The activity concept applied to shales: consequences for oil, tunnelling and civil engineering operations

Abstract

The concept of activity is commonly applied to shales to explain their behaviour when contacted with various fluids (oil and water based). The paper will review this concept and the physical mechanisms affecting the measurements of activity. From this, it will be concluded that the low activity values reported in the literature are due to capillary effects that happen in partially desaturated shales. A review of evidences from oil, tunnelling and civil engineering will be reported and reinterpret as being the consequences of capillary forces. More specifically it will show that: I. the so-called strengthening of the shale drilled with oil based muds cannot occur under downhole conditions; II. the different swelling pressures measured around tunnels and in the laboratory are completely explainable considering core dehydration; III. the consequences of partial saturation are most representative in the world of civil engineering when it comes to the problem of heave of the earch surface after serious periods of drought.