Experimental Study of High Temperature Combustion for Enhanced Shale Gas Recovery

Abstract

As an alternative to hydraulic fracture for shale gas recovery, high temperature treatments such as combustion and pyrolysis are being used to remove organic matters and decompose minerals from shale to increase shale rock permeability for gas recovery. In this study, shales obtained from Shang Gu Basin, Shangxi, China, were burned in an air environment at a temperature of 800 °C for 45 min to oxidize the organic matters to improve shale physical properties. Energy dispersive spectrometer (EDS), scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) tests were conducted to identify minerals and functional group inside the shale samples. The shale samples were also tested in a thermogravimetric analysis FTIR (TGA-FTIR) experiment to study the combustion characteristics and gas emission. The TGA curve shows that two weight loss peaks existed in the combustion process. The first peak occurring between 400 and 500 °C was found to be caused by oxidization of organic matter, and the second peak appearing between 600 and 800 °C was due to the decomposition of carbonate minerals and oxidization of organic matter. CO 2 was the main emission gas generated during the combustion process. The low temperature nitrogen adsorption method was employed to study the pore structure changes before and after the combustion. It was found that the diameters of the raw shale pores were mainly in the range of 4−8 nm. After combustion, the volume of small pores decreased and large pores increased; the mean pore diameter of the shale samples increased from 5 nm to greater than 10 nm after combustion. Scanning electron microscopy (SEM) images also show the volume of pores increased on shale surface. As a result, high temperature combustion not only resulted in oxidization of organic matters but also led to decomposition of carbonate minerals. Combustion may be one of the effective ways to increase shale permeability for gas recovery.

Thermally enhanced shale gas recovery: microstructure characteristics of combusted shale

“…During the combustion, some kerogen on the shale surface was removed and released as gas through oxidization (Chen et al 2017b). To investigate the changes in shale morphology at the beginning of the rapid oxidization stage, the shale sample (RS1) was combusted at 400 °C for 30 min.…”

“…Recently, the combustion treatment has been proposed for shale gas recovery (Chapiro and Bruining 2015;Chen et al 2017b). Combustion treatment not only provides energy for gas desorption from pore surfaces but also promotes the gas transport by improving the permeability (Chen et al 2017b(Chen et al , 2018a(Chen et al , 2019c. However, pore system evolution during combustion process involves complicated physical and chemical processes.…”

“…The pyrolysis of kerogen may enlarge the pores with a corresponding increase in pore volume and specific surface area (Han et al 2006). It was also found that the mean pore diameter of shale increased after combustion (Chen et al 2017b). During combustion, the organic matter was decomposed, and thermal cracking hydrocarbon gases were generated.…”

H2O2-Enhanced Shale Gas Recovery under Different Thermal Conditions

“…Organic-rich shale has high chemical reactivity and it is sensitive to oxidation reactions [14,15]. Chen et al have used a high-temperature combustion method to remove organic matter in order to improve the shale matrix diffusivity [16,17].In addition to high-temperature oxidation, low-temperature oxidation has also been applied to improve shale effective porosity in experiments. Oxidizers such as sodium hypochlorite (NaOCl), sodium peroxodisulfate (Na 2 S 2 O 8 ), and hydrogen peroxide (H 2 O 2 ) have been widely used to remove organic matter from soil [17][18][19].…”

“…Organic-rich shale has high chemical reactivity and it is sensitive to oxidation reactions [14,15]. Chen et al have used a high-temperature combustion method to remove organic matter in order to improve the shale matrix diffusivity [16,17].…”