Development and Successful Testing of a Continuous-Wave, Logging-While-Drilling Telemetry System

Abstract

Patton, B.J., SPE AIME, Mobil Research and Patton, B.J., SPE AIME, Mobil Research and Development Corp. Gravley, W., SPE-AIME, Mobil Research and Development Corp. Godbey, J.K., SPE-AIME, Mobil Research and Development Corp. Sexton, J.H., Mobil Research and Development Corp. Hawk, D.E., Mobil Research and Development Corp. Slover, V.R., Mobil Research and Development Corp. Harrell, J.W., Mobil Research and Development Corp. A logging-while-drilling telemetry system, using a digitally encoded, phase-modulated, continuous-pressure-wave signal in the mud inside the phase-modulated, continuous-pressure-wave signal in the mud inside the drillstring, was tested successfully. In addition to transmission capability, the tests provided information to confirm signal attenuation and depth-range theory. Introduction Mobil Research and Development Corp.'s development of a logging-while-drilling (LWD) telemetry system for sending information from down hole to the surface was concluded with a series of successful field tests during 1971. The field tests were a direct result of many years of exploratory research and 4 years of intensive research and development. This paper describes the development and field testing of a prototype system equipped to transmit 15 channels of information, and one channel for frame synchronization. Most of the information transmitted was related to internal conditions of the down-hole system and was used for operational diagnostics. System diagnostics included internal temperatures and voltages, mud turbine speed, motor slip (torque), a measure of the transmitter coherence, and alternator field current. Other than down-hole mud temperature and tool vibration, the system did not include measurement of conventional logging parameters such as hole direction and inclination, formation properties, drilling variables, and bottom-hole conditions. A later generation of the system that includes those measurements is now being field tested by a major service company. A successful telemetry system for use while drilling will satisfy a long-standing industry need. As early as the 1930's, attempts were being made to solve the many problems involved. Recent reviews list the many problems involved. Recent reviews list the many publications and patents issued during the last 40 years and publications and patents issued during the last 40 years and discuss the principal telemetry methods and the developers involved. Information obtained from down hole while drilling, generally can be placed in two categories: drilling variable or hole information, and formation characteristics. Included in the former might be hole direction and inclination, tool-face heading, weight and torque at the bit, temperature, mud properties, tool vibration and acceleration. Formation logs might include resistivity, natural gamma ray, etc. Data rate specification is critical in this system and is a trade-off between more logs, higher accuracy, and depth range. We concluded that a good compromise would be to specify the minimum data rate necessary to transmit five logs with a data point each foot of depth and with an accuracy of 0.1 percent of full scale (10 data bits plus two bits overhead). This leads to a requirement of 60 bits/ft. The data rate in bits per second required by this specification is ...................(1) JPT p. 1215