"Emerging Technologies Introduce New Possibilities, Open New Applications for Underbalanced Drilling"

By Rick Stone

The American Oil & Gas Reporter
January 2004 - Page 99

"The biggest growth drivers will likely come from integrating of underbalanced solutions with emerging technologies such as casing drilling, concentric drill pipe and expandable tubulars...The combination of UBD techniques and casing drilling, for example, promises to have a far greater impact and generate considerably more value than either technology could when applied individually."

HOUSTON-With the potential to improve drilling costs, underbalanced drilling (UBD) is the most significant advancement in drilling science since the advent of rotary tools. But given UBD’s rapid expansion around the globe over the last decade or so, will the evolution and adoption of UBD technology slow?

Hardly. The underbalanced market appears poised for continued impressive growth. With the application of UBD technology still emerging in some parts of the world and a number of key complimentary technological advances taking shape on the horizon, the future looks brighter than ever for UBD.

That growth will result partly from the development of new tools and system components designed to address the current limitations of underbalanced operations. For example, real-time data telemetry systems, bottom-hole pressure sensors and automated chokes are available, but real-time bottom-hole pressure/temperature acquisition and surface control capability has not yet been developed. The next logical technological progression in UBD is the development of an automated bottom-hole pressure controlled choke.

As we drill deeper, hotter and higher-pressure wells using UBD techniques, the ability of the driller to manage surface pressure and downhole events is reaching its limit. These limits exist because today’s drillers are utilizing surface parameters to gauge downhole conditions. This reactive mode will eventually cause safety issues with UBD techniques. With the advent of realtime bottomhole pressure and temperature, the driller could become proactive in his decision making. The impact to the system is that surface pressure containment and separation equipment will not be subjected to the limit of capacity. Therefore, unsafe conditions can be addressed downhole as they occur and with plenty of time before an event migrates from the bottom of the hole to the surface. Bringing automated bottom-hole pressure control together with real-time telemetry using the drill string to transmit data and underbalanced hydraulic simulators would give UBD a new level of downhole pressure control and safety, especially in well bores with pressure-sensitive zones in the open hole. Ultimately, these capabilities could allow drilling engineers to address multiple pressure regimes in the same bore hole differently by alternating between underbalanced, near-balanced or overbalanced modes as drilling proceeds according to each zone’s specific characteristics.

The impact of these kinds of equipment advances would certainly be significant for UBD’s future, but the biggest growth drivers will likely come from the integration of underbalanced solutions with emerging technologies such as casing drilling, concentric drill pipe and expandable tubulars.

Technology Integration

Technology integration will clearly be a key issue, because the combination of underbalanced techniques and casing drilling, for example, promises to have a far greater impact and generate considerably more value than either technology could when applied individually.

In fact, the merging of casing drilling and UBD may eventually prove one of the most powerful combinations of drilling technology the industry has ever seen. Advantages of casing drilling include reduced per-well drilling costs and shorter time to a cased bore hole, while the primary advantages of underbalanced operations include reduced formation damage and faster penetration rates. When applied together, I believe per-well cost savings on the order of 50 percent are attainable on the right candidates. That is a huge prize. Moreover, the combination of casing drilling and UBD would also deliver wells with increased formation productivity versus conventional drilling.

These two technologies are a perfect match. The majority of the risk-and cost-of UBD occurs during pipe trips, but because casing drilling places casing in the role of drill pipe, wells are drilled without tripping pipe to set casing. Also, because the annulus in casing drilling is smaller than in conventional drilling, the increased friction and equivalent circulating density experienced during casing drilling is actually a friend of UBD and assists in well control.

Another technology that has obvious potential when combined with UBD is concentric drill pipe, which is essentially one string of drill pipe within another string of pipe. Concentric drill pipe provides the driller with two conduits to total depth instead of one. Gas can be injected into the well annulus at any point during drilling for specific control of well bore pressures as the drilling formation changes. Conversely, gas can also be injected at any point into the internal string of pipe to affect reverse circulation. Adding “smart” pipe capabilities, which builds real-time data telemetry into the wall of the pipe string, to the combination of concentric drill pipe and UBD would be an even more powerful system, providing real-time data for circulation and formation pressures and temperature profiles, gas injection rates, etc.

Expandable casing is another emerging technology now moving into the drilling industry for which is it easy to imagine new possibilities when combined with underbalanced operations. By replacing conventional drilling liners with custom-engineered expandables, a larger sized hole is maintained. And again, not only are expandables an exciting fit for UBD that could help launch the technology to another level of usage, but its systematic combination with UBD techniques creates advantages that are potentially more powerful than what either can achieve when used on a stand alone basis.

Proper Application

Even with all its potential and the broad base of experience the industry has gained to date using UBD, many operators continue to misunderstand its role in the drilling toolkit, misapply underbalanced technology in the field, and underestimate its true economic benefits.

Underbalanced drilling-defined as drilling while maintaining well bore pressure less than the formation’s internal pore pressure, and allowing formation fluids to flow-is not a new concept. In fact, underbalanced drilling was granted a U.S. patent more than 100 years ago and it has been practiced since the 1940s. However, it has only been within the last couple decades that oil and gas drillers began to truly adapt and apply UBD techniques. The United States and Canada have been the focal points for UBD applications. Although it is unknown to me how many total UBD wells have been drilled in the United States, I do know that at least 14,000 have been drilled in the Austin Chalk of Texas and Louisiana. Canadian drillers have also drilled many thousands of UBD wells enjoying success in both oil and gas applications.

Underbalanced techniques and technologies are also finding widespread global application, especially in the former Soviet Union, Europe, the Middle East and Southeast Asia. That should continue, considering that the majority of the world’s reserves are produced from fractured shales and carbonates-formation types that exhibit characteristics that tend to be problematic for conventional drilling operations. UBD can often solve a number of these problems, most notably lost circulation while drilling.

Underbalanced operations help mitigate circulation losses by keeping the hydrostatic pressure of the mud column below pore pressure. This minimizes the density differential between drilling fluid and hydrocarbons in the formation. , Two interesting spin-off technologies of UBD are Mudcap Drilling (MCD) and Managed Pressure Drilling (MPD). Both of these technologies utilize UBD equipment and decision-making processes to drill rock at or very near its native pore pressure. They are most often applied when well bore pressures are too high for surface management, rock stability in the well bore is too fragile for UBD margins or H2S is contained in the productive zone. When UBD is planned and executed successfully, near-well bore formation damage is avoided, which not only minimizes well stimulation requirements during the completion phase, but optimizes well productivity. In addition, drilling underbalanced improves penetration rates, drill bit life and overall operational efficiency. Moreover, because the well bore is allowed to flow formation fluids during drilling, using underbalanced methodologies can even help operators find overlooked reservoirs as they drill through them to the target zone.

Overall, wells properly drilled using underbalanced or near-balanced techniques have the potential to be many times more profitable than conventional operations. In fact, some operators have estimated up to 800 percent improvements in their rates of return on drilling investments using UBD. Improved ultimate recovery of reserves is also a byproduct of properly applied and executed UBD.

UBD Adaptations

Underbalanced techniques have evolved as adaptations to solve specific problems (i.e., formation damage, lost circulation, deficient penetration rates, abnormal formation pressures, etc.). This evolution has been supported in no small way by the development of a variety of tools, ranging from high-pressure rotating blow-out preventers and control heads, to wireline wet connect systems, to surface pressure-controlled chokes.

The end result is a number of UBD variations-or spin-offs-adapted to address these problems. These include flow drilling (the hydrocarbon-bearing zone is allowed to flow during drilling), mud cap drilling (a highly viscous fluid is placed in the annulus with a sacrificial lighter fluid in the drill pipe to control downhole pressures, especially in wells where there is risk of a total loss of returns, high surface pressure or H2S), and low head drilling (lightweight or aerated fluids or foams are used to prevent damaging the formation in zones that do not have the reservoir energy to flow during drilling).

All types of UBD techniques have their own advantages, but to realize their full potential rewards, operators must apply them correctly. Industrywide, millions of dollars in operational and opportunity cost have been lost because of the misuse of underbalanced technology. It can be an expensive mistake to view UBD as a cookie-cutter, once-size-fits-all solution.

Operators should approach every well in a field as a new drilling project worthy of engineering on its own merits. The fact that UBD may have been successfully applied on Well “A” does not automatically translate into Well ““ being an appropriate UBD candidate. A multitude of variables distinguish good underbalanced candidates from bad ones, leaving no room for guesswork. Operators also have to evaluate the potential drawbacks versus benefits of applying UBD to a candidate well.

Candidate selection includes such considerations as whether UBD conditions can be achieved in the well, and how much underbalance margin could be applied before collapsing the bore hole. How does UBD compare to overbalanced drilling and subsequent stimulation to overcome formation damage, and do the benefits of drilling underbalanced justify the possible additional cost for the particular well? It is also important to analyze the effect that underbalanced operations could have on water or gas coning, and the danger of crossflow or damage caused by imbibition even though drilling in an underbalanced state.

UBD is at odds with one disturbing trend that has developed in the industry. Sometimes proper up front engineering is performed for the first well in a field including in-depth well-specific training but then the same design is applied to another formation drilled in the same field or the same formation in a different field. The end result can be costly and unsuccessful. An operator may drill 10 wells based on the engineering performed for the first well and end up with eight expensive and unsuccessful producers. Ultimately, whether it be UBD, coiled tubing or slim hole drilling, or some other drilling method, those kind of results tend to lead the operator to abandon the technique altogether, along with the potential bottom-line benefits of its proper application.

The investment made in up front engineering and candidate selection pays off at the end. When it comes to underbalanced operations, due diligence is not a luxury, but an essential requirement. As with a golfer’s swing, the time he spends setting up his shot will have a profound effect on whether the ball ends up near the cup on the green or lost somewhere in the rough.

If there could only be one truism applied to UBD, it is that there is no substitute for hands on experience. Planning and executing UBD operations are highly application-specific. There is not a generic procedure for the process, but there is a thought process through which one can become experienced. Proper application of UBD to a given application may simply require that one find help from a reliable, experienced source.

Subsequently, there can be no all-inclusive formula or equipment for successful UBD operations. The key to project success as it applies to UBD lies rather in the ability to create and maintain a dynamic knowledge base in the evolving realm of UBD processes, which can discuss the complex and various intricacies of each field requirements.

UBD is not the path of least resistance when it comes to planning and drilling a well, but it may potentially be the path of the highest returns. There are easier ways to drill, but depending on the specifics of the candidate well, there may be no better way.