Pro Petro Services, in Midland, Texas, found it saved as much as five to six working days per well by mixing up its fleet — using a smaller rig to drill the pre-sets and then a larger rig to finish the work.

Oil and gas wells are drilled in segments, each requiring different rig capacities. Drilling contractors can reduce operating costs by adopting a “mixed fleet” approach to get the maximum performance from equipment in each segment of the drilling process. Mixed fleets use lightweight, hydraulic top-drive rigs to perform pre-set casing work and drill shallow exploration and production wells. Then a larger rig can move on location and immediately begin drilling the deeper segments of the well.

Pro Petro Services has been using an Atlas Copco RD20 rig to drill pre-sets for larger rigs.

“We typically drill 200 feet to 1,500 feet cased at 13-3/8 inches, and 1,500 feet to 3,500 feet cased at 8-5/8 or 9-5/8 inches,” said Operations Manager W.D. Martin.

These wells are typically drilled with air, using the rig’s onboard compressor with an auxiliary compressor and a booster.

“Prior to using the Atlas Copco RD20, our conventional rigs were drilling down to the bird’s nest zone and losing 80 percent of their water or mud circulation pressure,” Martin said.

Using high-pressure air, the RD20 drills through the bird’s nest zone without losing circulation. When the surface and intermediate holes are cased and cemented, the conventional rigs come in to drill the wells to 6,500 to 8,000 ft. (1,981 to 2,438 m) total depth. Martin said his company has saved five to six days per well and drilled 10 to 15 more wells per year since using the mixed fleet approach.

Using lighter weight, mobile drills in the 700- to 900-hp (522 to 671 kW) class, contractors are able to drill and set surface with four to six people, in one to four hours, which is less time and fewer personnel than rigging up a conventional rig. Simple air drilling locations are usually turning to the right in one to two hours with a single crew. Pre-set rigs can set Range III casing at about the same rate as a conventional rig.

Today’s lightweight rigs handle 30- or 40-ft. (9 to 12 m) drill pipe and can be adapted to a wide range of pipe handling equipment. Lightweight rigs can rig down and move off location in one to four hours, including the related equipment.

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