1. Core sample may not be representative of the reservoir rock
because of reservoir heterogeneity.
2. Core recovery may be incomplete.
3. Permeability of the core may be altered when it is cut, or when it is cleaned and dried in preparation for analysis.
This problem is likely to occur when the rock contains reactive clays.
4. Sampling process may be biased.
There is a temptation to select the best parts of the core for analysis.