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The main design goal for all Class I wells is to deliver the waste to the permitted injection zone and keep it isolated without contaminating any USDW. Most problems with deep-well injection in the United States are attributable to poor well design, construction, and/or operation standards or requirements.
The general construction of a typical Class I injection well is illustrated in the accompanying diagram. The casing seals off formations above the injection zone and provides pressure control for the well operation. In some areas where bedrock is covered by thick unconsolidated deposits, a large-diameter conductor pipe is driven through these deposits into bedrock.
A large-diameter hole is then drilled to a depth below the base of the deepest USDW. Steel surface casing (approximately 2 inches narrower than the hole diameter) is run to the bottom of the hole, centered, and cemented in place. The cement job is tested for bonding throughout the length of the borehole.
Schematic diagram showing typical
Class I well construction.
The borehole is then extended to a predetermined total depth, and open-hole geophysical logs are run. These logs are necessary for determining the properties of the rocks encountered in the well and the condition of the borehole. In most instances, continuous, whole-rock core of the injection zone has been obtained for detailed analyses from at least one well from each class I site. The logs and most core from all Class I sites in the state are on file with the Division of Geological Survey. The "long string" casing is lowered through the surface casing to the prescribed depth in the hole, centered, cemented from the bottom to the surface, and tested. Depending on the material to be injected, special cements may be required. Typically, the lower portion of the long string casing is constructed of fiberglass, fibercast, or corrosion-resistant steel.
The injection tubing is placed inside the long string and sealed from the casing above by a packer at the top of the injection zone. The space between the injection tubing and the inner wall of the long string casing is called the annulus. The annulus is filled with an inert fluid (such as water and sodium chloride) and pressurized. The operator is required to constantly monitor the annulus pressure and report it to the Ohio EPA. Leakage from the tubing to the casing or from the casing to the surrounding rocks will cause either a pressure increase or decrease. The actual injection pressure also is monitored constantly. A fluctuation in either of these monitored pressures will automatically set off alarms and trigger shutdown devices to stop the injection pumps. The well operators are required to file monthly reports of their injection activities listing monitored pressures, injectate volumes, and injection rates.
A maximum allowable injection pressure, based primarily on the depth of the well, is set for each well. This limit must be below the fracture pressure for the well. This limit ensures that the operation of the injection well will not artificially initiate and propagate fractures in the injection-zone rock or the confining strata that protect the USDW's. Wells at three Class I sites in Ohio have had fracture treatment as part of their well-completion programs. Although well stimulation is not prohibited by state or federal law, the Division of Geological Survey believes that artificial fracturing in Class I injection wells is undesirable for several reasons. Most studies indicate that induced fractures at depth will be vertical. Once fractures are initiated they may be propagated further by operating the well at pressures close to the fracture limit. The integrity of injection sites is heavily dependent on modeling the flow of injectate away from the well bore and predicting the buildup of pressure as a result of continued injection. If the injectate is flowing along fractures it is not possible to reliably model the waste front or pressure front generated.
Proper well-treatment design and implementation are crucial to ensure that the rock units that make up the injection zone and confining strata will not be fractured in a way that will allow waste migration outside of permitted intervals. The Ohio EPA reviews plans for any well treatment to ensure that the integrity of the rock units will not be violated by the proposed treatment process.