An Experimental Investigation of Swelling and Elastic Property Changes Resulting from Carbon Dioxide Injection into Prismatic Coal Specimens

The world currently relies heavily on fossil fuels to meet its energy needs. The demand from a growing population for energy continues to increase and cannot be met solely by renewable sources of energy. As a result of this high dependence on fossil fuels, various methods of reducing emissions are under investigation. Carbon dioxide Capture and Storage (CCS) has been identified as a potential technique that can be employed to reduce emissions. Unmineable coal seams have been identified as a possible carbon dioxide geologic storage formation. The behavior of coal in response to CO 2 injection is not yet fully understood or well documented. There is a need to understand and quantify the physical changes of coal in response to CO2 injection. These physical changes include properties such as strain (swelling), sonic velocity (compressional and shear velocity), and elastic moduli. Understanding the physical changes undergone by coal during CO2 injection is crucial in evaluating the efficiency and integrity of coal as a potential geologic storage formation.;To study and quantify the effect of CO2 injection on adsorption induced strain (swelling) and coal elastic properties, specifically ultrasonic velocity (compressional and shear velocity) and elastic moduli, an experimental approach was followed. Two prismatic coal samples with approximate dimensions of 25cm (9.8 inches) x 17cm (6.7 inches) x 6cm (2.4 inches) were prepared. The coal samples used were from the Lower Sunnyside Coal Seam of the Books Cliffs Coalfields in Utah. A specially designed steel frame was used to contain the samples. Two extreme cases were tested using the samples. For case one, a high (4 MPa) injection pressure and high confining pressure (12 MPa) was used and in case two, both injection pressure and confining pressure were dramatically decreased. The conventional pulse transmission method was used to determine sonic velocities. This method uses transducers to send an ultrasonic wave through the sample, where corresponding transducers on the other end of the sample receive the signal. Pre-injection measurements of sonic velocity (Vp and Vs) were taken. From sonic velocities (Vp and Vs), other pre-injection properties such as Poisson’s Ratio, shear, bulk and Young’s modulus (dynamic) were calculated. Digital deformation gauges were also strategically placed on the samples to measure strain (swelling) during the injection phase. CO2 was injected into the first sample at high confining pressure and high injection pressure. Injection proceeded for five days for each of the samples (25 hours of actual injection). A maximum increase in compressional velocity of 21% was measured at 12 MPa confining pressure and 4 MPa injection pressure. Increases in elastic moduli were also observed due to CO2 injection. A maximum adsorption strain (swelling) of 0.9% was measured at 12 MPa confining pressure and 4 MPa injection pressure. At high injection pressure (4 MPa) and high confining pressure (12 MPa), the coal started to swell at 5 hours of injection. At low injection pressure (1.8MPa) and low confining pressure (1.8MPa), the coal only started to swell at 10 hours of injection.