How is cement casing made or inserted into an oil well drill hole?Is it expensive? There was a news story about BP only using partial casing. Are there safety or engineering standards which indicate how much casing is required or recommended?
The cement is injected at the bottom of the casing by using the drill pipe which lowered into the well. Drill pipe is hollow to allow drilling mud to be circulating during the drilling phase but it a can also take the cement and spot it at the bottom of the well. Knowing the depth of the well and the size of the drill bit being used along with the casing size, the cement company can calculate (with a safety factor) the amount of cement to inject to seal the space between the well bore and the casing. As to what BP did or did not do relative to their casing design, I am not sure but it does appear that the cement plug failed and let the oil reservoir pressure back into the well bore. Because the well bore was filled with salt water at the time and not drilling mud, the hydrostatic pressure of the salt water was not sufficient to counter balance the reservoir pressure. Casing designs vary with the conditions for a specific well or field and are set by the company owning the well. There are general industry best practices but I don't believe they are incorporated into any API standards.
