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Badrul Mohamed Jan - One of the best experts on this subject based on the ideXlab platform.
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Viscoplastic Modeling of a Novel Lightweight Biopolymer Drilling Fluid for Underbalanced Drilling
2016Co-Authors: Munawar Khalil, Badrul Mohamed JanAbstract:ABSTRACT: This paper presents a concise investigation of viscoplastic behavior of a novel lightweight biopolymer Drilling fluid. Eight different rheological models namely the Bingham plastic model, Ostwald−De−Weale model, Herschel−Bulkley model, Casson model, Sisko model, Robertson−Stiff model, Heinz−Casson model, and Mizhari−Berk model were used to fit the experimental data. The effect of concentration of clay, glass bubbles, starch, and xanthan gum on the fluid rheological properties was investigated. Results show that the fitting process is able to successfully predict the rheological behavior of the fluid very well. The predicted values calculated from the best selected model are in a good agreement with the experimental data both in low and high (1500 s−1) rate of shear. The result also indicated that the presence of clay, glass bubbles, and xanthan gum have significantly changed the fluid behavior, while the presence of starch has not. Results also showed that all of the tested fluid seems to follow pseudoplastic behavior except for the following three tested fluids: one is fluid with the absence of clay, second and third is fluids with no glass bubble or xanthan gum, respectively. The first fluid tends to follows a Newtonian behavior, while the other two fluids tend to follow dilatants behavior. Underbalanced Drilling (UBD) has been considered as one of the best methods to reduce formation damage during Drilling. UBD is usually preferred due to its many advantages durin
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lightweight biopolymer Drilling fluid for Underbalanced Drilling an optimization study
Journal of Petroleum Science and Engineering, 2015Co-Authors: Symm Nee Lim, Munawar Khalil, Badrul Mohamed Jan, Brahim Si AliAbstract:Abstract This paper presents an optimization of a lightweight biopolymer Drilling fluid for Underbalanced Drilling (UBD) using Response Surface Methodology (RSM). Concentrations of four raw materials (glass bubbles, clay, xanthan gum and starch) were varied to analyze their effects on three vital responses: density, plastic viscosity (PV) and yield point (YP) of the fluid. Based on the results, the optimum condition was achieved at concentrations of glass bubbles, clay, xanthan gum and starch of 24.46% w/v, 0.63% w/v, 0.21% w/v and 2.41% w/v, respectively. The results showed that it is possible to predict the three response parameters using models generated by RSM since the experimental values were found to be in good agreement with the predicted values (error 98.5%).
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viscoplastic modeling of a novel lightweight biopolymer Drilling fluid for Underbalanced Drilling
Industrial & Engineering Chemistry Research, 2012Co-Authors: Munawar Khalil, Badrul Mohamed JanAbstract:This paper presents a concise investigation of viscoplastic behavior of a novel lightweight biopolymer Drilling fluid. Eight different rheological models namely the Bingham plastic model, Ostwald–De–Weale model, Herschel–Bulkley model, Casson model, Sisko model, Robertson–Stiff model, Heinz–Casson model, and Mizhari–Berk model were used to fit the experimental data. The effect of concentration of clay, glass bubbles, starch, and xanthan gum on the fluid rheological properties was investigated. Results show that the fitting process is able to successfully predict the rheological behavior of the fluid very well. The predicted values calculated from the best selected model are in a good agreement with the experimental data both in low and high (1500 s–1) rate of shear. The result also indicated that the presence of clay, glass bubbles, and xanthan gum have significantly changed the fluid behavior, while the presence of starch has not. Results also showed that all of the tested fluid seems to follow pseudopla...
Glennole Kaasa - One of the best experts on this subject based on the ideXlab platform.
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state and parameter estimation of a drift flux model for Underbalanced Drilling operations
IEEE Transactions on Control Systems and Technology, 2017Co-Authors: Amirhossein Nikoofard, Ulf Jakob F Aarsnes, Tor Arne Johansen, Glennole KaasaAbstract:We consider a drift-flux model (DFM) describing multiphase (gas–liquid) flow during Drilling. The DFM uses a specific slip law, which allows for transition between single and two phase flows. With this model, we design unscented Kalman filter (UKF) and extended Kalman filter (EKF) for the estimation of unmeasured state, production, and slip parameters using real-time measurements of the bottom-hole pressure, outlet pressure, and outlet flow rate. The OLGA high-fidelity simulator is used to create two scenarios from Underbalanced Drilling on which the estimators are tested: a pipe connection scenario and a scenario with a changing production index (PI). A performance comparison reveals that both UKF and EKF are capable of identifying the PIs of gas and oil from the reservoir into the well with acceptable accuracy, while the UKF is more accurate than the EKF. Robustness of the UKF and EKF for the pipe connection scenario is studied in case of uncertainties and errors in the reservoir and well parameters of the model. It is found that these methods are very sensitive to errors in the reservoir pore pressure value. However, they are robust in the presence of error in the liquid density value of the model.
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Fit-for-Purpose Modeling for Automation of Underbalanced Drilling Operations
2014Co-Authors: Aarsnes Ulf Jakob F., Florent Di Meglio, Ole Morten Aamo, Glennole KaasaAbstract:Automation has the potential to improve efficiency, precision, and safety of pressure and flow control during Underbalanced Drilling (UBD). In addition, advanced estimation theory can be used to extract more information from existing measurements to increase knowledge of the downhole conditions during operation. An essential part of an advanced (model based) pressure control system is the hydraulic model. Even with high- bandwidth distributed downhole measurements, a calibrated hydraulic model is required to ensure robustness, e.g. to sensor loss, and obtain real time estimates of unmeasured quantities and reservoir characteristics. In UBD operations, in contrast to Managed-Pressure-Drilling (MPD) operations, the flow in the annulus is inherently a multiphase gas liquid flow, which severely complicates the modelling. Much effort has been put into developing multiphase flow models, however, to date; most of these are very complex, requires extensive configuration and are not well suited for real-time applications. Consequently, a major gap with respect to automation of UBD is the lack of a fit-for-purpose model capable of reproducing the main dynamics of the multiphase flow in the annulus, while being sufficiently robust and suitable for real-time applications. In this work, we present recent advances on the development of a simplified fit-for-purpose model of the distributed gas- liquid dynamics, suited for advanced control of UBD operations. Using an automated calibration procedure, the model is shown to retain accuracy in a realistic case study. It is also shown that its reduced complexity enables real time coupling with measurements to obtain estimates of unmeasured quantities such as gas distribution and perform reservoir characterization
Williams Thomas - One of the best experts on this subject based on the ideXlab platform.
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Use of Cutting-Edge Horizontal and Underbalanced Drilling Technologies and Subsurface Seismic Techniques to Explore, Drill and Produce Reservoired Oil and Gas from the Fractured Monterey Below 10,000 ft in the Santa Maria Basin of California
'Office of Scientific and Technical Information (OSTI)', 2006Co-Authors: Witter George, Knoll Robert, Rehm William, Williams ThomasAbstract:This project was undertaken to demonstrate that oil and gas can be drilled and produced safely and economically from a fractured Monterey reservoir in the Santa Maria Basin of California by employing horizontal wellbores and Underbalanced Drilling technologies. Two vertical wells were previously drilled in this area with heavy mud and conventional completions; neither was commercially productive. A new well was drilled by the project team in 2004 with the objective of accessing an extended length of oil-bearing, high-resistivity Monterey shale via a horizontal wellbore, while implementing managed-pressure Drilling (MPD) techniques to avoid formation damage. Initial project meetings were conducted in October 2003. The team confirmed that the demonstration well would be completed open-hole to minimize productivity impairment. Following an overview of the geologic setting and local field experience, critical aspects of the application were identified. At the pre-spud meeting in January 2004, the final well design was confirmed and the well programming/service company requirements assigned. Various design elements were reduced in scope due to significant budgetary constraints. Major alterations to the original plan included: (1) a VSP seismic survey was delayed to a later phase; (2) a new (larger) surface hole would be drilled rather than re-enter an existing well; (3) a 7-in. liner would be placed into the top of the Monterey target as quickly as possible to avoid problems with hole stability; (4) evaluation activities were reduced in scope; (5) geosteering observations for fracture access would be deduced from penetration rate, cuttings description and hydrocarbon in-flow; and (6) rather than use nitrogen, a novel air-injection MPD system was to be implemented. Drilling operations, delayed from the original schedule by capital constraints and lack of rig availability, were conducted from September 12 to November 11, 2004. The vertical and upper curved sections were drilled and lined through the problematic shale member without major stability problems. The top of the targeted Monterey was thought to be seen at the expected TVD of 10,000 ft where the 7-in. liner was set at a 60{sup o} hole angle. Significant oil and gas shows suggested the fractured interval anticipated at the heel location had been penetrated. A total of 2572 ft of 6{Delta}-in. near-horizontal interval was placed in the shale section, extending planned well length by approximately 470 ft. Very little hydrocarbon in-flow was observed from fractures along the productive interval. This may be a result of the well trajectory falling underneath the Monterey fractured zone. Hydrocarbon observations, cuttings analysis and gamma-ray response indicated additional fractured intervals were accessed along the last {+-}900 ft of well length. The well was completed with a 2 and 7/8-in. tubing string set in a production packer in preparation for flow and swab tests to be conducted later by a service rig. The planned well time was estimated as 39 days and overall cost as $2.4 million. The actual results are 66 days at a total cost of $3.4 million. Well productivity responses during subsequent flow and swabbing tests were negative. The well failed to inflow and only minor amounts (a few barrels) of light oil were recovered. The lack of production may suggest that actual sustainable reservoir pressure is far less than anticipated. Temblor attempted in July, 2006, to re-enter and clean out the well and run an Array Induction log (primarily for resistivity and correlation purposes), and an FMI log (for fracture detection). Application of surfactant in the length of the horizontal hole, and acid over the fracture zone at 10,236 was also planned. This attempt was not successful in that the clean out tools became stuck and had to be abandoned
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Use of Cutting-Edge Horizontal and Underbalanced Drilling Technologies and Subsurface Seismic Techniques to Explore, Drill and Produce Reservoired Oil and Gas from the Fractured Monterey Below 10,000 ft in the Santa Maria Basin of California
Temblor Petroleum Co Llc, 2005Co-Authors: Witter George, Knoll Robert, Rehm William, Williams ThomasAbstract:This project was undertaken to demonstrate that oil and gas can be drilled and produced safely and economically from a fractured Monterey reservoir in the Santa Maria Basin of California by employing horizontal wellbores and Underbalanced Drilling technologies. Two vertical wells were previously drilled in this area with heavy mud and conventional completions; neither was commercially productive. A new well was drilled by the project team in 2004 with the objective of accessing an extended length of oil-bearing, high-resistivity Monterey shale via a horizontal wellbore, while implementing managed-pressure Drilling (MPD) techniques to avoid formation damage. Initial project meetings were conducted in October 2003. The team confirmed that the demonstration well would be completed open-hole to minimize productivity impairment. Following an overview of the geologic setting and local field experience, critical aspects of the application were identified. At the pre-spud meeting in January 2004, the final well design was confirmed and the well programming/service company requirements assigned. Various design elements were reduced in scope due to significant budgetary constraints. Major alterations to the original plan included: (1) a VSP seismic survey was delayed to a later phase; (2) a new (larger) surface hole would be drilled rather than re-enter an existing well; (3) a 7-in. liner would be placed into the top of the Monterey target as quickly as possible to avoid problems with hole stability; (4) evaluation activities were reduced in scope; (5) geosteering observations for fracture access would be deduced from penetration rate, cuttings description and hydrocarbon in-flow; and (6) rather than use nitrogen, a novel air-injection MPD system was to be implemented. Drilling operations, delayed from the original schedule by capital constraints and lack of rig availability, were conducted from September 12 to November 11, 2004. The vertical and upper curved sections were drilled and lined through the problematic shale member without major stability problems. The top of the targeted Monterey was thought to be seen at the expected TVD of 10,000 ft where the 7-in. liner was set at a 60{sup o} hole angle. Significant oil and gas shows suggested the fractured interval anticipated at the heel location had been penetrated. A total of 2572 ft of 6 1/8-in. near-horizontal interval was placed in the shale section, extending planned well length by approximately 470 ft. Very little hydrocarbon in-flow was observed from fractures along the productive interval. This may be a result of the well trajectory falling underneath the Monterey fractured zone. Hydrocarbon observations, cuttings analysis and gamma-ray response indicated additional fractured intervals were accessed along the last {+-}900 ft of well length. The well was completed with a 2 7/8-in. tubing string set in a production packer in preparation for flow and swab tests to be conducted later by a service rig. The planned well time was estimated as 39 days and overall cost as $2.4 million. The actual results are 66 days at a total cost of $3.4 million. Well productivity responses during subsequent flow and swabbing tests were negative. The well failed to inflow and only minor amounts (a few barrels) of light oil were recovered. The lack of production may suggest that actual sustainable reservoir pressure is far less than anticipated. Temblor is currently planning to re-enter and clean out the well and run an Array Induction log (primarily for resistivity and correlation purposes), and an FMI log (for fracture detection). Depending on the results of these logs, an acidizing or re-drill program will be planned
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USE OF CUTTING-EDGE HORIZONTAL AND Underbalanced Drilling TECHNOLOGIES AND SUBSURFACE SEISMIC TECHNIQUES TO EXPLORE, DRILL AND PRODUCE RESERVOIRED OIL AND GAS FROM THE FRACTURED MONTEREY BELOW 10,000 FT IN THE SANTA MARIA BASIN OF CALIFORNIA
Temblor Petroleum Company (United States), 2005Co-Authors: Witter George, Knoll Robert, Rehm William, Williams ThomasAbstract:This project was undertaken to demonstrate that oil and gas can be drilled and produced safely and economically from a fractured Monterey reservoir in the Santa Maria Basin of California by employing horizontal wellbores and Underbalanced Drilling technologies. Two vertical wells were previously drilled in this area by Temblor Petroleum with heavy mud and conventional completions; neither was commercially productive. A new well was drilled by the project team in 2004 with the objective of accessing an extended length of oil-bearing, high-resistivity Monterey shale via a horizontal wellbore, while implementing managed-pressure Drilling (MPD) techniques to avoid formation damage. Initial project meetings were conducted in October 2003. The team confirmed that the demonstration well would be completed open-hole to minimize productivity impairment. Following an overview of the geologic setting and local field experience, critical aspects of the application were identified. At the pre-spud meeting in January 2004, the final well design was confirmed and the well programming/service company requirements assigned. Various design elements were reduced in scope due to significant budgetary constraints. Major alterations to the original plan included: (1) a VSP seismic survey was delayed to a later phase; (2) a new (larger) surface hole would be drilled rather than re-enter an existing well; (3) a 7-in. liner would be placed into the top of the Monterey target as quickly as possible to avoid problems with hole stability; (4) evaluation activities were reduced in scope; (5) geosteering observations for fracture access would be deduced from penetration rate, cuttings description and hydrocarbon in-flow; and (6) rather than use nitrogen, a novel air-injection MPD system was to be implemented. Drilling operations, delayed from the original schedule by capital constraints and lack of rig availability, were conducted from September 12 to November 11, 2004. The vertical and upper curved sections were drilled and lined through the problematic shale member without major stability problems. The top of the targeted Monterey was thought to be seen at the expected TVD of 10,000 ft where the 7-in. liner was set at a 60{sup o} hole angle. Significant oil and gas shows suggested the fractured interval anticipated at the heel location had been penetrated. A total of 2572 ft of 6.-in. near-horizontal interval was placed in the shale section, extending planned well length by approximately 470 ft. Very little hydrocarbon in-flow was observed from fractures along the productive interval. This may be a result of the well trajectory falling underneath the Monterey fractured zone. Hydrocarbon observations, cuttings analysis and gamma-ray response indicated additional fractured intervals were accessed along the last {+-}900 ft of well length. The well was completed with a 2 7/8-in. tubing string set in a production packer in preparation for flow and swab tests to be conducted later by a service rig. The planned well time was estimated as 39 days and overall cost as $2.4 million. The actual results are 66 days at a total cost of $3.4 million. Well productivity responses during subsequent flow and swabbing tests were negative. The well failed to inflow and only minor amounts (a few barrels) of light oil were recovered. The lack of production may suggest that actual sustainable reservoir pressure is far less than anticipated. Temblor is currently investigating the costs and operational viability of re-entering the well and conducting an FMI (fracture detection) log and/or an acid stimulation. No final decision or detailed plans have been made regarding these potential interventions at this time
Ulf Jakob F Aarsnes - One of the best experts on this subject based on the ideXlab platform.
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flow and pressure control of Underbalanced Drilling operations using nmpc
Journal of Process Control, 2018Co-Authors: Torbjorn Pedersen, Ulf Jakob F Aarsnes, Johnmorten GodhavnAbstract:Abstract This work evaluates the use of non-linear model predictive control (NMPC) for multivariate control of pressure and flow during Underbalanced Drilling (UBD) using a fit-for-purpose mechanistic model. Flow and pressure control are instrumental to the stability, safety, and successful Drilling of a well. Failure to keep within the pressure envelope may have severe economic and environmental consequences. A relevant Drilling scenario with different control objectives is evaluated to show the impact of the tuning and configuration of the control system. It is shown that with a sufficiently good model match, it is possible to achieve good performance with small deviations from the selected operational criteria, even when there are large changes in operational area. The multivariate control system manipulates choke pressure, pump flow, and separator set-points to simultaneously achieve both downhole and topside objectives. The optimization problem is solved within the relevant time dynamics of the system.
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state and parameter estimation of a drift flux model for Underbalanced Drilling operations
IEEE Transactions on Control Systems and Technology, 2017Co-Authors: Amirhossein Nikoofard, Ulf Jakob F Aarsnes, Tor Arne Johansen, Glennole KaasaAbstract:We consider a drift-flux model (DFM) describing multiphase (gas–liquid) flow during Drilling. The DFM uses a specific slip law, which allows for transition between single and two phase flows. With this model, we design unscented Kalman filter (UKF) and extended Kalman filter (EKF) for the estimation of unmeasured state, production, and slip parameters using real-time measurements of the bottom-hole pressure, outlet pressure, and outlet flow rate. The OLGA high-fidelity simulator is used to create two scenarios from Underbalanced Drilling on which the estimators are tested: a pipe connection scenario and a scenario with a changing production index (PI). A performance comparison reveals that both UKF and EKF are capable of identifying the PIs of gas and oil from the reservoir into the well with acceptable accuracy, while the UKF is more accurate than the EKF. Robustness of the UKF and EKF for the pipe connection scenario is studied in case of uncertainties and errors in the reservoir and well parameters of the model. It is found that these methods are very sensitive to errors in the reservoir pore pressure value. However, they are robust in the presence of error in the liquid density value of the model.
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A Methodology for Classifying Operating Regimes in Underbalanced-Drilling Operations
Society of Petroleum Engineers Journal, 2016Co-Authors: Ulf Jakob F Aarsnes, Florent Di Meglio, Robert Graham, Ole Morten AamoAbstract:This paper proposes an extension to an existing operating-envelope technique used for Underbalanced Drilling (UBD) to enhance control of bottomhole pressure and inflow parameters. With the use of an implementation of the drift-flux model (DFM) with boundary conditions typically encountered in Underbalanced operations (UBO), a steady-state analysis of the system is performed. Through this analysis, four distinct operating regimes are identified, and the behavior in each of them is investigated through steady-state calculations and transient simulations. In particular, the analysis reveals that a section of the operating envelope previously believed to be unstable/transient is, in fact, stable/steady when a fixed choke opening is used as an independent variable in place of a fixed wellhead pressure (WHP). This results in the steady-state operating envelope being extended, and gives an increased understanding of the well behavior encountered in UBO toward enabling the introduction of automated control. Finally, we investigate the mechanism behind severe slugging in UBO and argue that the cause is different from that of the slugging encountered in production and multiphase transport.
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Fit-for-Purpose Modeling for Automation of Underbalanced Drilling Operations
HAL CCSD, 2014Co-Authors: Ulf Jakob F Aarsnes, Di Meglio Florent, Aamo, Ole Morten, Kaasa, Glenn OleAbstract:International audienceAutomation has the potential to improve efficiency, precision, and safety of pressure and flow control during Underbalanced Drilling (UBD). In addition, advanced estimation theory can be used to extract more information from existing measurements to increase knowledge of the downhole conditions during operation.An essential part of an advanced (model based) pressure control system is the hydraulic model. Even with high- bandwidth distributed downhole measurements, a calibrated hydraulic model is required to ensure robustness, e.g. to sensor loss, and obtain real time estimates of unmeasured quantities and reservoir characteristics.In UBD operations, in contrast to Managed-Pressure-Drilling (MPD) operations, the flow in the annulus is inherently a multiphase gas liquid flow, which severely complicates the modelling. Much effort has been put into developing multiphase flow models, however, to date; most of these are very complex, requires extensive configuration and are not well suited for real-time applications. Consequently, a major gap with respect to automation of UBD is the lack of a fit-for-purpose model capable of reproducing the main dynamics of the multiphase flow in the annulus, while being sufficiently robust and suitable for real-time applications.In this work, we present recent advances on the development of a simplified fit-for-purpose model of the distributed gas- liquid dynamics, suited for advanced control of UBD operations. Using an automated calibration procedure, the model is shown to retain accuracy in a realistic case study. It is also shown that its reduced complexity enables real time coupling with measurements to obtain estimates of unmeasured quantities such as gas distribution and perform reservoir characterizatio
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an adaptive observer for hyperbolic systems with application to Underbalanced Drilling
IFAC Proceedings Volumes, 2014Co-Authors: Florent Di Meglio, Delphine Breschpietri, Ulf Jakob F AarsnesAbstract:Copyright © 2014 IFAC. Published by Elsevier Ltd. All rights reserved.. This is the authors' accepted and refereed manuscript to the article.
A A Dahab - One of the best experts on this subject based on the ideXlab platform.
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potential implementation of Underbalanced Drilling technique in egyptian oil fields
Journal of King Saud University: Engineering Sciences, 2011Co-Authors: K A Fattah, S M Elkatatney, A A DahabAbstract:Abstract The need to increase productivity and to reduce Drilling damage favors the use of Underbalanced Drilling (UBD) technology. In highly depleted reservoirs, extremely low-density fluids, such as foams or aerated mud, are used to achieve circulating densities lower than the pore pressure. In such cases, the induced modification of the in situ stresses has to be supported mainly by the rock, with little contribution from the Drilling fluid pressure. The application of Underbalanced Drilling depends on the mechanical stability of the drilled formation, among other factors. In general, poorly consolidated, depleted formations are not suited for that technology. In this paper, 23 UBD worldwide cases have been analyzed; two of which are from Egyptian fields and the others are from Iran, Algeria, Kuwait, Oman, Texas, Mexico, Indonesia, Canada, Libya, Middle East, Qatar, Saudi Arabia and Lithuania. From these analyses, the reasons of failure or success have been stated. The reasons of success included depleted reservoirs and highly fractured carbonates formation while, the reasons of failure include over pressurized shale, highly tectonic stress areas, and downhole failures. The main attractive application of this technology was proposed to be only in the reservoir section, and the target was to prevent the reservoir damage and hence increase the productivity and recovery factor. A proposed Underbalanced Drilling program is developed based on these analyses to be used in the three main regions in oil and gas producing Egyptian fields. The aerated mud was selected as a Drilling fluid to drill the reservoir section in Western Desert and Gulf of Suez region whereas the single phase fluid was selected as a Drilling fluid in the Nile Delta region.
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potential implementation of Underbalanced Drilling technique in egyptian oil fields
International Oil and Gas Conference and Exhibition in China, 2010Co-Authors: K A Fattah, S M Elkatatney, A A DahabAbstract:KEYWORDSUnderbalance Drilling;Drilling fluids;Drilling cost;Hole problemAbstract The need to increase productivity and to reduce Drilling damage favors the use of under-balanced Drilling (UBD) technology. In highly depleted reservoirs, extremely low-density fluids,such as foams or aerated mud, are used to achieve circulating densities lower than the pore pressure.In such cases, the induced modification of the in situ stresses has to be supported mainly by therock, with little contribution from the Drilling fluid pressure. The application of Underbalanced dril-ling depends on the mechanical stability of the drilled formation, among other factors. In general,poorly consolidated, depleted formations are not suited for that technology.In this paper, 23 UBD worldwide cases have been analyzed; two of which are from Egyptianfields and the others are from Iran, Algeria, Kuwait, Oman, Texas, Mexico, Indonesia, Canada,Libya, Middle East, Qatar, Saudi Arabia and Lithuania. From these analyses, the reasons of failureor success have been stated. The reasons of success included depleted reservoirs and highly fracturedcarbonates formation while, the reasons of failure include over pressurized shale, highly tectonicstress areas, and downhole failures. The main attractive application of this technology was pro-posed to be only in the reservoir section, and the target was to prevent the reservoir damage andhence increase the productivity and recovery factor.* Corresponding author.E-mail address: kelshreef@ksu.edu.sa (K.A. Fattah).1018-3639 a 2010 King Saud University. Production and hosting byElsevier B.V. All rights reserved.Peer review under responsibility of King Saud University.doi:10.1016/j.jksues.2010.02.001
