Experimental Study of High Temperature Combustion for Enhanced Shale Gas Recovery

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 degrees 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 degrees C was found to be caused by oxidization of organic matter, and the second peak appearing between 600 and 800 degrees C was due to the decomposition of carbonate minerals and oxidization of organic matter. CO2 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.