Evaluation of Injection-Well Risk Management in the Williston Basin
Authors: Michie Troy W. , Koch Charles A. 
Published: December 2007

Summary
A study of subsurface water-injection operations in the Willisten geologicbasin demonstrated the practicality of incorporating risk management proceduresinto the regulation of underground injection control (UIC) programs. Arealistic model of a computerized data base was developed to assess the maximumquantifiable risk that water from injection wells would reach an undergroundsource of drinking water (USDW). In the Williston basin, the upper-boundprobability of injection water escaping the wellbore and reaching a USDW isseven chances in 1 million well-years where surface casings cover thedrinking-water aquifers. Where surface casings do not cover the USDW'S, theprobability is six chances in 1,000 well-years. probability is six chances in1,000 well-years.

Introduction
In the continental U.S., the oil and gas industry operates 170,000saltwater-disposal and EOR injection wells that inject 60 million BWPD intosubsurface formations. By assessing the potential risk of USDW con tamination, operators and regulators can identify those areas where additionalcorrosion-protection measures and increased surveillance of injectionoperations can be most effective in reducing the likelihood of injection waterreaching a USDW. An API study of oil- and gas-industry subsurface waterinjection in the U.S. developed a methodology to quantify an upper limit forthe risk of water from waterinjection wells reaching a USDW. The methoddetermined the probability of simultaneous failures of an injection well'stubing, production casing, and surface casing going undetected, permittinginjection water to escape into the borehole and possible to reach a USDW. The API study also showed that the individual well and field data needed to assessrisk are available, but that the information often is kept by a number ofdifferent operators, regulatory agencies, and commercial computer data bases. The study recommended that a model of a UTC data base be implemented in ageologic basin to evaluate the practicality of collecting and usingrisk-assessment guidelines in managing underground injection programs. As aresult, a study was conducted to test the feasibility of constructing a modeldata base for the Williston basin and to use the risk approach to rank areasbased on their risk of contaminating a USDW. Recommendations on databasecontent solicited from selected regulatory agencies and operators were used todefine the basic engineering, geologic, and injection data, along with thedatabase queries and reports necessary to administer UIC programs andoperations effectively. As a result of this effort, a realistic model wasdeveloped of a data base containing the necessary elements for evaluating therisk of USDW contamination in underground injection operations in the Willistonbasin.

Overview of Risk Approach
The risk approach to managing underground injection operations in apractical method of prioritizing and/or redirecting personnel and prioritizingand/or redirecting personnel and funds toward activities that are most likelyto contaminate a USDW. Risk-assessment guidelines do not give scientificcertainty, but they can be useful in setting priorities, in designing corrosionprotection systems, and in formulating protection systems, and in formulatingregulations. Risk assessments can help distinguish realistic potential USDWcontamination threats from trivial ones. They also can give regulators theinformation they need to decide what degree of monitoring and testing is neededfor a given area. To be effective, the data elements used in defining risk mustbe an integral part of a basic UIC data base. Such a data base conwellidentifiers, well-completion data, mechanical-integrity test results, injectionvolumes and pressures, and the base and top of the deepest USDW. The dataelements needed to maximize the risk-based decision process include tubing andcasing failure process include tubing and casing failure history, well-workoverresults, and reservoir definition (including identification of corrosive zonesand saltwater aquifers).

UIC Regulations
A major driving force leading to increased emphasis on the protection of USDW's has been the regulations promulgated by the U.S. Environmental Protection Agency (EPA) under the Safe Drinking Water Act of 1974. That actgave the EPA or an EPA approved state agency the authority to regulatesubsurface fluid injection to protect USDW's. The EPA's UIC program addressesfive classes of injection wells. Class II wells, which the oil industry isprimarily concerned with, include those used for disposal of fluids brought tothe surface and in connection with oil and natural gas production, injection offluids for EOR, and storage of liquid hydrocarbons.
JPT
P. 737