T8 Environmental Issues Related to Oil and Gas Exploration and Production I

Sunday, 16 October 2005
8:00 AM - 12:00 PM

BOREHOLE INDUCTION CONDUCTIVITY AND NATURAL GAMMA LOGGING AT THE OSAGE SKIATOOK RESEARCH SITE, NORTHEASTERN, OKLAHOMA: LESSONS LEARNED

Paper number: T8-4
Program: Technical Sessions
Time: 8:40 AM – 8:55 AM

Authors

Abstract

Induction conductivity and natural gamma-ray logs collected from wells at the Osage Skiatook Research Site support ongoing geological, geophysical, geochemical, and hydrologic studies of the environmental impacts of oil production. Logged wells range in depth from 15 to greater than 100 feet and were completed with 2-inch PVC casing, bentonite grout, sand pack around the screened interval and steel well-head protectors. These logs can be obtained from cased wells and provide data both above and below the water table. The site lithologies include shales with high gamma activity and moderate conductivity, and sandstones with low gamma activity and low conductivity. Higher conductivity in the wells above the water table indicated the presence of conductive salts in the unsaturated zone. This was confirmed by high sodium chloride content in 1:1 aqueous extracts of selected samples from crushed core samples from the intervals.

The induction conductivity logs show the vertical distribution of soluble salts in the unsaturated zone and of saline ground water in the saturated zone. Induction conductivity logs augment chemical analyses of soil/rock extracts and ground-water samples, and improve estimates of the total volume of rock affected by salts. For example, higher conductivity values found in some shales indicated the extent of penetration of sodium chloride contamination into these low permeability rocks.

Gamma-ray logs were used to identify distinctive radioactive layers and these were used for lithologic correlations. When used in combination with conductivity logs, the gamma-ray logs provided additional lithologic information to supplement, or substitute for, core sample descriptions.

Vertical profiles of estimated electrical conductivity were used in calibration and interpretation of surface electrical conductivity surveys. The borehole logs have provided critical data to assist in other aspects of the study including geologic, geochemical, and hydrologic interpretations.