Similar to that of drilling mud we focused on in the previous lesson, well casing is another technology used to maintain well integrity. “Casing” is a generic term for steel pipe. Steel is used for casing due to its high durability and ability to maintain good condition over long periods of time, mitigating risks of weakened isolation.1US CHEMICAL SAFETY AND HAZARD INVESTIGATION BOARD. (2014, June 5). INVESTIGATION REPORT VOLUME 1 EXPLOSION AND FIRE AT THE MACONDO WELL. US CHEMICAL SAFETY AND HAZARD INVESTIGATION BOARD. REPORT NO. 2010-10-I-OS.
Formally, casing refers to pipe if it extends up to the wellhead. A section of pipe also can be hung from the bottom of a previous casing. In this case, it is technically called a “liner.” Liners can hang from casing or from another liner. The casing itself is sealed by cement within the wellbore, including cement poured between the several layers of casing constructed within the well. The many layers of casing help to further mitigate risks and confine the interior of the borehole from surrounding formations. Casing serves a number of purposes we will now examine in further detail.2US CHEMICAL SAFETY AND HAZARD INVESTIGATION BOARD. (2014, June 5). INVESTIGATION REPORT VOLUME 1 EXPLOSION AND FIRE AT THE MACONDO WELL. US CHEMICAL SAFETY AND HAZARD INVESTIGATION BOARD. REPORT NO. 2010-10-I-OS.
Stabilize and seal parts of the wellbore using cement
Casing is at least in part cemented in the ground. The cement provides the outside seal. Casing serves to strengthen the wellbore so that it does not fracture or collapse during drilling and production operations.
Protect shallow formations from high pressure gradients needed to drill in deeper zones
The casing essentially acts as a barrier between the interior of the wellbore and the outside formations. As the well drills further into the subsurface, the mud weight needed to keep the wellbore pressure at the bottom of the hole above formation pressure can increase. In some situations, the mud weight (or pressure gradient) needed to hold back formation fluids at the drill bit may exceed fracturing pressure gradients at shallower depths. To avoid formation fracturing that could cause lost circulation, steel casing is installed in the wellbore to cover all of the formations drilled thus far. Then, drilling can proceed at higher mud weights without worrying about fracturing at shallower depths because all of those formations previously at risk are protected “behind pipe”. This process of drilling and then casing may repeat several times until the total depth of the well is reached.
Seal zones containing corrosive water
Cement helps confer protection to casing against corrosive fluids, preventing possible contamination within the borehole.
Seal and protect fresh water bearing zones
The casing helps mitigate and prevent possible contamination outside of the borehole. It is extremely important that wellbore fluids do not leak into surrounding formations, especially freshwater aquifers, that are often tapped into for municipal purposes such as drinking water sources.
Isolate productive formations from other zones
The casing isolates the productive reservoir(s) from other zones. The wellbore is the conduit to bring hydrocarbons to the surface. Although the wellbore may need to pass through zones with saline water to get to the hydrocarbons, operators do not want to produce that water as it typically has no commercial value and would have to be disposed of if brought to the surface. Putting those zones behind pipe keeps them isolated from other production operations. In the case of multiple hydrocarbon reservoir, one zone may need to be produced before another (for technical or legal reasons), and the future production zone needs to be isolated from others until a later date.