Draining saturated ground

April 12th, 2012 | Posted by in T2W

Where the water table in the Bahamas is nearly at the surface, the way to evacuate rain water is a simple solution

Although the Bahamas are blessed with beautiful water, white and pink sand beaches and an abundance of sunshine, as a tropical island nation the islands do have spots of excessive rain. Land is little more than a meter or two above sea level in the Bahamas. Yet when it rains, water must be absorbed into the ground because the islands don’t have a storm drainage system. Drainage wells help displace water faster. For drilling contractor Bahamas Hot Mix (BHM), the Atlas Copco T2W is a good choice because of its ability to quickly drill and case these relatively shallow drainage wells.

In the past, drill rigs in the Bahamas didn’t need to have great depth or speed. Surface water is fresh, and most potable water for the islands comes from reverse osmosis well systems, from bottled water or from a 3-million gallon tanker that arrives daily. Within 20 feet of the surface, water becomes brackish, then totally salty by 30 feet.

“We drill 60 to 140 wells a year,” said BHM Technical Director Brian Davies. “We can do a well about every two days, or three a week.” This year the company is on schedule to drill 100 wells. But as Davies points out to drillers outside the country, they needn’t rush to paradise— the government protects such work for Bahamian companies.

Davies said they got into the work out of necessity. The company happens to be the chief supplier of asphalt to the islands and does much of the paving. Drainage wells are necessary in construction projects for roads as well as property development. In order to stay ahead of the projects BHM needed to increase its drilling capacity, which otherwise is done by cable rigs.

Driller Jimmy McAleenan shows that cuttings are not just stone, but large pieces of dead coarl and other remnants of an ancient seabed.

He said they bought the rig used and have been very happy with it. Service assistance comes from the U.S. Center of Excellence office in Milwaukee, Wisconsin, via phone with Technician Joel Kraft and Parts Manager Steve Matic.

Drill operator Jimmy McAleenen has also been happy with the used rig. “If I need something I can call Steve and he knows what I’m looking for most of the time without me even having to give him a part number. I just describe what I need.” This is important to the crew, who do all the maintenance themselves.

McAleenan said he was the shop maintenance foreman before becoming a self-taught-driller. “I needed the change and this is great for me.” He said they’ve never had a major problem with the rig, and at the most they were down just two days because they needed a part shipped from the United States. The problem in that instance was seals that gave out in the swivel because of the abrasiveness of the rock.

Currently BHM is four months into a 22-month project that includes drilling 60 drainage wells. The specifications for the wells require 150 feet of clean hole when complete. To get that, the crew drills a 170-foot bore hole, allowing for cave-in space during casing.

McAleenan said they never know what to expect in the formation and could get loose rock as easily as voids or caves. “No two wells are ever the same. A well right next to another could be totally different,” he said.

The rock is mostly oolitic limestone but they also see dolomite and some quartz. McAleenan says the most difficult material to drill through is clay-like sand. “The stuff looks almost like beach sand, but you can squeeze it in your hand and it’s sticky like clay.”

The sandy zones can be thin or thick and the only way to get through them is to pack it into the drill string and trip out, then clean the pipe. The zone is usually too thin to change drilling methods.

BHM has found the best way to drill the drainage holes in this formation is to use a reverse circulation method with both 4 ½- and 7-inch dual wall pipe.

The Atlas Copco T2W has a 750 cfm, 300 psi compressor, which provides more than enough air to raise the cuttings. The variation in the cuttings shows extremely hard, dark-colored limestone or crumbled, dead coral and seashells.

Cuttings from the discharge line are captured in a steel box so they can be hauled away or disposed of easily.

McAleenan has found that drilling in the limestone formation works best when rotating at about 80 to 200 rpm and feed pressure of 1,000 psi. They use a tricone bit with ballistic carbide tips and get pretty aggressive cuttings. To use this tricone bit McAleenan welded steel across the air opening, choking down the air flow. “We have to cut back have to slow feed pressure or I will choke up the pipe,” said McAleenan.

“We are very happy with the T2W and this way of drilling. It takes about seven minutes to change pipe, which I think is pretty good,” said McAleenan. Because each hole is different, it’s too difficult to estimate penetration rates. One day a hole may be completed in seven hours, but if voids or sandy clay-like material are encountered it takes much longer.

McAleenan said he likes the fuel economy of the T2W. When the rig is working hard it consumes 12 gallons per hour, but most of the time it uses 10.

It’s necessary to case the top 60 feet of hole to finish a drainage well. For that they use 10-inch schedule 40 casing in the hole. That takes care of the top 60-feet, while the bottom 9 7/8-inch hole will remain open. A collection box for water runoff will ultimately be built over each open well.

To finish a well in the Bahamas there is no perfect formula to cover every case. Each hole is unique and different. But as Technical Director Davies puts it, “We like challenges in the Bahamas.”

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