Qiao Lyu

Effects of coring directions on the mechanical properties of Chinese shale

This paper presents an experimental study of the effects of bedding planes on the mechanical properties of shale samples. Two specimens, sourced from Sichuan Basin in China, cored perpendicular and parallel to the beddings were tested using a uniaxial compressive strength (UCS) machine, and the corresponding crack propagation patterns were analysed using acoustic emission (AE) data. In addition, a digital camera system called ARAMIS was used to acquire the lateral and axial strains during the loading. It was observed that there was significant influence on the peak strength of shale when tested for various bedding planes. As expected, the sample with perpendicular load had nearly twice as high maximum axial strength than the sample with parallel load, and the peak cumulative energy release of the parallel load on the specimen was 1.5 times higher than that of the perpendicular load. The axial strain was higher than the lateral strain in perpendicular load, while under parallel load, the lateral strain was higher than the axial strain. When the axial stress was small, for the shale sample with perpendicular load, the axial strain increased gradually with the increase of stress. In contrast, the lateral strain remained near to zero during the loading period for the sample with parallel load, and both the axial strain and lateral strain were not affected by the increasing load and remained zero. The volumetric strain of the two samples indicated that shale with a perpendicular load shows a compaction behaviour, while parallel load leads to a dilatancy characteristic. The AE data showed that the compression period consists of three stages on both of the two tests and energy release occurs mainly in the third stage. From the ARAMIS results, no perpendicular cracking was observed in the normal stress test, while under parallel loading, the cracks were mostly parallel to the load direction.

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

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.