To study and quantify the effect of CO2 injection on sorption induced strain (swelling) and coal elastic properties, specifically ultrasonic velocity (Vp and Vs) and elastic moduli, an experimental approach was followed. A cuboid coal sample with approximate dimensions of 25cm (9.8 inches) x17cm (6.7 inches) x 6cm (2.4 inches) was prepared. The coal sample used was from the Lower Sunnyside Coal Seam of the Books Cliffs Coalfields in Utah. A specially designed steel frame was used to contain the sample. To provide some degree of confinement, the sample was loaded bi-axially to about 12 MPa (1800psi) using hand (pressure) pumps. The conventional pulse transmission method was used to determine P-wave and S-wave velocities. This method uses transducers to send an ultrasonic wave through the sample, where corresponding transducers on the other end of the sample then pick up the signal. Pre-injection measurements of ultrasonic velocity (Vp and Vs) were taken using the OYO New Sonic-Viewer. From 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 sample to measure strain during injection phase. CO2 was injected into the sample at 4 MPa (600psi) injection pressure and 12 MPa (1800psi) confining pressure. Injection into the sample proceeded for five days (25 hours of actual injection). An increase in compressional velocity (as high as 21%) was measured at 12 MPa confining pressure and 4 MPa injection pressure. Since the dynamic elastic moduli are influenced by velocity, increases in elastic moduli were also observed with velocity increases. A maximum adsorption strain of 0.9% was measured at 12 MPa confining pressure and 4 MPa injection pressure. The coal stiffened during active injection due to the possible adsorption of CO2 into the coal matrix.
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