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Clay minerals in reservoir sandstones may react with drilling and completion fluids. Reactions are influenced also by clay mineralogy, clay form, and location of clay minerals. Smectite and mixed-layer illite/smectite clay minerals may swell to produce blockage, or occur as loose particles that migrate. Illite has a diverse morphology, but commonly occurs as a fibrous "spiderweb" form that may migrate when subjected to high-velocity flow, trap fine particles or water, or be virtually inert. Loose particles of kaolinite may migrate, but when water chemistry is controlled, this problem is minimized. Chlorite may serve as a trap for fine particles, as a habitat for microporosity, and has the potential to react with acids to create a ferric-hydroxide precipitate. Clays in high-permeability "channels," through which the fluid is flowing, are particularly prone to produce formation damage.

Laboratory flow-test results show that there may not be a correlation of permeability damage from the flowing fluid with the quantity of clay, even authigenic clay, reported to occur in the rock. Scanning electron microscopy and X-ray diffractometry are not independent methods for predicting clay-reaction problems. However, these techniques are helpful, when combined with flow testing and interpretation experience, for predicting clay reactions.

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