Core quality: quantification of coring-induced rock alteration

Abstract 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.

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