The reservoir scale gas distribution and its accurate seismic response

The main concern in the monitoring of gas injection, exsolution and dissolution is the exact spatial distribution of the gas volumes in the subsurface. In principle, this concern is addressed by the use of 4D seismic data. However, it is recognised that the seismic response still largely provides a qualitative estimate of the moved subsurface fluids; exact quantitative evaluation of fluid distributions and associated saturations remains a challenge still to be solved. It is widely believed that a few percent of gas makes the pore fluid mixture very compressible, so that it cannot be distinguished from a more complete gas saturation. However, because of the fact that a gas distribution viewed at the reservoir scale is distinctly different from that observed at the laboratory scale, conclusions from laboratory measurements may not, in fact, be wholly applicable. Indeed, it is found in this study that the main factor controlling the seismic response is gas thickness, whilst gas saturation per se remains approximately constant. Modelling studies show that 4D seismic attributes have a linear trend with gas volume. It is expected that this linear trend is applicable for most of reservoirs that is going under gas injection projects. Reservoir heterogeneity is observed to affect these results by less than 2%.This project reveals the ability of 4D seismic to quantitatively monitor the gas injection, and highlights the fact that laboratory measures are not directly applicable at the reservoir scale.