Drilling fluid is the life line of safe and economic drilling operations to explore for oil and gas resources beneath the earth's crust. It is, however, also the root cause of various mud related drilling problems that make safe and economic drilling operations extremely challenging. Problems such as shale-drilling fluid interactions, borehole instability, loss of circulation, differential pipe sticking, etc. are frequent events associated with drilling mud and mudcake characteristics. As differential pressure sticking is one of the major drilling problems that frequently occurs in the presence of a permeable formations, every step should be taken to reduce the probability and likelihood of this occurring.

Drilling muds that produce mudcake with strong sticking bonds are more prone to cause differential sticking. On the other hand, drilling muds that deposit mudcake with weak sticking bonds significantly reduces the scope of differential sticking due to easy shearing and tearing of the sticking bonds. In the event of a stuck pipe situation, the recovery is much easier in the presence of a mudcake of lower sticking bond strength than a mudcake of higher sticking bond strength. Hence, the selection of a drilling mud that has the capability to produce a mudcake of weak sticking bonds is one of the most effective preventive approaches in avoiding or mitigating differential sticking. This paper describes a novel test apparatus and method for screening and evaluating various mud systems to select the most suitable drilling mud for a differential sticking prone zone. Based on the experimental results, two index parameters were defined to provide quantitative information and valuable technical guidelines to select a suitable mud system. The method and apparatus will also guide the mud engineers in selecting mud additives and designing new mud systems to lower the risk of differential sticking.

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