C8
Page 1: Overview
Transformation Context: Discusses the transition of Universiti Malaysia Sabah towards an Industry 4.0 campus.
Title: Lecture 8: Underbalance Drilling (UBD).
Page 2: Lecture Outcomes
Outcomes:
Introduction to UBD
Purpose of UBD
Advantages vs. Disadvantages
UBD Classification
Reservoir candidates for UBD
UBD Design
Limitations of UBD
Page 3: Introduction to UBD
Definition: Underbalanced drilling (UBD) is when the hydrostatic head of drilling fluid is less than the formation pressure.
Fluid Dynamics: UBD can occur via natural low hydrostatic pressure or by adding gases (e.g., natural gas, nitrogen).
Flow Management: Influx of formation fluids must be controlled and managed at surface while drilling.
Effective Downhole Pressure: Equal to hydrostatic pressure + friction pressures + surface applied pressure.
Page 4: Conventional Drilling vs UBD
Well Control in Overbalanced Drilling (OBD): Wells drilled with overbalanced fluid have pressure designed higher than formation pressure to prevent influx.
Fluid Column: A solid column of fluid ensures well safety and control in OBD.
Page 5: UBD Pressure Dynamics
Pressure Conditions in UBD: In UBD, lighter fluids lead to pressure at the bottom of the well being intentionally lower than formation pressure.
Page 6: Mechanisms of Well Control in UBD
Primary Mechanisms:
Hydrostatic Pressure: Passive pressure due to fluid density.
Friction Pressure: Dynamic pressure from fluid movement.
Choke Pressure: Active pressure due to surface sealing.
Page 7: Formation Fluid Influx
Challenges in UBD: Inflow of formation fluids during UBD needs careful control.
Filter Cake Mitigation: UBD prevents filter cake formation and mud invasion into formations.
Circulatory Control: Well fluids returned to a closed system at the surface ensure safe handling and blowout prevention.
Page 8: Purpose of UBD (1)
Maximizing Hydrocarbon Recovery:
Reduced formation damage.
Early production upon reservoir penetration.
Reduced need for stimulation processes.
Page 9: Purpose of UBD (2)
Minimizing Pressure-Related Issues:
Prevents differential sticking by avoiding filter cake formation.
Reduces risk of losses during drilling.
Enhances penetration rate and bit life due to less hydrostatic pressure.
Page 10: UBD Advantages vs Disadvantages
Advantages:
Increases Rate of Penetration (ROP)
Decreases formation damage
Reduces risk of lost circulation
Disadvantages:
Possible wellbore stability problems
Increased drilling costs
Higher associated risks
Page 11: UBD Classification
Role of IADC: The International Association of Drilling Contractors aids in assessing UBD risks.
Page 12: Reservoir Candidates for UBD
Applicability: Most reservoirs are suitable for UBD, but some face geological or technological issues preventing it.
Page 13: UBD Effects on Reservoir Types
Will Benefit from UBD: Low permeability, high-production reservoirs, formations vulnerable to damage.
Will Not Benefit from UBD: Very high permeability, low stability wells, those with extreme geological conditions.
Page 14: UBD Operation Design
Four-Step Process:
Determine Bottom Hole Pressure (BHP)
Identify Fluid Options
Establish Well Design and Flow Models
Select Surface Equipment
Page 15: BHP Requirements
Overbalanced vs. Underbalanced:
OBD requires fluid weight > reservoir pressure.
UBD necessitates fluid weight < reservoir pressure.
Page 16: Drilling Fluid Options
Fluid Classification: Different types based on equivalent circulating density include gas, mist, foam, gasified liquid, and liquid.
Considerations for Fluid Selection: Includes reservoir characteristics, safety, compatibility, and maintenance of pressure conditions.
Page 17: Gaseous Fluids
Use of Gases: Air ineffective in hydrocarbon formations; nitrogen is preferred for safety.
Technical Performance: Fast penetration rates and effective solids management.
Page 18: Mist Systems
Mist Technique Usage: Appropriate when water is present; similar to gas drilling but adds liquid to enhance performance.
Operational Challenges: Managing gas/liquid ratios to prevent slugging.
Page 19: Foam Systems
Benefits of Foam: Excellent properties at low hydrostatic densities, good for cuttings transport.
Operational Dynamics: Stable throughout circulation, absorbing formation water effects.
Page 20: Gasified Fluid Systems
Operational Characteristics: Gas injected into liquid to maintain controlled density; requires careful ratio management for stability.
Hazards: Incorrect ratios lead to operational problems.
Page 21: Well Design Process
Drill Pipe Injection - Simple but requires stopping to bleed pressure.
Annular Injection - Common offshore; stable pressure but size restrictions.
Parasite String Injection - Complex and typically for vertical wells only.
Page 22: Surface Equipment Requirements
Categories of Equipment: Includes drilling systems, gas-generation systems, well-control, and surface separation.
Page 23: Gas Generating Systems
Types of Systems: Air compressors and nitrogen generators for effective gas supply in UBD operations.
Page 24: UBD Limitations
Challenges: Formation strength, spontaneous imbibitions, water inflow, and geological variations present significant challenges to UBD efficiency.
Page 25: Technical Limitations of UBD
Problems: High permeability reservoirs, unsuitable for UBD due to substantial inflows; health and safety complications may hinder operations.
Page 26: Conclusion
Final Notes on UBD: Careful consideration of fluid dynamics and operational design is crucial to avoid pitfalls in underbalanced drilling practices.