Microhole coiled tubing drilling re-energizes a mature gas play in Kansas and Colorado.

Natural gas was first discovered in the Niobrara formation in 1912. Since then, the Niobrara gas play has seen several episodes of activity driven by gas prices and improvements in technology. Recently, microhole coiled tubing (CT) drilling has helped reenergize activity in this mature gas play.
The current play extends through Southwestern Nebraska, Northwest Kansas and Eastern Colorado. Gas-bearing chalk of the upper Cretaceous Niobrara formation is encountered at depths from 1,000 ft to 3,000 ft (305 m to 915 m). Gas accumulations in the Niobrara formation generally are related to low relief structural features found along the eastern margins of the Denver geologic basin.
Niobrara gas fields are characterized by high porosity, low permeability and low reservoir pressure. At greater depth, porosity and permeability decrease causing a reduced total pore volume and higher water saturation at a given structural position. Reported values for porosity in the Niobrara formation range from 30% to 50%, with lower values found at greater depths. Despite the high porosity of the chalk, permeability is inherently low because of the fine grain size. Values for permeability range from 0.01 to 0.3 md. The Niobrara is an underpressured gas reservoir with geostatic pressure gradient ranges from 0.06 to 0.24 psi/ft. In the Goodland, Kansas area, at a depth of 1,000 ft, the pressure is only 50 to 60 psi.
Thin payzones (sometimes near water), low reservoir pressures and low in-situ formation permeability (requiring wells be hydraulically fractured) combine to create a challenging environment for successful field development. Certainly, an efficient low cost approach to well drilling and completion is needed.
DOE microhole drilling program
The United States Department of Energy (DOE) is implementing a research program to develop marginal oil and gas resources using microhole well bores. The overall approach is to develop a portfolio of tools and techniques that will allow the drilling of 35?8-in. holes and smaller, which enables the development of marginal oil and gas resources through better economics. The field testing and demonstration of a "fit for purpose" CT drilling rig is one project within the program. The objective is to measure and document the rig performance under actual drilling conditions.
The CT drilling rig is trailer-mounted, with the coil and derrick combined to a single unit. The rig has been operating for approximately 1 year drilling shallow gas wells in western Kansas and eastern Colorado. Rig operations have improved to the point where it now drills 3,100-ft (945.5-m) wells in a single day. Cost savings of approximately 30% over conventional rotary well drilling have been documented. Well performance is improved because of less formation damage as
a result of minimizing formation exposure to drilling fluid through fast drilling.
Efficient rig mobilization
The rig moves with four trailer loads, reducing mobilization and transportation cost while meeting US Department of Transportation limitations for highway transport. This gives access to smaller roads and remote well locations. The rig contains all the equipment needed for drilling operations, including a zero-discharge mud system. It has pipe-handling capacity for casing up to 75?8-in. and can support a rotary and top drive.
Small environmental footprint
The small size of the rig provides several environmental advantages:
• A small drilling pad (1/10th acre) or no pad under some conditions;
• No mud pit. Tanks contain the required drilling fluids and are moved with the rig;
• Reduced air emissions and low noise engines;
• Smaller volume of fluids to be treated and drill cuttings; and
• Continuous length CT mitigates the risk of connection spills.
Rapid drilling
Very high rates of penetration (ROP) have been achieved with bit and downhole motor combinations and by using the advantages of CT drilling. ROPs as high as 500 ft/hr (152.5 m/hr) have been realized, with an average ROP per well of approximately 400 ft/hr (122 m/hr). This performance and other rig efficiencies allowed the drilling of a 2,850-ft (869.3-m) well in 19 hours including all rig moving time, logging, casing setting and cementing (Figure 1).
Good hole quality and cement
All the wells drilled have resulted in a gauge hole with very little hole deviation (1° to 2°, well within requirements) despite high ROP. Good cement job quality and well-bonded cement also derive from the gauge hole. The Niobrara is an under-pressured reservoir susceptible to formation damage because of fluid loss from drilling operations. Both the rapid penetration rate through the pay zone and the lack of pressure surges caused by conventional drillpipe connections help to mitigate fluid loss and formation damage.
No auxiliary equipment required
No auxiliary equipment is required to run casing, log wells or handle drill collars and bottomhole drilling assemblies. With its derrick, traveling block and rotary table components, all required drilling processes can be performed without additional equipment. While not currently equipped with a top drive, the rig can accommodate one if needed. CT eliminates connection time, and allows a smaller crew.
Zero-discharge capable
The rig has the capability to drill a well with zero discharge of any fluid or other materials if required. The procedure is as follows:
• Move the rig in and rig up on a sealed/booted tarp to contain any overflow or accidental spill.
• No earthen pits are prepared; all cuttings and drilling fluid are confined to tanks.
• A hole is augured for conductor pipe and a boot is placed around the conductor pipe.
Using this process, the ground is protected from any inadvertent spills and all fluids and cuttings are removed from the location. While obviously an added expense, this procedure may be required for drilling in sensitive environmental areas. The small size, efficiency and zero discharge capability enables drilling in sensitive areas.
Improved safety
The ADT CT rig significantly reduces drill pipe handling and has less equipment to mobilize. This creates a much safer operating environment.
Microhole CT drilling concerns
Operators have identified the following concerns that must be addressed:
• The ability to workover wells;
• Handling significant fluids in small boreholes;
• Limited space for downhole equipment;
• Lack of experience and familiarity with microhole and CT drilling;
• Depth limited by current coil metallurgy and coiled-tubing procedures; and
• Limitations of CT in overcoming drilling problems.
Technology development
Today, the evolution of lateral and horizontal drilling technology is beginning to allow the development of unconventional resources through the placement of smaller (microhole) well bores into exactly the area and location required for optimum production. Hydraulic fracturing remains an important and necessary well stimulation procedure, but is being done in a highly optimized manner, integrated with unique well completion procedures. Figure 4 illustrates the evolution of these technologies over the past 40 years.
The trend overall has been from large to small. Hydraulic fracture jobs pumped today are significantly smaller in size, but more effective than those in the past. Microhole technology will enable efficient placement of well bores while minimizing the surface and other environmental impact. The evolution of "fishbone" well drilling patterns and the ability to identify, drill, and produce very thin pay zones all add to the "lighter and smaller" and more efficient approach.

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