Underbalanced Drilling

Underbalanced drilling (also called Managed Pressure drilling) is the term most commonly used when drilling through hydrocarbon-bearing payzones with a circulating system that exerts a hydrostatic pressure less than or equal to the reservoir pressure. Depending on the application, the circulating system can be identical to that used for gas (air, nitrogen, natural gas), mist, foam, or an aerated or nitrified fluid drilling application. However, considerable precautions need to be taken since the underbalanced down hole conditions will result in flows of oil and/or gas into the wellbore.

Underbalanced drilling is gaining increased popularity as operators realize how wells drilled underbalanced result in a marked reduction or even elimination of formation damage. This increases the production from these wells, resulting in faster payback and earlier positive cash flow.

3600 SCFM Air Drilling Package

Types of Underbalanced Drilling

Dust Drilling is a technology that began in the United States in the 1930s. The primary purpose for air drilling is to reduce hydrostatic pressure in the wellbore. The technology is limited to use in mature sedimentary basins that have consolidated hard rock formations with little to no remaining formation water. High volume / low pressure air replaces conventional mud systems and circulates the wellbore clean of cuttings. Air volumes are determined by hole size, depth and penetration rates. Drilling with only air is commonly referred to as "dusting" because of the dust cloud that is made from the finely crushed cuttings that are carried out of the wellbore through the return flow line also called the "blooey line." The average air densities are equivalent to mud weights of 0.01 – 0.1 ppg.

Advantages of dusting are:

  • Increased penetration rates. Assessments of gas technology indicate that rates of penetration can be increased as much as 10 times over conventional mud systems, depending on the type of bits used. Dusting is the circulation medium that maximizes penetration rates because of specific gravities of close to zero.
  • Elimination of lost circulation when drilling into low pressure formations. Because of the low specific gravity of air, cuttings and fluids are carried out of the wellbore and injected into the formations.
  • Minimized formation damage. Because air drilling technology has virtually no hydrostatic pressure, cuttings are less likely to be injected into formations.
  • Prolonged bit life. Because of minimal hydrostatic pressure on the formation, the bits teeth can fragment the rock formations easier. Air also allows the cuttings to become entrained much quicker because the bit is not regrinding cuttings that are already fragmented.
  • Provides a continuous well test when drilling into gas zones; a real time indication of contact with the pay zone.
  • Benefits the environment. Air drilling eliminates potential pollution of drilling mud to the environment during and after drilling.

Disadvantages of dusting are:

  • Downhole fires. When drilling with dry air into formations with hydrocarbons, the risk of downhole fires increases dramatically.

Air (Dust) Drilling Layout

Drilling with membrane nitrogen is a technology that became necessary to eliminate the risk of downhole fires or explosions when drilling with air. Studies show the efficiency of nitrogen membrane units is approximately 50%. So if 1500 scfm of air is supplied to membrane unit the output will be approximately 750 scfm of 95% to 99% pure nitrogen product. The oxygen content is well below the level needed to support fire or explosions. Because of the low oxygen content, corrosion is minimal.

Advantages of drilling with membrane nitrogen are:

  • The same as with conventional air drilling except the risk for downhole fires or explosions are eliminated and corrosion is minimal.
  • The nitrogen membrane unit generates its own nitrogen on the location, which is ideal for remote locations where logistics is a concern. Cryogenic nitrogen has to be delivered in trucks. Nitrogen membrane production typically costs 50% less than cryogenic nitrogen.

Mist drilling is used when liquid influxes occur when drilling with air. Air volumes are increased and a small Triplex pump (mist pump) is utilized to inject small volumes of water, foamer and corrosion inhibitor into the air stream. The reason for this is to assure that the water and the cuttings will be carried out of the wellbore without causing a mud ring. The average densities equivalent to mud weights of mist vary from 0.1 to 0.3 ppg depending on gas volume, surfactants and liquid volumes. Air/mist also reduces the risk of downhole fires dramatically while drilling into hydrocarbons.

Advantages of mist drilling are:

  • The same as for dust drilling except penetration rates are decreased but still exceed that of drilling with conventional mud systems.
  • The risk of downhole fires are greatly reduced with mist drilling.
  • Formation damage is minimized since the lack of hydrostatic pressure prevents rock cuttings from being injected into the rock matrix.

Mist or Foam Drilling Layout

Aerated fluid drilling is used when high volumes of water influxes occur while air drilling and circulation cannot be established with conventional mud systems. Small amounts of air and foamer are injected into the drilling fluid to reduce the mud or fluid weight so that circulation can be established. The volume of air needed is determined by the injection rate of the mud pumps, weight of the drilling fluid and the depth of the hole. The average densities equivalent to mud weights of aerated fluids varies from 4 to 7 ppg depending on gas volume, pressure and temperature.

Advantages of aerated fluid drilling are:

  • Lost circulation is minimized or eliminated.
  • Penetration rates are high compared to fluid drilling.
  • Water hauling costs are reduced.
  • Formation damage is reduced dramatically.

Aerated Fluid Drilling Layout

Foam drilling is used when drilling large holes in low pressure formations where circulation cannot be established. Surfactants are mixed with water and injected from the mist pump into the air stream to generate a foaming solution with a consistency that resembles shaving cream. The cuttings are entrained in this solution and carried out of the wellbore. The cleaning capabilities of this foam are far superior to other drilling fluids. Foam quality is closely monitored and adjusted by fluid/gas ratio and with a back pressure valve on the Blooey Line. Foam density varies from 0.3 to 7 ppg depending on foam quality.

Advantages of foam drilling are:

  • It does not require any additional downhole equipment.
  • The entire annulus is filled with foam, thus low bottom hole pressures can be achieved.
  • Since the foam has a high density, the foam can support the cuttings in suspension when drilling operations are stopped to make connections.
  • The foam has 7 to 8 times the cutting carrying capacity of water.
  • Samples can be retrieved easily for analysis.

Percussion hammer drilling has been used for decades in shallow well drilling. In the past decade, however, hammer drilling has been used extensively in deep well drilling for the oil and gas industry. Percussion hammer drilling is used for drilling in hard rock formations. The hammer works by an internal piston that is actuated by differential air pressure. The drill string is rotated at low RPM with very little weight on the bit (WOB). As a result of minimal WOB, the well deviation is minimal to none. Percussion hammer drilling achieves very high penetration rates that can average as high as 165 ft./hr. in some locations. Percussion hammers are primarily used for vertical well applications. However, there are several percussion hammer companies trying to develop percussion hammer directional drilling.

Percussion Hammer and Bit

Advantages of percussion hammer drilling are:

  • All the advantages of conventional air drilling apply except the penetration rates are usually very high and hole deviation is minimal.

Directional drilling has become more prevalent in the past decade with the development of positive displacement motors that have sealed, grease-lubricated bearing assemblies. The dimensional tolerances between the rigid helical lobe shaft and the helical lobe flexible sheath are also relaxed to provide a looser fit between these elements. With the development of the Electromagnetic MWD (EM) directional drilling with air/gas is more common. However, there are limitations to depth and types of rock in which the EM signal can work.

Advantage of directional drilling with air/nitrogen is:

  • All the advantages of conventional air drilling apply.

Payzone drilling is advantageous when gas reservoirs or "payzones" experience very low bottom hole formation pressures and gradients, and are marginal producers when drilled with a fluid based drilling system (overbalanced drilling). The economics of producing these wells becomes greatly hampered when done with conventional drilling fluids. The application of underbalanced drilling to drill through these payzones prevents skin damage and overbalancing, both common with mud.

The major advantage in underbalanced drilling is that the sensitive payzone is not exposed to any fluids. In addition, a continuous drill stem test is conducted without pulling the drillstring out of the wellbore. The "payzone" systems supplied by Omni Compressed Air, Ltd. are readily adaptable to drilling and workover rigs to drill out of a cement shoe and into the payzone.

Overbalanced Drilling

Underbalanced Drilling