High Resolution Visualization of Near Wellbore Geology using While-Drilling Electrical Images

Abstract

Electrical borehole images offer a unique view of the subsurface to geologists and petrophysicists. Images from wireline electrical imaging tools are readily interpreted in terms of key geological characteristics such as structural and stratigraphic features of the formation. Today, advances in logging-while-drilling (LWD) technology allow high-resolution electrical imaging to be successfully applied in water-base. mud drilling environments. Key acquisition advantages for imaging-while-drilling include a better shaped borehole at the time of drilling and 100% circumferential borehole coverage (unlike the pad coverage of currently available wireline resistivity imagers). An important advantage is the opportunity for real-time decision making and related rig-time savings. Images sent to the surface, albeit limited in definition given telemetry restrictions, give an early indication of the angle of entry into a given formation and allow for more accurate/precise geosteering. We present field test results of a new high resolution, electrical borehole imaging-while-drilling tool. We demonstrate its field worthiness and show examples of the quality and accuracy of the images in conductive mud. In a series of controlled runs we have compared the response of the while-drilling tool with its wireline counterpart and with core. We show that the while-drilling images are comparable to the wireline images. In addition, a greater understanding of the geological features is possible because of their full circumferential coverage. Electrical images recorded while drilling show clear occurrences of laminated and disturbed mud rock, cross-bedded and bioturbated sandstone, and composite fractures as well as fracture swarms. This new LWD instrument has been quality-assured through mathematical and experimental modeling. In a laboratory setup, we have simulated the logging of a set of artificial formations with known dip, rugosity, fractures and different mud resistivities. Good agreement is obtained between the mathematical and experimental models. The electrical diameter for the LWD instrument is comparable to that of wireline electrical imaging tools.