Drilling, Pressure and
Pumping Solutions

Under-balanced Operations

A Brief History

The first wells drilled in the 1800s were drilled underbalance with insufficient pressure in the annulus. This resulted in uncontrolled flow.

UBD advanced through the 1900s as engineers began to understand the use of mist and multiphase fluids. In 1930 multiphase fluids (gas or air with water or oil) became popular in the US. Foam became popular in the 1960s because of its hole cleaning ability compaired to air or mist although by 1970 environmental concerns arose because of the large amount of waste generated in single circulation systems. Although most wells UB drilled before 1985 were to increase ROP, new technology led to a renewed interest in UBD with better rotating heads, recycling foam systems and Electromagnetic MWD tools. Improved technology and the introduction of horizontal drilling boosted UBD in particular as it limited the damage typically found in horizontal wells.

UBD is no longer a niche technology and is being successfully used throughout the world both on and offshore.

The Benefits of Under-balanced Drilling

Reduced Formation Damage

Reducing the formation damage caused by conventional drilling can dramatically increase productivity from a reservoir ias has been noted in several large projects. Most notable was the Exxon/Mobil ARUN project where production increased between two-to-six times compared to conventionally drilled wells.

Wells damaged by overbalanced drilling often require expensive stimulation programs to fix the associated problems although as Exxon/Mobil have found out, in many cases the original formation productivity is never regained.

Minimized Loss Circulation

Loss circulation can be defined as the loss of mud in to the formation. Losses occur when the hydrostatic pressure of the mud exceeds the fracture gradient of the formation. Due to the nature of conventional drilling fluids, loss circulation is always a risk.  However if an underbalanced state is maintained, lost circulation cannot occur.  In severely depleted reservoirs with high permeability the ability to remove drilled solids from the wellbore is lost.  If pore spaces are not large enough to take the drilled solids, solid buildup and mechanical sticking of the drill string results.  For this reason, severely depleted fields or under-pressured reservoirs cannot be drilled with conventional drilling fluids. ......read more

Underbalanced\Managed Pressure Drilling Techniques

Term Description
Gas Drilling Drilling process using only gas as the drilling medium; no intentional fluid added.
Mist Drilling Drilling with liquid entrained in a continuous gaseous phase; typical mist systems have <2.5% liquid content.
Foam Drilling Drilling with a two-phase fluid and a continuous liquid phase generated from the addition of liquid, surfactant, and gas; typical foams range from 55% to 97.5% gas.
Gasified Liquid Drilling Drilling with a gas entrained in a liquid phase.
Liquid Drilling Drilling with a single liquid phase.
Low-head or near-balanced drilling (MPD) Condition where the hydrostatic head of the wellbore fluid column is reduced to be either in balance or slightly greater than the formation pressure, thus not planning to induce hydrocarbons or formation fluids into the wellbore.
Underbalanced Drilling (UBD) Planned condition where the bottom-hole pressure exerted by the hydrostatic head of the fluid column is less than the formation pressure being drilled.

The Benefits of Underbalanced Drilling (continued...)

Better Formation Identification/Evaluation

Another significant advantage of UBD is that it allows continuous reservoir evaluation and characterization.  Not only can fluid types, flow rates, and pressures be identified but also reservoir parameters such as static pressures can also be estimated while drilling underbalanced.  Also natural fractures and the resulting flow/pressures may be identified during UBD.  Most importantly underbalanced drilling allows formation pore flow into the wellbore and therefore allows detection at the surface that would not be seen with conventional overbalanced drilling.

No Differential Sticking

In conventional drilling operations, all the necessary ingredients are always present, and differential sticking is always a concern.  With UBD, there is no hydrostatic pressure differential to the formation and no filter cake.  It is impossible to get differentially stuck while drilling underbalanced.

Increased ROP

It is well known that heavy muds combined with drilled solids impact penetration rate. Conventional drilling is a process of grinding where re-circulated drill solids that are not removed by the shakers are re-introduced into the wellbore and subjected to re-grinding.
In UBD, there is no pressure on the rock to hold the solids in place and create a filter cake.  And because the UBD fluid is free of solids, they cannot be re-introduced into the circulation system for re-grinding.  Furthermore, since the formation pressure is greater than the wellbore pressure, less energy is expended in breaking the rock, which can mean extraordinarily high rates of penetration (ncreases of penetration rates by a factor of ten are not uncommon) . 
The graph belows shows the ROP correlation developed by Bourgoyne and Young.  This correlation shows the relationship of ROP and mud weight as compared to the pore pressure.  As shown, the greatest ROP % increase occurs with harder formations.

Bourgoyne and Young ROP Correlation
......read more.

Key Drivers for UBD Candidate Selection

In general, with the current state of the technology, the key drivers for the selection of UBD have been:

  • Severe lost circulation or differential sticking problems while drilling conventionally.
  • Highly depleted reservoirs, which typically present the problems listed in the previous item during conventional drilling.
  • Hard rock formations that result in very low rates of penetration and poor bit life during conventional drilling.
  • Formation damage resulting in wells with productivity below potential.
  • Ability to evaluate formation productivity while drilling.

Generally the presence of one or more of the above is sufficient to consider UBD drilling.

Under-balanced Operations


Underbalanced drilling is usually more expensive than conventional drilling and has safety issues of its own.

Technically the well is always in a blowout condition unless a heavier fluid is displaced into the well.

Air drilling requires a faster up hole volume as the cuttings will fall faster down the annulus when the compressors are taken off the hole compared to having a higher viscosity fluid in the hole.

Because air is compressible mud pulse telemetry measurement while drilling (MWD) tools which require an incompressible fluid can not work. Common technologies used to eliminate this problem are either electromagnetic MWD tools or wireline MWD tools.

Downhole mechanics are usually more violent also because the volume of fluid going through a downhole motor or downhole hammer is greater than an equivalent fluid when drilling balanced or over balanced because of the need of higher up hole velocities.

Corrosion can also be a problem, but can be largely avoided using a coating oil or rust inhibitors.

Notwithstanding these potential disadvantages, UBD may serve as an additional tool for an operating company to drill and produce from those reservoirs that cannot be exploited by conventional drilling methods. UBD has proven to be not only safe, but also cost effective and can result in an overall more efficient drilling program.

Under-balanced Operations Classification

The IADC Underbalanced Operations Committee has adopted the following standard classification system for UBO and a set of standard nomenclature:-

Level 0

Performance enhancement only; no hydrocarbon containing zones

Level 1

Well incapable of natural flow to surface, inherently stable, and a low-level risk from a well control point of view.

Level 2

Well capable of natural flow to surface but enabling conventional well control methods and has limited consequences in the case of catastrophic equipment failure.

Level 3

Geothermal and non-hydrocarbon production. Maximum shut-in pressures are less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.

Level 4

Hydrocarbon production. Maximum shut-in pressures are less than UBD equipment operating pressure rating. Catastrophic failure has immediate serious consequences.

Level 5

Maximum projected surface pressures exceed UBO operating pressure rating but are below BOP stack rating. Catastrophic failure has immediate serious consequences.

IADC UBD/MPD Resources

The Benefits of Underbalanced Drilling (continued...)

Increased Bit Life

The considerable heat generated by friction at the bit, between the drill string and wellbore is removed by the circulating drilling fluid. Additional frictional heat is also generated by the inert solid content of the mud.  Heat removal from the bit is more efficient in underbalanced operations.  Since there is no additional force holding the formation in place (less frictional force), the bit does less work to cut the formation. By using UBD, the fraction of retained solids is maintained at a minimum. 
As UBD also requires less weight on the bit to obtain optimum ROP, the reduced load on the cutters and bearings increases bit life.  During the Exxon/Mobil PASE Project, a single bit drilled the entire (production) hole section of 754 feet. In contrast, the average bit life for this field was 150 feet per bit for conventionally drilled wells.

Reduction/Elimination of Expensive Drilling Fluids

Conventional drilling fluids consist of a mixture of chemicals that are added to control fluid properties such as viscosity and fluid loss.  In the case of loss return zones, additional chemicals and sized particles are added to control losses.  These systems can be very costly.  Because simple fluids (such as, KCl water or produced oil) are typically used in UBD, costly drilling fluid programs can be eliminated for the hole section drilled in an underbalanced mode.  Additionally, significant cost savings may also be realized by not losing expensive drilling fluids to the formation.

Improved Safety and Reduced Environmental Impact

Conventional drilling is designed such that reservoir fluids do not enter the wellbore. But if they do, the system relies on personnel to recognize the inflow and control the well pressures correctly. Most blowouts occur, not because of poor engineering or planning, but due failure of personnel to correctly recognize an inflow and properly handle it.
A properly designed UBD system is less reliant on personnel recognizing an unplanned event.  The system is designed to safely handle a continuous inflow from the formation and give a continuous positive BHP reading throughout the drilling operation. 
Conventional drilling fluids are heterogeneous mixtures of organic, inorganic, and inert constituents. Regardless of the efficiency of the solids removal equipment, continued dilution is necessary to maintain a usable drilling fluid.  Cosequently on a well it is not uncommon to generate thousands of barrels of waste material ranging in various levels of toxicity.  On the other hand, UBD requires naturally occurring constituents, such as air and water plus fairly harmless foamers plus perhaps some corrosion inhibitor. The concentration of foaming additives are very low while the majority of surfactants used for foaming agents are biodegradable.  As air/gas is the largest component, very little waste is generated as compared to conventional drilling fluids.  Therefore disposal problems are a fraction of that for conventional drilling.

UBD Design Variables

Variables that influence the final configuration of a UBD system.

1502 Contact Form


Your privacy is important to 1502.  This privacy statement provides information about the personal information that 1502 collects, and the ways in which 1502 uses that personal information.

Personal information collection

1502 may collect and use the following kinds of personal information:

Using personal information

1502 may use your personal information to:

Where 1502 discloses your personal information to its agents or sub-contractors for these purposes, the agent or sub-contractor in question will be obligated to use that personal information in accordance with the terms of this privacy statement.

In addition to the disclosures reasonably necessary for the purposes identified elsewhere above, 1502 may disclose your personal information to the extent that it is required to do so by law, in connection with any legal proceedings or prospective legal proceedings, and in order to establish, exercise or defend its legal rights.

Securing of your data

1502 will take reasonable technical and organisational precautions to prevent the loss, misuse or alteration of your personal information.

1502 will store all the personal information you provide on its secure servers.

Information relating to electronic transactions entered into via this website will be  protected by encryption technology.

Cross-border data transfers

Information that 1502 collects may be stored and processed in and transferred between any of the countries in which 1502 operates to enable the use of the information in accordance with this privacy policy.

In addition, personal information that you submit for publication on the website will be published on the internet and may be available around the world.

You agree to such cross-border transfers of personal information.

Updating this statement

1502 may update this privacy policy by posting a new version on this website. 

You should check this page occasionally to ensure you are familiar with any changes. 

Other websites

This website contains links to other websites.

1502 is not responsible for the privacy policies or practices of any third party.

Contact 1502

If you have any questions about this privacy policy or 1502's treatment of your personal information, please contact 1502.

This privacy statement

This privacy statement is based on an original template created by website-contracts.co.uk


This Internet Website is provided by 1502 International Pty Ltd ("1502"). Your use and access to this Website is subject to the following terms and conditions. Please read the following terms and conditions carefully and be sure you understand them. Your use of the Website constitutes acceptance of the terms and conditions outlined in it. From time to time the terms of this Website may change, so it is important that you review these terms every time you use the Website.

This website provides general information only and does not take into account customer’s particular circumstances. The website content has been prepared for general information purposes only and may not be complete or accurate for your purposes.

These Terms and Condition can be modified at any time by 1502 and you agree to continue to be bound by these Terms and Conditions as modified. We will give you notice of these changes by publishing revised Terms and Conditions on this website - we will not separately notify you of these changes.

You should be aware that the internet is not a completely reliable transmission medium. We shall not have any liability for any data transmission errors such as data loss or damage or alteration of any kind, including, but not limited to, any direct, indirect or consequential damage, arising out of the use of the services provided herein.

Messages that you send to us by e-mail may not be secure. We caution you on sending any confidential information to us by e-mail. If you choose to send any confidential information to us via e-mail you do so at your own risk with the knowledge that a third party may intercept this information.

All information residing on this website is copyright to 1502. You may only use the information for your own personal use. You may not otherwise use, modify, copy or distribute the information (or any part of it) without the express written permission of 1502.

These terms and conditions apply to the use of this website, which is accessible over the internet at www.1502.com.au.  By using this website, you agree to these terms and conditions. If you do not accept these terms and conditions, you must not use this website.
Information on this website is provided for general information only and is prepared without taking into account any person’s individual needs