Mixing Oil and Water

Rig crews from oil patch help water well contractor bring drought relief to California farms.

By Joe Bradfield

All along California’s Central Valley, water well drillers have been racing to keep pace with demand and getting rigs capable of performing the task.

Atlas Copco salesman Joe Beloso got an inquiry from Scott Belknap, owner of Scott Belknap Well Drilling in Dinuba, California, about trying an Atlas Copco RD20 for water well drilling. Belknap’s customers needed up to 28-inchdiameter wells drilled to depths as great as 1200 feet in complex soil environments.

The mobile, self-contained deep hole RD20 rig is engineered with 120,000 pounds of pullback capability for the oilfield, but compact enough for agricultural and municipal water wells. Belknap already operated Atlas Copco T2W, T3W, and TH60 water well rigs.

Beloso called his colleagues Tom Moffitt, Atlas Copco business line manager of deep hole rigs, and Ray Kranzusch, product manager for oil and gas drill rigs. Kranzusch, who works closely with RD20 top hole contractors in the oil and gas drilling industry, recognized a mutually beneficial solution and introduced the Belknap group to an RD20 crew freed up by the slump in oil drilling.

Although RD20s were developed for presetting casing in oil and gas projects, Kranzusch says their use in other applications is not unusual, ranging from ventilation shaft drilling and creating grouted pilings, as well as drilling water wells.

Belknap’s crews were able to put the RD20 to work immediately without the usual commissioning and training period required for a new, unfamiliar rig. For the RD20 crew, they would be making the transition to reverse circulation technique while working under an established water well contractor. The Belknap family has been working in California agriculture for almost 100 years. They have a close relationship with the land and with their customers. They are an integral part of the region’s business community.

Belknap says his history in the area helps him do his job better. “We don’t need a map to tell us 7 miles that way, you’ll be drilling in ‘Old Faithful,’ getting 1000 gallons per minute, but 10 miles this way, you’ll be lucky to see 20 gpm.”

The Belknaps are also farmers, so they identify with their customers’ water needs.

Reverse Circulation

Scottie Belknap, a fourth generation well driller who manages the company’s drilling operations with his father Scott Belknap and mother Marilyn Belknap.
Scottie Belknap, a fourth generation well driller who manages the company’s drilling operations with his father Scott Belknap and mother Marilyn Belknap.

The switch from drilling in the oil patch to drilling water wells didn’t require a major change in tooling or technique, and Kranzusch recommended adapting the RD20 for reverse circulation.

First designed for exploration using air rotary in the 1970s, a burst of tooling innovations made reverse circulation possible for nearly every kind of drilling in any kind of drilling conditions. In some areas of California, it means using rotary mud in the first 50 to 150 feet of unconsolidated sediment through granite bedrock at its bottom.

Kranzusch explains the benefits of using reverse circulation (RC) for water wells vs. conventional mud drilling.

“When you drill for oil using what we call ‘direct drilling technique,’ you actually want mud to cake the hole walls,” he says. “It helps maintain the hole walls as fluid and cuttings flush up the annulus until you can case it off from the aquifer to isolate it from the bore. Reverse circulation lets you drill in an aquifer without reducing the inflow of water. This keeps a clean and undisturbed borehole as desired for water wells.”

The group worked together to analyze the target formation geology, the drilling methods, and the well design objectives to establish the optimum method and process for completing the highest quality and performing wells in the region.

As for maintaining wall stability, RC can actually be more effective in certain formations than conventional mud drilling. Kranzusch explained that is because cuttings go up the pipe, not against the borehole wall. This also can lead to less time spent on developing the well.

Fluid return filled with cuttings is a prime cause of hole wall deterioration. But the drilling fluid in an RC situation has no cuttings in it. It is merely present in the annulus, migrating toward the bottom of the hole where it will be siphoned up the pipe, so there is also much less agitation.

Siphoning is assisted by injecting air at the hole bottom. The air reduces fluid density in the pipe. The difference in hydrostatic pressure between the fluid column outside the pipe and the less dense fluid inside it causes the fluid to percolate up the pipe, carrying cuttings with it.

Air Injection

The most common way to introduce air is dual wall RC pipe, available in a variety of sizes indicated in its naming convention. For instance, 7×4 dual wall pipe consists of a 4-inch-diameter tube inside a 7-inch-diameter drill pipe.

Cobble in the formation can be a problem for RC pipe, Kranzusch says, but the crews haven’t run into much cobble here.

“The Valley has some areas of really hard clay within 5 feet or so of the surface—but with the RD20 we drill right through them.”

Kranzusch says RC drillers using dual wall pipe here do not have much trouble with clay balling up on the bit and plugging the returns, a problem encountered by some drillers using a conventional flooded reverse system.

The capabilities of the RD20 coupled with an experienced drilling crew allow the company to provide a level of well design and completed performance for the California agricultural water market.

Those who drill deeper holes as Belknap’s customer required here would also typically run into trouble with cutting evacuation.

“A thousand feet or more of rock cuttings is a lot of stacked weight for RC to hold up,” Kranzusch says. “Chips begin to fall back on themselves, pile together, making heavier masses. Then they choke off circulation.”

But Kranzusch had some ideas how to overcome clay and move cuttings in the deepest holes. He consulted with engineers, Belknap, and industry drilling experts to refine the tool string. The modifications successfully eliminated the clay and cuttings-weight issues.

The drill string adaptations also sped things up. Kranzusch was on hand as one of the crews drilled an initial 17½-inchwide bore to a depth of 1200 feet and then opened it to 28 inches. Time from start to finish was less than 40 hours.

By comparison, conventionally drilled wells here at that diameter and depth take 60 to 70 hours—when drill strings don’t clog, and if they don’t have to sit on the hole in development for several days to get formation permeability back.

Flooded RC

The technique Belknap is using is actually a variation of RC called flooded reverse circulation. It’s a “closed-loop” system, in which the rig recirculates drilling fluid without the need for a mud pump or shaker box.

It begins with excavation of a U-shaped pit capable of holding 20,000 to 30,000 gallons of water. One side is dug as a narrow, shallow channel to serve as an intake trough. The deeper side of the pit is for collecting fluid as it returns from the hole. Cuttings settle out to the bottom. Clear water nearer the surface of the pit migrates to the intake trough, where it flows back to the wellbore again.

Turning Down the Air

An RD20 rig’s 1250 cfm at 350 psi (590 L/s) compressor package greatly exceeds requirements in this application. So Kranzusch instructed the driller how to lower the air compressor’s output flow to 600 cfm (283 L/s) and turn the pressure down, running it at 150 to 170 psi (10 to 12 bar).

“That means now they’re also saving a lot of fuel while drilling these holes,” Kranzusch says.

After the success Belknap had with flooded RC on their job sites with the RD20 rig, they have incorporated the method into drilling with one of their TH60s.

One particular example had them using a TH60 DH with 5½-inch Matrix RC drill pipe for a 17½-inch hole to a depth of 400 feet. Their TH60 DH (deep hole) is equipped with 70,000-pound pullback and 37-foot 6-inch tower to support deeper drilling holes up to 20 inches diameter.

Belknap has been happy with how the smaller footprint of the TH60 works around established trees in orchards. The cooperation between an oilfield contractor and a water well driller was good for the agricultural customers in California’s driest areas and more drillers have shown an interest in flooded RC drilling.

“With the urgent need and high demand, Belknap has shown they can roll with the changes,” Kranzusch says. “They’ve adapted in a way that better serves their customers. They led the way for two different drill rigs to use this method of flooded RC drilling.”

Joe Bradfield is senior writer of Ellenbecker Communications, an international communications firm specializing in the construction, mining, and drilling industries.