The determination of maximum injection pressures is an important component of planning and permitting for deep well disposal of liquid wastes. In the United States, safe injection pressures are defined as those that do not initiate or propagate fractures. The maximum injection pressures are often based on the closure pressure at the wellbore; however, higher pressures can sometimes be demonstrated to be safe. This was the case during the recent preparation of a no-migration petition and permit applications for deep well injection at the BP Chemicals Inc. Lima, Ohio, facilities. In this case, a substantial database existed, which included core mechanical properties, in situ stress tests and transient pressure tests on a specially drilled stratigraphic test well, as well as about 20 years of injection flow and pressure history. These data were used to provide the required assurances to the regulatory agencies that injection over the closure pressure could be adequately defined. This analysis was complicated by the stress dependance of the injection formation permeability and the pluggage of the near wellbore region due to fines in the injection stream. This paper describes the database used and the analyses conducted to demonstrate that injection above closure pressure at BP Chemicals' facilities at Lima, Ohio, does not initiate or propagate fractures in the injection zone and that the injected fluids will be contained in the injection zone for at least 10,000 years.
BP Chemicals is the world technology leader in the manufacture of acrylonitrile, a key ingredient in synthetic fibers, plastics, and nitrile rubbers. Production facilities include those at Lima, Ohio. The BP process is a partial oxidation reaction that produces water as a reaction product. Approximately 2.5 pounds of water are produced per pound of acrylonitrile product. The reaction water is separated from the products and forms the wastewater stream from the acrylonitrile process. This wastewater stream is approximately 95 percent water, 4 percent salts, and 1 percent organics.
BP Chemicals maintains and operates three Class I hazardous injection wells at the Lima site for the disposal of acrylonitrile wastewater. Deepwell disposal was determined to be viable for the site in 1967 and also was determined to be the most environmentally responsible method for handling the wastewater stream at this site. Disposal started in 1968. The wastewater is injected approximately 3,000 feet below the earth's surface into the Mt. Simon formation (Figure 1). The typical sitewide injection rate from 1968 to present has been approximately 350 gpm. The historic injection volume is approximately 4.6 billion gallons.
The Mt. Simon formation is approximately 345 feet thick at the Lima site, where it unconformably overlies the precambrian Middle Run formation, which has a very low permeability. The Eau Claire formation, consisting of impermeable shales, overlies the Mt. Simon and serves as an upper confining boundary to flow. The Knox formation consists of alternating permeable (aquifer) and impermeable (aquitard) layers of dolomite which provide redundant layers of protection between the injection zone and the lowermost underground source of drinking water (USDW).