There is a growing awareness in the drilling industry that many cases of borehole instability may be caused by the changes in shale pore pressure resulting from fluid invasion. A hydraulic pressure differential, arising when the mud-column pressure exceeds the pressure in the shale pore fluids, drives fluid from the wellbore into the surrounding shale and propagates an expanding pressure front. The resulting changes in the effective stresses around the wellbore may then be sufficient to promote rock failure.

A new generation of water-based drilling fluids are being designed to minimise such pressure-penetration effects in pores by reducing either the shale's permeability to aqueous fluids or the rate at which fluids can flow into shales. The new thermally activated mud emulsion (TAME) system, utilising a combination of a surfactant solution and emulsion phases to reduce fluid entry into shales, is one of the first field-tested drilling fluids designed specifically to reduce borehole instability arising from the penetration of pressure into shale pores.

The first prototype TAME system comprises a conventional aqueous KCl-polymer drilling fluid containing a solution of an alcohol alkoxylate additive, components of which can undergo a phase transformation at downhole temperatures to form a fine emulsion. Both the solution- and emulsion-phase elements of the alkoxylate appear to make some contribution towards reducing rates of fluid flow into shales, although their exact mode of action is still unclear.

The prototype TAME system has been successfully field tested during 1992/93 in 17.5", 12.25", 8.5" and 6" sections of five deviated development wells and one exploration well in the UK North Sea. The overall conclusion from this test campaign is that the TAME system is superior to conventional KCl-polymer and polyglycerol muds by virtue of its better borehole-stabilising qualities, higher rates of penetration, significant waste reduction and lower mud time rates (the total time spent drilling on bottom, tripping, conditioning the hole or mud and circulating the mud).

Following these encouraging field tests, TAME formulations have evolved to become Shell Expro's preferred fluid system for drilling reactive shales in standard exploration, appraisal and development wells. Moreover, building on the information obtained from field experience with TAME, even more effective shale drilling-fluid formulations are currently under development.

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