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

Abstract

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.

A Damage Constitutive Model for the Effects of CO2-Brine-Rock Interactions on the Brittleness of a Low-Clay Shale

“…The variation trends of UCS and Young's modulus values are similar to samples soaked in subcritical CO 2 + NaCl solution. According to the previous study [11], the change of shale's mechanical properties is because of the physical and chemical reactions during the imbibition. Therefore, it is necessary to manifest the effect of such physical and chemical reactions on shale's brittleness, which is of great importance for formation fracturing.…”

“…According to the authors' previous work, the pH of water soaked with subcritical and supercritical CO 2 under the same pressure and temperature will be smaller than 3 [11].…”

“…Shale samples used in this study were Longmaxi shale obtained from Sichuan Basin, China, as shown in Figure 2 [11]. The shales in these areas are characterized as having high thickness, suitable burial depth, and effective fracture development [47].…”

“…Because of the unique properties, CO 2 presents a great feasibility in the utilizations of drilling industry [6,7] and nonaqueous fracking for shale gas recovery [8][9][10]. During the drilling and fracturing process, CO 2 together with ground water or other fluids will interact with rock and change rock's mechanical properties [11][12][13][14].…”

“…Therefore, the investigation of shale's mechanical properties after fluidrock interactions is very important for drilling and fracturing industry. Many scholars have done a brilliant research about the variation of shale's strength and Young's modulus after the adsorption of fluids, such as water [16,17], saline solutions [18,19], CO 2 [20][21][22], CO 2 -dissolved water [11], and CO 2 -dissolved brine [23,24]. However, the studies of the brittleness of shale after fluid saturation are limited.…”