Experimental Investigation of Mechanical Properties of Black Shales after CO₂-Water-Rock Interaction

Abstract

The effects of CO₂-water-rock interactions on the mechanical properties of shale are essential for estimating the possibility of sequestrating CO₂ in shale reservoirs. In this study, uniaxial compressive strength (UCS) tests together with an acoustic emission (AE) system and SEM and EDS analysis were performed to investigate the mechanical properties and microstructural changes of black shales with different saturation times (10 days, 20 days and 30 days) in water dissoluted with gaseous/super-critical CO₂. According to the experimental results, the values of UCS, Young's modulus and brittleness index decrease gradually with increasing saturation time in water with gaseous/super-critical CO₂. Compared to samples without saturation, 30-day saturation causes reductions of 56.43% in UCS and 54.21% in Young's modulus for gaseous saturated samples, and 66.05% in UCS and 56.32% in Young's modulus for super-critical saturated samples, respectively. The brittleness index also decreases drastically from 84.3% for samples without saturation to 50.9% for samples saturated in water with gaseous CO₂, to 47.9% for samples saturated in water with super-critical carbon dioxide (SC-CO₂). SC-CO₂ causes a greater reduction of shale's mechanical properties. The crack propagation results obtained from the AE system show that longer saturation time produces higher peak cumulative AE energy. SEM images show that many pores occur when shale samples are saturated in water with gaseous/super-critical CO₂. The EDS results show that CO₂-water-rock interactions increase the percentages of C and Fe and decrease the percentages of Al and K on the surface of saturated samples when compared to samples without saturation.

Keywords: CO2-water-rock interaction; crack propagation; mechanical properties; microstructure; shale.

Figure 1

A sketch map of Lower Silurian Longmaxi shale in Southeast Chongqing.

Figure 2

High-pressure adsorbing system.

Figure 3

SEM images of of samples without saturation (a,b); samples with water + gaseous CO₂ saturation (c,d) and samples with water + super-critical CO₂ saturation (e,f). (a,c,e) have a magnification of 200×; (b,d,f) have a magnification of 500×.

Figure 4

EDS images of samples without saturation and samples with water and gaseous/super-critical CO₂ saturation. (a,c,e) are chemical mapping and X-ray spectra; (b,d,f) are compacted SEM results.

Figure 5

Chemical element composition by EDS analysis. (A) is for sample without saturation; (B) is for sample with water and gaseous CO₂ saturation and (C) is for sample with water and SC-CO₂ saturation.

Figure 6

Variation of UCS (a) and Young’s modulus (b).

Figure 7

Variations of brittleness index for shale samples.

Figure 8

Variation of cumulative AE energy with axial strain for sample without saturation (a), samples saturated in gaseous and super-critical CO₂ for 10 days (b); 20 days (c) and 30 days (d).