A specially designed hybrid microhole coiled tubing rig recently concluded the drilling of 25 test wells to penetrate a particularly intractable natural gas formation called the Niobrara in western Kansas and eastern Colorado. The effort delivered cost savings of 25 percent to 35 percent per well drilled, compared with conventional drilling equipment.

As a result, about 1 trillion cubic feet of shallow gas that had been bypassed by conventional drilling now has been made economic. That volume equates to about 5 percent of America’s annual natural gas consumption. The research project was funded by the U.S. Department of Energy (DOE).

In 1994, Los Alamos National Laboratory advanced a concept for drilling deep, small-diameter holes for sensor deployment to conduct long-term monitoring. The idea quickly expanded to include exploration holes for formation logging, wireline or drill stem-deployed logging, and micro drill stem production testing. Subsequent to that, microholes began to be considered for production of shallow- and medium-depth gas and shallow oil in special situations.

Some of the specific advantages of microhole drilling:

• Equipment is smaller and weighs less than conventional systems, thereby reducing equipment costs and manpower to operate equipment.
  
  
• Materials required for drilling and well completion are reduced.
  
  
• Coiled tubing saves time and money because it requires fewer trips in and out of the wellbore than conventional drilling techniques.
  
  
• Volumes of drilling fluids and cuttings can be reduced by one-fifth, reducing disposal costs.
  
  
• Drill rigs and associated equipment have smaller footprints, reducing environmental impact, and making the system particularly advantageous when operating in environmentally sensitive areas.

Drilling holes as small as 1 inch in diameter is nothing new. Relatively deep holes with diameters as small as 1.175 inches have been drilled using mining coring rigs for at least 50 years. Small-diameter coiled tubing is readily available.

The technological challenge, as DOE describes it, is to develop an entire drilling system. Suitable drill bits and pipes are available today, but subsurface sensors, motors, logging tools and other borehole instruments small enough to fit into the micro-wellbores, and rugged enough to withstand the rigors of underground environment still must be developed.

The commercial Niobrara drilling program – in which 3,000-foot wells were drilled in as little as 19 hours, from move-in to move-out – followed a DOE-funded research project undertaken by the Gas Technology Institute (GTI). In that effort, GTI partnered with two firms – Advanced Drilling Technology LLC (ADT), Yuma, Colo., and Rosewood Resources Inc., Dallas – to adapt a conventional coiled tubing rig for drilling exploratory and development wells with ultra-small diameters.

The GTI project received funding from the DOE Microhole Technology Initiative. Managed by the Office of Fossil Energy’s National Energy Technology Laboratory, the initiative seeks to develop the tools and techniques for drilling ultra-small boreholes and related downhole micro-instrumentation using coiled tubing drilling rigs that are small and easily transportable.

GTI’s microhole project was meant to pioneer the use of an experimental, purpose-built coiled tubing rig designed to drill exploratory and development wells with ultra-small diameters in the lower 48 states. ADT and its predecessor, Coiled Tubing Solutions Inc., designed and fabricated the rig specifically for microhole coiled tubing drilling to depths as great as 5,000 feet. Earlier DOE research had proven this capability to only a few hundred feet.

GTI and its partners field-tested this state-of-the-art hybrid coiled tubing rig by drilling a few inexpensive microbore wells to about 1,000 feet to 1,400 feet in the Niobrara chalk formation along the Kansas-Colorado border. The results far exceeded expectations, with drilling cost savings averaging 38 percent.

The project’s initial success and strong commercial follow-up also demonstrated the potential for coiled tubing drilling. For the first time, a Canadian coiled tubing drilling company, Xtreme Coil Drilling of Calgary, Alberta, is drilling grassroots wells in the lower 48 for an American company, using its newly patented coiled tubing design to drill wells in the aging, marginally economic, shallow oilfields of Colorado’s Denver-Julesburg Basin. By the end of this year, Xtreme is expected to deploy its coiled tubing drilling to depths as great as 10,000 feet to 12,000 feet in deep natural gas fields.

This technology could be applied to bypassed resources in thousands of oil and natural gas reservoirs across the United States, particularly for shallow reservoirs in mature or even apparently depleted fields.
ND