A novel quantitative petrophysical model for the stress sensitivity of tight sandstones

During the production life cycle of a reservoir, absolute permeability can change in response to an increase in the effective stress. In general, stress-dependent permeability has been measured using experimental methods with core samples. This paper focuses on presenting a petrophysical model of unequal diameter grains, based on the Hertz Theory, in that the true reservoir rocks are composed of closely packed grains of different sizes. The purpose of the model is to quantitatively evaluate the features of stress-dependent permeability in theory and to enhance the productivity of stress-dependent reservoirs. Control factors including the grain sizes, grain sorted behavior, mineral composition and cementation degree, which influence on the variation of stress-dependent permeability, can be calculated quantitatively and directly through the model while experiments cannot. Experiments were conducted on core samples of tight sandstones to verify the established model. Measurements were made at different confining pressures. The theoretical and experimental results obtained from the investigated rock were analyzed and compared. The results of the model show that control factors including the previous parameters have different influences on the stress-dependent permeability. The stress-sensitive curves calculated by the model have the desired behavior and are consistent with the laboratory data well, providing a more rigorous confirmation of the validity of the model and a new petrophysical insight into the significance and the role of the grain properties and grain contacts. The obtained theoretical results can exhibit the average features of stress-sensitive formation rather than the feature of only one point (a core plug sample) of a reservoir by laboratory test. Thus, the obtained formula through the model is of great benefit to being applied in reservoir numerical simulation. (C) 2014 Elsevier B.V. All rights reserved.