ATTENUATION PREDICTIONS AT EXTREMELY LOW-FREQUENCIES FOR MEASUREMENT-WHILE-DRILLING ELECTROMAGNETIC TELEMETRY SYSTEM

Extremely low frequencies (ELF) are employed to transmit data from und erground to the ground surface for Measurement-While-Drilling Electrom agnetic (MWD-EM) telemetry system. Based on electromagnetic field theo ry, the work is aimed at predicting the receivability of the signals o n the ground surface. A unified analytic method that is suitable for v ertical well, directional well, and horizontal well is presented. Atte nuation properties are examined for various parameters, including the earth's conductivity, operating frequency and the length of the drill string. Frequency dependence of the receivability in reference to a no ise level is illustrated for different depths of well and different ea rth cases. It is also demonstrated that the electric field distributio ns on the ground surface have the same features for the three types of well, and the measurements should be carried out near the well heads for any type of well. A scale model experiment is made to test our the oretical results. The measured data and the computed results are compa rable.