Underbalanced Drilling Methods as an Effective Solution for Drilling Issues

Rodrigo H. Ruschel

Bridging Course
Centre for English Language Teaching, University of Western Australia

Semester 1, 2013

Introduction
The Oil and Gas industry is looking for new methods of exploration and production. Currently, the oil and gas reservoirs are becoming more difficult to access because the operators are required to deal with complex and depleted reservoirs, such as low pressure zones, sub salt formations and tar zones. This is a challenge for both the on onshore and the offshore environment. These complications are directly related to drilling engineering because these professionals have to deal with complex situations involved in risks and pressure in order to keep operations under control.
In order to avoid problems during drilling operations might causing financial and environmental issues, underbalanced drilling (UBD) methods have been implemented. These methods are based in which both hydrostatic and dynamic pressures in the circulating down-hole fluid system are intentionally kept below or the same as the pressure of the formation being drilled
UBD methods have been used in order to defeat issues such as avoid formation damage, lost circulation, differential sticking and reduction of non-productive time. Also, it might improve productivity, reduce drilling time, bit life and also indicate reservoir zone productivity.
UBD methods are appearing as an effective solution for drilling issues. However, this technique requires a great level of planning, involving a combination of knowledge and high technology. Accurate ability is required to operate in underbalanced condition because a single error, such as, lack of knowledge relative to reservoir characteristics or equipment controlling, might provoke instability of underbalanced condition, may causing operation problems. Overall, when drilling underbalanced, is necessary to be concerned about reservoir characteristics and conditions, to plan each step about how this formation should be drilled and also reveal real-time monitoring, which is required in order to analyse all drilling requirements during an operation, such as, wellbore conditions, parameters of fluids, porosity, permeability and pressure. It is important to guarantee operation’s safety, beyond avoid environmental damage and other issues related to drilling operation. There are notable advantages of using UBD techniques, even considering reservoir or drilling aspects.
Complex Reservoir: Candidates to UBD
The first challenge for the oil and gas industry is to find out a hydrocarbon reservoir. Currently, it is increasingly difficult because the easy hydrocarbon reservoir access were gone. It means the hydrocarbon reservoirs are found mainly in complex formations, such as low pressure zones, sub-salt formations and tar zones.
Low pressure zones are found below mature field already discovered. These zones contain unstable pressure. According to Srinivisan et al, variability pressure zones might disturb well planning and operations because can lead to lost circulation or formation damage (2010, pp. 01). Sub-salt formations are primarily found in deep offshore fields. Several problems are associated with drilling in these complex zones. Mainly because depth and unstable pore pressure, requiring special equipment to deal with it. Vibration problems are common and problems associated with operation risks are often found (Srinivisan et al, 2010, pp. 02). One of the increasingly common problems in subsalt formation is tar deposit zones. This type of zone is not consistent enough to be drilled and also can bring damage to the bit. It is not a problem just when drilling. After drilling, the difficulty to deal with it continues because this kind of deposit might flows into wellbore, causing extra problems during production time such as, increasing non-productive time and stuck equipment into well bore (differential sticking) (Srinivisan, 2010, pp. 02).
All these problems such as lost circulation, formation damage, non-productive time, bit life and differential sticking are common during drilling operations in complex environments. Related to that, underbalanced conditions are implemented in these complex environments, with certain reservoir characteristics. However, UBD is not a technique to apply in all formation. In addition, there are several types of formation likely to benefit from application of this technique such as, low permeability reservoirs, low permeability reservoirs with potentially open micro fractures, highly fractured reservoirs, horizontal reservoirs and hard/competent overburdened reservoirs (WDLP, pp.19). These kinds of formations are often related to these drilling problems previously mentioned, such as lost circulation, formation damage, non-productive time, bit life and differential sticking.
To conclude this section, is remarkable to do the correlation between complex environment and increase number of problems encountered during drilling operations. It means these complex reservoirs are responsible for the great amount of problems due difficulty to access and requires a decent well planning. A decent planning needs to be acknowledged before starting an UBD operation.

UBD Planning – Process and Requirements
As mentioned before, underbalanced drilling methods are implemented in order to reduce problems during drilling operations. In one hand, it is a really useful technique and also helps to improve oil production. On the other hand, to drill underbalanced, is required a rigorous planning, being concerned about all phases during whole process of drilling. It is the reason of treating underbalanced drilling carefully, the planning and costs are important aspects to evaluate whether is suitable or not for a certain operation.
Design an underbalanced drilling operation requires attention to thorough details. According to Rafique, from University of Engineering & Technology Lahore in Pakistan, it is essential to acknowledge that “underbalanced drilling conditions may be achieved either by utilizing non-compressive drilling fluids or by the addition of gas to reduce drilling density” (2008, pp.02). Also, he states that in order to design a drilling circulating system is necessary to attend all requirements which satisfy the objectives of each specific operation (2008, pp.02). Three aspects are taken into account during when brainstorming an underbalanced drilling operation. First, bottomhole pressure, which should be controlled all the time in order to avoid problems and maintain underbalanced conditions during whole operation. Second, hole cleaning, which is responsible of the drilling fluid system “to guarantee the hole cleaning at any depth or inclination” (Rafique, 2008, pp. 02). Third aspect is surface separation equipment which is accountable to keep operation safely, controlling pressure and pipe work.
Another relevant aspect when designing an underbalanced drilling operation is drilling fluid selection phase. Drilling fluid selection varies depending on reservoir features and might be abstracted into three groups: single phase (gas), single phase (liquid) or two phase (gas+liquid) (Rafique, 2008, pp.02). This selection of drilling fluid is essential to the successful of operation.
In summary, it is essential to plan an underbalanced drilling operation carefully. As UBD being a complex operation, a great knowledge is required for the operators in order to execute it well. In addition, as a common knowledge, the budget of a drilling operation is substantially huge and might cause a financial problem and environmental issues when not designed and operated accurately. Because of that, an UBD operation requires a real-time monitoring in order to avoid those complications.

UBD Real-Time Monitoring
A successful underbalanced drilling operation requires control of downhole pressure, formation pressure and management of the fluids flowing from the well. However, it is yet a great challenge because requires real-time monitoring in order to avoid risks and problems during operations.
A real-time monitoring system allows measuring wellbore conditions, such as, parameters of fluid, drilling fluids and drilling pressure. Fluid parameters enable “to calculate multiphase flow in wellbore” (Li G. et al, 2011, pp.02). Also, the main aim for monitoring drilling fluid parameters is because of density, rheological behaviours and content of air inside wellbore. According to Li. G. et al, it is allowed to measure drilling fluids by liquid monitoring or by on-line gas monitoring (2011, pp.2). During liquid monitoring, the aim is measure the velocity of the liquid which returns from the well, and also learn about liquid makeup. However, under on-line gas monitoring is possible to comprehend the concentration and composition of hydrocarbons and non-hydrocarbons that have been found in the reservoir. Another aspect could be evaluated is drilling pressure, by wellbore pressure and formation pressure monitoring (Li. G. et al, 2011, pp.02).
Well pressure analyses it is one of the most relevant information because indicates general well conditions. There is a computational model enable to calculate wellbore condition and the objective is to extract data related to “wellbore flow pressure, circulating pressure loss, hydrostatic fluid column pressure and relative flow parameter at any depth” (Li. G., 2011, pp. 02). In addition, it is allow to measure in real-time monitoring drilling fluid density, viscosity, aeration rate and casing pressure of the well.
Other important information monitored momently are formation pressure and bottom hole pressure. During the measurement of formation pressure is required to use current drilling technology in order to find out about formation fluid production (FFP), wellbore liquid column pressure and equivalent circulation pressure (ECP) (Li. G., 2011, pp. 03). About bottom hole pressure, is acknowledged that during UBD operation is necessary to be kept lower than formation pressure in order to avoid overbalanced conditions (Li. G., 2011, pp.03).
In summary of this section, real-time monitoring is crucial to drilling operation development. It is essential because allows operators to acquire information from wellbore during drilling operation, such as, drilling fluid, drilling pressure, density, viscosity, circulating pressure loss and hydrostatic fluid column pressure. Because of drilling operations are complex and require a great knowledge, all these data is important to prevent future environmental problems and might guarantee operator’s safety, against any type of risk. Even though, real-monitoring improve efficiency during UBD operations, it needs to be better designed, with further studies, in order to increase productivity.
Conclusion
In conclusion of this research paper, we can understand the importance of underbalanced drilling (UBD) methods in the oil and gas industry. However, UBD methods are not simply to be implemented. It requires several prerequisites such as a reservoir with specific characteristics, and a combination of well planning and real-time monitoring in order to keep operation under control, making UBD methods appear as an effective solution for drilling problems.
In addition, the approach of this research paper is slightly superficial. It means explanations were given about important ideas, however did not provide us an entire interpretation about the topic. Future research should be done in order to acquire extra information, primarily about complex reservoirs and real-time monitoring. Complex reservoir characterisation should be better defined and classified in order to prove which kind of complex reservoir is more suitable for implementation of UBD methods. Although real-time monitoring should be investigated deeply, in order to find out evidence about how essential it is to UBD methods and which types of problems would be prevented by using a combination of real-time monitoring and UBD methods.

Bibliography: * Li, G. et al (2011). Study on Pressure Measuring and Formation Evaluation Methods While Underbalanced Drilling. SPE, Southwest Petroleum University and PetroChina Co.Ltd * Rafique, M. A. (2008). Underbalanced Drilling: “Remedy for Formation-Damage, Lost-Circulation, and Other Related Conventional-Drilling Problems”. SPE, University Of Engineering & Technology Lahore, Pakistan * Srinivasan A. et al (2010). New Directional Drilling With Liner System Allows Logging and Directional Control While Getting Casing Across Trouble Zones. Baker Hughes Incorporated * WDLP – Manual of Underbalanced Drilling