The first hole section is drilled with a drill bit, which has a smaller diameter than the inner diameter ( I.D. ) of the conductor. Since the I.D. of the conductor is approximately 28”, a 26” diameter bit is generally used for this hole section. This 26″ hole will be drilled down through the unconsolidated formations, near surface, to approximately 2000′.
If possible, the entire well, from surface to the reservoir would be drilled in one hole section. However, this is generally not possible because of geological and formation pressure problems which are encountered whilst drilling. The well is therefore drilled in sections, with casing being used to isolate the problem formations once they have been penetrated. This means however that the wellbore diameter gets smaller and smaller as the well goes deeper and deeper. The drilling engineer must assess the risk of encountering these problems, on the basis of the geological and formation pressure information provided by the geologists and reservoir engineers, and drilling experience in the area. The well will then be designed such that the dimensions of the borehole that penetrates the reservoir, and the casing that is set across the reservoir, will allow the well to be produced in the most effi cient manner possible. In the case of an exploration well the fi nal borehole diameter must be large enough to allow the reservoir to be fully evaluated.
Whilst drilling the 26” hole, drilling fluid ( mud ) is circulated down the drill pipe, across the face of the drillbit, and up the annulus between the drillpipe and the bore hole, carrying the drilled cuttings from the face of the bit to surface. At surface the cuttings are removed from the mud before it is circulated back down the drillpipe, to collect more cuttings.
When the drill bit reaches approximately 2000’ the drill string is pulled out of the hole and another string of pipe ( surface casing ) is run into the hole. This casing, which is generally 20″ O.D., is delivered to the rig in 40ft lengths (joints) with threaded connections at either end of each joint. The casing is lowered into the hole, joint by joint, until it reaches the bottom of the hole. Cement slurry is then pumped into the annular space between the casing and the bore hole. This cement sheath acts as a seal between the casing and the borehole, preventing cavings from falling down through the annular space between the casing and hole, into the subsequent hole and/or fluids fl owing from the next hole section up into this annular space.
Institute of Petroleum Engineering, Heriot-Watt University