Characterization of major overburden leakage pathways above sub-seafloor CO2 storage reservoirs in the North Sea – CHIMNEY

Industrial emissions of carbon dioxide (CO2), including fossil fuel power generation, are recognised as a likely agent of global climate change and acidification of the oceans, but most economies will remain dependent on these technologies for the next few decades. Carbon dioxide Capture and Storage (CCS) has been identified as an important way of reducing the amount of CO2 added to the atmosphere. CCS is seen as making a key contribution to reducing mankind’s greenhouse gas emissions by 80-95% by 2050 and keeping climate change derived temperature increases below 2 degrees C, as outlined in European Commission policy. In addition, CCS is considered an important way of reducing the cost of mitigation measures around the continued use of fossil fuels. Offshore storage of CO2 in depleted oil and gas reservoirs and saline aquifers is the option of choice for most European nations, and there is currently one operational storage complex (Sleipner, Norway), and several other commercial scale demonstration projects are in late stages of development (e.g. ROAD-Netherlands, Peterhead and White Rose-UK), and expected to be in full operation by 2020. A key element of CCS offshore is that there is confidence that the risks of any leakage are understood. The location and potential intensity of any possible CO2 leakage at the seafloor are critically dependent on the distribution of fluid (dissolved and gaseous CO2) pathways in the rocks overlying the reservoirs in which the CO2 is stored, and on the ability of these pathways to transmit fluid (termed permeability). Recent studies of the structure of marine sedimentary rocks in the North and Norwegian Seas have revealed that near-vertical structures, which resemble chimneys or pipes, cross-cut the sedimentary sequence. These structures may be pathways for fluid flow. Natural fluids from deeper rock layers have migrated through these structures at some point in geological time. If CO2 leaking from sub-seafloor storage reservoirs reaches the base of these structures, and if their permeability is sufficiently high, they could act as CO2 leakage pathways towards the seafloor and overlying water column. To provide a reliable prediction of potential seafloor seep sites, the degree to which these pathways are continuous and especially their permeability needs to be better understood. In this project (CHIMNEY) we will collect new data over a chimney structure within the North Sea by using a ship to make new and unusual measurements with sound waves. We will use several different marine sound sources to make images of the chimney, using receivers at the sea surface, and also record the sound arrivals on sea bed instruments known as ocean bottom seismometers. By looking at the sound travel paths through the sub-surface from a range of directions and frequencies we will obtain information about fractures/fluid pathways in the chimney as well as the surrounding rocks. We will calibrate and understand our marine seismic results using laboratory studies of materials (synthetic rocks) that mimic the sub-surface rocks. By understanding the propagation of sound through synthetic rocks with known fluid pathways we can understand the results of the marine experiment. We will also drill into the chimney and collect core samples which we will analyse for core geology and fluid chemistry. A computer model of the sub-surface chimney will be constructed combining the results of the seismic experiment, rock physics, and chemistry. We will work with companies involved in CCS to build realistic computer models of fluid flow that tell us about the risk of leakage from chimney structures generally within the North Sea that are relevant to Carbon Dioxide Capture and Storage.