Design Considerations for a New High Data Rate LWD Acoustic Telemetry System

Abstract

Abstract The first commercial acoustic telemetry system was successfully introduced in 2000 as part of a drill stem testing system. However, environmentally challenging LWD offshore applications and a need for higher throughput required a complete redesign of that system. Thus, the new system incorporates a new high power acoustic transmitter driven by an advanced signal processor that maximizes input energy into particular frequencies and therefore minimizes dispersion and distortion effects. Low-frequency acoustic attenuators were specifically designed to attenuate in-band noise entering the communication system. In addition, two other features were introduced as a part of the complete communication system. They were (1) an in-line wireless link to pickup, digitize, and transmit signals off the pipe while rotating and (2) a high data rate acquisition and telemetry processing system. The system is designed to work solo or in conjunction with a mud pulse telemetry system. This paper will discuss the system design considerations and the data rates associated with different modes of operation as well as results of initial field tests. Introduction The industry has been searching for a faster wireless method to communicate downhole data to the surface for over 50 years. Transmitting information via acoustic stress waves propagating through the pipe metal was identified as a potential method of high speed communication as early as 1948. Significant theoretical development by Barnes and Kirkwood in 19721 followed by Drumheller in 19892 laid the foundation for analyzing acoustic wave propagation in drill strings. Lee3 and Ramarao's4 analysis of wave propagation in fluid loaded drill strings furthered the understanding of acoustic attenuation processes in drill strings. At Halliburton Energy Services Group (ESG), an effort was initiated to develop a wireless telemetry system for non-drilling applications. The commercialization of the Acoutstic Telemetry System (ATS®) in 2000 clearly demonstrated value for non-drilling applications and provided a basis for applications in more challenging environments. Motivation There is a need for significantly higher uplink telemetry rates during LWD/MWD operations. Critical geosteering, wellbore pressure, and temperature measurements enable tight control of bore-hole trajectories and allow optimizing the drilling fluid program to enhance wellbore stability and to remove cuttings. Gas kicks can be more efficiently prevented in near-balance or underbalanced operations. Wireline quality LWD information would save operators trip-in and trip-out times while providing information about near-virgin formation conditions. Wireless transmission of information would reduce health, safety, and environment related risks by elimination of wire or cable in the tubing or borehole annulus. Traditional wireless MWD telemetry systems such as mud pulse and EM telemetry are rate limited systems due to low carrier frequencies. Mud pulse carrier frequency is typically below 100 Hz, while EM telemetry operates at lower than 30 Hz. The operating frequency band of acoustic telemetry is much higher and broader, ranging from 400 Hz to 2 KHz. This range of frequencies enables acoustic telemetry to operate at significantly higher telemetry rates, even when employing simple telemetry algorithms. Design Considerations The successful development of a commercial LWD acoustic telemetry system (LAT) required resolving two critical hurdles - dynamic attenuation and non-stationary noise. Dynamic variations in attenuation occur due to various phenomena associated with drilling processes. Borehole conditions affect acoustic wave attenuation in a drill pipe. These conditions include characteristics of the borehole/casing, the deviation of the borehole, physical properties of the drilling mud, and the extent of contact between pipe and the borehole wall In addition, attenuation depends on the characteristics of the drill string including its mechanical properties, construction, and the type of mechanical connection between pipes. Any instantaneous variation in one or more of the properties changes the attenuation.