Modelling of a Knife-Based Micro-Imager: Resolution and Depth of Investigation
Since the mid-1980s micro-resistivity imagers have provided borehole images using pad-type resistivity measurements. Geological features can be identified from borehole-image data, leading to improved reservoir characterisation and formation evaluation. Based either on conductive or resistive drilling fluid environments, different principles are applied to measure the near-borehole resistivity. As a result, the available technologies differ in terms of electrode arrangements, operating frequencies, and imaging pad design. Nevertheless, all micro-resistivity imagers are characterized based on borehole coverage, resolution, and depth of investigation (DOI).
This study investigates the synthetic responses of a resistive fluid-based imager, to determine the resolution and DOI, using COMSOL Multiphysics® with the AC/DC Module and the CAD Import Module to generate simulation results for already established CAD models of the imaging tool. The imaging tool operates at low frequency with imaging electrodes designed as knife blades. When the tool is open and its pads are pressed against the borehole wall, the knife blades cut through the mud cake layer and create a path for current to flow between the electrodes and the formation. This eliminates the need for mud cake correction.
The AC/DC Module was used to assess the tool response through modelling thin beds, electrical penetration, and mud-filtrate invasion into the formation. Despite being a low frequency AC tool, the resistive mud-based imager was modelled using DC, as the comparison between low frequency AC and DC for our model showed a negligible difference. The single bed and multiple bed responses were generated for various formation thickness and resistivity contrasts to determine the tool resolution. The electrical penetration length was computed by modelling different dipping angles and resistivity contrasts to estimate the DOI. The resistive mud-filtrate invasion radius was varied to obtain pseudo geometric factors for DOI determination.
The results of COMSOL® modelling indicated that the imaging tool can distinguish beds as thin as 0.5 in (12.7 mm). However, beds thicker than 2-in (25.4-mm) can be detected with significantly smaller error in measured resistivity (3 - 11%). In addition, a minimum shoulder bed effect was observed for beds thicker than 0.5 in (12.7 mm). The computed DOIs were 0.6 in (15.24 mm) and 0.625 in (15.88 mm) from electrical penetration and pseudo geometric factors, respectively. Finally, there was no significant difference in terms of resolution and DOI between the imaging electrodes.