Jeremy Lofts

High-resolution Electrical Borehole Images While Drilling

Electrical borehole images offer a unique view of the subsurface to geologists and petrophysicists. Images from traditional electrical imaging tools are readily interpreted in terms of key geological characteristics such as structural and stratigraphic features of the formation, and today, high-resolution electrical images are available while drilling. The drilling environment surprisingly offers an ideal platform for electrical borehole imaging. At the time of drilling, the borehole wall rugosity is commonly minimal, and electrical images generated by sensors that rotate with the drill string provide a full coverage of the borehole (when compared to the pad coverage observed on conventional wireline borehole images). The drilling environment also provides an opportunity for real-time geological analysis and decision making that is unavailable with wireline. Images sent to the surface can offer an early indication about the optimal angle of entry into a given formation and allow for more accurate and precise geosteering and possible geomechanical information that may mitigate drilling hazards. This chapter reviews the advances in resolution (sensor technology) and application of while-drilling electrical (resistivity) images and a variety of case examples. Examples include: Structural and geomechanical features, including micro- to macrofaulting, composite fractures, fracture clusters or swarms, drilling-induced fracturing, and breakout. Features imaged while drilling are commonly comparable to wireline resolution and typically offer a greater understanding of the geological features because of the full circumferential coverage. Real-time structural and geomechanical data provide the information needed for faster and better drilling-related decisions. A broad range of sedimentary features in high resolution allows textural analysis, facies discrimination, and feature orientation for sedimentological application (not unlike traditional image interpretation). This analysis is extended to the real-time environment where the concept of sedimentary steering is introduced. This entails the ability to use detailed, high-resolution features for advanced reservoir navigation (geosteering) with a predictive and real-time interface. This analysis focused around reducing uncertainties related to and improving geosteering within the desired sweet spot of a variety of reservoirs. The minimum useful resolution for sedimentary steering and appropriate feature recognition is discussed.