Changes in mineralogy and porosity during the reaction of shales with fracking fluid, CO2-fluid and brine
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Shale gas is an unconventional geo-resource, which had lots of attention in the last decade. This is mostly negative attention about the dangers of the chemicals in industrial fracking fluid and its impact on the environment. The methane emissions into the atmosphere are also a large concern when producing shale gas, as it is much higher than those of conventional resources. The challenge of today is to combine the continued use of fossil fuels, while simultaneously also reducing the associated CO2 emissions. In this study four different type of samples were put under the microscope (using the tabletop-SEM and FIB-SEM). These four samples were all Gothic shales: unreacted Gothic shale; Gothic shale reacted with industrial fracking fluid; Gothic shale reacted with CO2-fluid; and Gothic shale reacted with brine. The samples investigated in this study were already produced and provided by the group of Dr. Kaszuba at the University of Wyoming who are currently conducting experiments on samples from a US based shale gas site. The aim was to observe the changes in porosity and mineralogy of each sample at the nanoscale and to find out if CO2-fluid would be a suitable substitute for industrial fracking fluid, while at the same time also reducing the CO2 emissions into the atmosphere. The unreacted Gothic shale and brine reacted sample showed the least amount of pores. The Gothic shale reacted with CO2-fluid had larger pores in comparison to the Gothic shale reacted with fracking fluid. This was evident in the larger amount and size of cracks found in the CO2-fluid samples, but also in a larger increase in pore surface area compared to the fracking fluid samples. At a magnification of 600x the CO2-fluid contained a total pore surface area of 1581.4 μm2, in comparison with 1252 μm2 of the fracking fluid sample (see table 2). The total pore surface area of three different magnifications added up shows that the CO2-fluid has a total pore surface area of 3831.35 μm2, compared to a total pore surface of 3722.71 μm2. This study demonstrates that CO2-fluid could therefore be suitable as fracking fluid at the nanoscale, as it generates an increase in porosity in the Gothic shale rock, while at the same time it is a more environment-friendly way of producing shale gas as it reduces CO2-emissions. The reduction of CO2-emissions can take place in two different ways by injecting CO2-fluid into a shale gas site. The CO2 can either react with the shale, forming carbonates, or it reacts with the clays present in the shale, causing the clays to swell. The carbonates are formed when the CO2-molecules dissociates into different components and reacts with the host rock. The swelling of clays due to CO2-fluid causes the shale to increase its sealing capacity, resulting in no leakage to the Earth’s surface. Both ways of CO2-reduction due to injection into the subsurface, causes CO2 to remain in the ground. Unfortunately each shale is different in mineralogy and content, resulting in different reactions with the fluid used. This means that while CO2-fluid reacts well with the Gothic shale used in this study, it could react differently with another type of shale, clogging up the pores and resulting in a decrease in porosity. The difference in clay content forms the biggest problem for the clogging of the pores, as clays can be non-swelling or swelling clays (forming an increase or decrease in porosity, respectively). Therefore each shale must be investigated separately, before introducing CO2-fluid as a new and more environmental-friendly fracking fluid.