Despite the drilling industry’s shift toward managed pressure drilling (MPD) over the past half-decade, underbalanced drilling (UBD) continues to be a desirable option for many operations today, especially when drilling into pressure-depleted formations and/or attempting to reduce skin damage for better productivity. Because of this sustained demand, largely by operators willing to pay top dollar for skilled UBD personnel, a fundamental knowledge of the technique can benefit young petroleum engineers looking to excel in their field.
UBD Overview
The philosophy of UBD is diametrically opposed to traditional/conventional drilling methods and MPD. Conventional drilling uses an equivalent mudweight (EMW) that results in a bottomhole pressure (BHP) greater than that of the pore pressure exposed to the open hole to prevent formation fluids (oil, gas, or water) from entering the wellbore. MPD uses equivalent circulating density—a combination of hydrostatic pressure, circulating friction pressure, and applied surface pressure—to create a BHP equal to or greater than formation pore pressure.
Conversely, UBD utilizes an EMW maintained deliberately below the openhole pore pressure in at least one point of the open wellbore. The lower hydrostatic pressure exerted by the fluid column enables formation fluids to flow to the surface and through specialized surface equipment, thereby earning UBD the nickname “flow drilling.”
Perhaps the most beneficial aspect of UBD is the reduction or elimination of formation damage. When drilling traditionally, overbalanced drilling fluid can clog up or otherwise damage the naturally porous/permeable areas of the formation in a process called invasion. If the reservoir does not have sufficient energy during production to force this mudcake out, it can cause a significant decrease in—or total shutoff of—the natural flow of oil and gas from the formation to the surface.
In true UBD, the drilling fluid is much less likely to invade the formation; rather, it allows the formation to flow naturally. If the driller maintains an underbalanced state through the completion stage, damage can be minimized or avoided, which can optimize production rates and greatly increase the value of the well. Additionally, drillers can eliminate sophisticated drilling/completion fluid systems and costly remedial stimulation techniques historically used to overcome damage and increase production.
Fluids employed in UBD also vary with each operation. They may be composed of single-phase liquid or mud, gasified fluid (a two-phase fluid containing air, nitrogen, or natural gas mixed with water, oil-based fluid or mud), foam (a gasified fluid containing a surfactant), mist (gas with liquid droplets suspended in the mixture) and even air (using pure gas as the drilling fluid). The one constant is that the EMW is less than the pore pressure throughout the drilling process. Circulating these fluids at the proper rate is imperative for a successful UBD project to ensure adequate removal of the drilled cuttings from the wellbore.
Because of the decreased bottomhole pressure in the wellbore, there are further benefits of UBD such as increased rate of penetration (ROP) and reduced likelihood of differential sticking (where the drillstring becomes stuck to the wellbore wall). Another big advantage of UBD is reduction of lost circulation (LC), which occurs when overbalanced drilling fluids flow into the formation. Sometimes lost circulation creates irreparable formation damage or becomes so severe that the well is lost. With UBD, the inherently low-pressured drilling column does not fracture the formation needlessly, providing a likely remedy for formations where LC has been a problem.
However, it must be stressed that not all UBD applications lead directly to these benefits. As with all drilling techniques, there is the possibility of damage mechanisms that might counteract the applicability of UBD. A detailed analysis of all the well parameters and the project objectives is recommended before deciding whether to choose UBD for a given project.
UBD Fundamentals
Petroleum engineers performing at all levels during application of UBD will be expected to communicate with a wide variety of specialists. As such, a beginning engineer (with no field or design experience) should still expect to have a fundamental understanding of the basic components of UBD, such as hydraulics, fluid rheology, pressure regimes, well design, casing design, equipment layout, rig operations, and chain of command. As an individual’s field responsibilities expand, expectations will also increase. After three years, engineers should know whether UBD is viable for their formation, have good interactions with vendors, and also a good understanding of contingency planning. By the seventh year, the UBD engineer should have basic knowledge of all UBD facets, such as hydraulics planning, designing and procuring the necessary equipment, writing UBD procedures, writing contingency plans, and executing/overseeing the entire drilling operation.
Perhaps the most important aspect of successful UBD is a properly designed hydraulics plan, created and tested before the well is drilled. The driller can determine if the project needs to be designed for underbalanced drilling and how much margin exists between fluid pressure in the wellbore and pressure in the formation.
A proper hydraulics design is usually done using robust modeling software and once completed provides a “road map” that shows the driller the optimal parameters for drilling the well. These parameters should account for surface equipment, circulation rate, and fluid density at a minimum. Understanding this UBD road map is essential to a successful UBD operation. Young petroleum engineers should attend training courses on at least two hydraulics modeling software systems and use them frequently enough to become proficient in both. In addition, mastery of spreadsheets and associated graphical data representation (such as Microsoft Office Excel) is essential.
For undergraduates interested in UBD, it is important to study courses that provide a solid understanding of drilling disciplines. Basic drilling engineering courses help students understand standard rig equipment, casing and well design, basic hydraulics design, pressure drop calculations, and pressure management. Students must be able to identify indicators of drilling problems, such as kick-loss cycles, differential sticking, and surge/swab problems.
Knowing how to mitigate these issues is just as important, so students should also attend well control courses (methodology and calculations classes) that focus on training both novices and experts to maintain a safe workplace environment. Most of the employers and regulatory agencies now require such well control certifications from their employees. Typically, courses teach 1) How to react in such a well control situation, 2) Different tested procedures to solve the well control situation and resume drilling, and 3) Theory and calculations behind these methods. Courses also typically provide hands-on experience by means of a simulator. Hands-on experience on an operating rig is a great teacher as well, if the student can get permission to work on a rig.
Once these fundamentals have been established, there are several industry training courses that the young UBD engineer should consider. The classes focus on UBD problem solving and situations typically encountered in UBD operations. They are most often taught by licensed petroleum engineers and include curriculum requisite for prospective UBD engineers, such as equipment design, hydraulics design, well control operations when using UBD, logging while underbalanced, contingency planning, and UBD completions.
Candidate Identification
The first step in any successful UBD application is to ensure whether the formation is a good candidate for underbalanced drilling. It requires proper evaluation in great detail to make this determination. The engineer will need the following data: pore pressure/fracture pressure gradient plots, rate-of-penetration records, production rate or reservoir characteristics to calculate/estimate production rate, core analysis, formation fluid types, formation integrity test data (casing shoe pressure integrity test [pit] data), water/chemical sensitivity, lost circulation information, sour/corrosive gas data, location topography information, well logs from area wells, and triaxial stress data on any formation samples.
Lithological description is a good starting point to determine a UBD candidate, although not absolute. For instance, most UBD has occurred in carbonate rocks, but this is not a requirement. UBD has been applied successfully in limestone, dolomite, sandstone, clay, shale, and highly laminated and variable formations. The best candidates for UBD are formations where the full benefit of reduced fluid density, compared with conventional drilling, can be exploited.
Conversely, many formations are not suitable for UBD. Wildcat wells, or exploratory wells, so called because of the lack of offset well data, are generally poor candidates because the geologic drilling environment (pore pressure/fracture gradient) is usually unknown. There is also the question of hole stability, either chemical or mechanical. Development wells are much better candidates for UBD because the formation pressures and characteristics are well known. Also, development wells are more likely to be pressure-depleted and require a lower density fluid to drill successfully.
As with any unconventional technology, young engineers should remember that UBD must not be applied simply because it appears to be a good solution. Proper planning, candidate selection, design, and economic analysis must be performed to ensure UBD applicability and success. If an analysis shows that extensive stimulation applications will be necessary to make the well productive, even with the use of UBD, then the extra cost of UBD planning and surface equipment will typically outweigh the benefits of using UBD.
UBD on the Horizon
Fields mature every day and “difficult to reach” reserves are being discovered regularly. Operators are constantly seeking new, improved technology solutions. There is increased emphasis on safer and environmentally friendly operations, as well. These factors have pushed unconventional techniques such as UBD and MPD to the forefront of drilling engineering, which in turn has created a need for more experienced and skilled engineers. Learning to apply UBD properly can lead to a rewarding career for young engineers who enjoy challenges and solving problems before they occur. And since the technique is always evolving, there is always room for advancement and learning—even for experienced hands.
| Patrick B.B. Reynolds is a technical writer and registered project management professional at Signa Engineering. He has been a professional writer for 12 years and regularly contributes articles to oil and gas industry magazines. He can be reached at preynolds@signa.net. |
| Sagar Nauduri is a managed pressure drilling (MPD) engineer at Signa Engineering. He is a graduate of Texas A&M University in petroleum engineering. His interests are MPD and underbalanced drilling operations. He can be reached at snauduri@signa.net. |