another announcment, page-36

No Probs Grangeh

BTW you may like to check these out - from Google:

http://images.google.com.au/imgres?imgurl=http://www.kgs.ku.edu/Publications/Oil/gifs/fig24.gif&imgrefurl=http://www.kgs.ku.edu/Publications/Oil/primer12.html&h=378&w=606&sz=61&hl=en&start=5&um=1&tbnid=n9j70NHrxX1xnM:&tbnh=85&tbnw=136&prev=/images%3Fq%3Doil%2Bdrilling%2Band%2Bspudding%2Bdiagram%26um%3D1%26hl%3Den%26client%3Dfirefox-a%26rls%3Dorg.mozilla:en-GB:official%26sa%3DX

Drilling the surface hole
When a well is spudded in, a large-diameter drill bit is used to drill to a predetermined depth. This is for the purpose of drilling the surface hole. The surface hole is lined with casing. The casing protects aquifers which may contain freshwater, provides a mounting place for the blowout preventer, and serves as the support for the production casing that will be placed in the well bore if the drilling program is successful. The surface hole may be several hundred or several thousand feet deep. When the predetermined depth is reached, the drill string will be removed from the well bore. Steel casing of the proper diameter is inserted. Sufficient cement is pumped down the surface casing to fill the space between the outside of the casing and the well bore all the way to the surface. This is to insure the protection of freshwater aquifers and the security of the surface casing (see fig. 24). The casing and the cement are tested under pressure for a period of 12 hours before drilling operations may be resumed. A piece of equipment known as a blowout preventer is attached at the top of the surface casing. This device is required to control the well in the event that abnormal pressures are encountered in the bore hole that cannot be controlled with drilling fluid. If high-pressure gas or liquid blows the drilling fluid out of the well bore, the blowout preventer can be closed to confine the gas and fluids to the well bore.

Figure 24. --A casing is installed in the surface hole to prevent the contamination of freshwater zones and to support the production casing. A) The conductor pipe has been cemented into place. A predetermined amount of casing has been inserted into the well bore below the deepest freshwater zone. Cement is pumped down the inside of the casing until cement flows to the surface through the annulus. B) The cement in the bottom of the casing has been drilled out so that drilling can be resumed.