Coal cleat network stimulation through a combination of acidizing and liquid nitrogen fracturing

Abstract

Gas extraction from Coal Bed Methane has proved challenging mainly due to the low permeability of coals. This study investigated a combination of 2 stimulation techniques, Liquid Nitrogen (LN2) fracturing and acidizing, to increase pore structure and cleat network connectivity in coals. Microcomputed Tomography (μ-CT) scanning results suggested that a combination of those 2 applications, in either sequence of implementation, has substantial potential in recovering mineralized fractures and inducing new fractures; thus, leading to wide-open cleat networks. X-ray images show that if LN2 fracturing of coal is preceded by acidizing, the stimulation efficiency increases from more than double the initial value to over 700% of it, where a porosity of 12% can be achieved. Alternatively, the efficiency of acidizing in porosity enhancement can increase from 300% to 530% when a preceding application of LN2 freezing provides an open cleat network to facilitate acid access to remote areas. Acid diffusion and digital microscopy images also determined that in fractured coal, HCl concentrations of both 1% and 3% can result in desirable demineralization (13% porosity increment), whereas in the acidizing of original coals, 1% HCl might not be promising. Further, the latter experiments created a wide and connected cleat network after LN2 fracturing of efficiently acid-stimulated coal. Therefore, a combination of two enhanced recovery techniques, acidizing and LN2 fracturing, was used in this study to achieve a dramatically boosted cleat network in coals. Additionally, an analytical model was developed to predict the acid reaction rate with coal minerals; it had a good match with experimental results.