Borehole casing pipes or otherwise known as polyvinyl chloride (PVC) pipes are large cylinder-shaped pipes that are manufactured and inserted deep into a drilled section of a borehole immediately after drilling.

Similar to the human anatomy where bones of a spine protect the spinal cord, pvc casings are set inside the borehole to protect the integrity of the well stream.

Before drilling commences, a driller, usually with information gathered from geologists and surveyor/siter, will decide the depths at which the hole will need to be cased in order for drilling to reach the desired total depth. This strategy is determined by subsurface data such as ground formation pressures and strengths, borehole integrity, and also taking into consideration the cost objectives and desired drilling strategy.

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When all that is fully determined, hole diameter and casing diameter must follow. The hole drilled for each casing string must be large to create a perfect annular enough for the casing to be placed inside it. Taking into consideration also, sizes of bits that will continue drilling obviously must pass through existing casing strings. Thus, each casing string will have a subsequently smaller diameter i.e 140mm Casing can fit perfectly inside a 180mm Casing hence what is know as double casing.

Double Casing & Special Casing

Double Casing is when we use low pressure casing with smaller thickness known as Class 4 or Class 6. This is usually recommended in soil formation that are solid and have less ground movement. For soil formation that are weak and have high ground movement Special Casing is recommended. Special Casing is when only high pressure PVC Casings are inserted. These are Class 9/10 for both the inner 140mm and the outer 180mm, in sometimes extreme cases where there's high risk were hole integrity can be compromised and collapse, drillers can recommend and advise to case the borehole with steel casing. This Special Casing will be done until the drilling reach solid hard rock.

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Casing design for each size of designed PVC pipies is done by calculating the worst conditions that may be faced during water well construction and over the producing life of the borehole. Mechanical properties such as longitudinal tensile strength, and burst and collapse resistance, must be sufficient at various depths.

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Pipe of differing strengths often comprises a long casing string, which typically will have the greatest axial tension and perhaps highest internal burst pressure differentials in the upper parts, and the greatest collapsing loads deeper in the well from external pressure vs lowered internal pressure.

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