Understanding Chimneyin CO2 Plume at Sleipner Injection Site Through Pre-stack Elastic Forward Modeling

Abstract

CO2 produced at the Sleipner natural gas field is currently injected back into the Utsira Fm., a major sand formation in the North Sea.Since the injection started in 1996, seven time lapse surveys have been performed. The growth of the CO2 plume is evident in the 4D seismic data, where it is imaged as several reflections. A vertical, cylindrical feature can be observed in the data. Different authors have described it as a CO2 filled chimney, feeding the sub-horizontal layers connected to it. The aim of this project is to examine the seismic response from athought chimney in CO2 plume by creating plausible geological models of the injection site, and compare it to the real data. Then, possible geological explanations of the chimney will be suggested and discussed. First, two geological models were made and modeled. Two different temperature and pressure distributions were used as basis for calculat-ing velocities and densities of the CO2 in these models. The velocities within the plume were computed by a Gassmann fluid substitution. Even though the physical properties of the CO2 differ between the models, only minor differences are observed in the corresponding syn-thetic seismic diagrams. The geometry of the plume in the synthetic seismic correlates fairly well with the geometry of the real seismic. Differences are presumed to be caused by local variation of shale to-pography and variable distribution of CO2 saturation. In the synthetic seismic a chimney is clearly visible. It shares the size and shape of the chimney present in the real seismic. It is proposed that the chimney extends from the point of injection and up to the cap shale with a width of approximately 40 m. Assumptions and simplifications have been done throughout the modeling process, but because of similari-ties with the real seismic this results are considered to be transferable to the real situation. Conclusively, this study demonstrates the ability of a CO2 chimney to be imaged as a chimney in seismic. Views on the creation of the chimney sort into two main categories: pre-injection made or syn-injection made. It is proposed that the chimney may be a sand injection, which may form as a response to over pressure createdby intra-sand body shales. Indications of this in pre-injection seismicis also presented, but to little is known in order to conclude about its origin.