December 2004 Supplement    Future Rig Designs

Microhole technology gets more serious

The idea that, when it comes to wellbore diameter, smaller is cheaper has been understood since drilling began. But only recently has the idea of drilling truly tiny holes (< 2-in. dia.) been seriously considered. The reduction in well fluids, cement and steel could result in cost savings as dramatic as 50%, depending on what is being compared. Small drilling footprints and less fluid volumes have beneficial environmental impacts as well. Early attempts to modify mining coring rigs to the task proved technically successful, but did not yield significant cost savings because drilling depths remained relatively shallow and infrastructure costs remained high. More recent efforts at reducing hole size – in the slimhole range of about 4 to 5 in. – while successful, have been met with lukewarm acceptance, but slowly growing acceptance nonetheless, Fig. 1.

Fig. 1. Hybrid slimhole or (derrick tilts back) coiled tubing drilling rig. This rig has drilled 4-3/4-in. hole, from surface, using 2-3/8-in. CT. A design for the future? Photo courtesy Coiled Tubing Solutions, Inc.

In 1994, Los Alamos National Laboratory (LANL) advanced the idea of using coiled tubing to drill deep, small holes with 1-3/8-in to 2-3/8-in. diameters, up to 10,000 ft, for exploration wells and reservoir monitoring (see World Oil, Oct. 1999. pg. 21). More recently, LANL has expanded the idea for production as well, for development drilling up to 5,000 ft, including reservoir monitoring.

A significant increase in ROP is not likely with microhole drilling, absent a breakthrough in drilling technology or technique, because just achieving a conventional rate is difficult. This is due to the narrowing of the annulus around the drillstem. Problems with pressure (including equivalent circulating density), low annular flow rates, drill-cutting particle-size and increases in required hydraulic power all act to defeat attempts to increase ROP. The researchers believe that the savings must be primarily derived from two areas: 1) reduced mass of the required system components and materials by an order of magnitude, as well as fabrication and mobilization costs by a lesser factor; and 2) reduced labor costs due to automation and decreases in size/ weight, with the handling of small, non-automated components by not more than two people.

Experience to date. So far, LANL scientists have drilled over 2,000 ft of microhole using a commercially available coiled tubing drilling system (Fig. 2). Wells 1-3/4- and 2-3/8-inch in diameter have been drilled to 700 ft.

Fig. 2. LANL coiled tubing microdrilling rig. Mud-cleaning system on left; logging/ data acquisition van on right.

As part of the effort, microhole logging tools and seismic sensors are being developed. This is one area that everyone agrees should not be, and has not been, a problem, given the advances in electronic miniaturization.

This summer, six new areas were awarded partial funding by US Department of Energy within the microhole project:

•  Schlumberger will develop and build a microhole coiled tubing drilling rig that is purpose-built for shallow oil and gas reservoirs. The aim is to improve the economics of shallow well drilling. 
•  Gas Production Specialties will develop a gas well artificial lift system that can unload downhole liquids which prevent wells from flowing by natural pressure. 
•  Stolar Research will develop radar navigation and radio data transmission for microhole coiled tubing BHAs. Radar will be used to determine the drillbit location. 
•  Baker Hughes Inteq will design and fabricate microhole drillbit steering and electrical resistivity measurement of the formation. Both the bit steering device and the motor will be 2-3/8-in. dia. to serve a 3-1/2-in. or smaller hole size. 
•  Western Well Tool’s role is to create a downhole tractor tool that transports the drillbit and BHA into long (3,000+ ft) horizontal well sections. By using this drilling tractor, costs are expected to be 25 – 50% less than conventional drilling methods. 
•  Bandera Petroleum Exploration has been given the task to develop a mud system compatible with a microhole coiled tubing drilling system. The mud system must be able to mix, circulate, clean and store the returned fluids, while in an underbalanced condition with zero discharge.

For microdrilling to be accepted by industry, especially by independent, small service companies, a host of new equipment – from cementing and perforating to logging tools and artificial lift pumps – will have to be developed at microhole sizes. LANL has the industry off to a good start.