Al-Yaarubi, Azzan (Schlumberger) | Al-Mjeni, Rifaat (Petroleum Development Oman) | Bildstein, Joachim (Petroleum Development Oman) | Al-Ani, Khaled (Petroleum Development Oman) | Mikhasev, Maxim (Petroleum Development Oman) | Legendre, Fabienne (Schlumberger) | Hizem, Mehdi (Schlumberger)
We document the results of dielectric dispersion logging conducted in the Sultanate of Oman in wells drilled with oil-based mud. No conductive mud in the borehole complicates the tool response and requires careful job planning and the application of precise environmental corrections.
The borehole-corrected data are interpreted to deliver water-filled porosity, an estimate of salinity, a textural exponent usually designated mn, and the resistivity of the invaded zone. The invasion of oil-based mud filtrate generally displaces mobile fluid within the few inches of the tool’s depth of investigation. The water-filled porosity can then be interpreted as irreducible. By making permittivity and conductivity measurements over a wide range of frequencies and applying a dispersion model we derive a very accurate micro-resistivity log in boreholes drilled with oil-based mud. Formation water salinity and the water tortuosity factor mn can be derived from the dielectric dispersion measurements in favorable conditions.
Applications of dielectric dispersion logging in oil-based mud systems are demonstrated in various carbonate and clastics formations with known pay-evaluation challenges. The same interpretation techniques were then applied to improve the evaluation of a deep tight gas formation. The primary objectives were the determination of saturation, location of the water, and definition of intervals with mobile water. The results were compared with those from nuclear magnetic resonance and conventional formation evaluation. Overall, dielectric dispersion logging provided accurate water-filled porosity and microresistivity measurements. The interpretation of these results provided a robust distinction of gas-bearing intervals and saturation evaluation. Mud logs and well test results further supported the dielectric log interpretation.