Table Of Content

APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Introduction To Underbalanced Drilling Prepared by: Steve Nas Asia Pacific Regional Engineering Manager Weatherford Underbalanced Systems 238a Tompson Road #16-01/04 Novena Square Tower A Singapore 307684 Tel +65 6511 3688 Email: [email protected] Ref APR-WUBS-WFT-001 Page 1 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Table of Contents INTRODUCTION..................................................................................................................................5 OBJECTIVES..........................................................................................................................................5 PERFORMANCE DRILLING.....................................................................................................................6 MANAGED PRESSURE DRILLING...........................................................................................................7 WHAT IS UNDERBALANCED DRILLING?....................................................................................8 UNDERBALANCED RESERVOIR DRILLING...........................................................................................10 HISTORY OF UNDERBALANCED DRILLING.............................................................................11 WHY DRILL UNDERBALANCED?.................................................................................................12 UNDERBALANCED VERSUS OVERBALANCED......................................................................................14 DISADVANTAGES OF UNDERBALANCED DRILLING.............................................................................15 LIMITATIONS......................................................................................................................................15 CLASSIFICATION SYSTEM FOR UNDERBALANCED DRILLING.........................................16 HOW TO DRILL UNDERBALANCED?..........................................................................................18 DATA COLLECTION............................................................................................................................19 Reservoir Data..............................................................................................................................19 Reservoir Screening Tool (RST)....................................................................................................20 SURE Phase II..............................................................................................................................22 Reservoir Damage Assessment.....................................................................................................22 Drilling Data.................................................................................................................................24 Analog Data..................................................................................................................................24 EVALUATION......................................................................................................................................25 Risk Assessment............................................................................................................................26 CANDIDATE SELECTION......................................................................................................................29 UBS Well Type Matrix..................................................................................................................30 HIGH LEVEL COST ESTIMATES...........................................................................................................31 UBS FEASIBILITY...............................................................................................................................31 DETAILED WELL PLANNING........................................................................................................33 CIRCULATION SYSTEM DESIGN..........................................................................................................33 Fluid Selection..............................................................................................................................33 Formation Damage.......................................................................................................................38 Gaseous Fluids..............................................................................................................................40 Mist Systems..................................................................................................................................42 Foam Systems................................................................................................................................43 Gasified Systems...........................................................................................................................46 Single phase fluids........................................................................................................................47 Gas lift systems.............................................................................................................................48 Drillpipe injection.........................................................................................................................49 Annular injection..........................................................................................................................51 Parasite string injection................................................................................................................52 Gases for Underbalanced Drilling................................................................................................53 Air.................................................................................................................................................53 Natural Gas...................................................................................................................................53 Cryogenic Nitrogen.......................................................................................................................54 Membrane Nitrogen......................................................................................................................57 Exhaust Gas..................................................................................................................................58 FLOW MODELING...............................................................................................................................59 Pressure calculations....................................................................................................................59 Flow modeling..............................................................................................................................67 Ref APR-WUBS-WFT-001 Page 2 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Equivalent Motor Throughput versus Gas Injection Rate.............................................................75 Hole cleaning................................................................................................................................76 Annular Friction Pressure versus Gas Injection Rate..................................................................78 Annular Liquid Hold Up versus Gas Injection Rate.....................................................................79 Drillstring Injection Pressure versus Gas Injection Rate.............................................................80 Drillstring Liquid Hold Up versus Gas Injection Rate.................................................................81 Reservoir Inflow............................................................................................................................82 DRILLSTRING AND DOWN HOLE TOOL DESIGN..................................................................................83 Pressure While Drilling (PWD)....................................................................................................83 Conventional MWD Tools in Underbalanced Drilling.................................................................83 Electromagnetic Measurement While Drilling (EMWD)..............................................................83 Non Return Valves........................................................................................................................86 Wireline Retrievable Float Valves................................................................................................87 Down Hole Isolation Valves.........................................................................................................88 Drillstring Design.........................................................................................................................89 Drillpipe........................................................................................................................................91 Hard banding................................................................................................................................92 Drillpipe Rubbers..........................................................................................................................92 Jars...............................................................................................................................................92 Down Hole Motors........................................................................................................................93 EQUIPMENT SELECTION......................................................................................................................94 Gas Injection Equipment...............................................................................................................94 Air compressors............................................................................................................................94 Nitrogen Generation System.........................................................................................................95 Booster Compressors....................................................................................................................97 Well Control Equipment................................................................................................................99 Rotating Diverters.......................................................................................................................100 Snubbing systems........................................................................................................................105 Separation Equipment.................................................................................................................106 Horizontal separators.................................................................................................................107 Vertical Separators.....................................................................................................................108 UBD Choke manifold..................................................................................................................109 Data acquisition..........................................................................................................................110 Flares..........................................................................................................................................112 WELL CONTROL STRATEGY.............................................................................................................113 Well Kill Strategy........................................................................................................................113 Well Control................................................................................................................................113 Erosion........................................................................................................................................115 Corrosion Management..............................................................................................................116 Corrosion Inhibitor Types...........................................................................................................118 PERSONNEL SELECTION....................................................................................................................119 TRAINING AND COMPETENCY...........................................................................................................120 OPERATIONAL PROCEDURES............................................................................................................121 COMPLETING UNDERBALANCED DRILLED WELLS...........................................................................123 Snubbing.....................................................................................................................................123 Workover of an Underbalanced Drilled Well.............................................................................127 Underbalanced Drilled Multi-Lateral Wells...............................................................................127 SUBSURFACE SERVICES....................................................................................................................128 PROCESS FLOW DIAGRAMS..............................................................................................................129 RIG AND LEASE LAYOUT..................................................................................................................130 HEALTH SAFETY AND ENVIRONMENTAL PLANNING.........................................................................131 Environmental Aspects................................................................................................................131 Safety Aspects..............................................................................................................................131 DETAILED COST ESTIMATES.............................................................................................................133 UBS PROGRAM................................................................................................................................134 UNDERBALANCED RECORDS.....................................................................................................136 Ref APR-WUBS-WFT-001 Page 3 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 REFERENCES...................................................................................................................................138 SUPPLIERS OF UNDERBALANCED DRILLING SERVICES..................................................153 WEATHERFORD................................................................................................................................153 HALLIBURTON..................................................................................................................................153 SHAFFER...........................................................................................................................................153 TESCO...............................................................................................................................................153 LEADING EDGE ADVANTAGE..........................................................................................................153 BLADE ENERGY PARTNERS..............................................................................................................153 SCANDPOWER...................................................................................................................................153 NEOTEC............................................................................................................................................153 ABBREVIATIONS.............................................................................................................................154 Ref APR-WUBS-WFT-001 Page 4 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Introduction This introduction is intended to provide an overview of current underbalanced drilling technology and is therefore by no means exhaustive. It should serve as a guide to the current UBD technology, explaining how, when and why underbalanced drilling is carried out. Objectives The objective of this introduction is to provide an awareness of underbalanced reservoir drilling technology and the associated operations. It also provides a starting point and basic orientation for identification of hazards and corresponding risk assessment. This introduction covers a very brief introduction to managed pressure drilling and performance drilling to ensure a complete overview of the technologies associated with underbalanced drilling. Weatherford underbalanced drilling division currently offers three sub product lines under the underbalanced umbrella and these can be listed as : • Performance Drilling (PD) This technology is intended to achieve maximum penetration rates through reducing the well bore pressure to a minimum possible value. • Managed Pressure Drilling (MPD) With MPD the intention is to precisely manage and control the annular pressure to allow the bottom hole pressure to be within close limits to “walk the line” • Underbalanced Drilling The intention of underbalanced drilling is to reduce formation damage, discover potential bypassed pay, and increase reserves by allowing access to these reserves thus ultimately increasing net NPV. In underbalanced reservoir drilling the well is designed to allow the reservoir to flow to surface whilst drilling. Ref APR-WUBS-WFT-001 Page 5 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Performance Drilling This is the original air drilling technique to increase penetration rate. In performance drilling, the bottomhole pressure is as low as possible to increase drilling performance. Performance Drilling P >>>P = P + P + P formation bottomhole hydrostatic friction choke The objective of performance drilling is to reduce the drilling costs by drilling faster. This is normally achieved by using gas or air as a circulation medium. Reducing the bottom hole circulation pressure significantly increases the penetration rate. Fig 3 Performance Drilling Definition Ref APR-WUBS-WFT-001 Page 6 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Managed Pressure Drilling A number of related technologies have now been established in the industry as a result of tools used originally in underbalanced drilling. One of these techniques is Managed Pressure Drilling, which has been defined as follows: Managed Pressure Drilling is an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly.” It means that the annular pressure profile is controlled in such a way that the well is balanced at all times, or in mathematical format: Managed Pressure Drilling P = P = P + P + P reservoir bottomhole hydrostatic friction choke Fig 2 Managed Pressure Drilling Definition Ref APR-WUBS-WFT-001 Page 7 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 What is Underbalanced Drilling? Underbalanced reservoir drilling is defined by the IADC UBO committee as: Drilling with the hydrostatic head of the drilling fluid intentionally designed to be lower than the pressure of the formations being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid. Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids which must be circulated from the well and controlled at surface. This in effect means that in underbalanced reservoir drilling, the effective downhole pressure within the wellbore is always maintained to be lower than the reservoir pressure and it is intended to have reservoir inflow into the wellbore. Underbalanced Drilling P > P = P + P + P reservoir bottomhole hydrostatic friction choke Fig 1 Underbalanced Reservoir Drilling Definitions Ref APR-WUBS-WFT-001 Page 8 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Equipment used for Managed Pressure Drilling Underbalanced Drilling Performance Drilling UBD PD RCD Gas Handling Choke Manifold Float Subs MPD RCD = Rotating Control Device Fig 4 Shared Equipment for MPD, UBD and PD Technology These three techniques have equipment in common but are all applied in different circumstances. This introduction concentrates on underbalanced reservoir drilling and the associated equipment and techniques, not on the performance drilling or managed pressure drilling aspects. Ref APR-WUBS-WFT-001 Page 9 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Underbalanced Reservoir Drilling This introduction concentrates on underbalanced reservoir drilling and the associated techniques and equipment used in this drilling method. In underbalanced reservoir drilling, the wellbore pressure is maintained below the reservoir pressure at all times, and the resulting inflow from the reservoir is carefully controlled during the entire drilling process. Underbalanced Drilling P > P = P + P + P reservoir bottomhole hydrostatic friction choke The well is still controlled by controlling the wellbore pressure, but this pressure is maintained to be always below the reservoir pressure. Primary well control is no longer an overbalanced barrier of a column of fluid but is replaced by flow control using a combination of hydrostatic pressure, friction pressure and surface choke pressure. The BOP stack remains as the secondary well control barrier. It must be pointed out that a UBD well operates on a single barrier. The bottom hole circulation pressure is a combination of hydrostatic pressure, circulation friction losses and surface pressure applied at the choke. The hydrostatic pressure is considered a passive pressure and is a result of the fluid density and the density contribution of any drilled cuttings and a small contribution of any gas in the well. The friction Pressure is a dynamic pressure (It changes with pumps on or off) and results from circulating friction of the fluid used. The choke pressure arises from annular back pressure applied at surface. These three pressures are controlled at all times and ensure that flow control is maintained whilst drilling underbalanced. The lower hydrostatic head avoids the build-up of filter cake on the reservoir formation and avoids the invasion of whole mud and drilling solids into the formation. This helps to improve productivity of the wellbore and reduces any pressure related drilling problems. Ref APR-WUBS-WFT-001 Page 10 of 154

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APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Introduction To Underbalanced Drilling Prepared by: Steve Nas Asia Pacific Regional Engineering Manager Weatherford Underbalanced Systems 238a Tompson Road #16-01/04 Novena Square Tower A Singapore 307684 Tel +65 6511 3688 Email: [email protected] Ref APR-WUBS-WFT-001 Page 1 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Table of Contents INTRODUCTION..................................................................................................................................5 OBJECTIVES..........................................................................................................................................5 PERFORMANCE DRILLING.....................................................................................................................6 MANAGED PRESSURE DRILLING...........................................................................................................7 WHAT IS UNDERBALANCED DRILLING?....................................................................................8 UNDERBALANCED RESERVOIR DRILLING...........................................................................................10 HISTORY OF UNDERBALANCED DRILLING.............................................................................11 WHY DRILL UNDERBALANCED?.................................................................................................12 UNDERBALANCED VERSUS OVERBALANCED......................................................................................14 DISADVANTAGES OF UNDERBALANCED DRILLING.............................................................................15 LIMITATIONS......................................................................................................................................15 CLASSIFICATION SYSTEM FOR UNDERBALANCED DRILLING.........................................16 HOW TO DRILL UNDERBALANCED?..........................................................................................18 DATA COLLECTION............................................................................................................................19 Reservoir Data..............................................................................................................................19 Reservoir Screening Tool (RST)....................................................................................................20 SURE Phase II..............................................................................................................................22 Reservoir Damage Assessment.....................................................................................................22 Drilling Data.................................................................................................................................24 Analog Data..................................................................................................................................24 EVALUATION......................................................................................................................................25 Risk Assessment............................................................................................................................26 CANDIDATE SELECTION......................................................................................................................29 UBS Well Type Matrix..................................................................................................................30 HIGH LEVEL COST ESTIMATES...........................................................................................................31 UBS FEASIBILITY...............................................................................................................................31 DETAILED WELL PLANNING........................................................................................................33 CIRCULATION SYSTEM DESIGN..........................................................................................................33 Fluid Selection..............................................................................................................................33 Formation Damage.......................................................................................................................38 Gaseous Fluids..............................................................................................................................40 Mist Systems..................................................................................................................................42 Foam Systems................................................................................................................................43 Gasified Systems...........................................................................................................................46 Single phase fluids........................................................................................................................47 Gas lift systems.............................................................................................................................48 Drillpipe injection.........................................................................................................................49 Annular injection..........................................................................................................................51 Parasite string injection................................................................................................................52 Gases for Underbalanced Drilling................................................................................................53 Air.................................................................................................................................................53 Natural Gas...................................................................................................................................53 Cryogenic Nitrogen.......................................................................................................................54 Membrane Nitrogen......................................................................................................................57 Exhaust Gas..................................................................................................................................58 FLOW MODELING...............................................................................................................................59 Pressure calculations....................................................................................................................59 Flow modeling..............................................................................................................................67 Ref APR-WUBS-WFT-001 Page 2 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Equivalent Motor Throughput versus Gas Injection Rate.............................................................75 Hole cleaning................................................................................................................................76 Annular Friction Pressure versus Gas Injection Rate..................................................................78 Annular Liquid Hold Up versus Gas Injection Rate.....................................................................79 Drillstring Injection Pressure versus Gas Injection Rate.............................................................80 Drillstring Liquid Hold Up versus Gas Injection Rate.................................................................81 Reservoir Inflow............................................................................................................................82 DRILLSTRING AND DOWN HOLE TOOL DESIGN..................................................................................83 Pressure While Drilling (PWD)....................................................................................................83 Conventional MWD Tools in Underbalanced Drilling.................................................................83 Electromagnetic Measurement While Drilling (EMWD)..............................................................83 Non Return Valves........................................................................................................................86 Wireline Retrievable Float Valves................................................................................................87 Down Hole Isolation Valves.........................................................................................................88 Drillstring Design.........................................................................................................................89 Drillpipe........................................................................................................................................91 Hard banding................................................................................................................................92 Drillpipe Rubbers..........................................................................................................................92 Jars...............................................................................................................................................92 Down Hole Motors........................................................................................................................93 EQUIPMENT SELECTION......................................................................................................................94 Gas Injection Equipment...............................................................................................................94 Air compressors............................................................................................................................94 Nitrogen Generation System.........................................................................................................95 Booster Compressors....................................................................................................................97 Well Control Equipment................................................................................................................99 Rotating Diverters.......................................................................................................................100 Snubbing systems........................................................................................................................105 Separation Equipment.................................................................................................................106 Horizontal separators.................................................................................................................107 Vertical Separators.....................................................................................................................108 UBD Choke manifold..................................................................................................................109 Data acquisition..........................................................................................................................110 Flares..........................................................................................................................................112 WELL CONTROL STRATEGY.............................................................................................................113 Well Kill Strategy........................................................................................................................113 Well Control................................................................................................................................113 Erosion........................................................................................................................................115 Corrosion Management..............................................................................................................116 Corrosion Inhibitor Types...........................................................................................................118 PERSONNEL SELECTION....................................................................................................................119 TRAINING AND COMPETENCY...........................................................................................................120 OPERATIONAL PROCEDURES............................................................................................................121 COMPLETING UNDERBALANCED DRILLED WELLS...........................................................................123 Snubbing.....................................................................................................................................123 Workover of an Underbalanced Drilled Well.............................................................................127 Underbalanced Drilled Multi-Lateral Wells...............................................................................127 SUBSURFACE SERVICES....................................................................................................................128 PROCESS FLOW DIAGRAMS..............................................................................................................129 RIG AND LEASE LAYOUT..................................................................................................................130 HEALTH SAFETY AND ENVIRONMENTAL PLANNING.........................................................................131 Environmental Aspects................................................................................................................131 Safety Aspects..............................................................................................................................131 DETAILED COST ESTIMATES.............................................................................................................133 UBS PROGRAM................................................................................................................................134 UNDERBALANCED RECORDS.....................................................................................................136 Ref APR-WUBS-WFT-001 Page 3 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 REFERENCES...................................................................................................................................138 SUPPLIERS OF UNDERBALANCED DRILLING SERVICES..................................................153 WEATHERFORD................................................................................................................................153 HALLIBURTON..................................................................................................................................153 SHAFFER...........................................................................................................................................153 TESCO...............................................................................................................................................153 LEADING EDGE ADVANTAGE..........................................................................................................153 BLADE ENERGY PARTNERS..............................................................................................................153 SCANDPOWER...................................................................................................................................153 NEOTEC............................................................................................................................................153 ABBREVIATIONS.............................................................................................................................154 Ref APR-WUBS-WFT-001 Page 4 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Introduction This introduction is intended to provide an overview of current underbalanced drilling technology and is therefore by no means exhaustive. It should serve as a guide to the current UBD technology, explaining how, when and why underbalanced drilling is carried out. Objectives The objective of this introduction is to provide an awareness of underbalanced reservoir drilling technology and the associated operations. It also provides a starting point and basic orientation for identification of hazards and corresponding risk assessment. This introduction covers a very brief introduction to managed pressure drilling and performance drilling to ensure a complete overview of the technologies associated with underbalanced drilling. Weatherford underbalanced drilling division currently offers three sub product lines under the underbalanced umbrella and these can be listed as : • Performance Drilling (PD) This technology is intended to achieve maximum penetration rates through reducing the well bore pressure to a minimum possible value. • Managed Pressure Drilling (MPD) With MPD the intention is to precisely manage and control the annular pressure to allow the bottom hole pressure to be within close limits to “walk the line” • Underbalanced Drilling The intention of underbalanced drilling is to reduce formation damage, discover potential bypassed pay, and increase reserves by allowing access to these reserves thus ultimately increasing net NPV. In underbalanced reservoir drilling the well is designed to allow the reservoir to flow to surface whilst drilling. Ref APR-WUBS-WFT-001 Page 5 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Performance Drilling This is the original air drilling technique to increase penetration rate. In performance drilling, the bottomhole pressure is as low as possible to increase drilling performance. Performance Drilling P >>>P = P + P + P formation bottomhole hydrostatic friction choke The objective of performance drilling is to reduce the drilling costs by drilling faster. This is normally achieved by using gas or air as a circulation medium. Reducing the bottom hole circulation pressure significantly increases the penetration rate. Fig 3 Performance Drilling Definition Ref APR-WUBS-WFT-001 Page 6 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Managed Pressure Drilling A number of related technologies have now been established in the industry as a result of tools used originally in underbalanced drilling. One of these techniques is Managed Pressure Drilling, which has been defined as follows: Managed Pressure Drilling is an adaptive drilling process used to more precisely control the annular pressure profile throughout the wellbore. The objectives are to ascertain the downhole pressure environment limits and to manage the annular hydraulic pressure profile accordingly.” It means that the annular pressure profile is controlled in such a way that the well is balanced at all times, or in mathematical format: Managed Pressure Drilling P = P = P + P + P reservoir bottomhole hydrostatic friction choke Fig 2 Managed Pressure Drilling Definition Ref APR-WUBS-WFT-001 Page 7 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 What is Underbalanced Drilling? Underbalanced reservoir drilling is defined by the IADC UBO committee as: Drilling with the hydrostatic head of the drilling fluid intentionally designed to be lower than the pressure of the formations being drilled. The hydrostatic head of the fluid may naturally be less than the formation pressure or it can be induced. The induced state may be created by adding natural gas, nitrogen or air to the liquid phase of the drilling fluid. Whether the underbalanced status is induced or natural, the result may be an influx of formation fluids which must be circulated from the well and controlled at surface. This in effect means that in underbalanced reservoir drilling, the effective downhole pressure within the wellbore is always maintained to be lower than the reservoir pressure and it is intended to have reservoir inflow into the wellbore. Underbalanced Drilling P > P = P + P + P reservoir bottomhole hydrostatic friction choke Fig 1 Underbalanced Reservoir Drilling Definitions Ref APR-WUBS-WFT-001 Page 8 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Equipment used for Managed Pressure Drilling Underbalanced Drilling Performance Drilling UBD PD RCD Gas Handling Choke Manifold Float Subs MPD RCD = Rotating Control Device Fig 4 Shared Equipment for MPD, UBD and PD Technology These three techniques have equipment in common but are all applied in different circumstances. This introduction concentrates on underbalanced reservoir drilling and the associated equipment and techniques, not on the performance drilling or managed pressure drilling aspects. Ref APR-WUBS-WFT-001 Page 9 of 154 APR-WUBS-WFT-001 Revision 001 Issue Date: Introduction to underbalanced drilling January 2006 Underbalanced Reservoir Drilling This introduction concentrates on underbalanced reservoir drilling and the associated techniques and equipment used in this drilling method. In underbalanced reservoir drilling, the wellbore pressure is maintained below the reservoir pressure at all times, and the resulting inflow from the reservoir is carefully controlled during the entire drilling process. Underbalanced Drilling P > P = P + P + P reservoir bottomhole hydrostatic friction choke The well is still controlled by controlling the wellbore pressure, but this pressure is maintained to be always below the reservoir pressure. Primary well control is no longer an overbalanced barrier of a column of fluid but is replaced by flow control using a combination of hydrostatic pressure, friction pressure and surface choke pressure. The BOP stack remains as the secondary well control barrier. It must be pointed out that a UBD well operates on a single barrier. The bottom hole circulation pressure is a combination of hydrostatic pressure, circulation friction losses and surface pressure applied at the choke. The hydrostatic pressure is considered a passive pressure and is a result of the fluid density and the density contribution of any drilled cuttings and a small contribution of any gas in the well. The friction Pressure is a dynamic pressure (It changes with pumps on or off) and results from circulating friction of the fluid used. The choke pressure arises from annular back pressure applied at surface. These three pressures are controlled at all times and ensure that flow control is maintained whilst drilling underbalanced. The lower hydrostatic head avoids the build-up of filter cake on the reservoir formation and avoids the invasion of whole mud and drilling solids into the formation. This helps to improve productivity of the wellbore and reduces any pressure related drilling problems. Ref APR-WUBS-WFT-001 Page 10 of 154

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