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ABSTRACT: Induction logs in favourable and awkward borehole environments

Elkington, Peter1 and Christopher Skelt2
(1) Reeves Wireline, Loughorough, United Kingdom 
(2) LASMO Plc, London, United Kingdom

Array induction logs run in wells drilled with oil based mud frequently show evidence of filtrate invasion. Assuming step invasion profile, invasion diameters are traditionally computed from a tornado chart solution. Generally, this requires input from a shallow reading resistivity device such as a Laterolog-3 or MicroLaterolog, but these measurements are not available in non-conductive boreholes. A new technique using induction only measurements circumvents the problem; it has the additional twin advantages of being very simple yet extremely robust.

The technique has also been applied to data from wells drilled with conductive muds. In these cases, a significant part of the total induction signal may be due to the borehole environment, and this must be backed-out in order to derive the formation conductivity. This introduces a degree of uncertainty in the computed curves that increases as the mud resistivity decreases, and is also a rather complex function of borehole size and tool standoff. We quantify this by looking at the relative magnitudes of signals due to formation and borehole contributions.

The approach has particular application in low conductivity formations where received signals are of low amplitude. Conventional wisdom, largely inherited from first generation logs, is that resistivities above one or two hundred ohm metres are unreliable, and can indicate only that the formations are highly resistive. We demonstrate that, subject to appropriate processing, the dynamic range can be trusted to at least 1000 ohmm. This has helped us characterise an oil reservoir with fresh formation water.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia