energy systems with various techniques used to enhance its recovery
Aggregates of Ottawa sand conforming to three separate size fractions (40/60 mesh, 60/80 mesh, and 80/120 mesh) were used as artificial rock core of variable pore size to examine the effectiveness of microbially induced carbonate precipitation (MICP) on permeability reduction for different durations of microbial treatment
We examine the micromorphology of the generated CaCO3 as well as its distribution within the pore space using scanning electron microscopy (SEM)
Summary
Oil has and continues to play a dominant role in global energy systems with various techniques used to enhance its recovery. Oil reservoirs are developed through a series of production stages involving primary, secondary, and tertiary recovery techniques [1]. The successive use of primary secondary recovery in oil reservoirs may produce only ~20% to ~50% of the original oil in place [2]. Termed enhanced oil recovery (EOR), uses sophisticated techniques to recover oil which is locked within the reservoir and cannot be extracted within the primary and secondary stages of recovery [3]. The purpose of EOR is to restore formation pressure and to improve oil displacement or fluid flow in the reservoir [4]. The four major types of EOR operations are chemical flooding, gas displacement (miscible flooding and immiscible flooding), thermal recovery (steamflood or in situ combustion), and microbially enhanced oil recovery [5]
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