The effect of confining pressure on the hydraulic conductivity of capillaries in cylindrical samples is examined. The three-dimensional-printed samples were made from photopolymer resin. Capillaries in the samples were modeled by grooves of various geometric shapes. The mechanism of capillary deformation in the samples under increasing confining pressure has been identified. The change in capillary conductivity depending on their location (central and lateral) and configuration (sinuous) has been revealed. Based on correction functions for the geometric dimensions of the capillaries, it has been mathematically confirmed that under confining pressure, a capillary deforms primarily along the contact plane due to the sliding of the sample’s halves against each other. The width of a capillary is more sensitive to confining pressure than its depth. It has been established that the exponent in the conductivity (permeability) equation of the samples under cyclic loading is determined by the hydraulic area of the capillary. The obtained values of the width and depth correction factors allow for predicting changes in the filtration resistances of capillaries in various materials. Capillary deformation manifests as a change in its geometric dimensions (height and width), i.e., the crushing of the capillary banks is observed, leading to a reduction in the capillary’s hydraulic area, which causes a decrease in sample conductivity with an incomplete hysteresis.

This study investigates the causes of permeability decline in porous reservoirs under decreasing reservoir pressure by comparing laboratory experiments with well test data. Well tests indicate a greater sensitivity of permeability to pressure changes in reservoir formations compared to laboratory conditions and for this remain unclear. Field studies of permeability changes in northern Perm oil fields were conducted alongside laboratory experiments on core permeability under pressure. Results showed that highly permeable samples exhibited the greatest decline in permeability during elastic deformations, with reductions of 6% for limestones and 20% for sandstones. The relationship between permeability and purely elastic deformations for both rock types was accurately described by a power law. By comparing coefficients from field and lab studies, the mechanism of permeability decline in field conditions was established. A model incorporating elastic and plastic deformations of porous reservoirs was developed. The model considers the localization of plastic deformations in horizontal and vertical low-permeability deformation bands. Findings indicate that highly permeable formations are more susceptible to deformation band formation, particularly in thicker layers. The decrease in permeability was found to correlate strongly with the formation thickness, likely due to the formation of transverse deformation bands in pore layers.

The study of the influence of cyclic loading on the permeability of rocks has been conducted for a long time. Despite the extensive research database, the actual reasons for the decrease in permeability during loading have not been fully revealed. One of these reasons, as described in the research, is the migration of colloids. This paper presents the findings of a study on colloid migration as one of the causes of permeability degradation in porous rocks under cyclic loading. Permeability is measured by injecting nitrogen at a constant pressure. The cyclic loading program is designed to eliminate the effects of residual deformations, creep, and gas slippage. Direct and reverse nitrogen blowings with increased injection pressure were performed between loading cycles. These blowings promote colloidal movement within the porous medium, blocking pore throats and changes in permeability. A notable aspect of this work is that cyclic testing was performed before and after the saturation and drying procedure. Stuck colloids that cannot be moved by blowing are mobilized during saturation and drying. Comparative tests of cores after saturation and drying confirm the effect of colloid migration on permeability and enable the examination of whether plastic deformations caused permeability degradation in previous loading cycles. Additionally, it was observed that when saturated, new colloids can detach due to the Rehbinder effect, significantly reducing permeability.