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GEOPHYSICAL LOGS

Electrical Resistivity and Porosity measurments used to calculate water saturation

Geophysical logs, more commonly known as electric logs, map the location and thickness of water-bearing formations along depths of the subsurface. Subsurface samples collected during Exploratory drilling can be compromised by a variety of conditions within the borehole that reduce accuracy of grain size composition measurements as well as the indicated depths at which they were cut.

Electric logging provides continuous electrical voltage and resistivity records of the test hole fluid and surrounding formation regions. This information is used to differentiate formations composed mostly of clay and silts (indicated by low resistivity) from more “productive” zones made up of sand or gravel with interstitial water (indicated by high resistivity). Logs accurately depict the location and size of water-bearing formations with a precision to a fraction of a foot. When combined with driller’s descriptive logs and composition reports of the sieved cutting solids, electric logs inform where to place perforations in the well casing and where to place blank casing and sanitary seals during construction of the production well to follow. Eaton Drilling owns and operates digital geophysical logging equipment. We can also call on a reliable subcontractor who performs Geophysical logging to our standards.

logs show voltage differeces between the borehole fluid and fluid in formations surrounding the test hole. Shifts in spontaneous potential between high- and low-yield water-bearing formations are used to calculate the total dissolved solids in formation water, at corresponding depths, provided that the electrical resistivity and temperature of the borehole fluid (drilling mud) is known. The total dissolved solids are used to inform predictions about the overall water quality of the proposed well.

logs record the difference in electric potential between a given depth of the borehole and a reference (ground) electrode at the surface. Because the driving current is constant, electric potentials remain directly proportional to the resistance between these regions. Resistance increases with increasing grain size and decreases with increasing borehole diameter, fracture density, and concentration of dissolved solids present in formation water. Single-point resistance logs reflect lithological properties of the well, including locations of fracture zones, and can be used to gauge water quality.

logs use variably-spaced electrodes to record the resistivity of the local subsurface environment surrounding the logging probe. The method uses two electrodes for driving current as well as two electrodes for measuring resulting differences in electrical potential. Typical spacing between the two measuring electrodes are 16” and 64”; the greater the spacing, the further the radial extent of the resistivity reading. Bed thickness, borehole diameter and ionic composition of the borehole fluid all affect normal resistivity logs.

Eaton Drilling Co LLC in Woodland, Contractor license (133783), A General Contractor, C-57 Well Drilling and C-61 Specialty services in California, also other trades Nevada and Oregon. Our equipment conforms to current air quality standards.