Prediction of CO2 leakage from reservoirs during large scale storage

This project will investigate mechanisms for CO2 leakage from large-scale CO2 storage sites in reservoirs and aquifers. The CO2 is less dense than water and it is kept in place by a thick cover of tight sediments. Leakage through these tight sediments is possible due to the formation of a so-called chimney- or pipe-structures. Such pipe structures (chimneys) are currently observed most places in sedimentary basins, especially due to improved seismic imaging. It is important to understand leakage of CO2 through chimneys with respect to the stability and the safety of long-term CO2 storage.

Natural hydraulic fracturing is a possible explanation for the formation of the observed pipe- and chimney-structures. This kind of fracturing happens when the pore fluid exceeds the least compressive stress and the rock strength. The force that drives the fracturing through the overburden is the high pore fluid pressure in the reservoir or aquifer underneath the overburden. The compressive stress in rocks and sedimentary layers decreases towards the surface. When the fracturing propagates upwards, it encounters less and less compressive resistance. Therefore, the high pressure in a reservoir or aquifer is potentially an unstable situation.

We have started the development of a model for this type of natural hydraulic fracturing. One of the subgoals is to understand how an anisotropic stress field in the rock controls the direction of fracturing and the form of the pipe structure. A particular feature of the model is that it takes the rock strength as a random variable. That is because rocks are strongly heterogeneous. The model will handle how weakness zones in the rock control fracturing together with stress anisotropy.