M/LWD (Measurement or Logging While Drilling) data transmission from downhole to surface can be accomplished using several different technologies, but the most popular methods include mud pulse telemetry and EM (electromagnetic) telemetry. Mud pulse uses a downhole valve to restrict fluid flow and create a pressure pulse through which data is sent to the surface via the mud column. EM telemetry uses a downhole transmitter and surface receiver to transmit data through the formation using electromagnetic waves. EM telemetry is often the preferred telemetry method when drilling on land or in an underbalanced environment. Advantages of EM telemetry over mud pulse include: reliability (no moving parts downhole), speed (can often be configured to exceed six bits per second and to communicate independent of flow rates), and tolerance to lost-circulation-materials (not dependent on drilling fluid characteristics). A key disadvantage is depth limitation due to high formation resistivity and signal attenuation.
A new technique has been developed that reduces signal attenuation and enhances the signal-to-noise ratio to increase the operating depth of an EM telemetry system. This method employs an insulated wire that is externally attached to a standard casing string; a borehole receiver typically located downhole and connected to the casing, and a surface transceiver. The borehole receiver picks up the EM signal at the casing connection terminal and transmits it via the external signal wire to the surface transceiver, which decodes the EM signal. The wire exits the casing near the surface and passes through a wellhead modified to accept the cable pass-through. There is negligible attenuation within the signal wire.
This technique extends EM-M/LWD applications into well profiles, including long laterals, where high resistivity and attenuation has previously prevented its use. Of particular interest is pad drilling, where one insulated wire can be installed on the casing string of an anchor well and serve as a receiving antenna for the other satellite wells drilled on multi-well pads.
This paper will describe the engineering concepts underlying the new technique and how it improves EM drilling technology. It also documents testing and summarizes actual field results of multiple lateral wells successfully drilled in highly resistive and attenuative formations with the new technique.