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Many geologic factors must be considered in choosing a suitable subsurface location for liquid-waste injection. Some of the major factors are: (1) the capacity of the geologic units to accept and confine the waste, (2) structural geology of the setting, and (3) presence or absence of valuable economic mineral resources within the potential area of influence.

Injection reservoir capacity

The three main parameters affecting the storage capacity of a geologic unit are its porosity (percentage of open pore space in the rock), permeability (degree of connectivity of pore spaces), and thickness. Another factor that must be considered is the lateral extent and consistency of a geologic unit. A thick, laterally continuous geologic unit with high porosity (storativity) through which liquids may pass easily (transmissivity) is most desirable.
 
The Division of Geological Survey has determined that the Cambrian-age Mt. Simon Sandstone is, throughout most of the state, the most suitable unit for emplacement of waste through Class I wells. Statewide, the Mt. Simon is the lowest Paleozoic sedimentary reservoir rock known; depth to the Mt. Simon ranges from 2,500 to over 13,000 feet below the surface. In western and central Ohio, the Mt. Simon is a fine- to coarse-grained, feldspathic to arkosic quartzose sandstone containing minor amounts of interbedded dolomite and shale. Its porosity averages about 13 percent and overall permeability is about 40 millidarcys, which are very high compared to most of Ohio's oil and gas reservoir rocks. For comparison, a "good" producing "Clinton" sandstone well (the "Clinton" is Ohio's leading oil and gas producing unit) averages approximately 8 percent porosity and 5 millidarcys permeability.
 
In eastern Ohio, the Mt. Simon is a dolomitic sandstone and is less suited for use as an injection unit; porosity is about 8 percent and permeability is about 10 millidarcys. The boundaries or characteristics of this west-to-east facies change in the Mt. Simon have not been mapped in detail.
 
In general, the thickness of the Mt. Simon Sandstone is fairly consistent throughout any particular area considered for injection purposes. Statewide, the thickness of the Mt. Simon ranges from about 44 feet to more than 300 feet, although there are areas where the Mt. Simon is absent because of depositional or structural irregularities.

Characteristics of the confining strata

In order to keep the waste from moving vertically toward a USDW or potentially valuable mineral resources, the reservoir rock for the waste should be underlain and overlain by strata with opposite flow characteristics, that is, a thick, continuous rock unit with very low porosity and permeability which will impede the flow of the injectate. Such rock units are called aquitards.
 
The Mt. Simon Sandstone is underlain by Precambrian crystalline rocks (billion-year-old granites, gneisses, gabbros, and metasedimentary rocks) of the Grenville Province in central and eastern Ohio and by Precambrian Granite-Rhyolite Province and East Continent Rift Basin (ECRB) assemblages in western Ohio. The Grenville Province rocks are all mostly impermeable. The ECRB has been found to contain thick sequences of sandstone and siltstone (Middle Run Formation) interlayered with extrusive igneous (mostly basalt) strata. Initial analyses of cores from two wells (Warren and Allen Counties) show that the Middle Run Formation has very low porosity and permeability but may locally contain a high degree of fracturing. The Granite-Rhyolite Province rocks are mostly igneous (andesite, basalt, granite) and metasedimentary. The ECRB, Granite-Rhyolite, and Grenville rocks should provide adequate seals to downward migration of wastes, although additional research is needed on Precambrian lithologies, contacts, and structures.
 
Overlying the Mt. Simon Sandstone in western Ohio are rocks of the Cambrian Eau Claire Formation. The Eau Claire is composed of interbedded shales, siltstones, fine-grained sandstones, and argillaceous dolomites. Overall, this unit normally has porosity of less than 4 percent and permeability of less than 1 millidarcy. However, some individual shale, siltstone, and dolomite layers within the unit act as very effective aquitards, having porosities of less than 1 percent and permeabilities of less than 0.1 millidarcy. Interbedded with these layers are siltstone and sandstone units of higher porosities and permeabilities. As a whole, the Eau Claire provides a very good confining layer for wastes disposed of in Class I wells; any fluids that might migrate through the less permeable units are "absorbed" by the higher permeability units.
 
In west-central Ohio the Eau Claire changes laterally eastward to the Rome Formation and the overlying Conasauga Formation. The Conasauga is very similar to the Eau Claire. In central Ohio the Rome is composed of a lower dolomite and an upper dolomite separated by a sandstone interval. Farther east, the sandstone disappears and the Rome is composed almost wholly of dolomite. The Rome dolomite is fairly impermeable, but the Rome sandstone has relatively high permeabilities. As with the Eau Claire Formation, the overall character of the Rome creates a good system of aquitards and buffers.
 
Overlying the Eau Claire/Conasauga in much of the state is the Kerbel Formation. The Kerbel is a medium- to coarse-grained sandstone and sandy dolomite that, for the most part, has excellent porosity and permeability. Where present, the Kerbel should provide another good buffer zone, storing injectate rather than transmitting it vertically.
 
The Knox Dolomite overlies the Kerbel Formation in its area of occurrence and the Eau Claire or Conasauga throughout the rest of the state. The Knox also contains zones of low porosity/permeability interlayered with thick, vugular zones of high porosity/permeability.

Diagrammatic stratigraphic chart of Precambrian through lower Ordovician strata in Ohio (modified from Janssens, 1973).

In eastern Ohio the Rose Run sandstone within the Knox Dolomite provides another set of porous beds. A thick, porous zone may be present at the top of the Knox as a result of a regional erosional unconformity. The Knox unconformity may, at least locally, provide another avenue for lateral fluid migration.
 
Above the Knox unconformity, in ascending order, are the Wells Creek Formation, the Black River Group, the Trenton Limestone, and the Cincinnati group. Although the Trenton and the Black River may contain locally porous zones, these Ordovician strata may be viewed as a thick (>1,500 feet), low porosity/permeability succession which will impede the vertical flow of effluent upward toward any USDW. Some limestones in the upper portion of the Cincinnati group are, in some areas of southwestern Ohio, the stratigraphically lowest USDW in the state. Above the Cincinnati group lie the Silurian carbonates, which are the primary aquifers for much of western Ohio. Therefore, the Ordovician carbonates and shales may be viewed as the last inhibitor of upward migration of wastes into any sources of fresh water in western Ohio.
 
As can be seen in the accompanying table, the total thickness of confining strata, from the top of the active injection interval to the base of the lowest USDW, at Class I facilities in Ohio (excluding Cargill Incorporated) ranges from a minimum of 1,900 feet to 5,130 feet. Using the Mt. Simon as the primary receptor of Class I wastes insures the maximum protective thickness of strata below the lowest USDW.

Structural setting

In terms of structural geology, three areas of concern stand out when investigating the suitability of a Class I injection site: (1) the elevation of the injection interval relative to the surrounding structural setting, (2) the presence or absence of faults and/or fracture systems, and (3) the potential for injection-induced earthquakes.
 
In the subsurface environment, the natural flow of fluids, in general, follows the most direct path from areas of higher pressures to areas of lower pressures. Because the amount of overlying rock is the primary pressure-loading factor, this concept translates into flow from deeper environments to shallower environments along the path of least resistance. These normal flow rates are low, on the order of several inches per year at the depth of the Mt. Simon.
 
The injection rate and pressure will yield an approximate hypothetical radial flow of the injectate laterally away from the well into the injection formation. Continued injection creates an area around the well where pressures are higher within the injection formation. The longer the injection continues the larger the radius of this pressure front becomes. Thus, ideally, the fluid seeks to escape this higher-pressure area in a radial pattern. Once away from the area of pressure influence caused by injection, the fluid resumes its natural flow pattern. This flow pattern is dependent upon the relationship of the site to the local structural setting and hydrodynamic characteristics of the reservoir and confining layers.
 
If the site is located at the lowest portion of a downwarp or synclinal depression, the flow should continue to approximate a radial flow pattern, seeking lower pressure areas on the surrounding highs. If it is on the flank of a rise, the natural flow should be asymmetric toward the higher elevation. If the site is located at the top of an arch or anticlinal feature, lateral flow away from the injection site would be impeded by the natural flow (toward the injection well) of native formation fluids. This latter situation is undesirable because the injectate will then be inclined to migrate vertically should vertical routes exist.
 
Flow away from the injection site should follow the general principles given above unless:

• The integrity of the well construction fails.

• A permeability barrier is encountered, such as a nontransmissive fault or fracture, a sedimentary facies change in the reservoir rock, or the thinning of the injection unit against a unit of lower permeability.

• An avenue of higher permeability is encountered, such as a transmissive fault or fracture, artificial fractures induced in the rock, intersection with an unplugged well bore, or a facies change in the reservoir rock that results in a higher flow rate.

Aside from failure of a well's construction and the possibility of encountering an unplugged well bore, the potential for upward migration along a naturally occurring fault plane is probably the most serious threat to loss of integrity of waste confinement at a Class I site. Although fluid movement along a fault plane that is transmissive may proceed faster than through the confining strata, it may still proceed very slowly, from a human perspective. Injectate moving along a naturally occurring fault may take tens, hundreds, or thousands of years to reach a USDW, but it could still have undesirable effects when it does arrive. Because of this potential threat, the Division of Geological Survey investigates every site as thoroughly as possible for any indication of the presence of faults.
 
Earthquakes induced by injection of fluids are a well-known phenomenon. Probably the most widely publicized and documented instance was the series of earthquakes triggered by military waste injection at the Rocky Mountain Arsenal near Denver, Colorado. Several earthquakes in northeastern Ohio caused public concern that they too may have been triggered by deep injection wells.
 
When discussing the possibility of induced seismic events it is important to understand that injection activities do not cause earthquakes; rather, injection may trigger earthquakes. Earthquakes are caused by the accumulation of crustal elastic-strain energy in a rock body that raises the stress level of the body to critical levels near rupture. An earthquake occurs due to the sudden release of energy as stress is released, generally along pre-existing faults. Therefore, injection of fluids into the subsurface cannot, by itself, establish the conditions necessary to cause an earthquake. But the injection of fluids into the subsurface can increase the pore pressure along a pre-existing fault, which may already be at, or near, a critical level of stress, to trigger a local release of seismic energy.
 
To reduce the risk that an injection well may trigger a seismic event, the U.S. Geological Survey has recommended that a review of the site should include: (1) a survey of recent and historic seismicity in the area, (2) measurement of stress in the reservoir rock, (3) assessment of the presence or absence of faults, and (4) determination that the intended injection zone has adequate porosity and permeability to store and transmit the waste at pressures well below the failure pressure of the rock. If the area appears to have any risk of seismic activity, a seismic monitoring program should be established.
 
To insure the integrity of the waste reservoir and to protect our mineral resources, the environment, and human well-being, the Division of Geological Survey began requesting seismic-reflection profiling and seismic-activity monitoring on Class I sites in the mid-1980's. Ohio legislation now includes authorization for the Director of the Ohio EPA to require such data collection on all Class I sites. Seismic profiling is the least expensive and best available technology to reveal geologic structures and details of stratigraphy over the area of potential influence. The use of seismic profiling is not foolproof; at best its resolution is normally about 30 feet (in terms of vertical offset or bedding thickness) at the depths under consideration. However, it is much better than having data from only one or a few wells to evaluate an area of pressure buildup that may reach 50 square miles or more. These data are proving to be a valuable tool in Class I site evaluations. Approximately 250 line miles of 2D seismic reflection profiles related to the Class I wells have been submitted to the Ohio EPA. Copies of these data are on file at the Division of Geological Survey.

Mineral resources

With time and the continued growth of our industrialized society, more and more natural resources are thought of not just as routine commodities, but as essential commodities. When oil was first found in salt wells in 1812 it was viewed as a nuisance; now our culture is heavily dependent upon its availability and price. Furthermore, mineral resources are not renewable; some may eventually become so scarce that worldwide shortages and socioeconomic disruptions could result. A conservative approach seems wise when protecting mineral resources from contamination by wastes. To protect mineral resources, the Division of Geological Survey believes that the Mt. Simon Sandstone is the most appropriate formation for Class I waste injection where it is present as a porous, permeable, unfractured reservoir.

For more information about Ohio's Underground Injection Control Program, visit the Ohio EPA Web site.

FURTHER READING

Clifford, M. J., 1975, Subsurface liquid-waste injection in Ohio: Ohio Division of Geological Survey Report of Investigations 43, 27 p.

Drahovzal, J. A., Harris, D. C., Wickstrom, L. H., Walker, Dan, Baranoski, M. J., Keith, B. D., and Furer, L.C., 1992, The East Continent Rift Basin: a new discovery: Ohio Division of Geological Survey Information Circular 57, 25 p.

Healy, J. H., Rubey, W. W., Griggs, D. T., and Raleigh, C. B., 1968, The Denver earthquakes: Science, v. 161, p. 1301-1309.

Hsieh, P. A., and Bredehoeft, J. S., 1981, A reservoir analysis of the Denver earthquakes: a case of induced seismicity: Journal of Geophysical Research, v. 86, p. 903-920.

Janssens, A., 1973, Stratigraphy of the Cambrian and Lower Ordovician rocks in Ohio: Ohio Division of Geological Survey Bulletin 64, 197 p.

Owens, G. L., 1967, The Precambrian surface of Ohio: Ohio Division of Geological Survey Report of Investigations 64, 9 p.

Seeber, Lawrence, and Armbruster, J. G., 1988, Recent and historic seismicity in northeastern Ohio: reactivation of Precambrian faults and the role of deep fluid injection: Preliminary report to the U.S. Nuclear Regulatory Commission, Lamont-Doherty Geological Observatory, unpaged.

Shearrow, G. G., 1987, Maps and cross sections of the Cambrian and Lower Ordovician in Ohio: Ohio Geological Society, 31 p.

Shrake, D. L., 1991, The Middle Run Formation: a new stratigraphic unit in the subsurface of southwestern Ohio: Ohio Journal of Science, v. 91, p. 49-55.

Shrake, D. L., Wolfe, P. J., Richard, B. H., Swinford, E. M., Wickstrom, L. H., Potter, P. E., and Sitler, G. W., 1990, Lithologic and geophysical description of a continuously cored hole in Warren County, Ohio, including description of the Middle Run Formation (Precambrian?) and a seismic profile across the core site: Ohio Division of Geological Survey Information Circular 56, 11 p.

Wesson, R. L., and Nicholson, C., 1987, Earthquake hazard associated with deep well injection: report to the U.S. Environmental Protection Agency: U.S. Geological Survey Open-File Report 87-331, 72 p.

Wickstrom, L. H., Drahovzal, J. A., and Keith, B. D., eds., 1991, The geology and geophysics of the East Continent Rift Basin: Cincinnati Arch Consortium, Indiana Geological Survey Open-File Report OFR92-4, 130 p.

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