oa Vug Porosity Estimation using Acoustic Images in Oil or Water-based Mud Systems

Many productive carbonates show complex porosity systems with widely varying proportions of primary porosity (from the matrix) and secondary porosity (from vugs and open fractures). Until now vugs present at the surface of the borehole could be quantified only using micro-resistivity images logged in water based mud. For the first time, Schlumberger will present textural analysis using ultrasonic images. The image analysis, not dependent on a specific type of mud, is based on the variation of acoustic amplitudes within the geological formation. From amplitude histograms, the vug porosity quantification is then computed through: • Breakout / Background removal • Normalization of acoustic amplitude histograms • Determination of an amplitude threshold that limits host acoustic amplitude of the matrix from lowest acoustic amplitudes for vugs. • Calibration with the total porosity log. The technique presented here is applied and validated in a carbonate reservoir of Shell Brazil, logged in a well filled with oil based mud. The new vug porosity (0.2inc vertical resolution) captures better the variation of porosity of the formations than the conventional logs. This paper will also present different applications of the vug porosity: • To calibrate the volume of macropores derived from NMR. • To construct a Reservoir Rock classification scheme and lithofacies