Abstract

Biological formation of calcium carbonate, using purified urease enzyme or urease enzyme produced by a bacterium isolated from a Canadian oil field, were studied in a batch system. The extent of enzymatically produced CaCO3 increased with increases in enzyme and reactant concentrations and temperature. In the presence of bacteria the quantity of produced CaCO3 was dependent on urea concentration, with the highest achieved with 15 g/L urea. Bacterial production of CaCO3 was less sensitive to temperature. Plugging studies in unconsolidated porous media indicated that in situ formation of CaCO3 could lead to a significant decrease in permeability, with the extent of plugging being dependent on the concentration of enzyme and reactants, as well as temperature.

Introduction

The injection of water into oil reservoirs (water flooding) is common for enhancement of oil recovery. When water is injected into a reservoir, flow is preferentially diverted into high permeability zones from which oil has already been recovered during the primary production. Increased permeability variation decreases volumetric sweep efficiency of injected water and cross flow complicates this problem by allowing flow between contrasting layers, leading to an unacceptably high ratio of produced water to oil (1). The excessive production of water can be controlled by selective plugging of high permeability areas in the reservoir. Selective plugging of the reservoir by bacterial biomass, or insoluble biopolymers produced by microorganisms has been suggested as a means to improve microscopic and volumetric sweep efficiencies within the geological formation(2–4). Recently, the use of bacterially precipitated inorganic compounds such as calcium carbonate and silica has been suggested as an effective method for selective plugging of reservoirs(2). The controlled biological formation of CaCO3 is based on the decomposition of urea to carbonate and ammonium ions by the catalytic action of urease enzyme, either introduced into the system or produced in situ by a bacterium with urease activity, and the reaction of the produced carbonate ion with calcium chloride. In the present work enzymatic and bacterial formation of calcium carbonate were studied, using urease enzyme and a bacterium isolated from a Canadian oil field. The results of plugging tests, using enzymatically formed calcium carbonate, in unconsolidated porous media system are described.

Theory and Experimental Procedures

The efficiency of enzymatically formed calcium carbonate in plugging of porous media was studied in a model core system. This was a modification of the system used by Ferris et al.(2), for mineral plugging studies. A schematic diagram of the system used in the present study, allowing testing of three different reaction conditions simultaneously, is shown in Figure 1. Glass columns (ID 2.5 cm and H 30 cm), packed with a mixture of sand (Dave 225 micron) and glass beads (Dave 75 micron), were used as model cores. A glass tube (ID 0.5 cm and H 55 cm) with overflow open to the atmosphere was devised in the liquid pathway to create a constant head pressure. Before initiating the plugging experiment, the designated column was connected to the peristaltic pump and distilled water was pumped into the column at a specified rotation speed of the pump.

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