Two Geologists = Three Interpretations
Geosteering: Like Landing In Fog
By DAVID BROWN EXPLORER Correspondent (source) December, 2000
(The following is a tongue-in-cheek description of the difficulties of horizontal drilling. Now we have much more control. Peter)
Ed Stockhausen knows the strains, the pains and the gains that come from guiding a drill bit through a zone with geosteering.
"First there is the 'landing the well in the reservoir,' then there is the drilling of the lateral section," he said. "It's kind of like landing a plane on a runway in the fog, when the runway is moving up and down."
Officially, Stockhausen is a senior research scientist at the Drilling Technology Center of Chevron Petroleum Technology in Houston. More to the point, he serves as geosteering advisor for Chevron operating units worldwide.
He defines geosteering simply as "the use of real-time geological and directional data to help guide or place a well."
So where does the pain come in?
Try geosteering the bit on a 1,000-foot lateral, with a drill rate of 30 feet an hour, and you've been awake for 24 hours assessing continuous downhole information, with more than nine hours left to go.
Hello, coffee. Hello, mental overload.
Horizontal wells have become an important production tool in the Middle East, according to Walid Kholeif, operations geologist for Abu Dhabi Marine Operating Co. (ADMA-OPCO). The company is 60 percent owned by Abu Dhabi National Oil Co. (ADNOC) and 40 percent by minority partners, he said.
An internal survey brought out requests for information on well placement and steering, Kholeif recalled.
"The feedback was mainly directed toward horizontal well activity," he said, "and, also, directed toward the drilling techniques instead of the geology."
In response, AAPG helped arrange and sponsored the Abu Dhabi Geosteering Workshop, which was held in April. Kholeif coordinated the workshop and called it "perfect" for the company's needs.
"It was really practical information," he noted, "not just academic, or only a few papers you could get into."
Thinking in a Real (Time) Way
Geosteering involves "a different kind of thinking from what a geologist normally gets into," Stockhausen said. "We're used to taking weeks and months to think about the data and adjust our maps."
In geosteering, the geologist gets real-time data from downhole tools and gives the driller feedback to direct the bit.
"You're trying to predict the geology just ahead of the bit, particularly the dip of the beds," Stockhausen said. "You're making decisions on the spot, continuously, and you do the best you can.
"Many times you get lost in these wells," he continued. "You don't know if you're going out the top of the zone, or the bottom of the zone. You're looking for a marker bed."
Some horizontal laterals may drill at a rate of only 15 feet per hour, and Stockhausen described those wells as "cruel, because you're getting data so slowly, there is too much time for second-guessing yourself."
Other 1,500-2,000-foot laterals may drill in less than a day.
"Those wells are actually a blessing," he explained, "because you can say, 'OK, this is going to take 15 hours and I can stay awake, I will make my decisions and it will be over soon.'"
Is it possible to use two geologists taking turns on a longer project? Sure, Stockhausen said - but then you run into the traditional problem: Whenever you have two geologists, you get three interpretations.
In Abu Dhabi, according to Kholeif, geosteering helps to place wells - or "land" wells - in the reservoir layer to facilitate water or gas injection. It's a useful tool in efforts to maintain reservoir pressure.
"We have not yet done this for exploration," he said. "It's strictly for production."
The typical geosteering project begins with well planning, and in particular a drilling model that includes geology, resistivity and other important reservoir elements, according to Kholeif. And a good project places the geologist at the drill site "to direct the well to the best reservoir quality.
"There should be some experience in the field," Kholeif said. "If you have a geologist who is not experienced in the region, it would be a little difficult for him to begin.
"The key to the success of geosteering is really in the preplanning stage. You're trying to plan around your uncertainty."
Usually, Stockhausen said, "your targets are smaller than the accuracy of your maps.
"You try to use marker zones that are above the beds you're trying to get into," he continued. "You plan the design around those marker beds and make sure you enter them at the angle you want. This allows you to then continue forward to softly land the well in the reservoir.
"Getting too far ahead or behind on the plan leads to lost footage in the lateral section and lost reserves."
Stockhausen jokingly added that "you aren't allowed to have any of those unanticipated (geologic) faults along the well path. Little 10-foot faults can throw everything out the door."
He admits that geosteering initially earned a bad name for being too expensive - but, he added, geosteering actually involves a comparison of costs and benefits.
For instance, tools that provide inclinational measurement near the bit cost more to use than tools with these instruments further back, Stockhausen explained. The difference may be a three-foot drilling window from near-bit readings compared to a 10-foot window from other tools.
If the smaller drilling window leads to more accurate well placement, and the difference is capturing 100,000 more barrels of oil, the cost of near-bit information, he said, may be well justified.
Displays of downhole information in geosteering would be familiar to most geologists. Data is sent uphole and displayed in the normal way, said Ted Bornemann, principal geologist for Schlumberger's Center for Advanced Formation Evaluation in Houston - better known as the "SCAFE" (and pronounced as "S-Café").
"The commonly used tools are the gamma ray tools and the resistivity tools," he said. "Some new technology that's emerging now is the use of image data while drilling - real-time imaging."
Those imaging tools "show very graphically whether we are drilling up or down, or staying within the layer of the reservoir," said Bornemann, who specializes in forward modeling in drilling.
Imaging tools are located some 35 feet behind the bit, he said, while gamma ray and resistivity tools can be located right behind the bit. In every case, data is transmitted uphole by mud telemetry - pulses sent through drilling mud.
"That's why the data transmission is somewhat limited," he said. "It's not the bandwidth and the signal type you have in a wire, like you can get with wireline tools."
What might not be familiar to geologists are the downhole readings from a lateral well. Resistivity anisotropy produces anomously high resistivity readings different from those seen in a vertical well, according to Bornemann.
"In essence, the resistivity measurements look a little unusual," he said.
Where Am I?
Geologists who want to be successful in geosteering need to learn a foreign language:
"Drillers want target information," Stockhausen said. "This is one of the biggest challenges, how to communicate to the driller.
"I like to have the geologist on the rig site, because they can talk to the driller in person and negotiate."
To help guide the well, a geologist should know how the driller targets the hole and what information is needed to make adjustments. It's also important to understand the basics of directional drilling, Stockhausen said. Ask a driller if it's possible to lower the hole 10 feet in the next 10 feet of forward drilling, and the always-polite driller will respond:
"I don't believe so, sir."
Or words to that effect.
"Targets in horizontal wells tend to be lines - actually, moving lines - and well plans need to be flexible," he said.
"As we get information on the geology, we may have to admit that our line's in the wrong place and ask if we can please move it. But if you make too many kinks in the well you're going to stick the drill pipe," Stockhausen said.
And the geologist has to exercise restraint in making adjustments as downhole geology and measurement differ from the predrill model.
It's easy to oversteer a hole.
"There's always a constant issue of losing the well," Stockhausen said. "I try to preplan my decision points around a few key marker beds: These decision points should be communicated to the entire drilling team prior to drilling.
"When these beds are encountered while drilling, then it is time to make a decision - and then I'm going to talk to the driller about how to adjust the well path," he said.
"Preplanning key geosteering decision points allows wells to be drilled without as much alarm."
"Also, you've got to do everything in a very site-specific way," he continued. "That's one of my favorite terms, site-specific. You can't use the same geosteering techniques for every well in the world."
"Drilling a well is not like playing with a joystick on the computer," Bornemann noted. "There are a lot of restrictions on drilling a well."
In geosteering, the geologist must "work out the geology as detailed as possible along the planned azimuth of the well," he said, while knowing the characteristics of each layer above and below the hole.
"The real trick in geosteering is to know where you are geologically," Bornemann continued. "Through this whole process of landing the well, we will encounter a number of these formation tops. If we can recognize this marker on this new well we are drilling, we can say, 'Now we know where we are.'
"That does not mean we know in absolute depth from the surface where we are, but we know that 15 feet below this marker we have identified is our target zone."
Awareness and Accuracy
Even with good knowledge and reliable measurements, location can be difficult to pin down. That's when the driller is waiting for instructions and instead of breaking out in a smile, the geologist breaks out in a sweat.
"Everyone on the rig knows if the geologist is lost," Bornemann said.
"The people who tried to do horizontal well drilling without geosteering techniques drilled to a line drawn on a sheet of paper. You'd give a piece of paper to a driller and say, 'Drill these XYZ coordinates' without putting any geology on there," Stockhausen said.
The problem with that approach, Bornemann noted, is that a good driller could drill a perfect hole that matched the coordinates exactly, and the well might not be in the reservoir. There's too much uncertainty in mapping.
Stockhausen recalled his frustration the first time he tried geosteering, not having the tools and techniques he needed. He said Chevron recognized the need for a company-wide approach to geosteering instruction.
"In team environments, geologists become severely isolated from other geologists. We were all making the same kind of mistakes," he said. "That's when we began to see that there was some teaching needed."
Because of their reputations and experience, Stockhausen and Bornemann were asked to conduct the Abu Dhabi Geosteering Workshop. One challenge in Abu Dhabi, Stockhausen said, is to keep a directionally drilled hole in a thin, high-porosity zone for as much as 2,000 or 3,000 feet.
"You can imagine trying to stay in a 10-foot zone for 2,000-3,000 feet, how accurate you have to be. Some of this sounds somewhat impossible, but the zone helps you. A lot of times, the zone you're drilling in is the softest zone, and the bit is very content to stay there," he said.
Bornemann said he might have changed a few things about the workshop held in April, but not very much. He rated it highly successful for a first-of-its-kind effort.
"One of the biggest things was that everybody in the class came away with the feeling that he was well aware of the problematics of geosteering," he said, "and that is, principally, that you have to be aware of the uncertainties involved and what you can do to minimize them, thus drilling a successful horizontal well."