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Salinity Of Water In The Tulare Formation At The Elk Hills Field: Identification Of Protected And Non-Protected Water For SB-4 And Other Regulatory Reporting


Recent California regulations require monitoring of groundwater in non-exempt formations wherever reservoir stimulation operations are untaken. However, monitoring is not required if groundwater is absent, or if groundwater is of a non-protected status where it meets and/or exceeds the minimum threshold of 10,000 mg/l total dissolved solids (TDS). When direct sampling of groundwater is not practical, calculation of salinity using wireline logs is a valid alternative. The Resistivity Porosity (RP) or Humble method, described by Davis (1988), is favored over the SP method because of higher accuracy in calculating TDS. Salinity calculation has four steps: (1) converting measured density to formation porosity, (2) calculation of apparent water resistivity, (3) correcting apparent water resistivity to a standard temperature, and (4) converting temperature corrected apparent water resistivity to salinity. Calculated salinity in the Tulare Formation at Elk Hills ranges from about 4,000 mg/l to over 20,000 mg/l TDS and shows increasing salinity with depth. Actual water samples from the Tulare range from 1 percent under to 30 percent over the calculated values. The difference is attributed to two causes: (1) water samples are generally collected from a large completion interval and may represent a mixture of waters with several difference salinities, and (2) calculated salinity is based on a NaCl-only fluid, and a correction of salinity for a TDS composition more likely to be present will cause calculated salinity to approach measured salinity. Caution should be used with application of the RP method in thin bedded formations, where deep resistivity is affected by bed boundaries. Calculated values are also more variable in clayey sands, the RP method assumes calculations for clean sand. Good hole conditions are important for valid density values washouts or large holes may yield anomalously low salinity values. In areas of Elk Hills where groundwater is absent from the Tulare Formation, or areas containing only non-protected groundwater and unsaturated Tulare sands, a Letter of Concurrence may be requested from the State Water Resources Control Board which acknowledges that protected waters (<10,000 mg/l TDS) are absent. This process has been successfully used at Elk Hills to identify several areas that are now exempt from groundwater monitoring.