Borehole data gives formation realities
Image Gets to the Core of the Issue
Mention imaging to folks in the E&P business, and they’re likely inclined to think of seismic-derived images.
Yet there’s a whole other world of imaging out there – it just doesn’t get the ink devoted to seismic.
It’s called borehole imaging technology, which evolved from dip-meter technology beginning in the late 1980s and continued to be refined through the decade of the 1990s – and beyond.
“In the early 21st century, this technology advanced significantly with the introduction of electrical borehole imagers that are capable of acquiring formation images through both salty fluids and non-conductive fluids in the wellbore,” said AAPG member Paul Elliott, global product champion borehole imaging at Halliburton.
Outcrop Middle Miocene rocks at northern rim of the LA Basin. These strata lie within the same stratigraphic interval as the Moynier, Bradna, Nodular Shale and Sentous. Note scale, complexity and the fold that has been refolded.
“Instead of three, four, maybe six points around the borehole using dipmeters, with our fresh mud imager we have 150 points around the borehole,” Elliott said.
“That’s what creates the image you see and work with.”
The electrical borehole imager actually is a wireline tool with six arms, or calipers. Once it’s down the hole where logging begins, the logging engineer opens the calipers, and the pads – or electrodes – have to touch the borehole wall.
The imager acquires near-field measurements, meaning it takes very shallow measurements into the formation.
Fresh-mud imagers and oil-based mud imagers represent two different worlds, according to Elliott, who noted fresh mud imagers won’t work in oil-based mud, or vice-versa.
A portion of an image log within the Nodular Shale. Nose of a small fold lies at roughly 7558 feet (one third distance from top). This is very analogous to the outcrop above.
Imaging in the non-conductive fluid environment became a challenge to the industry when it opted to use oil-based mud in the 1990s, prompting development of new tools, including Halliburton’s OMRI™ (oil-based reservoir imager), which debuted a couple of years ago. In contrast, the company’s XRMI™ (extended range micro imager) for salty borehole fluids and highly resistive formations has been around for approximately seven years.
Among other commercial field applications, the XRMI recently was used in the 80-plus-year-old Inglewood Field in the Los Angeles Basin. The tool played a key role in operator Plains Exploration & Production Co.’s (PXP) large-scale development program targeting deeper horizons than previously produced, according to AAPG member Dalton Lockman, senior geologist at PXP (Related Story).
“What I did with borehole imaging in the ‘90s was not near what I do today as far as geological applications, petrophysical applications, asset management and more,” Elliott noted. “There’s a lot of value there.”
The Price is Right
This value is particularly apparent when comparing borehole imaging to the pricey task of coring. In fact, imaging technology often is used in place of coring.
“I like to say borehole imaging creates a core-like image,” Elliott said. “If you can’t have the whole core on your hands, the next best thing is a high resolution image of the borehole.”
The cost savings can be sizeable.
Consider, for example, a hypothetical scenario where an offshore rig is running a tab of $800,000 daily. The cost for three days of coring would tally $2.4 million – and this doesn’t include the significant add-on cost to crate the retrieved core, ship it to a land location and on to a lab for analysis.
In contrast, many thousands of feet of borehole imaging can be run for the cost of a short interval of core. The wellbore can be imaged top to bottom versus acquiring only a section of core.
Highly fractured interval interpreted as a fault zone extending from 5603 to 5613.. Note the offsets visible at 5608.5 and 5610.5. Dips on these surfaces are 25-30 degrees; strikes are about N70E.
Even when acquiring a whole core, it’s desirable to run imaging in order to calibrate that image back to the real core – or ground truth – and then apply this to offset wells so they don’t require as much core.
“For reservoir studies, you still like rock properties coming from cores, so people are still coring” Elliott said, “but never as much as the thousands of feet we run with image tools.”
Declaration of Independents
The electrical borehole imagers are used worldwide both onshore and offshore, and their application is not limited to the Big Guys. Even the mom ‘n’ pop-size independents have latched onto this technology in a big way.
“There are plenty of independents that run imaging because they don’t have the funds for the whole core, and they need to see the geometry of the sand, or whatever,” Elliott said. “They know the value of getting an image log with everything else.”
Image logs have proven especially popular in some of the unconventional resource plays, such as shales.
“It’s probably unusual not to have to have an image log in any of the Barnett Shale wells,” Elliott noted. “There are a lot of ways it’s used to deal with these reservoirs, whether it’s fracture identification or texture you’re looking for, or other geological information the geoscience team might need.
“I’d say for smaller players there’s a lot more imaging than not,” he said, “especially with new plays where they’ll need it until they get a better handle on what some of these real, real fine-grained reservoirs are capable of.”
Identification of thinly laminated formations is another instance where the electrical borehole imagers rise to the occasion.
It’s all about vertical resolution.
Elliott noted the fresh mud imager can measure down to 1/10 inch, while one inch or less is doable with the oil-based imager, making it possible to see thinly laminated formations.
“You get much greater vertical resolution than any conventional logging tool available,” Elliott said. “This is as good as it gets for vertical resolution.”
It’s a given that asset managers always want to save money any way possible, in addition to making appropriate decisions. Borehole imaging can contribute significantly to this goal given that applications are not limited to geological and/or petrophysical interpretations.
For starters, the images can help refine perforation targets, fine-tune frac jobs, assist with decisions related to production facilities at the surface, and lower completion costs.
“These,” Elliott said, “are a part of using imaging to its ultimate degree.”