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The optimum utilization of underground space for the emplacement and storage of waste and surface water can be achieved through an understanding of the geohydrologic environment. The emplacement of liquid waste and the artificial recharge of aquifers generally requires the drilling of exploration, injection, and monitoring wells. Some geophysical logs are run on almost all wells drilled for deep disposal. Even more information could be obtained, however, by utilization of all available logging techniques. To date, geophysical well logging has not been applied widely to artificial recharge projects; however, borehole geophysics is being used by the U.S. Geological Survey to study geohydrologic parameters related to recharging the Ogallala Formation on the High Plains of Te as.
Geophysical well logs provide preinjection data necessary for the selection of environments for liquid waste or water storage. Logs provide data on the location, thickness, and lateral continuity of storage zones and confining beds, percent and distribution of total or effective porosity, and the relative magnitude of permeability. Intergranular and fracture porosity can be discriminated by crossplotting acoustic velocity and neutron or gamma-gamma logs. The distribution and orientation of preinjection fractures can be determined by acoustic televiewer logs. Logs provide data on the chemical quality of the native fluids and the mineralogy of the aquifer, which are necessary to predict chemical reactions with injected fluids. The temperature and conductivity of the interstitial fluids ay be measured directly and their specific gravity and viscosity may be calculated from log data.
Because aquifers overlying injection zones can be polluted by improper well construction or well failure, geophysical well logs should be used to guide the design, construction, and maintenance of injection and monitoring wells. It is important to answer questions such as: are the casing strings and screens properly installed; are they plugged or corroded; does the grout fill the annular space and is it properly bonded to the casing; and, are there leaks through the casing, between pipe strings, or through the annulus?
After waste injection or artificial recharge has started, logs provide in-situ measurements of changes in the system. We have found that an increase or decrease in porosity caused by solution or precipitation in pore spaces or cavities may be detected. This type of information not only explains changes in well efficiency
but provides the basis for selecting remedial treatment. Accidental hydraulic fracturing caused by drilling or injection can be the reason for vertical leakage through confining beds, and acoustic televiewer logs can locate these fractures. The distribution and velocity of injected water and the location of chemical or thermal pollution may be determined by means of temperature logs. We have used temperature logs to map the horizontal and vertical distribution of injected fluids. Diurnal thermal changes in injected water provide the basis for measuring the velocity of flow and its change with time.
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