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Core samples retrieved from deep boreholes may be permanently damaged as a result of the stress-release occurring during drill-out. This paper presents work performed to quantify the effect of this damage on subsequent rock mechanical and petrophysical laboratory measurements. Using synthetic rocks manufactured under stress, we have measured material properties in ‘virgin’ (simulated in situ) conditions, and compared these to properties of ‘cores’ that have been unloaded to simulate coring and subsequently reloaded to in situ conditions. For the three synthetic sandstones manufactured, the compaction modulus was permanently reduced as a result of an unloading–reloading cycle, wave velocities were also permanently reduced, and for the weakest materials, porosity was reduced as well. The unconfined strength was found to be strongly affected by the simulated coring. Correction schemes have been developed to correct core data, laboratory test procedures are proposed, and a novel coring tool for field use has been developed in conjunction with this work, to reduce core damage effects.

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.

An experimental study is presented, where the unconfined strength (UCS) is compared to the strength obtained by a CMS (constant mean stress) path terminating at zero confinement. The CMS values are found to be 20–50% in excess of the UCS values. In this study, we have utilized synthetic sandstones, formed under controlled stress conditions, in order to have a reproducible material. With this technique, we can also estimate the “virgin” strength of the material, which is found to be close to the CMS strength. Experiments with natural sandstones confirm the behaviour seen with the synthetics.