Making Interpretable Images from Image Logs
Generating a borehole image from the original measurements of rock properties involves a series of steps. The purpose of this chapter is to explain these steps systematically. For wireline (WL)-based imaging instruments, the measurements may come from sensors placed at different depths and orientations at the borehole imaging tool. When composing the image, the position of each measurement ideally has to be calculated at the size of a pixel, a procedure involving the use of downhole accelerometers and magnetometers, which are also part of the borehole imaging tool. For logging while drilling, the imaging is based on a rotating sensor that is part of the drilling assembly: the measurements are assigned to sectors oriented in relative bearing, and image generation is far simpler. Once the measurements have been oriented in space and placed on a rectangular grid, they have to be mapped to a color scale so features of geological interest become visible. A variety of filters may be applied to remove information of a nongeological nature, and pattern recognition or thresholding methods may be used to quantify imaged rock properties. Guidelines to correct faulty input data and detect defective processing are given. Within the context of a typical image processing and interpretation workflow, data verification and image generation and processing are discussed. Aspects of image interpretation, particularly the mechanics of dip picking, are also briefly considered.
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Borehole imaging is among the fastest and most accurate methods for collecting high resolution subsurface data. Recent breakthroughs in acquisition, tool design, and modeling software provide real-time subsurface images of incredible detail, from the drill bit straight to a workstation. Associated interpretation workflows offer the high level of detail that is needed to make operational decision and to increase the predictability of subsurface models. Many exploration and production companies have acquired a wealth of dipmeter and image log data. The data are readily available and provide, for example the orientation of fractures and fluvial channels in space. Further applications of borehole imaging technology include matrix and fracture characterization, pore-type partitioning, geosteering, and in-situ stress determination. Exciting new applications are found in enhanced oil recovery, carbon dioxide sequestration, and geothermal projects. In addition, borehole image data are paramount to unlocking unconventional plays such as shale gas and coal-bed methane. AAPG Memoir 92 portrays key applications of dipmeter and image log data across the exploration and production life cycle. (Continued)