Time-lapse seismic interpretation of injected CO2 plume at the Sleipner Field, North Sea

Abstract

One of the methods to cope with the increase of emitted greenhouse gases has been to capture CO2 gas from a point source and storing it within the Earths subsurface; “Carbon Capture and Storage” (CCS). Since 1996, Statoil and its partners have injected CO2 into a saline aquifer called the Utsira formation located in the North Sea. About 0.9Mt of CO2 is injected into the formation yearly, with a total injection volume estimated to be approximately 25Mt of CO2. The reservoir conditions of the formation have been interpreted to be good, with porosities in the range 35-40% and permeability in the range 2-5 Darcy’s. The injection is occurring at a vertical depth of approximately 1000m below the sea level, where the storage formation has a thickness of approximately 300m, and is overlain with sediments interpreted to constitute enough seal for the planned injection. The most widely used monitoring technique at the CCS project is to compare 3D seismic datasets obtained at different times, where the evolution of the CO2 can be monitored in a 4D seismic time-lapse sequence. In this thesis, four 3D seismic datasets have been interpreted for detecting and mapping the injected CO2 at the Sleipner facility. One of the dataset was acquired before the injection began and the three others were acquired after 5, 8 and 10 years of injection. The 4D seismic time-lapses have proven to be suitable for detection of injected CO2, where small CO2 thicknesses of only one meter can be visualized. The CO2 from the three post-injection datasets is observed as a series of bright sub-horizontal reflections, where they have been observed to accumulate at nine specific layers within the storage formation. The accumulations are interpreted to be located beneath shales within the reservoir. Amplitude anomalies of the internal layers, and for the whole plume, have shown to have a general elliptical planform with the long axis in a NNE-SSW direction. This is presented to be a response of the topography of the capping layers. Reflection amplitude-changes have been observed on the time-lapse seismic data, where the lower layers seem to be dimming and the upper layers seem to be brightening as more CO2 is being injected. The dimming is proposed to be caused by several factors, including energy reduction of overlying brightening CO2 layers and upward migration from deeper to shallower layers. Storage security within the injection site have been examined, both the integrity of the caprock situated above the CO2 plume and changes of the seabed surface. Neither show any signs of threat to storage security.