Wells and reservoirs are like keys and locks: any key won’t do; it has to fit perfectly.
To unlock the stacked sands of the Orion oil pool within the Schrader Bluff, a geologic formation on Alaska’s North Slope, engineers at BP needed a special key. So this spring and summer they drilled L-205: the first hexa-lateral well in the state.
A hexa-lateral is one well with six underground branches—i.e. sidetracks. For L-205, this means six parallel wells, one below the other, each stretching horizontally into six different oil-bearing sands in the Schrader Bluff formation.
Although drillers around the world have used multi-lateral technology for decades, economic conditions have only recently made it preferable in Alaska, where companies have used it to develop portions of the wide Schrader Bluff formation.
Still, with eight months of planning, almost two and a half months of drilling, 29,000 feet of mostly horizontal pipe and a nearly $19 million price tag, the L-205 hexa-lateral well is among the most expensive and complex wells ever drilled in Prudhoe Bay.
The company drilled it without any spills or injuries.
What is it about L-205?So why was L-205 the right key? It’s all about the lock, said Rob Tirpack, engineering team lead of the Prudhoe Bay rotary drilling group for BP.
“The design of the reservoir is really what led to driving the technology to have multi-laterals,” Tirpack said. “The reservoir wasn’t developed prior to that.”
The Schrader Bluff formation stretches across the North Slope, touching parts of Prudhoe Bay, Kuparuk and Milne Point, among other fields.
The Orion oil pool running below the western half of Prudhoe Bay is a relatively shallow reservoir, where low temperatures contribute to the thickness of the oil.
Orion is part of the Western Region Development Project, BP’s multi-billion dollar and multi-year effort to tap the vast heavy and viscous oil resources west of the Kuparuk River in Prudhoe Bay. With oil prices rising globally and oil production declining locally, those tricky deposits have become a priority for BP in Alaska this decade.
Geologists have known about the Schrader Bluff resources of western Prudhoe Bay for decades, but only began to pursue them in earnest within the last 15 years, according to Scott Kolstad, senior drilling engineer of the Prudhoe Bay rotary drilling group for BP.
“Initially, in the early development of the Schrader Bluff, we tried vertical wells and tried horizontal wells, and noticed an increase in production from the horizontal wells, and then an even bigger increase in production from the multi-lateral wells,” Kolstad said.
The Orion oil pool section of Schrader Bluff in the western region of Prudhoe Bay is a series of stacked oil-bearing sands separated by non oil-bearing rock and shale.
A vertical well can cut through all these sands perpendicularly, but would only touch a small piece of each one. A horizontal well running through these flat sands touches ten times as much of each reservoir, meaning more “exposure,” in drilling terms.
But drilling six separate horizontal wells creates a lot of overlap. By using one mother bore with separate branches, a multi-lateral reduces the surface footprint of the well and uses fewer feet of pipe, while theoretically increasing production.
A combination of the horizontal and vertical options, where a well snakes through the ground and taps all various oil-bearing sands, would run the risk of flooding the reservoirs with water from surrounding reservoirs.
That’s why multi-lateral technology has become the standard for developing Schrader Bluff sands over the past five years. BP drilled its first tri-lateral in 2003, its first quad-lateral in 2004 and its first penta-lateral in 2005. The company now has around 15 multi-laterals in operations in the Schrader Bluff formation of Prudhoe Bay.
Possibilities and challengesThat first penta-lateral, the S-213A well drilled in the Polaris oil pool just east of Orion, showcased both the possibilities and the challenges of multi-lateral technology.
An earlier vertical well drilled through the sands hit about 200 to 250 feet of reservoir, and produced around 200 to 300 barrels of oil per day. The five lateral wells of S-213A hit 27,000 feet of reservoir, and produced around 1,000 to 1,500 bpd.
But at $10 million, S-213A cost roughly four times as much as the vertical well. And because multi-lateral wells are drilled from the top down and then constructed from the bottom-up, a mistake anywhere along the line can jeopardize all the previous work.
The costs don’t end after construction, either.
The numerous horizontal wells are difficult and costly to maintain. Because each reservoir produces oil at different rates, ratios and pressures, engineers need the ability to shut off or isolate the distinct branches, as well as isolating sections along each branch.
The sand-filled reservoirs require corrosion resistant materials down hole. To prevent re-entering the wrong well during work-over and cleaning, each branch is specifically “keyed” to the tools that will be used on it.
To manage the multiple streams of fluid coming to the surface all at once through a single pipe, engineers have “fingerprinted” the oil from each reservoir.
“It definitely is reservoir management intensive after we leave,” Tirpack said.
Nancy sands spurred sixthBecause it targets sands similar to those found at S-213A, BP originally intended for L-205 to be another penta-lateral well, targeting five Orion sands in Schrader Bluff.
These sands stretch across the entire Orion oil pool, and form the basis of the Western Region Development Project. But on top of those Orion sands sit a younger and thinner interval called the Nancy sands, where the oil is even more viscous than Orion.
With those five Orion sands, all economically viable, and the Nancy sand on top proving to be oil bearing, “that’s how we got to six,” Tirpack said.
L-205 is only the third well in Prudhoe Bay to target the Nancy sands.
Opportunity-driven technologyThe future of multi-lateral technology in Alaska all depends on the reservoirs, said Gary Christman, BP’s director of Alaska drilling and wells.
The drilling team at BP is planning a multi-lateral to target lighter oil in the Ivishak formation in the eastern part of Prudhoe Bay next year. In Orion, the team is working on what they call “The Palm Tree,” where the branches would fan out in every direction.
The team is also considering plans to build a well with mirroring branches, each running in opposite directions through the length of the sands. Using that option to its fullest would turn a potential hexa-lateral into a dodeca-lateral: a well with 12 branches.
“Even though multi-lateral technology has actually been deployed on the slope for a while — it’s been deployed globally for a while — if you look at who’s deployed it to its greatest extent and its greatest variation, no one’s actually done this before,” Christman said about the current workload at Prudhoe Bay.