Michael Wilt

Crosshole electromagnetic tomography: A new technology for oil field characterization

With the advent of crosshole seismic technology in the 1980s, a new generation of high resolution geophysical tools has become available for reservoir characterization. The chief improvement is simply that the tools are deployed in boreholes so measurements take place much closer to the region of interest.

A simple method for calculating apparent resistivity from electromagnetic sounding data

Frequency‐domain electromagnetic (EM) sounding is becoming an increasingly useful deep‐exploration tool with recent applications to crustal sounding and geothermal exploration (Tripp et al, 1978; Sternberg, 1979; Duncan et al, 1980; Stark et al, 1980). A major drawback of the method is that most field data are analyzed by computer after the data are returned to the laboratory; no intermediate parameters, such as apparent resistivity, are calculated to provide on‐site information. The following example illustrates this problem.

Experience with the EM-60 electromagnetic system for geothermal exploration in Nevada

Lawrence Berkeley Laboratory (LBL) conducted controlled-source electromagnetic (EM) surveys at three geothermal prospects in northern Nevada. Over 40 soundings were made in Panther Canyon (Grass Valley), near Winnemucca; Soda Lakes, near Fallon; and McCoy, west of Austin, to test and demonstrate the applicability of LBLs EM-60 system to geothermal exploration. The EM-60 is a frequency-domain system using three-component magnetic detection. Typically, +-65 A is applied to an 100-m-diameter four-turn horizontal loop, generating a dipole moment >10/sup 6/ MKS over the frequency range 10/sup -3/ to 10/sup -3/ Hz. With such a source loop, soundings were made, at transmitter-receiver separations of up to 4 km, providing a maximum depth of penetration of 4 km.

Comprehensive geophysics investigation of an existing dam foundation; engineering geophysics research and development

Part 1 of this paper (TLE, August 1989) reviews recent geotechnical investigations conducted at Beaver Dam, Arkansas. The problem addressed was anomalous seepage beneath Dike 1, adjacent to the main embankment dam. The paper presents a summary of the site geology, seepage history, and foundation grouting programs. Figure 17 is a plan map showing the north and south bounding fault zones of a graben structure beneath Dike 1. The foundation is a down‐faulted block of severely weathered limestone/dolomite of the Boone formation. Figure 4 of part 1 shows a simplified geologic cross‐section through Dike 1 showing the graben structure. Overall objectives of the geotechnical investigations were to assess the anomalous seepage and plan remedial measures to eliminate or significantly abate the seepage. The engineering geophysics investigations discussed in part 1 were designed to detect, map, and monitor anomalous seepage paths and delineate geologic structure beneath Dike 1.

A Numerical Feasibility Study of Three-Component Induction Logging for Three Dimensional Imaging About a Single Borehole

A theoretical analysis has been completed for a proposed induction logging tool designed to yield data which are used to generate three dimensional images of the region surrounding a well bore. The proposed tool consists of three mutually orthogonal magnetic dipole sources and multiple 3 component magnetic field receivers offset at different distances from the source. The initial study employs sensitivity functions which are derived by applying the Born Approximation to the integral equation that governs the magnetic fields generated by a magnetic dipole source located within an inhomogeneous medium. The analysis has shown that the standard coaxial configuration, where the magnetic moments of both the source and the receiver are aligned with the axis of the well bore, offers the greatest depth of sensitivity away from the borehole compared to any other source-receiver combination. In addition this configuration offers the best signal-to-noise characteristics. Due to the cylindrically symmetric nature of the tool sensitivity about the borehole, the data generated by this configuration can only be interpreted in terms of a two-dimensional cylindrical model. For a fill 3D interpretation the two radial components of the magnetic field that are orthogonal to each other must be measured. Coil configurations where both the source and receiver are perpendicular to the tool axis can also be employed to increase resolution and provide some directional information, but they offer no true 3D information.

Improved reservoir characterization and monitoring using electromagnetic geophysical techniques

This paper presents recent advances and case histories of two non-traditional electromagnetic geophysical techniques for oil reservoir characterization, and production/process monitoring. The crosswell electromagnetic induction method has seen rapid advancement since its inception in the early 1990s, and to this date numerous surveys have been completed in active oil fields as well as other sites. Here one example is given where a 2D electrical conductivity image derived from crosswell data is employed for reservoir characterization, and two other examples presented where the method provided valuable insight into the 2- and 3-D progress of water flood operations. The second, less mature method discussed involves 3-D imaging from multi-component induction log data. Here all three components of the magnetic field generated by a single or multiple sources are inverted to produce estimates of the electrical conductivity distribution surrounding the borehole. For this technology an example is given of three component data acquisition in an active oil field, as well as a proof of the 3-D imaging concept on a synthetic data set.

Cooperative projects to improve reservoir management

“The times they are a-changin’…” And we’d better keep up! It wasn’t too long ago that major oil companies kept their technical developments secret. Collaborative efforts were discouraged by management and the U.S. government (and perhaps others) threatened criminal charges against “anti‐trust” violators who dared share technology. Now many major technological developments are part of cooperative projects and the cooperation takes many forms such as consortia of industry, university, and government laboratories; teams of major companies only; partnerships between smaller companies and government labs; or service companies combining with independents. Nearly every conceivable combination is currently being tested with one technology or another.

Deep electromagnetic sounding in central Nevada

Sixteen shallow and deep controlled source electromagnetic soundings were performed in Buena Vista Valley, near Winnemucca, Nevada, to investigate an intra-basement conductor previously detected with magnetotellurics. The survey was carried out with the LBL EM-60 system using a remote magnetic reference for low-frequency geomagnetic noise cancellation, 100-m- and 2.8-km-diameter transmitter loops, and a minicomputer for in-field processing. EM soundings were made at distances from 0.5 to 30 km from three loops over the frequency range 0.02 to 500 Hz. Data were interpreted by means of 1-D inversions and the resulting layered models were pieced together to yield an approximate 2-D geoelectric model along the N-S axis of the valley. The EM soundings and one MT sounding show a 3 to 7 ohm-m zone at a depth of four to seven km. The conductor appears to be deepest at the northern end of the valley and shallowest beneath a basement ridge that seems to divide Buena Vista Valley into two basinal structures. Similar intra-basement conductors are also reported 50 to 75 miles south in the Carson Sink-Fallon areas, suggesting a common source, probably related to an anomalously hot, thin crust.