History is unfolding along the coast of California, according to Dennis Williams, president of GEOSCIENCE Support Services Inc.
His groundwater consulting firm has designed something like 1,000 municipal water supply wells in almost 40 years. But those were typical wells, vertical wells. What he’s working on now he’s only done once before.
“Necessity is the mother of invention,” he said, alluding to a well technology his firm is designing for California’s Monterey Peninsula. It’s called a slant well or subsurface intake, and while the technology has been used in Europe and tested in the United States, he says it’s still a very rare method.
“The evolution of the subsurface slant well technology,” as Williams calls it, is an outcome of California state regulators and environmental groups that prefer an environmentally friendly approach to desalination. Their goal is to avoid harming marine life like more traditional ocean pipelines tend to.
The slant well will be drilled close to the coastline at a diagonal and collect enough ocean water to produce about 100 million gallons of drinkable water daily.
That’s what California American Water hopes, according to Rich Svindland, its vice president of engineering. California American Water is a subsidiary of American Water Works Company Inc., the largest publicly traded U.S. water and wastewater utility company. They proposed the idea after California ordered reductions to the Monterey Peninsula’s current water sources, a local river and aquifer that are expected to lose more than half of their current supply in the next decade.
“The idea is that we’re trying to launch a well field out under the ocean floor to basically ensure that we capture ocean water as opposed to inland ground freshwater,” Svindland said. The local groundwater basin he’s referring to is protected and cannot be exported to residents across the peninsula.
Slant Well Design
In an effort to ultimately build a permanent series of slant wells, California American Water has proposed the building of a test well, already designed by GEOSCIENCE. Svindland said a specialized drilling firm will be selected once it is approved and permitted.
The test pilot hole insertion point is expected to sit approximately 450 feet inland of mean sea level at an elevation of about 25 feet. It is estimated to be built at a 19 to 20 degree angle, be 1,000 feet long and get about 300 feet deep, roughly parallel to the ocean floor. Actual screen depth is more likely to be 250 feet deep, according to Svindland.
“We’re probably pushing the max as far as how large they can get,” he said. It’s slated to be about three times longer than the first slant well GEOSCIENCE designed eight years ago in Dana Point, Calif. He said the technology’s limit is about 1,000 feet, the length needed to grab the 16,000 gallons of ocean water per minute he’s hoping for.
The well will link to a single pipe, which will lead to a desalination plant. From there, the filtered water will travel through about 20 miles of pipeline to customers.
The engineers and residents have many questions regarding what the final project would involve and what harm it might bring to groundwater supplies. Svindland said running the proposed test well for up to two years will fill those gaps.
Building the Slant Well
“It’s a perfect situation” for digging based on borehole samples from the proposed site, Williams said. The results show sand, some gravel and small clay pockets.
“We just build a drilling cradle at whatever angle we want and place the dual rotary drill rig on it and drill the wells,” Williams explained.
To prevent the hole from collapsing, temporary casing will be installed around the production string using
a telescoping method. “So you’re going to have one casing that’s pushing, say, 100 feet and then say you have 36-inch casing,” Svindland said.
“And then you’re going to go inside that casing and you’re going to
push another slightly smaller diameter—say 34—so it fits right inside there tight.”
The technique will be used all the way down, resulting in a diameter as small as 141⁄8-inches, where the screen will be located. The larger screen on top to hold the pump is still being designed and will likely span 18 to 20 inches. “All this has to be done with high quality steels that are not going to be subject to corrosion from ocean water, so we’re talking super duplex steals,” Svindland said.
The louver won’t be cut for the screen openings until the hole is drilled and the grain sizes can be determined. Otherwise, they risk having a lot of sand coming inside the screen.
Since the well rests at such a low angle, gravity doesn’t help, so everything has to be pressurized, including the artificial filter pack. Removing the temporary casing without destroying the filter pack is another difficulty not associated with vertical wells.
The special equipment needed to push the casing down and then pull it out isn’t so easy to come by since slant wells aren’t so common. “Then they’re only of a certain size, where vertical stuff there’s a lot bigger pieces of equipment out there that can do a lot more things,” Svindland said. “So the market is not as fully developed as a traditional vertical well.”
Another symptom of the slant involves the pump and motor, typically designed to either be truly horizontal or truly vertical. The slant well is somewhere in between, so the bearing and parts that help transfer load for rotating elements must be designed more robustly. Svindland says this can be done, but how well the bearings will hold up over time is an area of concern.
Something else they must consider is coastal erosion, an issue at the proposed site. The slant well can’t be drilled right at the water’s edge, meaning the digging could be pushed as far as 700 feet inland. This could make it more difficult to reach as far out under the ocean floor as is necessary.
The number one issue, according to both Svindland and Williams, is permitting. They admit that public skepticism is a typical and healthy occurrence with such large undertakings, but he said that’s the reason for this test. “The biggest challenge from my perspective is just to get folks to acknowledge, ‘hey, this is a test well, not permanent.’”
Getting approval soon is crucial because it ties into another obstacle, their timeline, which is only getting tighter. “We’ve been trying to do this well for two years now and we keep getting hung up in different permitting areas,” Svindland said.
The number one issue, according to both Svindland and Williams, is permitting.
They are still on track to build the slant well from November to March. They are limited to building in that window because a threatened bird that inhabits the beach does not nest there during those months.
Svindland doesn’t want the desalination plant built until the test slant well is finished so they know the water can be pulled in. Pushing the well construction to next year would delay the building of the plant too.
Despite some public skepticism, Svindland said it feels good to see how far the proposal has come. “We’re certainly kind of glad to be where we are and hopefully we’ll have this one last final push this last month and a half and we’ll get the permission to go ahead.”
At a time when water shortage is increasingly rampant and environmental awareness is growing, Williams said he’s confident this innovation will serve the need for new water supplies in the Monterey Peninsula and elsewhere.
Looking forward, he said he sees slant wells becoming more common. “I think as time goes on you’re going to see this technology developed. California’s probably the leader in groundwater development technique, so you’ll probably see other sections of the country follow suit.”