Stress-sensitivity analysis of geological confined pores with ultrasonics

Abstract

Ultrasonics could be a promising technology to modify the geological formations for geo-energy productions, carbon geological sequestrations, monitoring of crack formation with stress and volcanic activity prediction. However, immature understanding of the post-ultrasonic geological formations seriously restricts the further applications in practices. This paper initially focuses on the in-situ geological porous media of low-permeability sandstones, analysing their stress sensitivity and relevant influential factors post-ultrasonics. Basically, the original and treated in-situ cores with and without ultrasonics were characterized through a series of experimental approaches, including permeability stress sensitivity (PSS), high-pressure mercury intrusion (HPMI), inline nuclear magnetic resonance (NMR), X-ray diffraction tests (XRD), triaxial stress test (TS) and scanning electron microscope (SEM) to explore the stress-induced rock compression mechanisms and the associated effects of pore restructures and mineral re-compositions. The experimental results reveal that the ultrasonics cause desorption of clay minerals at relatively low pressures (<5 MPa), which enlarges the pore throat and increases the permeability by 2.4 times. As the pressure increasing to 5 MPa, the porosity decreases from 8.79% to 6.13% because the plastic deformations of rock become irreversible without the cementing support (i.e., clay minerals). A further pressure increases from 5 to 25 MPa results in more porosity reduction from 6.13% to 5.77% through the rigid compressions. Overall, under the same pore pressure, the core porosity with ultrasonic treatments is usually smaller than the original ones, which thus indicates ultrasonics to some extent augment the stress sensitivity of core with compression. Hence, it is suggested to control pore pressure if the ultrasonics are introduced for geo-energy productions and carbon geological sequestrations.