Measuring-while-drilling (MWD) and logging-while-drilling (LWD) systems gather data regarding the borehole and surrounding formations, and some of this information is most useful during the drilling process. For this reason, telemetry systems have been developed to transfer the information from downhole to the surface. One method of transferring the data from downhole to the surface is by encoding the data in pressure pulses of the drilling fluid within the drill string.
In ideal systems, each and every pressure pulse in the drilling fluid (also known as drilling mud or just mud) created downhole propagates to the surface and is detected by a pressure transducer or sensor and related electronics. However, drilling mud pressure fluctuates significantly and contains noise that tends to corrupt data transmission. There are several sources for these noise pressure fluctuations; the primary sources are: 1) bit noise; 2) torque noise; and 3) the mud pump.
Bit noise is created by vibration of the drill string during the drilling operation. As the bit moves and vibrates, bit jets where the drilling fluid exhausts can be partially or momentarily restricted, creating high frequency noise in the drilling fluid column. The industry's recent use of bi-centered drill bits has allowed for better extended reach drilling, but at the cost of higher downhole bottom assembly vibration and resultant pressure fluctuations and interference with LWD telemetry. Torque noise is generated downhole by the action of the drill bit sticking in a formation, causing the drill string to torque up. The subsequent release of the drill bit relieves the torque on the drilling string and generates a high-amplitude pressure event that is of significant duration compared to the LWD transmission. Finally, mud pumps themselves create cyclic noise as pistons within the mud pump force the drilling mud into the drill string. Aged, poorly maintained mud pumps, or pumps with a poor power source generate an inconsistent pump output. Thus, the drilling fluid pressure, upon which data is encoded, fluctuates, making pulse detection, and therefore data retrieval, difficult. Pulse detection also becomes more difficult as the distance from downhole to the surface increases because the propagated signal becomes smaller as the distance increases.
Current mud pulse telemetry systems have a set of parameters that can be adjusted and altered to optimize the data rate and telemetry accuracy of the system. Some of these parameters may control filter coefficients and others may control the system's ability to recognize and decode the telemetry signal. Some of these systems may be able to automatically monitor and change parameters while the system is operating but typically only one set of parameters can be used at a time. Accordingly, improvements in pulse detection and data retrieval are needed.
The drawings show illustrative invention embodiments that will be described in detail. However, the description and accompanying drawings are not intended to limit the invention to the illustrative embodiments, but to the contrary, the intention is to disclose and protect all modifications, equivalents, and alternatives falling within the spirit and scope of the appended claims.
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