A novel permeability reduction process has been developed to control fluid flow in porous media. The procedure generally, involves use of bacteria to actively precipitate calcium carbonate as a mineral plugging and cementing agent. This process may be suitably employed to enhance the recovery of oil from oil reservoirs, cement unconsolidated sand underground, or to control the flow of contaminants in an aquifer.
In heavy oil fields, water responds to pumping more readily than viscous oil, so wells on primary production commonly water out after low recoveries of the oil-in-place. This is a serious problem that can occur gradually over several years, or be a catastrophic event when water directly underlies the oil bearing zone. For both of these situations, control of excess water production may be accomplished by selectively plugging zones of water encroachment(1,2).
A number of selective plugging systems for reservoir conformance correction have been developed. Chemically cross-linked polymers are available for use, but are expensive in pure form and perform unpredictably under reservoir conditions(3). Similarly, insoluble biopolymers and biomass generated by injected bacteria or by indigenous micro-organisms can be used to selectively plug off zones of high water permeability(1,4,5). The concept has been demonstrated in laboratory experiments; however, poor results have been obtained in field applications of microbial plugging systems(6). More recently, work has focussed on inorganically precipitated mineral plugs using polysulfides(7) or colloidal silica(8). This is expected to improve plug stability and extend use of the technology to more sophisticated production modes. The same advantages may be afforded by a novel bacteriogenic mineral plugging system developed to control fluid flow in porous media.
Bacteriogenic mineral plugging involves using injected or indigenous micro-organisms to precipitate authigenic minerals in high permeability water channels. This can be accomplished by
using bacteria as passive nucleation sites while injecting an appropriate solution to oversaturate formation water with respect to a certain mineral phase,
stimulating specific bacteria whose metabolic activity will bring about a mineral oversaturation, or
a combination of the two processes. Precipitation of the mineral would be expected to selectively plug off the water bearing zones allowing oil production to continue (until further water breakthrough occurred).
Of the various minerals that bacteria can precipitate, calcium carbonate is a particularly good candidate for bacteriogenic mineral plugging. Bacteria have been implicated as the causative agents for carbonate precipitation in a number of laboratory and field investigations(9-11). In addition, most heavy oil reservoir formation waters in Western Canada are near saturation with calcite(12). Consequently, stimulation of bacteria that generate alkaline conditions would promote an increase in carbonate concentrations and induce calcite precipitation(13).
In this paper, experiments are described using bacteria to increase pH and precipitate calcium carbonate as a mineral plugging and cementing agent.
The bacterial strain used was Bacillus pasteurii NRS 673. Sand used for this work was number 16 silica. Whisling, Illinois. Analysis provided by the company indicated packed cores would have a 0.37 porosity with a permeability of 2.5 Darcies. Reservoir sand for enriched cultures of indigenous bacteria was also used.