Casing strings are connected sets of series of steel pipes which run through the wellbore from wellhead to the oil reservoir. Sections of pipe are connected with each other using couplings, to suit a particular wellbore. Normally length of a typical pipe section is 40ft or 12m. These sections are male threaded on both ends are connected to each other by couplings which are female threaded on both ends.
To keep fluids from different formations from mixing with each other, generally a well is not drilled by running the same bore through all the sections. Normally a well is drilled by in different sections, separating these sections by different casing strings as shown in the figure. For example, after the wellbore is drilled through the top formations containing fresh water, before going to deeper formations this section of the well has to be sealed off. This is achieved by running a casing string down the wellbore and filling the annulus between casing and wellbore by cement. Once a top section is sealed in this fashion, another smaller bore is drilled below this first section of the well. The new section is sealed using a smaller casing contained within the first section casing.
Thus subsequently smaller casing strings contained within other casings, seal off different formations until the smallest of them penetrates the oil reservoir.
Casing String Functions
- Various casing strings for different sections of the well protect various tubing carrying fluids.
- Casings maintain the wellbore wall in place and prevent movement of formation structure into the wellbore.
- Casing strings provide strength and pressure integrity for installing the wellhead equipments, including Blow Out Preventer (BOP).
- A casing string serves to isolate different formations from each other when it is cemented to the wall of wellbore. This way mixing of fluids from different formations through the wellbore is prevented.
- During drilling, casings help by sealing off low pressure, low strength formations from high pressure, high strength formations so that they can be separately and safely drilled.
- Casings protect fresh water aquifers from contamination from other fluids
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