Drilling in a mature reservoir can involve multiple added risks over time including reservoir depletion that may cause serious drilling problems and significant cost increase. Effective management of these problems during well planning stage and during execution is crucial to prevent issues with drill pipe, casing and logging tools that are at a higher risk for differential sticking,

Differential sticking has the potential to become a significant issue for drilling operations because of the wide variation in pressure in different parts of the Tengiz reservoir. The degree of depletion depends on the variability of the reservoir, which itself is dependent on the properties of the different geological facies. Reservoir hole section can have different pressure regimes, which require higher mud weight and thus high potential for differential sticking problems. Consequently, as the Tengiz reservoir pressures depletes, the risk of differentially stuck pipe, casing or logging tool increases. During current drilling campaign there were three wells that experienced major issues caused by differential sticking and lead to significant cost associated with sidetrack operations and lost in hole charges.

Pore pressure predictions and proposed well trajectory are some of the most important factors for prediction of differential sticking issues. For predictions during well planning "differential factor" was defined, which incorporates well angle and reservoir pressure. As a result, it helps to understand if differential sticking risk is high and use preventative measures if needed to reduce it. Guidelines were developed to help to mitigate potential problems while drilling across differentially pressured zones and include identification of signs of sticking, bottom hole assembly design considerations, preventative mud treatments and reactive measures to optimize ability to drill wells and reduce non-productive time associated with differential sticking. After implementation of above mentioned measures there were only two minor differential sticking events observed.

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