Abstract
One of the constant formation evaluation questions is that of selecting the optimal technique for determining formation resistivities. Two techniques are in common use, induction and laterolog. We review the latest developments in laterolog logging.
A recent array laterolog tool, HRLA (High-Resolution Laterolog Array tool), makes several focused measurements, with five depths of investigation, in a way similar to an array induction tool.
All measurements use bucking currents returning to the tool rather than to the surface. The proximity of the current return has considerable advantages; it eliminates Groningen effects and significantly reduces shoulder effects caused by strong bucking currents returning to the surface, in particular the well-known “squeeze” effect. It also eliminates the need for a bridle, resulting in improved operational efficiency.
The data is interpreted by inversion software using formation models of varying complexities to best approximate the true formation resistivity. At the wellsite, Rt is provided by a fast one-dimensional (1D) inversion that only takes into account invasion effects. Improved information content and vertical resolution results in more accurate 1D Rt determination. In thinly bedded formations, a two-dimensional (2D) model is inverted, which simultaneously accounts for both radial and axial resistivity variations. Significant improvements in Rt determination are seen when shoulder-bed influences are taken into account.
The improvements in tool design and in processing have resulted in significant increases in the operating range of the new laterolog. Examples from recent surveys clearly display the benefits of this new tool.