• Subsurface rock and fluid conditions.
• Depths of casings and well completion.
• Casing specification and cementing materials and programs
• Wellhead specification
• Drilling fluids, drill string assemblies
• The necessary drilling tools and equipment.
The Purpose of casing
Prevention of loose formation material from collapsing into and blocking the hole.
• Provision of anchorage or support for drilling and the final wellhead.
• Containment of well fluids and pressures.
• Prevention of ingress or loss of fluid into or from the well, and “communication” or leakage of fluids between different aquifers.
To counter losses of drilling fluid circulation during drilling.
• Protection of the well and formation against erosion, corrosion, fracturing and breakdown
The permanent wellhead components include:
- Casing Head Flange (CHF) usually, and preferably, attached to the top of the Anchor casing – but in some instances is attached directly to the top of the production casing. The casing head flange may incorporate side outlets to which side valve are attached.
- Double flanged Expansion / Adaptor spool. Side outlets may be incorporated in the expansion spool (as an alternative to those on the CHF).
- Master Valve
A typical wellhead assembly for a ‘Standard’ well completed with an 8½” diameter production hole section, 95/8” production casing and 133/8” anchor casing is illustrated schematically in Figure 3 below.
The wellhead should be designed to comply with codes of practice for pressure vessels or boilers, and in accordance with API Spec. 6A – and most importantly, rated for the maximum pressure / temperature exposure possible at the surface under static or flowing conditions. The fluid at the wellhead may be water, saturated steam, superheated steam, cold gas, or mixtures of some of these fluids. Due to the column of fluid in the well, surface conditions cannot equate to downhole values, but in some circumstances can approach downhole conditions closely.
Reference : Hagen Hole, 2008: Geothermal Well design-Casing and Wellhead, Petroleum Engineering Summer School