Laboratory study of the effect of chemical osmosis on the elastic rock properties of Pierre shale and implication for oil recovery

Abstract

The main objective of this research is to experimentally (1) measure the effect of chemical osmosis on elastic properties of shale, (2) determine chemical osmosis membrane efficiency under in-situ stress, and (3) evaluate the swelling tendency of Pierre shale resulting from chemical osmosis and its implication on oil production. Three relevant tri-axial experiments were conducted on Pierre shale core samples, before and after fluid invasion at reservoir pressure and temperature conditions, to determine rock properties and behavior. To accomplish the tasks, a new tri-axial cell was built to accommodate coupled acoustic and static measurements attributed to swelling during water intrusion into the Pierre shale matrix. Bulk modulus, Poisson's ratio, and shear failure envelope of Pierre shale were determined from measurements on several shale samples with varying clay content. The membrane efficiency of the shale samples was experimentally determined to be in the range of 10 to 30% depending on clay content and stress level. Swelling strains of the samples were determined from experiments to be 1% in high smectite content samples and 0.07% in a 3.5 wt.% TOC sample from Pierre shale. For the PI-LC-WY-H-01 Pierre shale sample with low smectite and high TOC, we obtained an internal friction angle of 48.4 degrees. The shale samples containing high swelling clays indicated a decreasing trend in Young's modulus when low salinity brine imbibed into the pore space by chemical osmosis. For relatively low smectite content and high TOC, the Young’s modulus increased when water saturation increased during osmosis pressure build-up. The aforementioned results were included in the numerical model to account for osmosis and clay swelling characteristics on oil recovery from the rock matrix. The correlation of the membrane efficiency with stress is dependent on the formation mineralogy, particularly clay content. A coupled mass transport-geomechanical mathematical model was developed (1) to simulate mass transport between the fracture and rock matrix amidst the occurrence of clay swelling, and (2) to evaluate how fluid and rock interactions could affect oil recovery from the rock matrix in unconventional reservoirs. The model was used to evaluate oil recovery from a single matrix block while accounting for the change of membrane efficiency with stress, swelling, and mechanical property of shales. Based on our laboratory observations, I recommend investigating the swelling tendencies of shale formations before any water injection operations, especially for low salinity water injection. I also recommend determining the increase in effective stress with clay content resulting from low salinity water injection.