Abstract
The shale fracture permeability is critical in determining gas production and deep CO2 sequestration performance. Moreover, how shale fracture permeability evolves after interactions with supercritical carbon dioxide (Sc-CO2) should be understood to constrain the shale reservoir permeability and evaluate the long-term sealing ability of shale formations. In this research, we conducted soaking experiments with shale fractures and Sc-CO2 at various times and then measured the shale fracture permeability and hydraulic aperture evolution under different stress states. Additionally, we quantify the chemical compositions, pore characteristics, fracture surface roughness alternation through X-ray diffraction, nuclear magnetic resonance, scanning electron microscope, and optical profilometry techniques. Our results indicate that soaking with Sc-CO2 will dramatically increase the shale fracture permeability and aperture due to the calcite and dolomite dissolution. This free-face dissolution process will remove the mineral particles in the fracture surface, resulting in larger pores, peaks, and valleys in the fracture surfaces. This process may last for seven days, and after that, chemical reactions may terminate, and the rock-Sc-CO2 system turns stable. Our results explain how Sc-CO2 alters the shale fracture permeability through the chemical dissolution of specific minerals from a microscale analysis.
Highlights
Shale gas production has boomed during the last decades due to the technological advancements in horizontal drilling and multistage fracturing in Sichuan Basin, China [1,2,3]
Samples and Experimental Method e experimental study consists of several components: (a) shale samples soaking with Sc-CO2 under the temperature of 50°C and 10 MPa for 1, 3, 5, 7, and 14 days; (b) shale fracture permeability measurement under the effective normal stress ranges from 1 to 20 MPa; (c) X-ray diffraction analysis (XRD) tests for mineralogy analysis, Nuclear magnetic resonance (NMR) tests for shale pore characteristics analysis, and surface profilometry scanning tests for fracture surface roughness analysis
Under the effective normal stress of 20.0 MPa, fracture aperture increases from 3.95 μm to 6.52, 6.52, 8.45, 10.20, 12.85, and 13.65 μm after soaking with Sc-CO2 for 1, 3, 5, 7, and 14 days. us, our experimental results indicate that the soaking with Sc-CO2 will largely increase the shale fracture permeability and effective hydraulic aperture
Summary
Shale gas production has boomed during the last decades due to the technological advancements in horizontal drilling and multistage fracturing in Sichuan Basin, China [1,2,3]. 2. Samples and Experimental Method e experimental study consists of several components: (a) shale samples soaking with Sc-CO2 under the temperature of 50°C and 10 MPa for 1, 3, 5, 7, and 14 days; (b) shale fracture permeability measurement under the effective normal stress ranges from 1 to 20 MPa; (c) XRD tests for mineralogy analysis, NMR tests for shale pore characteristics analysis, and surface profilometry scanning tests for fracture surface roughness analysis. K e2h , 12 where ef is the fracture hydraulic aperture; k is the fracture permeability; μf is the dynamic viscosity of water; L is the contact length of the fracture; W is the width of the fracture; Q is the measured flow rate; ΔPf is the fluid pressure difference between upstream and downstream reservoirs. After the Sc-CO2 soaking for various times, the fracture surface roughness was characterised by a ZygoTM NewView optical profilometer [47]. e parameter, root mean square roughness, is used to represent the fracture roughness, which describes the profile height deviations from the mean line
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