USING GEOPHYSICAL LOGS TO DETERMINE GROUNDWATER SALINITY TRENDS AND THE EFFECTS OF SALINE WATER DISPOSAL IN AQUIFERS

Geophysical logs can be useful in determining salinity variations within aquifers including determining the depth at which salinity exceeds 10,000 milligrams per liter (mg/L) and, with less precision, where salinity exceeds 3000 mg/L. These two values of salinity typically define accepted values for saline vs. brackish and brackish vs. fresh water respectively and are often used to make regulatory decisions regarding potential effects of practices such as surface or underground disposal of waste fluids into aquifers. The most accurate geophysical estimates of salinity are obtained from Archie’s equation which uses information from both porosity and resistivity logs along with temperature values. Salinity estimates made solely from deep resistivity values are less accurate because of the strong dependence of deep resistivity on porosity in some settings. For example, low porosity intervals in saline aquifers typically have high resistivity similar to that observed in fresh or brackish aquifers.

Geophysical logs can also be used to assess the effects of saline water disposal in areas such as oilfields where high salinity produced water is often disposed via water disposal wells and surface disposal ponds into shallow brackish aquifers overlying oil reservoirs. These assessments can be especially informative where well densities are high and the wells are drilled and logged in the same areas over decades. Near saline water injection wells, affected sands often show stratification of resistivity within the sands such that the lower part of the sand displays lower resistivity (higher salinity) than the upper part as the denser saline injected water settles to the base of the sand before it has had time to thoroughly mix with the relatively fresher ambient water. When saline water enters the vadose zone near disposal sites, perched aquifers with low resistivity are observed above high resistivity unsaturated sands. In some cases, logs from newer wells can also detect sands in which water salinity has increased, possibly due to mechanical integrity problems in nearby older wells. This usually manifests as sands that have markedly lower resistivity than underlying sands because resistivity tends to decrease with depth as salinity naturally increases with depth.