Field Testing of an Advanced LWD Imager for Oil-Based Mud Applications

Abstract

Microresistivity and acoustic-borehole imaging encounters many challenges in oil-based mud (OBM). Although several wireline OBM imaging tools are commercially available, thus far there has been no equivalent LWD device. In this paper, we introduce a new tool that addresses this gap in LWD formation-evaluation technology. Existing wireline oil-based imagers use four-point resistance measurement methods. These devices require good and stable coupling to the formation. This is typically achieved by mounting the sensors on pads, which are forced against the formation wall by hydraulically activated arms. Such a solution is impractical in the LWD environment in which rotation and lateral movement of the drillstring prevents reliable contact with the borehole wall. Moreover, the additional friction caused by firm pad contact with the borehole wall would reduce the rate of penetration and could result in tool sticking or failure. The tool presented in this paper addresses the challenge by combining the best characteristics of a medium-resolution electromagnetic resistivity imager with the advantages of the dynamically focused acoustic transducer. The resistivity imaging provides stratigraphic information, and the acoustic transducer is capable of borehole and fracture imaging with a resolution of 0.25 in. (6.35 mm) over a wide range of standoffs. The paper presents the tool performance during field tests juxtaposed against modeling and laboratory experiment results, and discusses tool specifications and limitations of the technology. It also provides suggestions of various ways in which the information-rich data stream can be used by geologists, petrophysicists, and geomechanics experts, in both recorded and real-time modes for formation evaluation and as input for drilling decisions.