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EMWD Systems
Conventional MWD and LWD systems rely on pressure pulse type telemetry in incompressible drilling fluids. When compressible fluids are used in low-pressure drilling (e.g., Underbalanced drilling), the pulses are absorbed by the drilling fluids, resulting in signals that are undetectable at the surface. In these situations, electric-magnetic induction (EMI) telemetry is used.
The MWD tool with (EMI) telemetry is called EMWD. Figure 1 depicts EMI telemetry through a formation.
Applications of this type of EMI are limited by depth, high resistance formations, anhydrite beds, and battery life. Deep wells require longer antennas and repeaters installed. High resistance formations require special antenna placement. Anhydrite beds cause signal complications due to reasons not well known, such as fractures. Battery life is still a limitation for long-time operations.
Figure 2 illustrates a Sperry-Sun Mercury (EMI) system, which relies on EMI telemetry through the drill string to derive better quality signals in deep wells.
Tool specifications are shown below:
Sperry Sun Mercury EMI Tool Specifications:
- Tool Length: 34 ft
- Tool outside diameters: 3.50 in, 4.75 in, 6.50 inch, 8.00 in., 9.50 in.
- Maximum pressure: 18,000 psi
- Maximum temperature: 125°C [257°F]
- Azimuth accuracy: +/- 1.75°
- Inclination accuracy: +/- 0.1°
- Pressure accuracy: +- 15 psi
- Power/Life: Lithium batteries / 140 hrs
- Maximum flow rate; Maximum Dogleg (Rotating/Sliding)
- 3.50 in: 250 GPM; 35/15
- 4.75 in: 350 GPM; 35/14
- 6.50 in: 650 GPM; 35/14
- 8.00 in: 950 GPM; 14/8
- 9.50 in: 1200 GPM; 14/8
- Transmission rate (channel dependent):
- TF: 15-20 seconds
- PWD: 15-45 seconds
- TF/Gamma: 15-20 seconds
- Survey: 40-220 seconds
