Abstract
Volumetric estimates are required at all stages of the field life cycle. One of the main parameters used to estimate the stock tank oil initially in place is the water saturation, which depends mainly on the true resistivity (Rt) of the formation. Correct log interpretation of LWD resistivity is a challenge in the Zuata Field due to the apparent conductive invasion profile phenomenom.
This study compares the LWD phase resistivity against Compact™ Dual Laterolog resistivity curves in the same well. The section of the well was drilled with water-based mud (WBM) in unconsolidated sandstones intercalated with shale and low conductivity formations.
The geological information for this area (core analysis and image log from two neighbor wells) indicates that there are sand/shale laminations in the surrounding formation. The area where significant differences were observed among the LWD propagation resistivity curves coincided with spiral-shaped wellbore. The interbedded formation causes the electrical anisotropy and the spiral zones amplify this effect. Compact Dual Laterolog tools on the contrary, although affected by anisotropy, can be easily borehole correted to yield an accurate Rt value.
In conclusion, when the LWD data can be correctly normalized the anisotropy effect could be reduced and the LWD propagation resistivity curves may be use for formation evaluation; however, another option is to run tools that can be properly corrected by borehole size and are less affected by anisotropy and polarization such as the Compact Dual Laterolog MDL.
