Higher Recoveries Are Pushing More Drilling

(Tech Connections Column, August 2001, American Oil and Gas Reporter)

As industry thinking evolves on how much conventional overbalanced drilling/completion operations restrict reservoir productivity, even with modern stimulation technologies, the drive toward underbalanced drilling and completion continues.

Applications in Canada and the United States dominate the underbalanced drilling market. Of the two driving forces—improving production and reducing costs—the potential for improving production is the more significant. With the greater formation exposure in horizontal wells, underbalanced drilling is logically considered most there. With proper equipment and training, underbalanced drilling in and of itself is not inherently dangerous. But all involved need to understand the equipment and operational procedures.
There are conditions where underbalanced drilling is not appropriate–for the most part through shale sections, with exceptions in some areas like the Eastern Devonian Shale; salt sections; shattered coal sections; or highly-unconsolidated sands/chalks. Accordingly, uphole sections may be drilled overbalanced, then switched to underbalanced conditions for the target formation(s).

Primary reasons for drilling underbalanced are stopping lost circulation, increasing drill rate, and limiting reservoir damage. As one moves into the underbalanced regime, drilling rates increase sharply, up to the point of bit flounder. The real advantage to protecting the reservoir comes with open-hole completions. Other potential reasons for drilling underbalanced include avoiding differential sticking, reducing/eliminating stimulation costs, and finding overlooked reservoirs.

Underbalanced drilling systems vary from liquid drilling with light muds/fluids, through aerated, to foam and foam/mist, and air or gas drilling. Densities decrease as one moves from light muds/fluids to air or gas drilling. A cardinal rule for all underbalanced drilling is to keep it underbalanced, since short-term excursions into overbalance can negate all of what one is trying to achieve. Therefore, the margin of underbalance (how many psi underbalanced) will vary depending on the stability of the underbalanced drilling system.
Aerated systems with gas/liquid ratios varying from 10-to-1 to 50-to-1 are quite simple and flexible, but because they are simple mixtures of gas and fluids, pressure control/gas surging can be a problem. Recognizing this, and wanting to ensure one always stays underbalanced, the margin of safety for aerated systems is typically larger than for more stable systems, like foams.

Conventional mud-pulse measurement while drilling systems can’t be used with gas injected down the drill pipe, so steering tools and the more costly/exotic electronic measurement systems are employed. With gas injection in the drill string, it is common to place a jet sub at 3,000 feet for control. Gas injection through a parasite string is another option, but it is costly and time consuming.

Foams, which are progressing toward very low gradients, are structured blends of gas in fluid. Foam quality, or the percentage of gas in the mixture, can be as high as 98 percent. Being structured, foams are quite stable, so surging or heading is not a problem. Since foams are structured, they exhibit very high carrying capacity. However, the structured aspect causes frictional pressure losses with foams to be very high. Since the gas is “locked” in the foam, corrosion problems are less than with aerated systems. Foams exhibit some sensitivity to hydrocarbons, so large inflows of hydrocarbons can destabilize them. Temperature limits of current foams, about 180 degrees F, restrict foam’s use to depths less than 12,000 feet.

Choices for the gas phase in underbalanced drilling include air, nitrogen and natural gas. When natural gas is available, and can be recovered and reinjected into the supply/sales line, underbalanced drilling can be quite cost effective. But drilling with natural gas is often not an option. While drilling with air injection is common, corrosion problems can be quite severe, and the potential for downhole fires always exists. For that reason, nitrogen is becoming the gas of choice for underbalanced drilling.

Although cryogenic nitrogen of high purity can be purchased, economics and logistics direct one toward generating nitrogen. The source for onsite-generated nitrogen is primarily the membrane unit, although a few exhaust gas units are available in Canada. Membrane units typically supply 95 percent nitrogen. At this purity, corrosion, although still requiring special treating, can normally be managed, and downhole fires are not a problem. Modern membrane units generate one part nitrogen for two parts air (i.e., 3 Mcf of air will generate 1.5 Mcf of nitrogen).

Major components of a membrane unit include air compressors, the membrane unit itself, and a booster compressor to increase pressure to required injection/drilling pressures. Units are now available in a range of capacities. Nitrogen cost is a function of feed-air pressure, desired nitrogen pressure and purity, and capacity. A typical 1.5 Mcf unit on duty 24 hours a day can provide nitrogen for $2.20 an Mcf. 

If underbalanced drilling is appropriate, make sure that completion operations also stay underbalanced, or you may well negate what you are trying to achieve through drilling underbalanced.

Editor’s Note: These insights were shared during a workshop on underbalanced drilling held July 11 in Norman, Ok., by Bill Rehm, Maurer Technology Inc.