Many companies are slow to adopt new technology. However, sometimes old technology used in new ways can boost production and cut costs. Underbalanced drilling (UBD) is not new technology. Air drilling was used in the Texas Austin Chalk region in 1978, with reports of the technique being used as early as the 1950s.

A September 1984 report by the Society of Petroleum Engineers stated that “using UBD is a sound and economical procedure with an average of [US] $500,000 being saved per well” and with “19 days being cut from the average drilling time per well.”

Shell Oil Co. uses UBD drilling and servicing on more than 80% of the wells it drills and estimates that UBD has the potential to improve well production by 800%. Thanks to the improved flow of oil and gas, fewer wells are needed to drain a reservoir, and the overall environmental footprint is smaller.

In China complex geological formations reduce drilling speeds. Using UBD has increased penetration rates by 2.6 times and cut the average drilling cycle from 138 days to 57.4 days. The yield from UBD horizontal wells is 3 to 8 times as much as vertical wells.

There are several benefits to using UBD:
• It eliminates formation damage routinely caused by using drilling mud and overbalanced drilling (OBD) and production techniques;
• It increases ROP with less pressure at the bottom of the wellbore, so it is easier for the drillbit to cut and remove rock;
• It maximizes hydrocarbon recovery since using OBD blocks the natural flow of oil and gas from the well. UBD allows hydrocarbons to flow freely. The well is producing as soon as the reservoir is penetrated; and
• Differential sticking is eliminated. Using OBD can cause drillpipe to stick. When using UBD, the pipe is pushed away from the wellbore walls, which eliminates differential sticking.

How UBD works

What is UBD and how does it boost production and save costs? UBD is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than the fluid pressure in the formation being drilled. As the well is being drilled, formation fluid flows into the wellbore and up to the surface. This alone enhances production because oil and gas flow begins immediately when the well is completed.

This is the opposite of the usual situation, where the wellbore is kept at a pressure above the formation to prevent formation fluid entering the well. In a conventional OBD well the invasion of fluid is considered a kick, and if the well is not shut in it can lead to a blowout – a dangerous situation.

In UBD there is a "rotating head" at the surface, essentially a seal that diverts produced fluids to a separator while allowing the drillstring to continue rotating.

If the formation pressure is relatively high, using a lower density mud will reduce the wellbore pressure below the pore pressure of the formation. Sometimes an inert gas is injected into the drilling mud to reduce its equivalent density and hence its hydrostatic pressure throughout the well depth.

This gas is commonly nitrogen since it is noncombustible and readily available. But air, reduced-oxygen air, processed flue gas, and natural gas have all been used in this fashion. The most common are:
• Dry air: This is also known as dusting. Air compressors combined with a booster (which takes the head from the compressors and increases the pressure of the air but does not increase the volume of air going downhole) are used, and the only fluid injected into the well is a small amount of oil to lubricate bit bearings or motor stators when downhole motors are used;
• Mist: A small amount of water and a foaming agent (soap) is added into the flow of air. Fine particles of water and foam in an atmosphere of air bring cuttings back to the surface;
• Foam: A larger amount of foaming agent is added into the flow. Bubbles and slugs of bubbles in an atmosphere of mist bring cuttings back to the surface;
• Stable foam: An even larger amount of foaming agent is added into the flow. This is the consistency of a shaving cream;
• Airlift: Slugs and bubbles of air form a matrix in water. Soap can or cannot be added into the fluid flow of air; and
• Aerated mud: Air or another gas is injected into the flow of drilling mud. Degassing units are required to remove air before it can be recirculated.

Drilling, workover services

Omni Air uses air and generated nitrogen foam-based UBD techniques to accelerate drilling and workover services while increasing production. Omni was originally created in 1978 and did air drilling. The company has since adopted the latest high-tech nitrogen UBD processes.

Nitrogen is inert, and it prevents downhole fires. It is not corrosive like mud and chemicals, and it can drive unrecovered hydrocarbons from an injector well to a producing well to boost or extend production.

In the past UBD was often more expensive, especially if nitrogen was used. Omni Air has lowered the cost by using onsite nitrogen membrane production units that provide unlimited quantities of nitrogen, eliminate transportation issues, produce nitrogen for half the cost of cryogenic units, and provide enhanced safety features.
High-tech control units also are used to select oxygen levels from 0.01% to 0.5%. These also control set points and include auto shutdown alarms that aren’t fixed and can monitor temperatures, liquid levels, and flows into and out of the well. Quality is assured by using nitrogen that is 95% to 99.9% pure, with a dewpoint of -80 and flow capacities ranging up to 2.8 cm/min (100 cf/min).

Newer air foam units deliver up to 42.5 cm/min (1,500 cf/min) at 2,500 psig. Air foam is ideal for sand cleanout, bridge plug drilling, fluid recovery, fishing, and deepening projects in lost circulation zones or pressure-depleted reservoirs. Air foam and N2 foam increase penetration rates while minimizing formation damage.
These types of drilling services are typically used in applications such as dust drilling, drilling with membrane nitrogen, mist drilling, aerated fluid drilling, foam drilling, percussion hammer drilling, directional drilling, and payzone drilling. Air drilling can increase drilling penetration rates by 5 to 7 times and eliminate lost circulation, differential sticking, and formation damage. Some customers have achieved drilling rates of up to 61 m (200 ft) per hour.

Fracing increases the need for UBD and underbalanced services. Fractured wells produced using OBD techniques naturally have sand and particles left in the well that reduce oil and gas flow and cause tubing to stick and pumps to fail. Using compressed air and nitrogen foam cleans out the well. Using UBD on existing wells has been proven to reduce maintenance issues and boost production.

UBD technology continues to evolve and improve. Often old proven technology can be updated and improved to produce amazing results. UBD is the perfect example.

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