Long Range Electromagnetic Telemetry Using An Innovative Casing Antenna System

Abstract

Abstract Wireless data transmission is an essential part of measurement-while-drilling (MWD) or logging-while- drilling (LWD). It can be achieved by several different technologies, such as mud pulse telemetry or electromagnetic (EM) telemetry. Compared with mud pulse telemetry, EM telemetry has been proven to have better reliability and relatively faster rates of data transmission. However, EM telemetry may suffer severe signal attenuation when conductive formation layers exist between bottom hole assembly (BHA) and the surface. An innovative casing antenna system has been developed to alleviate the signal attenuation issue of EM telemetry. A transceiver is set on the surface connected with two coaxial cables. One cable is externally attached to a casing string and connected to the bottom of the casing; the other cable is placed at surface connected to either the blowout preventer (BOP) or a metal stake driven into the ground with a certain distance away from the rig. Telemetry signals are propagating between the surface transceiver and the borehole transceiver via underground formation. The new system essentially measures the potential differences between the bottom of the casing string and one point near surface. On the contrary, conventional EM telemetry measures the potential differences between two locations on the surface. The innovative deployment of this casing antenna makes the new telemetry system be able to avoid heavy signal attenuation caused by any low resistivity layers above the bottom of casing, and consequently it has much larger operation depth than conventional EM telemetry. Numerical studies also show that this new telemetry technology has less signal drop as the working frequency increases; in other words, it can provide higher rates of data transmission. In this paper we will describe the setup of the casing antenna system, explain the working principle of this innovative method, and compare the new system with traditional EM telemetry system. Numerical simulations as well as filed data will be given and discussed to demonstrate the advantages of this new technology over conventional EM telemetry.