The effects of capillary forces on the constitutive behaviour of reservoir rocks are an item which has been poorly investigated but which could have potential important consequences from an operational point of view especially when considering the behaviour of reservoir rocks with very small pores such as chalks and shaley sandstones. Laboratory investigations have been carried out on an impure chalk in order to determine its mechanical properties and constitutive behaviour under different environment conditions; the variations of the moduli resulting from a set of experiments performed at different temperatures and pressures may be qualitatively interpreted in terms of capillary pressure variations. In the same way, uniaxial compressive strength of some sedimentary rocks seems to be affected by capillary effects: a qualitative interpretation is given and a simple model able to predict variations in strength with respect to saturation has been verified with good agreement between experimental and predicted data.

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