Evaluating the Effect of Capillary Pressure on Rock Mechanical Properties

Abstract Geo-mechanical models targeting downhole applications always consider effective stress in terms of pressure of pore fluid, typically single phase; dry or wet. To simulate reservoirs in situ where multiphase fluids co-exist, other effects such as capillary pressure, need to be taken into consideration. The objective of this study is to evaluate the effect of capillary pressure on the effective stress in a laboratory environment. Effective stress is mainly considered a function of pore fluid pressure and Biot's coefficient. To measure Biot's coefficient and effective stress, hydrostatic compression tests were performed on sandstone rocks saturated with a single phase fluid; brine or oil. To study effect of capillary pressure of multiphase systems on effective stress, a second fluid, air or oil, was introduced into the water saturated rock by centrifugation. When reaching irreducible water saturation, then capillary pressure was recorded. Effective stress was again measured on the multiphase samples and compared with that of single-phase systems, to quantify the effect of capillary pressure on effective stress. In multiphase systems (particularly air/brine), the Biot coefficient displayed complex stress-dependent evolution, initially decreasing to a minimum before increasing again. This nonlinear trend likely reflects pore compression followed by collapse. In contrast, oil/brine systems primarily showed a decreasing Biot coefficient with stress, only rising slightly at the end of the loading cycle. Compared to single-phase brine, multiphase systems (air/brine and oil/brine) exhibited lower Biot coefficients, suggesting capillary pressure enhances effective stress by compressing the rock matrix. These findings are critical for reservoir characterization, Geo-mechanical modeling, and production forecasting, particularly in gas reservoirs. Notably, the Biot coefficient's nonlinear response to applied loading—observed in Nugget and Berea sandstones—highlights its dependence on effective compression and porosity reduction. The novelty of the present workflow lies in challenging the current approach of estimating effective stress on rocks which traditionally assumes single phase fluid, with experimental results of evaluating and quantifying effect of capillary pressure on effective stress measurements and Biot coefficient. Future work should explore a wider range of fluid systems and integrate in-situ imaging to elucidate pore-scale mechanisms.