Some seismic experiments on supercritical CO2

Abstract

When CO2 is injected into a formation in its supercritical form, it acts as a gas having the ability to be compressed to a much lower volume than in its liquid form. During the Nagaoka site injection in Japan, CO2 was injected in its supercritical form into a saline aquifer and a cross-well tomography experiment was performed in which a seismic source was placed in one well and receivers in another. The objective of the cross-well tomography was to image the CO2 during injection, in order to track the progress of the CO2. In order to simulate this field experiment, we used a large synthetic sandstone core representing a physical model of the reservoir. The water-filled core was subjected to a confining pressure of 8.5 MPa, with a pore pressure of 8.2 MPa, and had ultrasonic transducers placed down opposite sides. As the supercritical CO2 passed through the core, the seismic system recorded transmission data, to allow the later production of a seismic tomogram, and the seismic transmission response as the supercritical CO2/water interface moved through the core. This paper presents an experiment to simulate the field response, and the results so far of the injection process. There was a velocity change of some 5% when supercritical gas replaced water, but there was also a major amplitude change with some 25% reduction in transmission amplitude when supercritical CO2 replaced water. The velocity tomogram is an important indicator of how the supercritical CO2 migrated while amplitudes provide an indication of state-of-mix. This has consequences for monitoring the state of phase of CO2 during injection, using seismic data.