ABSTRACT:

A new methodology is described in order to evaluate, on drilled cuttings, the effects of the interactions between shale and water based muds. This methodology can be applied for preventive and screening purposes. The objective of this technique is to have a quantitative tool able to discriminate between the most suitable drilling mud to be employed in a specific drilling context, in particular, to differentiate between muds that may induce geomechanical modifications inside the shale from a physico-chemical point of view. This methodology is included in the package of the techniques used by ENI-Agip to reduce the incidence of wellbore instability problems while drilling. The technical background of this procedure is the acoustic characterisation of drilled cuttings, which can be achieved with two different experimental devices. One device is called ‘PUC’ (pulsed ultrasound on cuttings) [Ref. 1] and it is able to acoustically characterise shales prior and after their interaction with different mud formulations. The second device is called ‘Marsonic’ and it allows to continuously monitor velocities on a cutting while it is immersed in a fluid, permitting in such a way the detection of eventual interactions and the time of their occurrence. The methodology is described in this paper along with case histories of more than 20 different fluid compositions. Results are, then, statistically analysed and confirmed with mechanical testing on the same rock formations before and after mud interactions. The main advantages of the proposed methodology are:

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    quantitative evaluation of rock-mud interactions,

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    low cost and time saving features,

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    non destructive testing,

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    while drilling utilisation for immediate operative application of the results.

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