--> --> Calculation of groundwater salinity using the resistivity-porosity (RP) method on digitized geophysical logs from California oil and gas fields

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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Calculation of groundwater salinity using the resistivity-porosity (RP) method on digitized geophysical logs from California oil and gas fields

Abstract

The recent drought in California has placed a greater demand on groundwater resources. Publicly-available data indicate that produced water from oil and gas extraction in California is commonly being injected into the subsurface for disposal at relatively shallow depths in some fields. This may conflict with California Senate Bill 4, which protects groundwater containing < 10,000 ppm total dissolved solids (TDS). Direct measurements of groundwater TDS are not common requiring alternative methods to quantify TDS. This study uses the resistivity-porosity (RP) method which relates formation water resistivity to TDS using a modification of the Archie equation. We use the RP method to calculate the depth structure of groundwater salinity from resistivity and porosity measurements recorded in borehole geophysical data. The RP method is only accurate within clean, wet sand intervals. Hydrocarbons and shale, if present, distort the TDS calculations rendering inaccurate results. Due to this limitation, and the large number wells needing to be analyzed, an algorithm was coded in Python to process digitized geophysical well data to identify clean sand intervals where TDS calculations are more accurate. Radioactive, arkosic sands within the study area make it difficult to distinguish sand and shale with gamma ray curves. Therefore, the algorithm detects clean sands through a combination of locating where the SP curve deflects from the shale baseline significantly, and the difference between deep and short resistivity, then automatically selects the calculated TDS values within those intervals. The calculated TDS values are used to construct maps of groundwater quality for the Fruitvale and Rosedale Ranch oil fields in the San Joaquin Basin of California. The maps will be used to compare the salinity profiles of the two fields. Also, the maps can serve as a guide for wastewater injection operations, and to assist in groundwater conservation efforts while maintaining oil and gas production.