The High-Resolution Dipmeter Reveals Dip-Related Borehole And Formation Characteristics

Abstract

Abstract Progress in dipmeter interpretation practice has shown the usefulness for stratigraphic studies of detailed dipmeter data. This has resulted in an increasing need for a dipmeter tool capable of accurate dip resolution even in the unfavorable environments of high resistivity and thick mud cakes, and in rough, deviated and oval-shaped boreholes. To fill this need the four-arm High-Resolution Dipmeter system was developed. This new system provides several data not previously recorded. For maximum utilization of the large amount of information obtained the dipmeter data are recorded digitally on magnetic tape as well as on the usual photographic paper or film. Computer handling of the curve correlation and dip computation is rapidly accomplished. The versatility and speed of the computer process can easily provide both high - density dip calculations for environmental studies and lower-density calculations for description of complex structures and near vertical bedding planes. The excellent resolution of dip provided by this system, combined with the dual-caliper feature of the tool, has made possible a better understanding of the interrelations among structural dip, hole drift and direction, and hole elongation. Furthermore, comparison of relative levels of resistivity of the four dip curves has indicated that some formations have a preferential direction of low resistivity. Finally, the effect on the appearance of induction-electrical logs in steeply dipping anisotropic shales shows good correlation with computed dip meter results and enhances the over-all interpretation of the structure. Introduction FOR MANY YEARS dipmeter surveys were run solely to determine structural dip. Improvements in the device made it possible to compute dips over shorter intervals and at higher densities, with the result that additional detail became visible, superimposed on the structural and regional dips. This detail was found to yield valuable information concerning sedimentary processes(1, 3). However, very salty muds, thick mud cakes, high resistivities, and deviated and oval-shaped boreholes still restricted the type of formation and length of interval from which reliable data could be extracted. The High-Resolution Dipmeter(3) was designed to solve such problems. Since its introduction the intervals being logged are much longer than was previously the practice. Now, the excellent resolution provided over the full range of dip magnitude permits a study of formation and borehole characteristics related to structural-dip trends. The High-Resolution Dipmeter The four-arm High-Resolution Dipmeter tool (HDT) features the recording of four microresistivity correlation curves with high vertical resolution, and two caliper curves. The log is recorded on paper or film for immediate viewing, and on digital tape for computer processing(4). The four-arm feature greatly improves pad contacts with the formation. To reliably record detailed microresistivity curves the sensor must be as close as possible to the borehole wall. Figure la shows diagrammatically how, with a three-pad system, poor pad contacts may occur in an irregularly shaped hole. Data taken from pad No.2 would be poor indeed, and dip computation difficult if not impossible, Figure lb shows how the four-pad system would produce good formation contact of all four sensors.