Mechanistic Understanding of Microbial Plugging for Improved Sweep Efficiency

Abstract

Microbial plugging has been proposed as an effective low cost method of permeability reduction. To optimize the field implementation, better mechanistic and volumetric understanding of biofilm growth within a porous medium is needed. In particular, the engineering design hinges upon a quantitative relationship between amount of nutrient consumption, amount of growth, and degree of permeability reduction. As a first step toward such a relationship, a Pseudomonas aeruginosa culture was inoculated into columns of glass beads and Berea sandstone cores. A growth substrate with acetate as carbon and energy source was injected continuously. Growth substrate utilization and permeability changes were used to track growth, and post-experiment in situ staining of biomass provided visual evidence of colonization and growth. Growth was observed as grainy coatings but in a spatially complex and unpredictable manner. Permeability was reduced noticeably in each experiment, but replicate experiments exhibited different growth rates and ultimate growth-induced permeability reduction. The experiments demonstrated that microbial growth is effective for reducing flow in porous media. Obtaining a mechanistic interpretation of the behavior will require a better understanding of the variability in microbe growth at the grain scale. 

Introduction

United States onshore oil fields have undergone extensive waterflooding. Many fields reach an economic limit at high watercuts because of the increased costs of lifting large amounts of fluids with only a small amount of produced oil. In many cases, high permeability thief zones contribute much of the produced water. Bypassed oil remaining in the lower permeability reservoir rock can be mobilized if injected fluids are effectively diverted from the thief zones to the remaining areas of the reservoir. Microbial plugging has been touted as a low cost, potentially high reward technology for increasing sweep efficiency by occluding thief zones in mature oil fields. Stimulating the growth of indigenous microbes is a particularly attractive version of this idea. This circumvents the twin challenges of cultivating a strain that will thrive at reservoir conditions and of propagating organisms to useful distances within the formation. This approach has been implemented with technical and economic success in the North Blowhorn Creek Unit (Vadie et al, 2002).

Quantitative modeling of the behavior of this and other microbial EOR techniques is difficult. In particular the relationship between the rate of consumption of microbial nutrients (e.g. acetate, dissolved oxygen) and the mechanism of permeability reduction (e.g. incremental growth of biofilms on all grain surfaces vs. local growth of colonies that block pores) is not well understood.

The aim of this study is to quantify biofilm induced permeability reductions for a model microbe in porous media columns. Permeability reductions were measured during flow experiments employing a single, well-characterized microbial species growing in a defined growth medium. Carbon source utilization and flow pressures were monitored concurrently. Images of the biofilms within the porous media were acquired by two methods to better understand biofilm location within the pores. Traditional batch growth experiments (no porous media) were conducted to determine growth kinetics independently of the flow experiments. 

Previous Work

Microbial induced permeability reductions in porous media have been verified on several fronts within many fields of science. Most experiments have shown that permeability reductions are normally between 65% and 95%. Cunningham and Characklis (1991) created a series of experiments in which several porous media of large permeability (between 98 and 2127 Darcy) were inoculated with Pseudomonas aeruginosa and permeability reduction was measured as biofilm grew. After growth was complete each sample had a permeability in the range of 3 to 7 Darcy. Their results indicated that final biofilm-altered permeabilities were similar, despite large differences in original permeability.