Using a Multiarray Induction Tool to Achieve High-Resolution Logs with Minimum Environmental Effects

Abstract

Over 30 years ago Poupon proposed using several simple induction arrays, each measured separately, and combining them to produce an induction log with improved characteristics. The response of each simple array becomes a "basis function" that can be combined with the others without introducing unwanted interactions. This approach has been used in the Array Induction Imager Tool (AIT*). The AIT tool consists of eight three-coil arrays, six of which are operated simultaneously at two frequencies. By treating the response of each measurement as a basis function, an optimization technique has been developed to combine all the raw measurements into logs with vertical, radial, and two-dimensional (2-D) response properties that are not achievable with conventional fixed-focus induction arrays. Using this approach, a set of logs has been developed with a 90% vertical resolution width of 1 ft and a reasonable rejection of borehole rugosity effects. Additional log sets are available that have different resolution-cave effect tradeoffs. A log set with a vertical resolution of 2 ft has less cave effect than conventional induction logs with 8-ft resolution, and a 4-ft resolution set has virtually no cave effect. The effectiveness of this response shaping technique has been demonstrated in case studies with both computed and field logs over a wide range of environmental conditions.

Rock quality assessment using the effect of mud-filtrate invasion on conflicting borehole resistivity measurements

“…In-phase and out-of-phase signal components are measured for each array at one or two frequencies suitable for that array length, to provide 28 raw borehole-corrected conductivity measurements. The processed logs from the arrayinduction tool are calculated as weighted superposition of the raw measurements at five median radial lengths of investigations: 10, 20, 30, 60, and 90 inches; and three vertical resolutions: 1, 2, and 4 feet (Barber and Rosthal, 1991). The inversion algorithm makes use of only the in-phase signals, which represent the apparent conductivity of the formation.…”

Dipping bed response and inversion for distance to bed for a new while‐drilling resistivity measurement

“…The bucking coil moment is adjusted such that the tool rejects all fields that decay as 1/r 3 , where r is the distance to the transmitter coil. This bucking coil system is commonly used in wireline induction tools (Barber et al, 1991;Beard et al, 1996) Figure 1.Schematic showing the magnetic field response to a shale bed above and a water zone below a cross-coil tool are in the opposite directions at the transverse-coil antenna. The direct coupling between the transmitter and receiver antennas is absent.…”

Imaging Radial Distribution of Water Saturation and Porosity Near the Wellbore by Joint Inversion of Sonic and Resistivity Logging Data

“…Here, we include responses for only the axial source and axial receivers at a single frequency of 26 kHz. This subset of data can also be acquired with earlier versions of the tool and is sensitive to formation resistivity at DOIs ranging from 1 to 7 ft and vertical resolution ranging from 1 to 4 ft (Barber et al 2004). The resistivity data are simulated with a semianalytical electromagnetic solver for cylindrically layered media on the basis of the numerical mode-matching method (Chew et al 1991).…”

“…Note that the resistivity logs in the traditional interpretation, labeled AT10 and AT90 in Fig. 7, are obtained by post-processing the raw, wellbore-corrected induction logs to produce depth-corrected and vertical resolution-matched logs at different radial depths (Barber and Rosthal 1991). The present inversion seeks to match the raw, wellbore-corrected induction logs rather than the logs from post-processing.…”