Biologically Formed Calcium Carbonate, A Durable Plugging Agent For Enhanced Oil Recovery
Authors: Nemati M. , Voordouw G. 
Published: March 2010

Abstract
Biological formation of calcium carbonate, catalyzed by purified urease enzyme or urease enzyme produced by a bacterium isolated from a Canadian oil field, were studied in batch systems. The extent of enzymatically produced CaCO3 increased with increases in enzyme and reactant concentrations, as well as temperature. In the presence of bacteria, the quantity of produced CaCO3 was dependent on urea concentration, with the highest amounts achieved with 15 g/L urea. The maximum concentration of CaCO3 produced enzymatically (57.6 g/L) was 2.6 times higher than that achieved in the presence of bacteria (21.5 g/L). Bacterial production of CaCO3 was less sensitive to temperature, whereas CaCO3 production rates with the urease enzyme were enhanced as temperature increased from 20 to 50 °C. Plugging studies in unconsolidated porous media and in a core-flooding system with Berea sandstone indicated that in situ formations of CaCO3 could effectively reduce the permeability of porous media and can be used as an efficient approach in enhanced oil recovery operations.

Introduction
The injection of water into oil reservoirs (water flooding) is common for enhanced 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 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(1,2). 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 have 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 the urease enzyme (reaction 1), 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 ion (reaction 2):
(1) N H 2 - C O - N H 2 + 2 H 2 O → 2 N H 4 + + C O 3 2 -
(2) C a 2 + + C O 3 2 - → C a C O 3 ↓
In the present work, controlled enzymatic and bacterial formation of calcium carbonate were studied, using a purified urease enzyme and a bacterium isolated from a Canadian oil field. The results of plugging tests in unconsolidated porous media and a core-flooding system with Berea sandstone are described.

Theory and Experimental Procedures
Biological formation of CaCO3, either catalyzed by the urease enzyme or by a bacterium with urease production ability, was studied in batch systems. The effects of enzyme concentration, reactants concentration (urea and calcium chloride), and temperature were investigated. With the bacterially mediated reaction, effects of urea concentration, ratio of calcium chloride and urea concentrations, as well as the availability of yeast extract on production of CaCO3, were assessed.