Underbalanced drilling (UBD) is known to be beneficial in hydrocarbon exploration and production: less formation damage, faster drilling, better control of lost circulation and stuck pipe situations, plus an enhanced ability to further develop depleted and marginal reservoirs and fields. However, UDB is inherently more complex than conventional drilling. Therefore it is essential that downhole tool and formation information be collected and delivered in real time to the driller during UBD. Achieving reasonable telemetry data rates for logging-while-drilling during UBD can be problematic with current telemetry techniques. Mud pulse telemetry may become ineffective when highly compressible UBD drilling fluid is used. Electromagnetic telemetry fails when the formation becomes too conductive. Acoustic telemetry, based on the propagation of stress waves through the drill string, is however particularly suited for operation in underbalanced drilling operations. It is relatively unaffected by the properties of the formation and operates even more efficiently in UBD with compressible low weight drilling fluids compared with conventional overbalanced drilling with heavy drilling fluids. This paper will discuss the drill string acoustic channel characteristics, with particular reference to signal attenuation and transmissibility. It will be shown that acoustic telemetry is potentially capable of transmitting 50 - 100 bits per second through the drill string channel while drilling. Finally, preliminary field test results will be presented as part of the goal of demonstrating the advantages of the acoustic telemetry over both mud pulse and EM telemetry. Introduction In order to safely and effectively manage the UBD process, it is imperative that essential information, such as wellbore pressure data, be available to the driller in real time. Unfortunately, achieving reasonable telemetry data rates for logging-while-drilling (LWD) during UBD can be problematic with the two commonly used LWD telemetry techniques. In most cases, the highly compressible drilling fluid used in UBD renders mud pulse telemetry completely inoperable due to the drastic increase in mud-pulse signal attenuation. As an alternative, Electromagnetic (EM) telemetry has been adapted for application in UBD. But it is well known that EM telemetry achieves reasonable data rates only in formations with resistivities in the range of 10 ohm-m or higher. When the formation resistivity is low or the resistivity profile too complex, EM telemetry can be rendered inoperable. There is another promising alterative for UBD that can potentially overcome the shortcomings of EM telemetry: acoustic telemetry. Based on the propagation of stress waves through the jointed drill pipes, acoustic telemetry is particularly suited for operation in underbalanced drilling conditions. It is relatively unaffected by the properties of the formation. It has the potential to operate even more efficiently in UBD with low weight drilling fluid since the loss of the telemetry signal to the mud column is lower compared to conventional overbalanced drilling with heavy drilling fluid. Another clear advantage of the acoustic telemetry is its potentially high data rate. Mud pulse and EM telemetry are both rate-limited due to their low carrier frequencies. Mud pulse carrier frequency is typically below 100 Hz, while EM telemetry operates at lower than 30 Hz. In contrast, the operating frequency band of acoustic telemetry is much higher and broader, ranging from 400 Hz to 2 KHz. It is this broad range in carrier frequency that makes it possible for the acoustic telemetry to operate at significantly higher telemetry rates. The concept of acoustic telemetry has actually been around for a long time. As early as 1948 [1], the transmission of data via acoustic stress waves propagating along the drill pipe was identified as a potential method for high speed communication. Theoretical works by Barnes and Kirkwood [2] as well as by Drumheller [3] analyzed acoustic wave propagation in drill strings. Lee [4] and Ramarao [5] further advanced understanding of the attenuation processes of acoustic attenuation for waves propagating along fluid-laden drill strings.
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