The Use of Bacteria to Reduce Water Influx in Producing Oil Wells
Authors: Robertson E.P. 
Published: December 2007

This paper reports the results of a laboratory investigation evaluating the use of bacteria to reduce water production from oil wells. The objectives are to show that bacteria can be used to form in-depth stable plugs in porous media, that the bacterial plug can effectively reduce the amount of water being produced in oil wells by preferentially reducing water relative permeability, and to briefly report on the economical value of such a treatment. Bacterial strains for tests in a physical model of a producing well and near wellbore region with flow in two dimensions were selected based on results in linear-flow sandpacks.

Results from the linear sandpacks showed that in-depth plugs can be formed in porous media by the injection of one pore volume of nutrient. The pre-treatment sandpack permeability of 6600 md was reduced to a stable 200 md after one treatment. Bacterial injection and incubation in the two-dimensional physical model resulted in a 47% decrease in water cut (from 95% pre-treatment to 50% post-treatment). The experiments also show that the bacterial plug preferentially reduces water relative permeability in the presence of both oil and water. A preliminary economic evaluation was performed using field data and laboratory results. Although no attempt was made to economically optimize the laboratory system, extrapolation to field values shows economic potential.

Introduction

Water production in producing oil wells can be a large problem in some fields. The most common method of controlling water production from the producing well is through the use of gelled polymers. A cross-linked polymer solution is injected down producing wells to treat the entire production zone. In successful gelled polymer treatments, water production decreases while oil production remains unchanged. Gels tend to reduce water permeability more than oil permeability because the pathways of oil and water through a porous medium are segregated. Water-based gels enter the water pathways because of different mobilities between the oil and water phases.

The use of bacteria to plug porous media and change permeability has shown recent success in the field. Bacterial processes have a potential economic advantage over chemical processes in that they can use inexpensive feedstocks and convert them to plugging agents such as polymers and biomass. An inexpensive bacterial process that reduces water production in oil wells would be useful in controlling field production and disposal costs and in extending the life of high water cut marginal wells.

In one field test, indigenous bacteria were stimulated to grow and produce polymer by injecting inexpensive nutrients (carbohydrate and organic phosphate). The resulting microbial metabolite breakthrough pattern suggested that the areal flow characteristics within the pilot area had changed although the oil production did not increase.

Other experimenters found that injecting a continuous flow of nutrients into small sandpacks for several days can cause a substantial decrease in permeability. However, the bacterial plug formed in this manner did not penetrate more than 2 to 3 cm. The heaviest plug formation occurs at the sand face because bacteria tend to grow at the point of nutrient introduction where nutrients are clean and metabolic waste products from other bacteria are least prevalent. The essential problem with bacterial plugging has been to create a substantial bacterial plug in-depth where desired.

Some bacteria form spores, which are cells in a state of suspended animation and are much smaller than vegetative cells. The spore's outside wall is not as sticky as that of vegetative cells and allows the spore to be transported more readily through porous media.