A set of experiments has been performed in order to quantify the extent of core damage caused by stress release during coring. Synthetic sandstones have been created under stressed conditions: The mechanical behaviours of identically prepared specimens are then compared in two types of tests. In one type, the rock is tested for mechanical properties directly. In the second test, the rock is first unloaded ("cored"), then reloaded, and finally tested in exactly the same way as in the first case. Acoustic velocity and acoustic emission measurements are performed during the tests, for possible future field use as core damage indicators. The results show that permanent damage occurs, leading to reduced stiffness, possibly to reduced mechanical strength, and also to a permanent volume decrease. This means that, within the petroleum industry, core measurements (unless properly interpreted) may lead to overestimated reservoir compaction and to underestimated volume of reserves.
Within the petroleum industry, the knowledge of rock mechanical pro- perties is essential for predictions of e.g. reservoir compaction during depletion, sand production, and stimulation by hydraulic frac- turing. Since all these predictions have a critical impact on field economy, it is of great importance that rock mechanical data are obtained with as good accuracy as possible.
Core measurements represent the only source for direct measurement of rock mechanical parameters. The reliability of the measurements is, however, limited by the quality of the core material. When a core is taken out of the ground at depth and brought to the surface, several possible sources of alteration exist (see e.g. Santarelli and Dusseault, 1991). The external stress as well as the pore pressure are released, the temperature is decreased, and the core is exposed to the drilling fluid. After having reached the surface, the core is handled by personnel on the rig, it is transported to the laboratory, it is stored, and finally prepared (cleaned, cut, polished, saturated) for laboratory testing. Clearly, all these processes may lead to permanent alteration of its mechanical behaviour.
In the work presented here, we have chosen to focus on only one core alteration mechanism, namely stress release. The observations of anelastic strain recovery (e.g. Teufel, 1982) and acoustic wave velocity anisotropy (e.g. Ren and Hudson, 1985) after coring imply that significant mlcrocracking occurs during the coring process. The use of these techniques (Ramos and Rathmell, 1989? Teufel, 1989) and differential strain curve analyses (Simmons, Siegfried and Feves, 1974) imply that the microcracking is related to the in-situ stress conditions. Thus, stress release during coring clearly affects rock mechanical measurements. The question that remains to be answered is to what extent this results in a permanent core damage, i.e. if the core behaviour after reloading to in-situ stress conditions deviates from the rock behaviour at depth.