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Dakota Aquifer Program--Petrophysics

Geophysical Log Analysis of the Dakota Aquifer

The resistivity log

Resistivity logs measure the ability of rocks to conduct electrical current and are scaled in units of ohm-meters. There is a wide variety of resistivity tool designs, but a major difference between them lies in their "depth of investigation" (how far does the measurement extend beyond the borehole wall?) and their "vertical resolution" (what is the thinnest bed that can be seen?). These characteristics become important because of the process of formation "invasion" that occurs at the time of drilling. In addition to its other functions, drilling mud forms a mudcake seal on the borehole wall of permeable formations. However, in doing this, some mud filtrate penetrates into the formation, displacing formation water and this is called "invasion". The replacement of formation water by mud filtrate involves a change of pore water resistivity.

Figure 12. Spontaneous potential (SP) spherically focussed (SFL) medium- (ILM) and deep- (ILD) induction resistivity logs from KGS Jones #1.


The difference between the resistivity log measurements and the invasion process can be seen on Figure 12, where separation between the curves can be seen in the more porous and permeable sandstones, but minimal separation in the shales which are effectively impermeable. From a hydrologic perspective, the multiple resistivity curves are therefore excellent discriminators of aquifer and aquitard units. The mud used in the example well was less saline than formation waters in the deeper units, as is common in many drilling operations. The shallowest reading resistivity device (in this case, the spherically focused log) therefore records the highest resistivity because it responds mostly to formation invaded by the higher resistivity mud filtrate. The two induction logs draw their responses from deeper in the formation, so that the deep induction log (ILD) probably records a reading close to the true resistivity of the undisturbed formation. Notice that the resistivities in the uppermost sandstone (depth, 100 feet) are contrasted with those in the lower sandstones by showing a much reduced separation. As observed already, the dampened deflection of this sandstone on the SP log shows that its contained water is only slightly more saline than the drilling mud, and much less saline than the lower sandstones. Therefore, invading mud filtrate is only slightly fresher than the connate water, so that invasion effects on the resistivity logs are masked.

The sensitivity of resistivity logs to water salinity can be used in an alternative method to SP log estimates of water quality. In a sandstone-shale sequence, resistivity variation is controlled by a variety of phenomena, including cation-exchange mechanisms by clay minerals within the shalier zones, conduction by metallic minerals, and the dissolved ions within the pore water of the sandstones. However, formation water resistivity may be calculated in shale-free sandstone zones that are logged by resistivity and porosity tools. The water resistivity (Rw) is calculated from the resistivity and porosity log readings by the Archie equation (Archie, 1942) that incorporates a "cementation factor" (m) expressing the tortuosity of the pore network as a modifier to the fractional volume of pore space (F):

      Rw = Ro x F**m 
where Ro is the resistivity reading of the zone when it is completely saturated with water whose resistivity is Rw. The method is widely used by log analysts in the oil industry and generally gives good estimates of water resistivity in deeper (more saline) formation waters. Results are less reliable in aquifers because of clay mineral effects as well as surface conduction on quartz grain surfaces.

A water resistivity/specific conductance curve was computed for the Dakota Aquifer in the Jones well using the Archie equation with a cementation exponent (m) of 1.6 (an appropriate value for a slightly cemented sandstone). The water resistivity curve is shown in Figure 13 and is indexed with two water sample measurements and a reference value from Rattlesnake Creek. The curve is shown only for zones of sandstone that are relative low in clay content as indicated by the gamma-ray log. The estimated specific conductance trace is a highly acceptable match with sample measurements and appears to show a transition zone between the fresher water of the upper sandstone and the more saline waters of the lower sandstones.

Figure 13. Spontaneous potential (SP) log and profile of specific conductance of formation water estimated from resistivity and porosity logs in KGS Jones #1. Note match between profile and conductances measured from well water samples.


Again, it must be emphasized that log estimates of water quality should only be used (and then with caution) where no samples are available for direct analysis. In each case, the log property is an indirect measure, because it records a physically dependent property, rather than water salinity itself. In addition, rock properties other than water salinity may contribute to overall conductivity effects. The accuracy of the estimates degrades as water salinity decreases, with a general rule of a bias to pessimism in overpredicting salinity in fresher waters. However, when used judiciously with water chemical measurements, log data estimates are valuable in extending knowledge of Dakota Aquifer water quality over larger geographic areas and greater depth ranges.

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Kansas Geological Survey, Dakota Aquifer Program
Updated July 5, 1996
Scientific comments to P. Allen Macfarlane
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