Geochemical Aspects of Michigan Waterfloods
Authors: Tinker G. E. , Barnes P. F. , Olson E. E. , Wright M. P. 
Published: December 2007

Waterflooding started in the carbonate oil reservoirs of the Northern Michigan Niagaran reef trend in 1978 with Shell's Chester 18 water-flood. Ten projects had been installed by the end of 1982 so that significant operational results are available for evaluation. The design and operating programs initially planned for the projects have been proven successful. Operating data from some of the more mature projects indicate that the understanding and proper management of the geochemical systems for these projects will be crucial to the success of the project. The intent of this paper is to present what is currently known and understood about the geochemistry of Michigan waterfloods.

The geochemical system is here defined as all the various interconnected fluid environments constituting the project, namely the fresh water source system, the injection well system, the reservoir, the production wells, the production facilities, and the produced water disposal or reinjection facilities. Problem areas have been identified and corrective action has been taken or planned to counteract the detrimental effects of disruptions to the geochemical system. These upsets are brought about by injection of water into the reservoir where an equilibrium condition had existed between the formation fluids and the rock.

Project monitoring procedures, established to control and optimize waterflood operations, have made it possible to develop the proper approach to these geochemical disruptions. The more important items in this program are the measurement of produced and injected volumes, transient pressure analyses, injection well profile surveys, chemical analysis of the injection and production fluid samples, radioactive injection tracers, and continuous bottom hole pressures from submersible pumps. The use of these surveillance techniques has identified the following situations that are the result of disruptions to the geochemical equilibrium: Incompatibility of formation water and the available fresh water source has resulted in CaCO3 scale formation in producing wells.Temperature dependent precipitation of CaCO3 in some fresh water sources has resulted in injection well plugging.Dissolution of CaSO4 (anhydrite), present in limestone-dolomite reservoirs, has greatly increased the SO4= concentration in produced waters to cause precipitation of CaSO4•2H2O (gypsum) scale at production wellbores.Dissolution of solid NaCl from limestone-dolomite porosity has affected the reservoir quality and solubility product of scale forming ions.Increased fresh water and SO4= concentration has resulted in the generation of H2S in the reservoir from bacterial activity.The dissolved O2 in some fresh water sources and favorable reservoir temperatures created an optimum environment for bacterial plugging of injection wells.Corrosion has been attributed to H2S and dissolved O2.A sludge material derived from the incompatibility of acid and some crude oils has developed after acid stimulation to cause well bore impairment.The use of high volume submersible pumps has resulted in reverse oil-in-water emulsions. This water, when used for reinjection, has created injection well plugging.The production and deposition of solid NaCl in perforations and production tubing has required continuous fresh water treatment of some producers.

Many of these conditions have been successfully counteracted by a combined program of chemical treatments and wellbore stimulation. The volumetric nature of the Michigan reef reservoirs and their generally small size has allowed geochemical field studies to be conducted over reasonable time periods. The Michigan waterfloods have offered unique models for the development of at least a partial understanding of the very complex chemical relationships that exist, both within and outside the oil reservoir.