Core damage is a more serious issue in petroleum engineering than in conventional rock mechanics because empirical on site design optimisation using direct observation is not possible. Thus, coring is a critical source of information, but it is not easy to obtain high quality specimens from core samples. This paper identifies, quantifies, and illustrates core property deterioration, and will propose means of quantifying damage in a systematic manner and controlling it.
In the petroleum industry, decisions to core are made on economical and technical grounds. The economical stakes are clear; coring interrupts drilling activity, and carries a penalty of about one full day of rig time for deep operations. To reduce costs, the industry is taking longer cores; 18, 27, 36 and even 55 m cores are now being taken and even continuous coring is occasionally carried out. Technical reasons for coring are twofold. Conventionally, coring provides qualitative geological and reservoir engineering evaluation. However, quantitative information may be vital, particularly in marginal fields where long appraisal phases with full-scale, long-term well tests are not possible. The second technical reason is therefore quantitative core measurements, such as permeability data under stress to assess Ak arising from compression, and so on. Good results cannot be achieved with poor core, yet major economic decisions may ride on the outcome of the core tests, affecting decisions as crucial as field development. In geomechanics, one cannot assume that samples are representative of downhole material; issues of possible core damage during coring and transportation become important. We discuss various issues such as discing, jamming, and core fracturing. We identify and illustrate several damage mechanisms and propose several means of quantifying core damage, or at least of systematically establishing some quality control measures. Physico-chemical issues such as wettability or saturation changes are deliberately ignored in this paper; we limit our discussion to rock mechanics issues, and damage is understood to consist of irreversible changes in mechanical properties because of microcrack formation, rupture of cohesive bonding, and disruption of fabric even to the point of total disturbance.
It has been recognized and proven experimentally (Obert and Stephanson, 1965) that stress concentrations near the coring front, including underneath the advancing core bit, are peculiar and can lead to a type of failure known as core discing. Discing particularly affects brittle rocks such as strong carbonates or basalts that are more prone to rupture in tensile parting than in shear. Interest exists in using discing geometry and frequency as method to determine in situ stresses (Panet, 1969 Perreau et al., 1989). Recent important contributions have been made in modelling the phenomenon for both vertical (Venet et al., 1989) and inclined boreholes (Dyke, 1989). As a direct evidence of rock failure at the coring front discing can take several forms (Figure 1). To study the process, the front created by two bits often recommended for soft formation coring were analyzed for an 8.5" well (Figure 2).
