Drilling and maintaining in-gauge hole remains one of the driller’s greatest challenges . in-gauge, stable hole not only means trouble-free drilling, it also helps ensure that logs are of high quality, that the cement job runs smoother, and that every subsequent action in the well occurs to the operator’s field starts with the drillhole , so ensuring the best hole possible is worth some though and expense.
Fighting for hole integrity , the driller must monitor and mud weight. Mud and formation must be balanced chemically, particularly in shales, to prevent the formation swelling agaist the drillpipe or sloughing into borehole. Simultaneously, a mechanical balance must be achived to prevent two well-known phenomena-breakouts and formation fracturing-and, as experts now suspect, maybe also a third phenomenon called shear displacement. This article focuses on how the mechanical balance is monitored andachived, and reviews the latest theories and measurement techniques in a case study from the newly discovered cusiana field in eastern coloombia.
Rock in its natural state is stressed in three principal directions –vertically from the overburden and horizontally in two orthogonal directions. The two horizontally in two orthogonal directions. The two horizontal stresses are generally not equal- the maximum and minimum horizontal stresses are expressed as SH and SH’ respectively.
as a borehole is drilled, hydraulic pressure of the drilling mud must replace the support lost by removal of the original column of rock . but mud pressure being uniform in all directions cannot exactly balance the earth stress. Consequently , rock surrounding the borehole is distoreted or strained , and may fail if the redistributed stresses rock strength.
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