Water based glycol (polyol) muds are becoming increasingly popular, and are now replacing oil based muds in many drilling operations. Apart from exceptional shale inhibition, other benefits include environmental friendliness, ease of handling, robustness and good lubricity.

This paper explores the mechanisms of shale stabilisation by glycol muds. Experimental data show shale dispersion, swelling and hardness behaviours for a model reactive shale (London Clay). The data suggest the high level of inhibition obtained is mainly due to a shale hardening effect - the shale actually becomes harder than in its original state. Measurements with a range of salt types show that salt (eg KC1) plays a major role in the inhibiting mechanism.

Adsorption measurements indicate that the glycol displaces water from the shale, resulting in a change in pore fluid composition. In effect, the glycol acts as a penetrating glue which, it is believed through hydrogen bonding effects, strengthens the shale. This mechanism contrasts with other water based mud additives (e.g. PHPA or polyamines) which form a protective or encapsulating layer on the surface of the shale.

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