Emphasis here is on understanding factors that influence wellbore stability in open holes, prediction of wellbore failures, and applications of rock mechanics concepts to control wellbore stability, A brief historical overview is followed by discussion of various types of wellbore instabilities and descriptions of studies of field wellbore stability problems. Some of the literature representative of this work is discussed in thisarticle. Since 1940 considerable effort has been directed toward solving rock mechanics problems associated with wellbore instabilities, and much progress has beenmade during the past 10 years toward providing predictive analytical methods. These problems can result in side tracked holes and abandoned wells. Hole instabilities can cause stuck drill pipe as well as casing or liner collapse. Excessive production rates can lead to solid particlein flux.
If the wellbore fluid pressure is too high, lost circulation can occur as a resultof unintentional hydraulic fracturing of the formation if it is too low, the hole may collapse. Hole size reduction can occur when plastic rock is squeezed into the hole, and hole enlargement can be caused by caving shales or hard rock spalling. Wellbore instabilities can take several forms (Fig.1).
Wellbore stability requires a proper balance between the uncontrollable factors of earth stresses, rock strength, and pore pressure, andthe controllable factors of wellbore fluid pore pressure, and the controllable factors of wellbore fluid pressure and mud chemical composition. Wellbore stability requires a proper balance between production. The shape and direction of the hole must becontrolled during drilling, and hole collapse and solid particle influx must be prevented during production. Maintaining a stable wellbore is of primary importance during drilling and production of oil and gas wells.
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