Nitrogen freezes unconsolidated formations; modifications minimal for existing drilling units
Leonard Le Blanc
- Nitrogen gas is inert and acceptable to the environment.
- Little waste and contaminated mud is left over for disposal.
- Rig modification is minimal.
- Full formation permeability is restored after thawing.
- Contamination between formations is halted by temporary formation impermeability.
- Bit balling in clay is eliminated.
- Open hole completions are possible.
Developers hope to use the process more effectively in high angle and horizontal oil and gas wells where consolidation problems are extensive. The "freeze while drilling" or cryo-drilling technique stabilizes the borehole wall and offers other advantages over conventional fluids such as low environmental impact, restoration of permeability, and minimum rigsite waste.
Another advantage is that the entire shallow section of a hydrocarbon well, where highly permeable formations are most often encountered, could be drilled with the cryogenic process. While the borehole walls are still frozen, casing could be run into the hole and cemented before active thawing begins. Thereafter, the driller would switch to conventional fluid systems.
The cryo-drilling method was developed by the Lawrence Berkeley National Laboratory (Berkeley, California) and the University of California-Berkeley. The method has been used to stabilize soil excavation sites for decades and only recently employed in environmental remediation.
Thus far, the developers have tested the process only in water wells of less than 100 ft in depth, and are unable to confirm the impact on a borehole and formation in deeper wells. Although attractive, there remain many questions about the process:
- Would the nitrogen temperatures in the borehole remain cold enough to freeze formations at depths up to 1,000 ft, which is the present limit of shallow water flows?
- Is the cryo-drilling process ideal for tophole only, or can it activated periodically when highly permeable formations are encountered deeper?
- Would 2,000-4,000 gallon nitrogen containers with a vaporizer be sufficient onsite to drill shallow permeable formations, or would it be necessary to station a nitrogen generator there?
- How do the costs of cryo-drilling an entire shallow section of an oil and gas well compare with present methods of using specialized drilling fluids and foamed cements to drill and case permeable or unconsolidated formations?
Despite cryo-drilling's attractions, the process has recognized drawbacks at the rigsite.
Nitrogen must be handled with the greatest degree of safety at the rigsite. A nitrogen spill can fracture thick steel decking and frame members, as occurred in one accident where nitrogen was being used in coiled tubing operations offshore.
In the borehole, conventional carbon steel drill pipe can experience brittle failure or fracture when exposed to high impact loads at cold temperatures. Metals with face-centered cubic structures - brass, bronze, copper, and stainless steel - remain ductile at low temperatures, unlike carbon steels, which have a body-centered cubic crystal structure.
Bronze is too soft for drill pipe and stainless steel threads frequently gall. Thread lubricants do not work well at low temperatures. The developers recommend using beryllium copper connectors between the drill pipe sections.
In addition to metal failure problems, rubber seals become brittle, crack and experience accelerated wear at nitrogen temperatures. The developers recommend use of graphite/PTFE chevron packings as sealing materials.
The process has been tried by Berkeley researchers on a mobile drilling unit with top drive used to drill water wells. The nitrogen was introduced to the drill string at the swivel point. The swivel and 2 5/8-in. ID drill string were made of 316 stainless steel joined together with Beryllium copper thread connectors using square threads. A conventional tungsten carbide insert 3-cone roller drill bit was used to drill the well.
Cuttings were returned through the annulus and collected through a stainless steel 10-in. diverter, and routed to the atmosphere through a steel hose. The driller was protected from the intense cold around the drill string and annulus return.
The nitrogen was supplied by a two-in. hose from a truck-mounted mobile vaporizer and pumping tanker unit capable of injecting nitrogen gas at a flow rate of 9,000 cu ft/minute with a pressure of 10,000 psi. The tanker had a capacity of 2,000 gallons, with an additional 5,000 gallons available through a standby tanker. In-line monitoring equipment kept track of the flow and pressures.
The site in which the equipment was tested had sand stringers with interbedded boulders and rocks. The ideal rate of flow for the gas was 1,000 cu ft/minute at 200 psi and -200 degrees F.
The drill test experienced failures with the swivel, which was not designed for high vibration and thrust, by the loss of cones from the bit, which also was not designed for rough drilling, and by annulus plugup with large rocks. Failure of the bit cones could have been attributed to the cold nitrogen, but there was apparently no way to identify the express cause.
The developers reported no problems with the nitrogen injection process. No safety hazards were identified and the drillers were able to use the equipment after some initial delays in stabilizing the injection rate.
The nitrogen use rate was double that anticipated because of the large lifting force required to clear the annulus. The nitrogen flow had to lift cuttings and rocks up to 2 1/2-in. in diameter. The developers estimated the nitrogen cost of the shallow onshore at $8-20/ft for sandy soil with rocks and as low as $3/ft when drilling in sand. The beryllium copper pipe joint connectors worked much better than the stainless steel connectors and showed no signs of galling.
The cryo-drilling process is now being used for environmental cleanup and geotechnical investigation. Next on the list for the developers of the process is to use it on high angle and horizontal wells and formations with high permeability and water influx in order to test the practical limits of the process.
Several large offshore producers have expressed interest in the process, however there are reservations about having so much liquid nitrogen on an offshore structure.
Reference:Pippin, C., Simon, R., Cooper, G, "A Successful Borehole Drilled by Cryogenic Drilling in an Arid, Unconsolidated Soil with Boulders," Energy Week - ASME, January, 1997.
Copyright 1997 Oil & Gas Journal. All Rights Reserved.