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 microdrill-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 are:

• Equipment is smaller (microdrilling systems could occupy a space roughly 1/20th that of a typical rig) and weighs less (less than one tenth as much) as conventional systems reducing equipment costs (by up to 90 percent) 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 not new. Relatively deep holes with diameters as small as 1.175-inch 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 must still be developed.

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

Bore size illustration compares conventional bottom hole well diameters to slimhole and microhole sizes to emphasize the potential for materials and equipment size reduction through the use of microholes. Click to enlarge. Source: Los Alamos

The GTI project received funding from the Energy Department’s 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 ultrasmall boreholes (generally, 1-3/4 to 4-1/2 inches in diameter) 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 ultrasmall 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 Energy Department research had proven this capability to only a few hundred feet.

GTI and partners field tested this state-of-the-art hybrid coiled tubing rig by drilling a few inexpensive microbore wells to about 1,200-1,400 feet in the Niobrara chalk formation along the Kansas-Colorado border. The results far exceeded expectations, with drilling cost savings averaging 38%.

The project’s initial success and strong commercial follow-up also demonstrated the potential for coiled tubing drilling of exploration and development wells in the Lower 48. For the first time, a Canadian coiled tubing drilling company, Xtreme Coil Drilling Corp., Calgary, AB, Canada, is drilling grassroots wells in the Lower 48 for an American company, Anadarko Petroleum Corp., The Woodlands, Texas. Xtreme Coil is 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 Coil also is expected to deploy its coiled tubing drilling at depths as great as 10,000-12,000 feet in deep natural gas fields such as Pinedale and Jonah in Colorado's Green River Basin for another operator, Denver, CO-based EnCana Oil & Gas (USA), Inc.

Microhole coiled tubing drilling technology could be applied to bypassed resources in thousands of oil and natural gas reservoirs across the US, particularly for shallow reservoirs in mature or even apparently depleted fields. The Energy Department estimates the volume of bypassed oil in US oilfields at less than 5,000 feet subsurface at more than 218 billion barrels. Recovering just 10 percent of this targeted untapped resource equates to an amount equal to 10 years of OPEC oil imports at current rates.

Resources:

• Road Map for a 5,000-Ft Microborhole

• Technologies for Tomorrow’s E&P Paradigms: Microhole Technology

• DOE Microhole Systems R&D