The industry is now becoming more knowledgeable concerning underbalanced drilling (UBD) and managed pressure drilling (MPD). These techniques are gaining popularity because of their capability to control severe fluid losses and other problems that are inherent to conventional overbalanced drilling. As a result, considerable increase in their usage has been noted; however, with these increases, it has become apparent that there is a great deal of confusion concerning the basic concepts of each technique and when each should be used. Generally speaking, UBD can be described as a reservoir performance improvement and characterization tool that also provides drilling benefits. Another advantage is that UBD can offer a unique well testing environment in which the properties of reservoir layers can be determined while drilling. MPD, on the other hand, primarily addresses drilling-related problems that result in nonproductive time (NPT) in drilling scenarios. Sometimes, however, there are well scenarios in which both techniques might be needed in different hole sections. Reservoir information gained during underbalanced drilling also can help reduce the time and cost associated with gathering and analyzing well-test data post-completion with conventional methods. Techniques have been developed to quantify reservoir properties and characteristics for homogenous and heterogeneous and/or fractured reservoir systems. This paper focuses on where each concept should be used and what benefits can be expected from their application. Results from UBD and MPD case histories are used to quantify the results from these operations. Differences between the two techniques concerning equipment requirements and reservoir characterization potential are also analyzed. Introduction Many operators have chosen UBD and MPD to curtail severe fluid losses and other problems related to conventional overbalanced drilling. During operations focused on addressing drilling problems, some reservoir benefits have become apparent, convincing operators that with full underbalance, they may be able to realize improved production benefits. During UBD, comprehensive characterization of the reservoir while drilling can be obtained, and in some cases, zones that were unseen during overbalanced operations have been discovered, characterized, and subsequently appraised. Underbalanced and managed pressure drilling are not necessarily new technologies, but their potential has yet to be fully realized by the industry. Historically, the petroleum industry has been very slow to adopt "game changing" technologies. Granted, a handful of companies have taken a lead position in the employment of UBD and MPD techniques. These companies have already experienced the learning curves and no longer fear the step away from conventional drilling. However, a large part of the oil and gas industry needs further qualification and quantification for UBD and MPD. This paper will investigate the challenges that each technique can address and will compare the two techniques, the benefits each offers, and their applications. All drilling from conventional to air drilling could be considered as a form of "Managed Pressure Drilling," since for a drilling project to be conducted in the safest manner, the pressure must be controlled or managed. However, for purposes of this paper, managed pressure drilling will be considered as a discrete method, and discussions will focus on applications that are considered as "low-head" drilling applications.

Depletion of the Kristin HPHT field (911 bar, 172º C / 13,213 psi, 342º F) is occurring, challenging the drilling of future increased recovery wells. The main concern is uncertainty of the reservoir pressures in development areas of the producing intervals. Initially, the operational window range for the bottomhole pressure (BHP) varied from a pore pressure of 1.97 SG to a fracture pressure of 2.14–2.16 SG. With depletion, the difference has become narrower since fracture pressure is decreasing. This paper describes the need to implement managed pressure drilling (MPD) techniques in the Kristin field to overcome the problems related to a narrow drilling window. MPD is a technique that allows the use of lower-density drilling fluid, minimizing the overbalance pressure. Thus, BHP can be easily controlled and changed by applying surface-back pressure using a closed and pressurized circulation system and an automated choke. Circulating and static BHP can be increased rapidly, by applying back pressure at surface, if a higher than expected reservoir pressure is encountered. Conversely, BHP can rapidly be reduced, by reducing surface back pressure, should losses be seen down hole. The paper will discuss the use of a drilling fluid with a density giving a hydrostatic pressure lower than the original pore pressure to facilitate drilling operations and allow better control of the BHP for drilling future wells in the Kristin Field. The Kristin development wells may be the first wells globally to use MPD techniques in a harsh weather offshore environment on a floating drilling installation. The paper will also discuss some of the new equipment which has been designed to allow the implementation of MPD techniques in a harsh weather offshore environment from a semi submersible rig. Introduction The initial drilling program for Kristin is finished with 11 wells drilled and completed. Due to the high initial reservoir pressure, the pore pressure declines rapidly with production. Looking ahead at the possible need to drill increased recovery wells, several challenges to drilling were identified as a result of depletion, particularly with respect to the conventional requirement of keeping the BHP during drilling above the original pore pressure. MPD allows the safe use of lower mud weights much closer to the conventional drilling window limits, but in order to implement such drilling techniques on a floating drilling installation in the harsh weather environment of the Norwegian Sea, several issues had to be addressed. The equipment utilized to date for MPD from floaters is not suitable for use on a HPHT well with Norwegian Sea types of weather conditions. Some new rig-up methods had to be developed which are described in this paper (Santos et al, 2003 & Lage et al, 2005). To ensure successful implementation of totally new technology, a step by step approach is outlined (William, 2003). Finally in the last section the methods that can be used with the MPD system are described and it is shown how the drilling of further wells can be safely achieved with ongoing depletion.

Underbalanced drilling (UBD) and managed pressure drilling (MPD) are gaining in popularity as drilling methodologies to overcome some of the problems faced in conventional overbalanced drilling. These techniques are complimentary technologies rather than completely separate techniques, MPD techniques at one time having been classified under UBD. Therefore, with the current terminology and the many similarities they are often confused with one another. Underbalanced drilling is a tool both for reservoir performance improvement and reservoir characterization as well as for addressing drilling problems. MPD, on the other hand, is primarily a solution for mitigating drilling related problems. Both result in a reduction of non-productive time (NPT). Sometimes a combination of both techniques may be required for the same well. Different operators have chosen UBD and MPD with the goal of curtailing severe fluid losses and other drilling-related problems associated with conventional overbalanced drilling. Often times, while applying this technique to solve these drilling problems, the reservoir benefits have become apparent and have convinced the operators to go to full underbalance to realize the full reservoir production benefits without any period of overbalance throughout drilling, tripping, and completion operations. They have often found that when using UBD for reservoir production improvement, it is possible to perform comprehensive characterization of the reservoir while drilling. In some cases, zones that were not seen as productive during overbalanced operations have come to light, and reservoir characterization has enabled appraisal of these formations. Reservoir information obtained during the drilling phase can significantly reduce the time and cost associated with gathering and analyzing "well test" type data post-completion with conventional methods and these methods have been field tested and results compared to conventional well testing with favorable results. Introduction Underbalanced drilling was initially adopted for resolving drilling problems, but it soon became evident that this technique could also minimize reservoir damage. As originally conceived, underbalanced drilling technology included techniques that were fully underbalanced with influx to the surface as well as methods called "low-head" and "at-balance" drilling, in which the bottomhole pressure was kept marginally above or approximately equal to the pore pressure. These techniques later became designated as part of a separate category called managed pressure drilling, which has been adopted by the IADC. Many would agree that all drilling from conventional to air drilling might be considered as a form of "Managed Pressure Drilling," since for a drilling project to be conducted in the safest manner, the pressure must be controlled or managed. However, for purposes of this paper, managed pressure drilling will be considered as a discrete method, referring to applications that are considered as at balance or "low-head" (marginally overbalanced). This paper will describe normalized data results from UBD and MPD case histories. It will quantify the differences between the two techniques in terms of equipment requirements, reservoir characterization potential as well as quantifying the technical and economic benefits/limitations of each.

Underbalanced drilling is considered a technique for improving productivity, since the target zone in the well is drilled with bottom-hole drilling pressure lower than pore pressure in the target zone. These differences in pressure may minimize the risk of formation damage if the bottom hole circulating pressure is maintained lower than reservoir pressure at all times. For this reason, it is very important to have good knowledge of pore pressures and bottom hole circulating pressures and their variations while drilling. The ability to accurately predict bottom hole pressure is critical for both designing an underbalanced drilling operation as well as for predicting the effect of variations during the actual drilling operation. Commercial models are available to calculate wellbore hydraulics for fluids used in underbalanced operations. Modeling is employed for designing and monitoring underbalanced drilling operations and for modifying operational parameters in response to changing conditions. It is also very important to realize that, in case the bottom hole pressure signal from the downhole tool is lost, it is necessary to rely on the model being used. If calibration has already been made and predictions agree with actual measurements there is more confidence in the model and the prediction that the system is underbalanced. Otherwise underbalanced conditions may not be guaranteed, and formation damage could occur. Field data from oil and gas wells are used in this paper for comparing the prediction of bottom hole pressure using different wellbore hydraulic simulators against the actual pressure while drilling underbalanced. Results show which simulator and correlations give better predictions based upon characteristics of the wells and fluids type. The source of differences in results is examined and recommendations are made. Introduction Preventing or reducing formation damage in the near wellbore can enhance production, reduce stimulation needs, and delay the onset of gas or water break-through. When drilling conventionally overbalanced, one of the primary mechanisms of damage in the near wellbore is solids and filtrate invasion in the formation pore spaces. The damage can be due to mechanical or chemical factors. The main mechanical mechanism is drilling solids/fines blockage of pore throats. Another major factor is the change in the relative permeability's due to a change in the residual saturation profile due to filtrate invasion. Chemical causes include permeability reduction due to chemical interaction between the mud filtrate and pore fluids. Such interaction can result in the formation of scales, sludge, or emulsions which block fluid flow. Interaction can also take place with any shales or clay present, causing them to swell and reduce permeability. Care must also be taken when drilling underbalanced to avoid periods of overbalance. It has been shown that going overbalanced when drilling underbalanced can be even more damaging than if conventional drilling fluids had been used. Essentially no mud cake is created while drilling underbalanced. Hence, no "barrier" is created between the productive interval and the wellbore; and invasion of the filtrate from a fluid used in underbalanced drilling can travel deeply into the target reservoir.1 Wellbore hydraulics modeling is key when designing the underbalanced drilling operation, when monitoring during the operation, and when modifying operational parameters in response to changing conditions. During the underbalanced drilling operation, the bottom hole pressure (BHP) can be estimated using hydraulics simulators to verify whether an operational change will allow the system to remain underbalanced. Reliable simulation is key when avoidance of formation damage is one of the objectives in drilling underbalanced. Prior to making a change in the operational parameters the new, intended change can be simulated to determine the effect on the pressures. It is important for the engineer to know the degree of accuracy of the predictions to gauge the impact of recommended changes and to calibrate the simulator results with actual downhole pressure measurements.

TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractUnderbalanced drilling (UBD) and managed pressure drilling (MPD) are gaining in popularity as drilling methodologies to overcome some of the problems faced in conventional overbalanced drilling. These techniques are complimentary technologies rather than completely separate techniques, MPD techniques at one time having been classified under UBD. Therefore, with the current terminology and the many similarities they are often confused with one another. Underbalanced drilling is a tool both for reservoir performance improvement and reservoir characterization as well as for addressing drilling problems. MPD, on the other hand, is primarily a solution for mitigating drilling related problems. Both result in a reduction of non-productive time (NPT). Sometimes a combination of both techniques may be required for the same well.Different operators have chosen UBD and MPD with the goal of curtailing severe fluid losses and other drilling-related problems associated with conventional overbalanced drilling. Often times, while applying this technique to solve these drilling problems, the reservoir benefits have become apparent and have convinced the operators to go to full underbalance to realize the full reservoir production benefits without any period of overbalance throughout drilling, tripping, and completion operations. They have often found that when using UBD for reservoir production improvement, it is possible to perform comprehensive characterization of the reservoir while drilling. In some cases, zones that were not seen as productive during overbalanced operations have come to light, and reservoir characterization has enabled appraisal of these formations. Reservoir information obtained during the drilling phase can significantly reduce the time and cost associated with gathering and analyzing "well test" type data post-completion with conventional methods and these methods have been field tested and results compared to conventional well testing with favorable results.