Combining electromagnetic and seismic methods to monitor carbon dioxide sequestration

SUMMARY Project description This project is concerned with developing a geophysical strategy, using seismic and electromagnetic (EM) methods, for monitoring sequestration of carbon dioxide in reservoirs buried hundreds of metres beneath the sea floor in areas such as the North Sea. The motivation for this research comes from the CASE partner, Petroleum Geo-Services (PGS) who wishes to develop a service in this area that is both operationally efficient and technically sound. When carbon dioxide is injected into saline aquifers, or into reservoirs that previously contained hydrocarbons, it occupies part of the pore space in the rock and changes the rock properties. The effect on the seismic response is a dramatic decrease in acoustic impedance with the early injection of a small amount of gas. Thereafter the seismic response is essentially unchanged. The resistivity of the reservoir rock is affected only slightly by early CO2 injection, but as the quantity of CO2 increases, the rock resistivity increases exponentially. Electromagnetic methods therefore have the potential to monitor the quantity of CO2 stored in a reservoir. The migration and inversion of seismic reflection data is now mature. This project focuses on the electromagnetic data. The objectives of the project are: 1. To relate the magnitude of the observed seismic and electromagnetic anomalies to the quantity of injected gas, the gas saturation, and the depth of the reservoir, and to determine the detectability of the anomalies in the presence of noise. 2. To develop a top-down modelling approach to invert the synthetic electromagnetic data for subsurface resistivities, based on a new ray-theoretical approach being developed in Edinburgh. This is totally new and relates the peak of the earth impulse response to a known subsurface 'ray path.' 3. To apply the same approach to relate the electromagnetic anomalies to subsurface changes in resistivity. 4. To apply the developed inversion method to real transient electromagnetic data. Workplan 1. Model-building. Build a three-dimensional (3D) physical and petrophysical model of a reservoir embedded in a background medium below a water layer of variable depth. The resistivity model will be anisotropic, with the vertical component of resistivity greater than the horizontal (3 months). 2. CO2 Sequestration Simulate changes in the seismic, electromagnetic (resistivity) and petrophysical properties of the reservoir by adding gas to the pore fluid in the reservoir (1 month). 3. Geophysical Monitoring Simulate both seismic and transient electromagnetic surveys over the reservoir using the 'Nucleus' modelling package provided by PGS (3 months). 4. Feasibility Study Determine the magnitude of the observed seismic and electromagnetic anomalies as a function of the quantity of injected gas. We expect large seismic anomalies for early gas injection and very little change thereafter, while the EM data should show small anomalies for early gas and then progressively larger anomalies with increasing gas, as indicated in the figure. The detection of the computed anomalies in real data with noise needs to be estimated using realistic noise measurements provided by PGS (6 months). 5. Inversion of EM synthetic data Develop a top-down modelling approach to invert the synthetic electromagnetic data for subsurface resistivities, based on a new ray-theoretical approach being developed in Edinburgh. This is totally new and relates the peak of the earth impulse response to a known subsurface 'ray path.' (1 year) 6. Inversion of EM time-lapse synthetic data Apply the same approach to relate the electromagnetic anomalies to subsurface changes in resistivity. (6 months). 7. Inversion of real transient EM data Apply the developed inversion method to real transient electromagnetic data in PGS's Edinburgh office. (6 months).