Through advancements in well control, the ability to more precisely control bottomhole circulation pressure (BHCP) and industry experience, underbalanced drilling (UBD) and its variations are gaining the industry's confidence
and support as a reservoir damage prevention tool and also as a conventional drilling add-on toolbox
to eliminate common drilling problems.
As advancements in UBD are finding their way back into more conventional drilling as a means of overcoming some of the traditional challenges associated with well control, terms such as "controlled-pressure drilling" (CPD) and "managed-pressure drilling" (MPD) have been coined to acknowledge the ability to manipulate the annular wellbore pressure profile to different degrees of underbalance or overbalance to suit a specific need.
CPD reflects a broader use of UBD technology to affect well control from an underbalanced condition through balanced conditions and "low-head" applications. UBD technology is designed to create drawdown to primarily minimize invasive drilling induced damage. The other CPD technologies are primarily designed to eliminate conventional drilling problems.
The IADC UBO and MPD Committee defines MPD as "an adaptive drilling process used to precisely control the annular pressure profile throughout the well bore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular pressure profile accordingly." MPD operations include rotating blowout preventer (BOP) technology, pressure-while-drilling (PWD) technology (optional), choke manifold, choke line, drillstring floats and a low-volume pump. Further specialized CPD and MPD equipment, such as a closed and pressurized separation system, may be used to enhance the ability to control pressure. Nevertheless, the major distinguishing factor between UBD technology and other processes is the addition of reservoir influx to the circulation stream. UBD is now widely referred to as a "reservoir exploitation" tool instead of simply a method to overcome drilling challenges, said Robert Graham and M. Culen of Precision Energy Services in their "The Evolution of Underbalanced Drilling," white paper.
Today, there are four common techniques available to achieve underbalanced conditions while drilling. These include using lightweight drilling fluids, injecting gas down the drillpipe, injecting gas into a parasite string and using foam. A common system is to simply drill with a base fluid and the necessary CPD equipment. However, in some cases severely depleted wells cannot be drilled using this method because reservoir pore pressures are well below the gradient of water and oil-based drilling fluid systems. The alternative is to introduce gas into the drilling fluid system to achieve the desired equivalent fluid density and BHCP. Although almost any gas will satisfy the density requirements of CPD, gases used for CPD operations typically include air, nitrogen (cryogenic, membrane-generated or exhaust gas) and natural gas, with nitrogen commonly being the gas of choice.
In applying UBD technology, operators and service companies are also finding that CPD techniques are not particular to onshore or offshore applications, oil or gas reservoirs, sandstone or limestone, but offer a versatile solution to depleting reservoirs globally. As an engineered system tailored geographically and geologically to a specific challenge and desired objective, CPD uses interchangeable equipment from a common stable. A properly engineered CPD solution should strive to balance safety, technical feasibility and environmental sensitivity against economic viability.
Successful CPD projects have been completed from geologically sensitive sandstone gas reservoirs in the North Sea to remote onshore jungle oil reservoirs in Asia Pacific. Although equipment required for these geographically diverse wells was different, similar engineering processes were used to achieve the final result.
Cost efficiencies
With 70% of oil produced today coming from reservoirs that have been in operation for more than 30 years and demand for energy increasing, efficient techniques for extracting hydrocarbons from mature reservoirs are necessary.
The application of underbalanced drilling has increased productivity of many oil and gas reservoirs throughout the world. The Middle East, for example, reports that when UBD horizontal wells are simultaneously compared to conventionally drilled wells located less than 328 ft (100 m) apart on the same geological horizon, the average flush production on the UBD wells exceed the conventional wells by 2.2 times. The sample data was taken from cumulative UBD length of 11,483 ft (3,500 m).
CPD and MPD are enabling operators worldwide to drill their wells to planned total depth, where conventional drilling problems would have prevented them from doing so.
Lessons learned
Recently, subsurface engineers have begun to realize the value of the technology from a reservoir characterization point of view (i.e., real-time permeability, reservoir pressure, logs, etc). The real-time attributes of UBD make it an ideal technology for identifying production features in a near-virgin state. The application of UBD as a reservoir characterization tool has led to research and development of several adjacent technologies. Such technologies include models to predict reservoir parameters while drilling, tools to interpret LWD data on a pseudo-real time basis and development of a borehole identification sensor to increase the resolution of water influx measurement.
The development of an electromagnetic (EM) telemetry system has been a major UBD technology enabler, as conventional mud-pulse systems typically lose signal at 15% to 18% gas volume fraction at the wellhead. EM technology uses the earth to transmit continuous, bi-directional communications via an EM field circuit between the downhole transmitter and surface receiver. The EM field is relatively independent of drilling fluid, thus multiphase injection is no longer considered problematic at higher gas volume fractions. An EM signal is affected by the resistivity of formations and overburden between the downhole transmitter and the surface receiver, and can suffer significant signal attenuation in extremely low resistivity formations. "Extended Range" EM solved this problem by hanging wireline antennas between the EM transmitter and a point higher up the drillstring. However, extended EM has some inherent risks such as wireline in the drillstring and the time needed to run and place the antennas. Recent innovations in EM include coated casing, or electrically insulated casing. Coating the casing with ceramic improves signal range by insulating the casing and providing a signal conduit through high attenuation, low resistivity formations. The application of EM technology and coated casing successfully set an EM transmission depth record at 14,502 ft (4,420 m) total measured depth and 6,496 ft (1,980 m) total vertical depth in the North Sea for Shell UK Ltd. That record is detailed by Graham and Peter J.C. Schermer with Precision Energy Services in their "Controlled Pressure Drilling in High-Pressure Reservoirs - A Dynamic Solution," paper prepared for presentation at the 2001 IADC Underbalanced Technology Conference & Exhibition/Aberdeen, Scotland.
With UBD and CPD markets expanding from the traditional declining-pressure or marginal reserves to higher-pressure reserves, new technology has been developed to meet this market expansion. The dynamic nature of underbalanced drilling has led to the concept of adopting dynamic pressure-control techniques for use in high-pressure wells. Dynamic pressure control provides opportunities for simplifying many pressure control challenges associated with high-pressure wells, providing an increased level of safety, particularly when combined with enhanced surface handling equipment typically associated with UBD.
The use of traditional UBD equipment to provide "kick control" for drilling applications where high-pressure zones may be encountered is growing. The dynamic pressure control method in combination with UBD surface separation equipment and EM-MWD technology makes it possible to circulate out kicks faster, with lower surface pressures and more accurate bottomhole pressure control, according to D. Kimery, SPE, and Matt McCaffery, Weatherford International Ltd. in "Underbalanced Drilling in Canada: Tracking the Long-Term Performance of Underbalanced Projects in Canada," SPE/IADC 91593.
This method can be used in either the reservoir section or the tophole section and it should be noted that using traditional UBD well control techniques do not apply to the tophole sections where casing does not exist. When flowing the well on choke, significant annular friction pressure (ACP) is placed on the exposed open-hole interval, according to D. Power, C.D. Ivan, S.W. Brooks, M-I, "The Top 10
Lost Circulation Concerns in Deepwater Drilling," SPE 81133. If this ACP exceeds the casing shoe fracture pressure, an underground blowout may result. Thus, when practicing tophole UBD techniques, ensure that casing designs and operational procedures consider ACP.
Conclusion
A step-change is occurring in our approach to drilling. Whether it is drilling optimization, enhanced well control, reservoir exploitation or reservoir characterization, the most significant paradigm shift has been acceptance of mechanical equipment as the primary means of well control.
Despite the fact that UBD has been used for more than 15 years to exploit hydrocarbon reservoirs and enhance drilling performance, more work is required to develop it into a universally accepted technology used on any competent reservoir. The way forward will be for operators and service companies working together to forge fit-for-purpose solutions through a dedicated approach to UBD.

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