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Underbalanced Drilling (UBD)
Lesson 10 Flow Drilling ,Mudcap ,Drilling Snub Drilling ,Closed Systems
Flow Drilling
 Flow drilling refers to drilling operations in which the well is allowed to flow to surface while drilling.
 All UBD operations are really flow drilling operations, but the term is usually applied to drilling with a single phase mud, and no gas is injected except by the formation.
Drilling Fluid Selection
 Density is determined by:
• Maximum pressure ≤ to formation pressure.
• Minimum pressure dictated by wellbore stability.
 Pressure limitations of diverter and BOP equipment.
Sizing Flare Line
Weymouth’s equation can be used to predict the pressure drop for a gas, in steady-state, adiabatic, flow along the pipe:…
Surface Pits
 Primary oil separation pit.
 Secondary oil separation pit.
 Skimmer system safety.
 Drilling fluid pit.
 Oil transfer tank.
Operating Procedures
 Mechanical objectives during flow drilling are:
• To control the well.
• Minimize differential sticking problems.
• Minimize drilling fluids losses.
Maximum tolerable surfaces pressures should be established before drilling starts.
Mudcap Drilling
 Utilized with uncontrollable loss of circulation during flowdrilling operations.
 Higher pressures than can be safely handled with the rotating head or RBOP.
 It is not strictly an underbalanced drilling technique.
 Driller loads the annulus with a relatively high density high viscosity mud and closes the choke with surface pressure maintained.
 Drilling is then continued “blind” by pumping a clear non-damaging fluid down the drillstring through the bit and into the thief zone.
 Applications:
o Sustained surface pressures in excess of 2,000 psi.
o Sour oil and gas production.
o Small diameter wellbores.