Reservoir Fracture Detection by LWD and Wireline Electrical Imagers

This paper includes an evaluation of fracture detection between logging while drilling (LWD) and post-drilling wireline images. The imaging tools were electrical “micro resistivity” imagers that were used to image a sub-horizontal borehole drilled with water-based mud in a fractured carbonate reservoir. The obtained images were interpreted side by side and compared for fractures and other structural discontinuities. We observed that the wireline tool detects a few fractures and misses a great number of fractures detected by the LWD tool, which is of a slightly lower resolution. We recognize that this reflects the dynamics of reservoir-borehole fluids. In particular, two distinctive borehole conditions can explain the detection of reservoir open fractures. Firstly, overbalanced drilling forces water-based mud into fracture apertures to be detected as strong conductive features against less conductive or rather resistive lithofacies. Secondly, thick mud cake masks rock features as the tool sensors make only shallow resistivity measurements. The two conditions are in favor of using LWD imaging tools to detect, and therefore describe, reservoir fractures. This is despite the fact that LWD imaging tools are yet of a lower resolution and can encounter more drilling challenges than wireline imaging tools.