What happens if you're drilling a well and you run into dips, folds and other deviations that are too small to be detected by your seismic data model, yet large enough to interfere with your drilling?
Historically, you either abandon the well or continue at the cost of considerable time and money.
But for the geoscientists working on one particular offshore California site, another option emerged - one that utilized real time well analysis that took place at the same time of actual drilling.
"This (was) a breakthrough concept that represents a large step in the technology of formation evaluation while drilling," said Steve Grayson, sales development engineer with Schlumberger Oil Field Services in Ventura, Calif.
Grayson won the 1998 Pacific Section H. Victor Church award for a poster session on the project, "Advantages of Real Time LWD Dipmeter Data." His co-authors included AAPG members Jim Ezell, Jon Schwalbach and Dalton Lockman, all with Exxon USA, Thousand Oaks, Calif., at the time of the project.
The "project" was in the Monterey formation, a Miocene age fractured reservoir, offshore California in an area that is dominated by irregularities and folds in the faults.
Conventional drilling techniques weren't working, Grayson said - the usual logging well data (LWD) that was available conveyed a hint of a changing structure through resistivity and gamma ray measurements, but was not able to allow for direct, real time observation of geologic structures.
Exxon and Schlumberger personnel, who were working together on the test, decided the project would be an ideal location to test a relatively new Schlumberger tool called Resistivity At Bit (RAB), which has button resistivity electrodes mounted on a rotating LWD sensor sub to provide the data for a resistivity image of the borehole.
These images provide the data for ongoing structure interpretation during drilling, and can be used to "geosteer" the wellbore to the optimal location.
Novel and Intense
The geoscientists, in testing the tool, worked to develop algorithms to extract the dip information from the images provided by the RAB.
Conventional wire line resistivity tools have resistivity buttons (sensors) to make a complete picture of the borehole. The improvement, over time, was to add more sensors to obtain increased borehole coverage.
"You can take advantage of the spinning of the drill string itself to get an actual image," Grayson said. "You already have spinning drill pipes that you can take advantage of without the need of building in additional sensors.
"The images that come out are a bit novel in the world of LWD, very visually intensive," he said, "and it takes a little learning curve to get used to them. When the bedding planes are intersecting with the well bore, the angle of intersection is apparent by the sign wave created on the image.
"And when the bore hole is approaching parallel to the bedding, the image has a very distinctive bull's-eye pattern," he continued.
"The pattern jumps right out at you."
Grayson added that "when you are drilling down the bedding plane you want to stay on the right path. Sometimes you want to cut through the parallel, and other times to stay on that path for awhile."
When it came to the guiding, he said, the RAB was a big help.
Grayson also referred to another somewhat involved case study where the plan was to cut through the zone of interest with a long exposure.
This well, drilled in the Santa Ynez Unit, had a highly deviated well path, 77 degrees from vertical. The plan was to intersect a gently dipping five-degree Monterey Formation target.
"This nearly parallel borehole-strata geometry provided little room for error when attempting to intersect reservoir intervals at specific elevations," he said.
This is what was found:
- The marker bed was discovered to be shallower than anticipated.
- The identification crisscrossed the fold and the dip was reversed from what it was thought to be.
- Then, below that, the marker bed was found "upside down."
All of these features were too small to be captured by the conventional methods, Grayson said. Drillers using conventional methods most likely would have proceeded along the wrong path - and the well might very well have been abandoned or sidetracked.
But in this case, the RAB tool provided all of the data that the geologist needed to revise his model.
"He had to modify the trajectory a few degrees," Grayson said, "then continued drilling and eventually dug into the zone 800 feet deeper than originally anticipated."
When the new system was first being used, it relied on a satellite delivery process to take the data from the well site, then transmit it to the processing center, with the results then transmitted within six hours back to the geologist.
But since that time the technology was developed to calculate the dips down the hole while the tool is operating. The formal name for this is Real Time LWD Dipmeter Data, or, more simply, Real Time Dip Calculation.