After drilling, there is a completion process needed to stabilize the wellbone opening before production can begin. The drilling process cuts through formations in the rock inorder to reach deep oil reservoirs. The hole that is drilled is not stable enough to support itself. Casing is used as a structural component to support and protect the well.
Casing Specifications & Benefits
Casing can represent a big percentage of the overall well cost. This makes material grade, size, and connector selection not only an engineering consideration, but an economic one as well.
Casing pipe is available in large diameters that are designed aid the borehole in withstanding forces and chemically aggressive brines. It is typically made from carbon steel, which gets its strength through a heat treating process. Casing can also be specially fabricated using aluminum, stainless steel, titanium, and other materials. Once the casing is assembled and inserted into a freshly drilled section of a wellbore it must be held in place using cement.
Casing adds several beneficial functions to the drilling process:
- Stabilization of upper formations preventing cave-in.
- Stopping contamination from reaching fresh water well zones.
- Providing zonal isolation for sections with different fluids or pressures to prevent blowouts.
- Increasing strength of upper foundation, allowing for use of high-density drilling fluid and deeper drilling.
- Prevention of fluid loss into production zones.
The casing string typically includes these basic elements: surface, conductor, intermediate, and production casing.
Surface casing provides protection from blowouts, prevents lost circulation and isolate water sands. It provides the added strength to drill high-pressure transition zones and prevent keyseating of the formation . This casing is cemented to the surface of an oil well.
Conductor casing is below the structural casing protecting loose formations near surface and enabling circulation of drilling fluid. Conductor casing isolates unconsolidated formations, water sands and protects against shallow gas.
Intermediate casing isolates unstable hole sections, low pressure zones and production zones. It is often set in the transition zone from normal to abnormal pressure. The casing cement top must isolate any hydrocarbon zones. Some wells require multiple intermediate strings. Some intermediate strings may also be production strings if a liner is run beneath them.
Production casing is typically exposed to fracking injection pressure, gas lift, and injection of inhibitor oil. It's used contain formation pressures in case of tubing leaks and in isolation of production zones.
It is critical for the production casing string to receive a excellent primary cement job for optimal function.