Temperature wellbore stability

Invert emulsion drilling fluids are commonly selected for their temperature stability and their ability to prevent the wellbore stability problems associated with the hydration of clays in shale formations. The thermodynamic activity aw of the water in the aqueous (dispersed) phase is controlled by the addition of a salt (usually calcium chloride) to ensure that it is equal to or less than the activity of the water in the drilled shale formations. The emulsified layer around the water droplets is claimed to act as a semipermeable membrane that allows the transport of water into and out of the shale but not the transport of ions (61). When the activities (or, more strictly, the chemical potentials) of the water in the shale and invert emulsion are equal, then no net transport of water into or out of the shale occurs (i.e., the drilling fluid does not hydrate or dehydrate the shale). This equality of water activity has lead to the development of so-called balanced activity oil-based drilling fluids. 

Choi, S.K. and Tan, C.P. (1998). Modelling of effects of drilling fluid temperature on wellbore stability. SPE 47304, Proc. SPErtSRM EUROCK 98, Trondheim, Norway, 8-10 July 1998, Vol. I, pp. 47M77. 

Geothermal cements are also employed to fix the steel wellbore casing in place and tie it to the surrounding rock (8). These are prepared as slurries of Portland cement (qv) in water and pumped into place. Additional components such as silica flour, perlite, and bentonite clay are often added to modify the flow properties and stability of the cement, and a retarder is usually added to the mixture to assure that the cement does not set up prematurely. Cements must bond well to both steel and rock, be noncorrosive, and water impermeable after setting. In hydrothermal applications, temperature stability is critical. Temperature cycling of wellbores as a result of an intermittent production schedule can cause rupture of the cement, leading to movement and, ultimately, failure of the wellbore casing. 

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