What are the 5 types of casing?
- Conductor casing
- Surface casing
- Intermediate casing
- Production casing
- Drilling Liner
What are the functions of casing?
- Keep the hole open and provide support for wear or fractured formations
- Isolate porous media with different fluid/pressure properties
- Prevent contamination of near surface fresh water zones
- Provide passage for hydrocarbons
- Provide connection for the wellhead equipment
- Provide a hole of known diameter and depth to facilitate testing and completion
What is a conductor casing used for?
- Protect near surface unconsolidated formations and prevent washout
- Seal off shallow water zones
- Supports a diverter to provide protection against gas flows from shallow gas formations
- Provide a circuit for the drilling mud to protect the foundations of the platform
- BOPs may be installed in on this casing
What is a shallow gas blowout?
Gas encountered at shallow depths for which the fracture gradients are low and do not allow kicks to be controlled with conventional shut-in techniques
What is a diverter?
An annular BOP with a low working pressure. Function not to seal the well but to divert the blowout to a flare therby protecting the well site from fire hazards
What is the surface casing used for?
- Secure formations near the surface to prevent them from caving in
- Seals off surface fresh water to stop it being contaminated with drilling mud
- Provides an anchor point for BOPs and hangers for further casing strings
What is the intermediate casing used for?
Used in transition zones (below/above high pressurised zones) and is used to
- seal off a severe loss zone
- protect against problematic formations ( ie salt sections)
- give increased strength
- allows higher blowout protection
What is the production casing used for?
- Isolate producing zones
- Provide reservoir fluid control
- Permit selective production in the multi-zone production
What are the advantages and disadvantages of using liners?
ADVANTAGES
- Total costs are reduced
- Running and cementing times are reduced
- Length of reduced diameter is reduced allowing completion with adequate size tubings
DISADVANTAGES
- Casing and cementing processes are more complex
- Possible leak across a liner hanger
What is the difference between TVD and MD?
True Vertical Depth is measured vertically down from the surface where the Measured Depth is the total length of the wellbore measure along the well path
What is a fracture gradient?
Minimum total in situ stress divided by the depth
What is the overburden stress?
Stress arising from the weight of rock overlying the zone under consideration
What is the formation pore pressure?
Pressure exerted by the formation fluids on the walls of the rock pores
What is normal pore pressure?
When the pore pressure equals the hydrostatic pressure of a full column of formation water
What is abnormal formation pressure?
Exists in zones which are not in direct contact with its adjacent strata. Pressure gradient is usually higher due to trapping of fluids lighter than water, therefore trapped fluid supports more of the overburden stress
What is the formation breakdown pressure?
Pressure required to overcome the well bore stresses in order to fracture the formation in the immediate vicinity of the well bore
What is the collapse strength?
The maximum external pressure required to collapse a specimen of casing
