Air-Drilling, Electromagnetic, MWD System Development

ABSTRACT Two-way communications, between sensors and other devices at the bottom of, or along, boreholes and the surface, have traditionally been performed by wirelines and stored-data technology. Telemetry (up-link) and command (down-link) func-tions have more recently been performed by use of real-time, mud-based methods. As more and more horizontal, slant-hori-zontal, arc, and other high-angle bore-holes are drilled (some with air), the need for non-wireline and non-mud-based communications becomes more important because of the difficulty of inserting tools and logs in the absence of a large gravitational force component, and the inability of mud-based systems to operate in air or for that matter in the absence of flow. The oilfield logging industry has developed tubing-conveyed-logging tools, but these too lack the flexibility required, because they still require a wireline. Even with side-entry subs, these tools are too costly or difficult to use in some cases. Air-drilling of directional wellbores is on the rise for a number of reasons, and wireline stee-ring tools and mud-based MWD tools are proving to be less than effective. In recent years new wireless electromag-netic-based real-time borehole communica-tions techniques have been developed with capabilities for supporting relatively-low data rates (compared with wireline) adequate for some applications. These are in commercial use for Measurement-While-Drilling (MWD) systems. Others are find-ing their way into non-drilling use in connection with oil and gas reservoir evaluation and completion work. While most of the work has been done in larger boreholes, some systems are being built into drill-strings as small as 2.5 inches (6.35 cm) OD. Wireless electromagnetic borehole communications technology holds out hope for providing reliable, two-way, high-data-rate data and command links in wellbores; regardless of hole size and angle, mud or air, flow and no-flow, cased or open-hole. This paper traces the development of wireless electromagnetic borehole commu-nications technology, and describes the state-of-the-art in oilfield and non-oilfield applications. Development of, and field results from, a recently-com-pleted air-drilling model are presented.