Module 7. Underbalanced Drilling Flashcards
What are the reasons for Underbalanced Drilling?
1) Minimize formation damage
2) Increase productivity index
3) Control lost circulation (LCM)
4) Remove the additional cost for the use of water and oil base mud phases
5) Minimize differentional sticking
6) Lower or decrease well completion/stimulation operations
7) Faciliate completion operations
8) Deplete reservoirs
What conditions to consider for UBD?
- Static formaton pressure
- Dynamic formation pressure
- Health, environment and safety issues
- Off-shore drilling
Risks of UBD?
- Wellbore instability
- Down-hole fires
- Water inflow
- Horizontal and directional tool issues
Limitations of UBD?
- Lack of drilling supervisors
- Directional survey methods
- Modified well control techinques and equipment
What is consideration needed to design UBD in terms of reservoir?
- Formation type
- Drive mechanism
- Reservoir energy levels
- Quality of formation
- Quality and availability of injection gases
- Pressure
- Target drawdown requirements
List a gases used in UBD:
- Produced methane
- Scrubbed exhaust gases
- Cryogenic nitrogen
- Nitrogen
There are basically two locations available for the injection of gas into the drilling in order to reduce the BHP? List advantages and disadvantages of each.
1) At the surface into the drill string
adv: cheap
dis: fluctuation in BHP
2) Down the annulus outside a string of intermediate casing
adv: maintain BHP closer to requirements
dis: high cost
List advantages and benefits of UBD?
1) we can evaluate (analyze) formation fluids
2) Reduced drilling time
3) Mud is cheap because less additives we add (e.g. weightening agents)
4) Less formation damage (due to mud invasion)
5) MACP is high
6) Annular velocity is higher
List disadvantages & risks of UBD?
1) poor gel strength
2) wellbore stabbility
3) risk of influx
4) downhole fires
5) risk of corrosion
How to choose the well to do UBD?
1) competent formation (not going cave in)
2) underpressure (depleted zone)
3) formation sensitive to fluid
4) high strength formation - hard rock - higher ROPs
5) high permeability - consolidated sands
Three types of fluid used in drilling
1) gas (compressible) - air, nitrogen, natural gas
2) multiphase fluids - foams, aerated mud, mist
3) liquid - WBM, OBM, saltwater
What is Underbalanced Drilling?
Bottomhole circulating pressure is intentionally reduced to be less than expected BHP
Advantages of UBD within drilling:
- deviation is minimized
- low RPM, low WOB
- prolong bit life
