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.
Highlights
Increasing attention has been given to the reduction of the emission of carbon dioxide (CO2 ), which contributes most of the greenhouse effect
For samples saturated in water with gaseous CO2, to 47.9% for samples saturated in water with super-critical carbon dioxide (SC-CO2 )
For shale samples saturated in water with SC-CO2, the uniaxial compressive strength (UCS) values show reductions of 33.66%, 47.77% and 66.05% with saturation times of 10 days, 20 days and 30 days, respectively
Summary
Increasing attention has been given to the reduction of the emission of carbon dioxide (CO2 ), which contributes most of the greenhouse effect. Shale gas reservoirs, which are characterized as having ultra low permeability and high storage potential, are suitable for CO2 sequestration [2,3,4]. As CO2 is injected into shale reservoirs, it dissolves into waters or brines and changes the acid-base equilibrium which triggers the dissolution and precipitation of minerals [5,6]. Observed the dissolution of feldspars and calcite cement and the precipitation of dickite, opal and calcite, and reported that the dissolution of Ca and Fe cations limited the precipitation of carbonate. Lu et al [8] found that concentrations of cations in groundwater presented two trends, and all the concentration variations were dominated by the precipitation of carbonate minerals. Liu et al [9]
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