Optimizing the reservoir using an underbalanced environment

Oct. 1, 1998
Underbalanced applications incorporate all operations throughout the life of the well. [31,185 bytes] Sub-hydrostatic drilling has come full circle, from the primitive cable-tool rig successfully used to drill the Drake 1 well, to modern underbalanced systems requiring multidisciplinary teams. Underbalanced applications (UBA) can greatly enhance reservoir exploitation as a real-time reservoir solution, surrounding the wellbore in an environment conducive to maximum reservoir performance.

Advantages growing for many types of reservoirs

Chan Daigle, James L. Hunt
Halliburton Energy Services
Sub-hydrostatic drilling has come full circle, from the primitive cable-tool rig successfully used to drill the Drake 1 well, to modern underbalanced systems requiring multidisciplinary teams. Underbalanced applications (UBA) can greatly enhance reservoir exploitation as a real-time reservoir solution, surrounding the wellbore in an environment conducive to maximum reservoir performance.

UBA benefits are maximized by use of multidisciplined, interdependent teams with expertise in reservoir geology, drilling, completion, and production. This advancement requires both the development and integration of new technology. Long-term working relationships must be cultivated among the teams and goals, completely aligned both internally and externally. Finally, it is critical that these teams correctly apply the technology integration.

As the millenium approaches, various parallel developing technologies will be linked to the underbalanced environment. Underbalanced solutions linked with other developing technologies will likely evolve into 21st-century technologies not yet envisioned or even considered possible.

In any event, the primary objective in upstream petroleum technological development continues to be maximum reservoir performance. UBA offers oil and gas operators a vitally important means of achieving this objective.

Most petroleum drilling personnel have been familiar with underbalanced drilling for years. Recent successes for some major operators in highly visible areas launched underbalanced drilling into conventional mainstream applications. High value created by, and benefits realized from, UBA have brought these operators to the forefront of integrated-technology development. UBA offers operators its ultimate benefits in a integrated system in which discrete disciplines are used interdependently in planning and operations.

This article summarizes the benefits, development, and foreseeable future of UBA. It uses the term underbalanced drilling (UBD) when referring to the past. Current and future advancements are called underbalanced applications (UBA), which are operations that include greater participation by disciplines in UBA. These disciplines definitely include reservoir geology, completion, and production in addition to drilling engineering.

Drilling wells underbalanced can benefit operators. But to maximize asset value, the well should remain underbalanced or near balance after necessary pressure balance is achieved and safely maintained during completion. Multidisciplined, interdependent teams as just described should facilitate the entire process.

Conventional structure

From the 1960s through the 1980s, many major operators had highly structured work environments, in which separate departments and even individual workers with discrete technical expertise were effectively maintained in "cubicles" or "silos". Reservoir geological, drilling, reservoir production, and well-completion engineering personnel were confined to departments of highly circumscribed work responsibilities. They simply did not function together well enough from outset to finalization of projects vital to upstream petroleum operations.

Instead, these specialists tended to work in isolation of one another. This situation also tended to exist within service companies. They seldom shared technical information. Finally most operators realized that this isolation paradigm's deficiencies were rooted in sequential transfers of responsibility for discrete plans and operations from one group to another.

Small operators who bought older, marginal fields were generally the first to realign activities into effective multidisciplinary teams because they could least afford compartmentalization costs. Now most major companies have adopted interdisciplinary-team concepts and organization as key to successful petroleum operations.

Relevant issues

Certain valid issues and questions about UBA must be addressed. First, how much value is created by
  • Increasing future reservoir recoveries by 30-50% or, in some cases, by 100% or more?
  • Producing hydrocarbons while drilling?
  • Increasing rates of penetration (ROPs) by 50%, or in some cases, by 100% and faster?
  • Reducing lost-in-hole bottomhole assembly (LIH BHA) costs by 50% or more?
  • Reducing LIH incident-associated rig time (for sidetracking) by 50% or better?
  • Reducing differentially stuck pipe-associated rig time by 50% or more?
  • Reducing lost circulation overbalanced drilling-associated costs by 25% or more?
There are further questions that must be addressed on this issue:
  • What is the value (or cost) of wells plugged and abandoned with hydrocarbons in place for lack of an available extraction method or mechanism at the time?
  • How many wellbores were abandoned because of formation damage in drilling and/or completion; and what is their potential value?
  • How much more production could have been realized on good or even prolific producing wells with less formation damage, and what value would have been created?

How it works

Hydrocarbons exist in the earth's subsurface in formations under varying degrees of pressure. When drill bits penetrate such formations, the contained pressure, if uncontrolled, may drive hydrocarbons to the surface through the wellbore and into the atmosphere.

In the initial flow stages, formation fluid influx into the wellbore is called a kick. Continued uncontrolled flow may cause a blowout and jeopardize wellsite personnel and the environment, especially if released hydrocarbons are toxic, ignited, or both. Under conventional drilling practices, drilling mud, chemicals, and weighting material are circulated into and out of the wellbore to, among other objectives, maintain well control by making the mud column's hydrostatic pressure exceed formation pressure. This state is referred to as overbalanced. Weighted drilling fluids are the primary means of controlling reservoir flows during conventional well drilling and completion.

A major disadvantage in drilling some relatively sensitive reservoirs overbalanced is the formation damage skin factor. Damage occurs when solids, fines, and chemicals, circulated into the wellbore penetrate the pay zone. Added drilling-fluid ingredients often impede or stop hydrocarbons that would otherwise flow.

The result is a reduction in the well's overall productivity. Often a costly well-stimulation program is then needed, and frequently the well's potential productivity is never realized. Formation damage is of particular concern in horizontal drilling, because drilling fluids contact producing formations longer and over a greater area.

Unlike overbalanced drilling, UBAs use drilling fluids with lower hydrostatic pressure than that of the formation. This difference allows hydrocarbons to flow when the formation is drilled. The hydrocarbon flow can prevent entry of drilling mud and associated contaminants to the producing zone and thereby minimize formation damage. Higher production rates are the typical result.

UBA also can prevent drillpipe from differentially sticking, and downhole circulation loss, and extend drill-bit life. Lighter drilling fluids used in UBA usually cost less than heavier ones. Higher rates of penetration achieved by UBA result in less rig time. Furthermore, the target formations and bypassed zones can be evaluated better while drilling proceeds. If hydrocarbons are flowing into the wellbore, they can be recovered and sold.

Considerable time to research, compile, and prioritize data is needed to determine a project's primary drivers and economic feasibility. These factors provide impetus for UBA; define the work's basis; and determine solutions to the technological and economic hurdles.

In fractured-carbonate (limestone and dolomite) reservoirs, drilling fluid can plug or reduce production from fractures in which some minerals in the mud precipitate out of solution and swell.

Horizontal wells drilled in fractured reservoirs also often intersect depleted fractures, causing lost circulation, differential sticking, or both. If the well is allowed to flow by using lightweight fluids, such as water or aerated fluids, formation damage and high costs of losing costly drilling fluids to fractures are minimized.

In horizontal drilling, long lateral wellbores are drilled in targeted formations for a considerable distance. Because of the extended periods spent drilling these formations, the damage from fluid invasion is more common than it is in vertical wells; and fluid loss is an added cost. Hence, horizontal wells are generally good UBA candidates, provided that hole stability is not a serious problem.

Marginal fields

Marginal fields are those barely economic under prudent well construction, completion and production scenarios. Value can be added to these fields by applying technology that results in an increase in production rates and recoverables whose value exceeds the cost of applying the technology.

Drilling a well underbalanced in these fields that results in an undamaged reservoir can add tremendous value over a well with even a moderate degree of reservoir damage. For a well in a moderately permeable reservoir, a decrease in skin factor from 10 to zero can result in a doubling of the initial production rate, increasing recoverables significantly during the reservoir's life.

By drilling underbalanced, the likelihood of achieving a zero skin factor is significantly enhanced. For reservoirs in which traditional drilling and completion practices result in serious formation damage and those that will not flow economically without stimulation, the potential impact of underbalanced drilling and completion can be tremendous.

Depleted fields

Many producing regions have old fields from which less than 35% of the oil reserves have been recovered. Production rates from these reservoirs are decreasing because drive mechanisms are seriously depleted. Recovery from pressure-depleted zones can be increased by drilling new wells, specifically horizontal wells that allow much greater reservoir contact with the wellbore than vertical wells.

Because depleted zones are highly susceptible to drilling fluid loss, these zones may be entered and drilled underbalanced. However, a grassroots horizontal well drilled underbalanced in a pressure-depleted zone may not be economical. One strategy to overcome the economic limitation is to enter an existing wellbore and drill a lateral underbalanced through the depleted zone to a predetermined length.

What's left of the reservoir energy can then be used more effectively by the lateral to increase recovery from the reservoir. A lateral, drilled and completed underbalanced through an existing wellbore is a viable strategy for enhancing recovery from pressure-depleted zones.

Lost circulation

Lost circulation occurs when drilling mud escapes into zones penetrated by the wellbore. Creating new mud volumes with special circulation-loss materials to replace losses can be very costly. Sometimes hydrocarbon-bearing zones are penetrated and bypassed as more productive, deeper zones are sought.

Overbalanced drilling mud systems prevent flows from bypassed zones. Should a lost-circulation zone be encountered and loss of drilling mud result, the bypassed zone could start flowing. Well control could be lost at this point, and a catastrophic well-control incident could occur.

Tight sandstone reservoirs should not be overlooked as UBA candidates merely because they can generally withstand significant formation damage during drilling. Tight, low-permeability reservoirs containing little or no swelling clay and that are not severely low-volume, high-pressure zones can be good UBA candidates.

Water-driven reservoirs and water-flooded fields lack value as UBA candidates because the pressure drawdown caused by UBA may cause premature water coning. These phenomena can affect the formation's natural cementation.

UBA process

The process of surrounding the wellbore with an underbalanced environment adds interactive elements to the drilling and completion process not normally associated with conventional well construction. In an underbalanced project, operations can be grouped into three main categories:
  • Upstream: Refers to specialized services and equipment employed before fluids are pumped downhole (i.e., drilling fluids, injection medium and associated equipment, and rotating control diverters).
  • Downhole: Refers to the specialized equipment run in the drillstring (i.e., pressure sensing equipment, specialized motors, electromagnetic MWD, check valves, etc.).
  • Downstream: Refers to the specialized services and equipment employed to process wellbore returns (i.e., specialized chokes and manifolds, separation equipment and flare systems).
Depending on the UBA project's scope, the amount of equipment needed in each category can vary significantly, ultimately affecting the project's economic viability. In many cases, an underbalanced environment can be achieved without an injection medium.

The effect of each category on operations should never be minimized (that is, injection-medium rate, drilling-fluid weight, separation system backpressure, formation influx). Thus it becomes critical that the integrated project team be involved in both the planning and execution of underbalanced operations.

Once project teams become operational, they must establish a process to ensure proper execution of four main elements: safety, environmental protection, execution credibility, and consistency of delivery.

The process must become part of the company's management system and should be linked to all aspects that could affect the project. It should clearly define project scope, responsibilities and procedures, identify path processes, establish delivery standards and competency levels of project personnel. Flexibility of operations is always necessary for adaptation to remote locations; but this flexibility should never compromise the original four elements of the underbalanced process.

Particular attention should be placed on safety and environmental issues, which could easily be dismissed amid the current interest in UBA. The UBA process must include an internal or external audit element to ensure conformance with health, safety, and environmental standards. The audit element should be part of the total UBA process and should afford operating entities the ability to respond as necessary with a fully integrated solution. The audit element should be designed to assist development of the conventional plan, establish specific operational procedures, audit the conventional plan, develop the contingency plan, and identify equipment and personnel to execute the contingency plan. The process should include a strategic event plan (STEP), a site-specific contingency plan that provides a detailed decision tree and actions to take for a host of possible events.

Lessening risks

With the continued development of underbalanced technology and the application of its delivery to offshore locations, it becomes critical that UBA operators lessen associated risks. The recent development of a product called WellsureSM, offered by Boots & Coots/IWC, Inc. permits not only UBA but conventional applications to use a unique insurance plan.

WellsureSM combines traditional control of well insurance, underwritten by Lloyd's of London and certain insurance companies, with well-control-preventative and post-event-response services provided by the Boots & Coots Group. WellsureSM helps operating companies manage risks, minimize insurance cost, and mitigate losses.

Under this insurance program, the underwriters recognize the benefits of the Boots & Coots/IWC alliance. They also recognize the significant drain on the operator's cash reserves of a catastrophe-management plan and have thus agreed to provide coverage on a pay-on-behalf-of basis.


Underbalanced operations will be used offshore increasingly as operators re-develop older fields. Where applicable, UBA will extend the producing lives of existing fields and will help reduce overall lifting costs. Once commonly dubbed "cowboy" operations, these will be mainstreamed by all operators.

Even in times of lower oil prices, UBA become a more viable alternative: the major benefit to the operator usually comes with greater well productivity. The primary impediment to further development of underbalanced technology will be catastrophic well-control incidents. A major operator in the North Sea has already proven this technology in an offshore environment. Attention to details in the planning and execution of operations must never be minimized or taken for granted.

As use of UBA increases in the offshore environment, the surface equipment needed to achieve the underbalanced state will need to be further developed. The "footprints" of feed compressors, membrane units, and boosters must be reduced; yet, outputs of these units must increase simultaneously to accommodate larger hole sizes.

The fuel efficiency of this equipment should improve to increase the economic viability of the underbalanced project. In addition, the separation packages to process produced fluids must be further developed.

The systems of the future will require smaller footprints, be modular in design and deployment, and provide for shorter cycle time. The number of personnel needed to operate this equipment efficiently and safely must decrease as well. These systems will be driven to more automation as the increased personnel on board will become an issue with underbalanced operations on smaller rigs.

Acknowledging that "underbalanced" is an environment in which the wellbore is placed, it will become part of many of the advanced applications now under development. Just as with the development of other technologies in this industry, operators will turn to UBA for better solutions.

At one time, operators drilled directional wells only if forced to do so. Several years ago, horizontal wells were not common; today, they are a mainstream application. In the near future, the decision process, or tree, may likely include a point at which operators need to choose between overbalanced and underbalanced.

This is not to infer that all wells will be drilled and completed underbalanced. It merely signals that consideration will be given to the selected well candidate. That very consideration will also cause more attention to be placed on drilling and completing wells at or near balance to minimize or avoid formation damage.

Underbalanced multilateral wells with openhole completions have been common in Canada and the continental U.S. In the very near future, further advancement, primarily in the completions area, will allow delivery of multilateral wells of Level 4 and higher in an underbalanced environment.

With this advancement, operators will be able to access multiple reservoirs in the maximum performance environment from a single wellbore. Completion technology will continue to provide services and equipment better described as reservoir solutions.

More attention will be placed on the development of underbalanced completions. Until now there has been little need for sophisticated completions in underbalanced wells.

In the future, downhole sensor technology will not only be able to read pressures along the wellbore but will also determine the composition and characteristics of formation fluids in real time as they enter the wellbore.

Greater use of coiled tubing UBA will occur as time goes on. In many areas of the world, these units have already proved effective and economical. Coiled-tubing and hydraulic workover units will not replace the need for drilling and workover rigs. They will only provide other viable alternative mechanisms for exploiting the reservoir.

As operators venture into deepwater and ultra-deepwater, underbalanced technology will not be far behind. The investments operators are making in these fields are substantial, and the stakes for fully exploiting targeted reservoirs are higher. Maximum well productivity is required to achieve the financial return expected to justify initial investment.

In addition, lost-circulation costs will be extremely high in these environments, and conventional technology will not provide the solutions needed for making these projects economical.

The technologies of expandable tubular goods and downhole separation are developing and are also linked to the underbalanced environment. Expandable tubular goods will place the mechanism of stabilizing or cementing casing and liners at the same time without any formation damage.

With greater well productivity will come increased oil, gas, and water production. Two of the three are very valuable in this business; the third presents a significant cost. The technology needed to separate produced water downhole and dispose of it without coming to the surface could significantly reduce operator costs.


Underbalanced applications can advance reservoir exploitation. It is a real-time reservoir solution that surrounds the wellbore in an environment conducive to maximum performance.

As the millenium approaches, achieving maximum reservoir performance will be the main objective in all technology development; Underbalanced applications will provide one of the mechanisms to accomplish this goal.


The authors would like to acknowledge the contributions of the following through general discussions: Evert Jan Mulder, Halliburton Energy Services, Rick Stone and Robert Davis, Signa Engineering, and Mike Tangedahl, Hydril. The authors would also like to thank Pat York, Halliburton Energy Services and Patricia Wright, COREComm for their assistance in preparing this paper.


Bennion, D. B., "Reservoir Screening Criteria For Underbalanced Drilling," Hart's Petroleum Engineer International, February 1997.

Butler, S.D., Teichrob, R.R., van Regen, N.C.R, "An Engineered Approach to Underbalanced Drilling," Presentation at the CADE/CAODC Spring Drilling Conference, Calgary, April 19-21, 1995.

Comeau, L., " Integrating Surface Systems with Downhole Data Improves Underbalanced Drilling," Oil & Gas Journal, March 2, 1997.

Foy, J., Brett, P., "First North Sea Underbalanced Drilled, Jointed Pipe Well," The 3rd International Underbalanced Drilling Conference & Exhibition, The Hague, October 7-9, 1997.

Gray, R., Bird, K., "Laboratory Evaluation of Under balanced Formation Damage Compared to Neutral and Overbalance Conditions," The 1st International Underbalanced Drilling Conference & Exhibition, The Hague, October 2-4, 1995.

Killalea, M., "Shell Expro Rocking the Charts with Top 10 Technology Picks," Drilling Contractor, July 1997.

Saponja, J., "Considerations for Underbalanced Drilling with Jointed Pipe," The 1st International Under balanced Drilling Conference & Exhibition, The Hague, October 2-4, 1995.

Tangedahl, M., "Well Control Issues of Underbalanced Drilling," The 1st International Underbalanced Drilling Conference & Exhibition, The Hague, October 2-4, 1995.

Taylor, J., "Offshore Application of Membrane Nitrogen Technology to the First North Sea Underbalanced Drilled Joint Pipe Well," The 3rd International Underbalanced Drilling Conference & Exhibition, The Hague, October 7-9, 1997.


Chan Daigle is the Product Manager for Underbalanced Applications for Halliburton Energy Services in Houston.

James L. Hunt is the Senior Reservoir Engineer for Reservoir Description for Halliburton Energy Services in Houston.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.