Abstract: Researchers disagree on the existence of high stress sensitivity in low permeability reservoirs, which has caused hindrance in the policy making in oil and gas production. The focus of these disagreement is to determine if the core undergoes plastic deformation during permeability stress sensitivity tests, if a micro-gap exists between the core and envelope, and if such a gap exhibits significant influences on the experimental results. In view of the incapability of traditional experiments to verify the plastic deformation of the core, the current experimental method is improved by performing mechanical tests on the core before and after conducting the permeability tests. Based on the theories of elastic mechanics and effective stress, the theoretical formula of stress sensitivity evaluation is deduced and quantitatively calculated. Experimental and theoretical results showed that during the stress sensitivity tests, the effective stress on the core is below the elastic limit, indicating that the core do not undergo plastic deformation. A micro-gap is found between the core and envelope, which has a significant effect on the permeability test results and their variations. The permeability of a low permeability core measured under a low effective stress incur considerable measurement errors. The existence of the micro-gap leads to an overestimation of stress sensitivity of the core. During the experiments, the core strain is diminutive, and the resulting change in permeability is extremely small. Therefore, it is concluded that there is no strong stress sensitivity present in the low permeability reservoirs, and in general, the lower is the permeability of the rock, the lower is the stress sensitivity.

Key words: permeability, stress sensitivity, micro-gap, core sample, theoretical computation