In downhole logging and resistivity measurement, we’re getting close to LALA land.
The latest advance in technology comes from researchers at Schlumberger and Statoil, who have developed and deployed a new electromagnetic look-ahead tool, or EMLA.
Still in the prototype stage, EMLA is a modular device that contains a low-frequency electromagnetic transmitter 1.8 meters (about 6 feet) behind the bit in a rotary steerable drilling tool.
The transmitter creates currents at multiple frequencies both around and in front of the bit. Then two or three receivers spaced along the drill string record the induced magnetic field.
Through inversion of the signals, the formation structure ahead of the bit can be interpreted by differentiating sensitivity around the tool from effects in front of the bit.
And, like magic, drillers can get a glimpse of the geology they’re drilling into.
“The EMLA tool provides a step change with regard to precision in detecting changes in resistivity properties ahead of the bit in vertical and low-angle wells. The ability to react to resistivity contrasts ahead of the bit has a direct impact on how wells are drilled,” the researchers wrote in an article published in the October issue of Petrophysics.
Toward Look-around/Look-ahead Capability
Whether or not it really is a step change, the EMLA tool does take another step toward a destination loggers have been targeting for a long time: downhole devices that allow drillers to see what they are drilling into and what they are drilling through, in real time, using look-around/look-ahead capability.
“The main reason for look-around and look-ahead capabilities while drilling is for effective well navigation — well geosteering — through reservoir pay and to avert and avoid drilling risk,” such as changes because of faults, said Carlos Torres-Verdin.
Torres-Verdin is a professor in the Petroleum and Geosystems Engineering Department at the University of Texas at Austin and a 2015 Distinguished Member of the Society of Petroleum Engineers.
“In the near future, with the technology already there, simultaneous look around and look ahead while drilling will be available,” researchers wrote in a paper presented at the Society of Petrophysicists and Well Log Analysts annual symposium in June.
“Interpretation of measurements in 2-D and 3-D environments is the main challenge to overcome to make LALA happen,” they noted.
One well-established benefit of look-ahead capability comes from reducing idle rig time and other nonproductive time by enabling drillers to make decisions and adjustments without halting or delaying drilling operations.
Expensive delays can occur when drilling has to be stopped for computation time using downhole data.
Earlier Steps Toward LALA
Some earlier versions of look ahead utilized seismic/acoustic tools, including seismic-while-drilling (SWD). SWD at first used energy from roller-cone bits as a seismic source, with geophones picking up the seismic waves. As bits changed, the industry switched to surface seismic sources.
Schlumberger developed a Seismic-Guided Drilling (SGD) service, using surface seismic and logging-while-drilling data to identify geological and geomechanical features.
“By predicting hazards and pore pressure ahead of the bit and establishing a 3-D velocity model that quantifies uncertainties and discrepancies between multiple models,” the SGD service improved both drilling safety and well costs, the company said.
Even when LALA doesn’t offer “real” — instantaneous — real-time capability, the goal is to allow the drilling team to make downhole adjustments without interrupting drilling operations.
Statoil described other benefits from use of the EMLA tool, one involving a subsalt-play well in the Gulf of Mexico. A main objective was to detect bottom salt before drilling through it.
For that well, three EMLA receivers were used to maximize look-ahead distance, deployed at 10.9 meters, 21.5 meters and 35.3 meters from the transmitter. Although seismic uncertainty was high in the salt, the salt formations were insulating and a nearly perfect environment for the EMLA device, the researchers noted.
“The bottom salt was detected 30 meters ahead of bit, which gave the drillers an early warning of the salt exit and potential drilling challenges,” the researchers wrote.
In another application, in the North Sea Statfjord field, the objective was “to stop and set casing close to the reservoir in order to allow drilling with a low mud weight in the depleted reservoir section to prevent fracturing of the formation,” the researchers said.
Statoil planned to detect and stop 3 meters into the Mime formation, a thin, calcareous marl that covers the entire Statfjord reservoir. Using EMLA, the resistivity increase above the formation was detected by inversion 9 to 10 meters ahead of bit, and the total-depth section was set as planned.
Geosteering involves directional control of drilling based on the results of downhole geological logging measurements. Landing a hole in a specified parameter or stopping overburden drilling at a point determined by information about downhole geology is a practice known as “geostopping.”
Look-ahead capability for geostopping drops off as angle-of-incidence increases, and falls to zero at angles beyond 40 degrees for the deepest tool configuration using previously existing technology, the researchers stated.
Using the EMLA tool, they said, “we for the first time in the industry can sense and evaluate resistivity contrasts from several meters to tens of meters ahead of bit, making geostopping feasible for all incidence angles.”
Having a preliminary understanding of downhole geology and calibrating the EMLA tool are essential to successful implementation. Accuracy also increases when detail about the formation already drilled is included in the inversion.
Ultra-high harmonic resistivity (UHR) responses are the key to look-ahead sensitivity and need to be fully calibrated, the researchers noted.
“Since calibration of deep directional resistivity tools such as EMLA is practically impossible on the surface, the calibration of UHR responses is done downhole, using the look-around inversion. The inversion is performed on a long data interval, at least three times the longest EMLA spacing,” they said.
With more accurate LALA capabilities, the industry can proactively react to changes in resistivity properties several meters ahead of bit, according to the researchers. New tools will have additional benefits, like selection of coring points, detection of base salt or oil-water contact.
At this point, in getting information about downhole geology while drilling, we might be just a few steps away from LALA land.