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QUESTION:

What are the drilling problems caused by gumbo/swelling shale? And how?
e.g. swelling shale can result in stuck pipe.

(Question asked by Sher)

ANSWERS:

Hi Sher, Some shale types react strongly with water based mud. This causes the shale to hydrate and swell; shale cuttings in the well become sticky so clump together, or can stick to the steel drillstring downhole, closing up the annulus and causing swabbing and stuck pipe.

On the surface, sticky hydrated shales can plug up the shale shaker screens, causing loss of mud over the shakers. The solutions include getting water based mud chemistry correct to prevent shale hydration or to use oil based mud.

(Answered by Fiona Webster)

The characteristic that makes shale most troublesome to drill is its water sensitivity, due in part to its clay content and the ionic composition of the clay. Shale absorption to water usually leads to dispersion and swelling. Dispersion occurs when the shale subdivides into small particles and enters the drilling mud as drill solids. Swelling occurs due to an increase in size of the silicate minerals increasing the clay structure (layers of clay penetrated by water) leading to hole destabilization : caving or sloughing shale.

In order to avoid such problems, the following measures should be taken in the sections containing shales * Change of the chemical composition of the mud by adding potassium for instance. This will reduce the chemical attraction between shale and water or by using an oil-based mud. MI-SWACO proposes special mud type ULTRADRILL * Minimize the time for which the section containing the shale uncased.

Answered by Mohamed BOUMEZRAG

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