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Home Solutions Drilling SolutionsUnderbalanced Drilling (UBD)

Underbalanced Drilling – Managed Pressure Drilling Technology Specialist

An underbalanced well is defined as a drilling process where bottomhole pressure is maintained below formation pressure, allowing formation fluids to flow into the wellbore and circulate to the surface. This technique is referred to as underbalanced drilling (UBD) or negative pressure drilling‌.

As a professional provider of drilling technologies and product services, we deliver comprehensive underbalanced drilling solutions, integrating core equipment such as rotating control devices and choke manifolds to precisely manage bottomhole pressure and effectively resolve challenges including narrow density windows and reservoir damage. Technical advantages: increases mechanical rate of penetration by 40%, reduces lost circulation incidents by 60%, preserves reservoir productivity, and has been successfully applied in the development of complex formations such as shale gas and tight oil.

Benefits of Underbalanced Drilling

The engineering benefits of underbalanced drilling come from preventing lost circulation, overcoming differential sticking, increasing the rate of penetration, and extending drill bit lifespan. However, the greatest benefits are realized in exploration and production: earlier production startup, increased production capacity, enhanced recovery efficiency, fewer intermediate tests, and more effective reservoir discovery. The most significant advantage is in reservoir discovery – specifically, avoiding reservoir damage and accurately evaluating the reservoir.

Key Challenges in Underbalanced Wells

Underbalanced drilling technology primarily differs from conventional overbalanced drilling systems in two aspects: the pressure balance relationship and the type of drilling fluid used.

Schematic diagram of the pressure balance relationship in underbalanced well

Pressure balance in Underbalanced drilling:

The goal is generally to maintain a controlled underbalanced state, meaning that the effective pressure of the fluid column inside the wellbore is lower than the reservoir pressure. As a result, movable fluids from the reservoir flow controllably into the wellbore and return to the surface. The pressure balance can also be in an uncontrolled underbalanced state, such as when pure air drilling is used to penetrate a high-pressure oil and gas reservoir.

Drilling fluid and downhole tools for underbalanced drilling

Drilling fluids for Underbalanced drilling:

The drilling fluid can be the same as in conventional drilling, such as a water-based or oil-based fluid. It can also be a gas-containing fluid (e.g., an aerated or foam gas-liquid mixture), a pure gas (e.g., air, natural gas, or combustion exhaust gas), or a fluid with solid-phase lightening agents to reduce fluid density. Different types of drilling fluids are used to match various formation pressure gradients.

Underbalanced Drilling Injection Equipment

Common injection systems for underbalanced drilling includes air compressors, boosters, nitrogen generators for drilling, triplex drilling pumps, and gas-liquid mixers.

Air Compressor

For gas drilling, the air compressors used is typically a screw-type compressor that draws air directly from the atmosphere and provides primary pressurization.

Booster

For gas drilling, the booster is generally a reciprocating piston-type booster that further compresses the gas that has already been pressurized by the air compressor to reach a higher pressure.

Nitrogen generator on a white background

Nitrogen Generator for Drilling

In gas drilling, the nitrogen generator is the main equipment for nitrogen drilling. It is installed between the air compressor and the booster and uses membrane separation technology to produce nitrogen.

High-Pressure Nitrogen Generation Truck

Typical basic parameters of high-pressure nitrogen generation trucks: gas output: 900 m3/h, 1200 m3/h, and 1800 m3/h, and other capacities; nitrogen purity: 95% – 99.5%; nitrogen pressure: 20–35 MPa.

Triplex Drilling Pump

Triplex drilling pump is a three-cylinder drilling pump used at the wellsite. It is recommended to use an electric drive pump with stepless speed regulation to allow precise adjustment of drilling fluid injection rates.

Gas-liquid Mixer

The gas-liquid mixer is used in aerated drilling operations to mix gas rapidly and uniformly into the drilling fluid, achieving a consistent gas-liquid mixture.

Underbalanced Wellhead & Surface Equipment

Underbalanced wellhead and surface equipment include rotating blowout preventer, separator, ignition device, drilling fluid four-phase separator, vacuum degasser, pressurized sampler, etc.

Rotating Blowout Preventer

The rotating blowout preventer (RBOP), also known as the rotating control head (RCH), can be categorized into three types based on the sealing method of the rubber core: passive sealing, active sealing, and hybrid sealing. The first two types are most commonly used in the field.

Separator

During Underbalanced drilling, when gas is mixed into the drilling fluid, the free gas in the form of large bubbles is removed using a gas-liquid separator.

Ignition device

In Underbalanced drilling operations, the gas separated from the gas-laden drilling fluid at the wellhead is discharged through a vent line and ignited and burned by an ignition device installed at the end of the vent line.

Drilling Fluid Four-Phase Separator

It can separate and process gas, crude oil, and cuttings from drilling fluid under sealed conditions, serving as an alternative to open systems such as liquid-gas separators and oil skimming tanks.

Vacuum Degasser

When small-diameter gas bubbles are mixed into the drilling fluid and invade the drilling fluid system, resulting in gas-invaded drilling fluid, a vacuum degasser should be used to remove the gas.

Pressurized Sampler

A pressurized sampler is recommended for conventional Underbalanced operations and is a required cuttings logging sampling device for Underbalanced drilling in sulfur-bearing formations.

Downhole Tools for Underbalanced Wells

Common downhole tools for underbalanced wells include check valves, air hammers and percussion drill bits, drill pipe, etc.

Since both the inside of the drill string and the annulus contain compressible two-phase fluids, when such fluids suddenly stop flowing, fluid inertia can create a 'vacuum' low-pressure effect within the pipeline, causing the backflow of the liquid column into the drill string. This backflow from the annulus into the drill string often causes the bit nozzles to become blocked, making it impossible to resume circulation after adding a single joint. To prevent this backflow, a check valve is installed above the drill bit (sometimes two are used in series for added safety). The check valve is typically used in conjunction with a Kelly valve, and then an additional set installed every 100 to 200 meters along the drill string, with the Kellyvalve below and the check valve above.

Underbalanced well drill pipe system on a white background

Drill pipe dart tyoe check valve on a white background

Drill pipe dart type check valve

A check valve is typically a spring-loaded dart-type check valve. For easier passage of survey instruments, some operators use a butterfly check valve; however, in practice, passing survey instruments through butterfly check valves requires considerable patience. There are also retrievable check valves that must be removed from inside the drill pipe before running a survey instrument downhole.

Kelly valve

A Kelly valve must be used; otherwise, operating with a high-pressure union is extremely dangerous. It is also recommended to install a separate Kelly valve at the lower end of the Kelly drill pipe. Close the Kelly valve before breaking out each connection to maintain pressure and prevent significant drilling fluid loss on the rig floor, which could impact operations.

The air hammer and percussion drill bit are downhole tools specifically designed for air drilling. They rely on the conversion of the compression energy of high-pressure gas into the hammer’s impact energy to achieve rock breaking by impact, rather than by drill bit weight on bit. Air hammer drilling offers the advantages of high penetration rate, long service life, and better deviation control. The air hammer used in oilfield air drilling is a center exhaust, valveless, full-bore drilling air hammer. It mainly consists of a back sub, check valve, distribution seat, cylinder, outer casing, piston, tail pipe, retaining ring, and drill bit. Air hammer drill bits used in air drilling can be classified into integral and split types. In terms of the cutter material, they can be divided into tungsten carbide type and diamond type. According to the cutter edge shape, they can be categorized as blade type, button type, or mixed blade-button type. Currently, the air hammer drill bits applied in petroleum drilling mainly adopt a tungsten carbide button structure.

Air hammer and percussion drill bit on a white background

The drill pipe used for underbalanced drilling is a drill pipe with an 18° tapered shoulder, designed to facilitate the smooth passage through the rubber element of the wellhead rotating control device. It is also essential to smooth out burrs from tong marks after each single joint is connected, to ensure the extended service life of the rubber element. If a top drive system is used in conjunction, it can significantly reduce the number of single joint connections and extend the service life of the rubber element. Although some manuals specify that drill string connections for underbalanced drilling should use gas-tight threads (e.g. Grant Prideco XTM type), in actual operations, no significant issues have been observed when using conventional threads. In underbalanced drilling, especially air drilling, the torque on the drill pipe and the friction between the wellbore and the drill string are greater than in conventional overbalanced drilling, and drill string vibration is also more severe. Therefore, special attention should be paid to the proper design, use, and inspection of the drill string.

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