During field testing of a logging-while-drilling (LWD) laterolog resistivity and imaging tool, formation resistivity differences were observed between the new laterolog and standard propagation resistivity. This paper compares the measurement acquired in the same borehole using different tools in both sand and shale formations.
The laterolog resistivities were higher in the shales and lower in the sands than the propagation resistivity values. The data was acquired while drilling in a water-based mud, sub-vertical exploration well in the South China Sea. While the main objective of the data acquisition in the siliciclastic formations was high-definition resistivity borehole images, the radial laterolog resistivity response was also of interest. An advanced wireline multi-frequency dielectric measurement was also acquired, and its response was used for comparison and validation.
In this paper, we associate the differences in resistivity response for varying formation properties to the tool physics, vertical resolution, depth of investigation, and time after bit between the measurements. In the sands, a resistivity inversion was applied to correct the logs for invasion effects and forward modeling used to resolve the resolution differences. The resistivity of the invaded zone obtained from the shallow laterolog measurements and inversion compares well with the dielectric measurements acquired at wireline time. The inverted formation resistivity from the LWD laterolog matches the deeper reading LWD propagation resistivity. The shale response was initially found to be more difficult to explain. It is commonly and historically accepted that due to resistivity anisotropy laterolog reads higher than propagation resistivity in low angle wells with laminated formations. Advanced forward modeling was used to investigate the laminations observed on the high-definition images and high-resolution laterolog resistivity curves. Although a model could be created to match both sets of resistivity measurements, the level of anisotropy required was considerably higher than expected, and supplementary information was required to validate the model. The wireline multi-frequency dielectric measurements provided the additional information required to confirm the anisotropy contrast observed by the resistivity modeling and confirm the LWD tool responses.