The Mattoon water flood, located in south Central Illinois, encompasses 450 acres and includes 28 injection and 32 producing wells. A pilot flood utilizing Pennsylvanian age brine proved the feasibility of flooding the Mississippian age formations at Mattoon pool. However, since usable subsurface brines were limited in availability, an expansion of the project could be achieved only after another source of compatible water was found. One such source was the effluent from the municipal sewage disposal system of the city of Mattoon. This effluent carries considerable suspended matter in addition to aerobic and anaerobic bacteria. Following a series of tests, a satisfactory treating method was developed, consisting of chlorination, settling, and filtration. A contract was negotiated with the city of Mattoon to purchase the sewage effluent and injection began in May, 1954.
During the summer of 1955, additional tests revealed that produced water from the oil wells could be mixed with the effluent without additional treating and, during September, 1955, the produced water was introduced into the injection system. Currently, some two and one-half million bbl of sewage effluent have been injected with no permanent plugging of input wells being evidenced. Phosphates, principally of iron, resulting from the presence of detergent soaps in the sewage effluent continue to form after filtration but are readily removed by periodic acid treatments of the wellbores. Although the cost per barrel of water injected is greater than for similar floods using subsurface brines, and considerable time is required to control quality, sewage effluent has proven to be a satisfactory injection water.
The Mattoon pool, located along the west side of the city of Mattoon, Illinois, was discovered in June, 1940. Development of the pool proceeded at a moderate pace and was not essentially completed until 1948. Final development included approximately 420 producers and 90 dry holes drilled on 10-acre spacing. The principal producing horizons are the Cypress and Rosiclare sands of the Mississippian Chester series.
During May, 1952, a 70-acre pilot water flood was initiated to evaluate the feasibility of flooding the Cypress and Rosiclare sands. Product ion increases were realized in November, 1952, and by late 1953, results of the pilot indicated that large-scale flooding would be economically attractive. Water requirements for the proposed expansion of the project were in excess of that available from the Pennsylvanian brine used in the pilot. The only other subsurface water source was the Devonian limestone, found at a depth of approximately 3,000 ft. However, since the Devonian brine was sour, its use would be accompanied by relatively high lifting and treating costs, and its availability in sufficient quantities to meet ultimate requirements was uncertain.
