Experimental study of low-temperature combustion characteristics of shale rocks

Abstract

Relatively low temperature (500 °C) combustion has been applied to shale rocks collected from Lianmuqing, Xinjiang Province, China, to improve the permeability of shale without pore structure change caused by mineral decomposition. The shale rocks were firstly grinded into small particles, then burned in a furnace at a constant temperature of 500 °C for 5 min, 15 min, and 30 min, respectively. A cylindrical shale sample was also subjected to the combustion experiment. It was found that thermal cracking occurred along the height of the shale as combustion propagated from bottom to top. From the low-temperature nitrogen adsorption test, it was found that the pore diameters of shale samples were in the range of 2–50 nm, which were less than the mean free path of an oxygen molecule. Thus, the diffusion of gas inside the shale was Knudsen diffusion and the Knudsen number (Kn) was between 8 and 25. Moreover, the mean diameter of shale pores and the effective diffusion coefficient increased with increasing oxidization time. Whereas the surface area decreased after combustion, the diameter of the shale particles remained constant. So the density of the shale decreased with increase of combustion time. According to the porous media combustion model, the oxidization of shale particles was considered to be in Regime I, which is under kinetic control at isothermal combustion condition (500 °C). Furthermore, the effective diffusion coefficient was in a range of 3 × 10−6–6 × 10−6 m2/s. It increased with increasing combustion duration, especially during the first five minutes. The experiment results showed that low temperature combustion can effectively improve shale permeability to facilitate gas extraction from shale reservoir.