Assessment of well cement risk leakage on CO2 geological storage : the Goldeneye reservoir (North Sea, UK)
Recasens Diaz, Montserrat
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There is currently a great deal of concern regarding the wellbore integrity, particularly of existing abandoned wells. These wells represent the greatest potential risk of CO2 leakage through its engineered structure, more specifically, through well cement as was studied in this PhD thesis. The aim of this Thesis is to study how the internal microstructure of cement undergoes a series of transformations that change its initial properties, particularly its porosity and permeability, by the effect of geochemical reactions that occur in the CO2 injection process but also when the wells are no longer operative. A series of experiments were conducted in order to simulate the degradation process in which Class G cement representative of the Goldeneye reservoir (North Sea) can be exposed during and post injection of CO2 into the storage site, using high-pressure and high-temperature reactors. Reacted cement samples were examined for porosity, permeability and mineralogical changes. The internal structure of cement samples has been highly affected by CO2 injection. The alteration depth observed in the different set of samples varies depending on factors such temperature, pressure and salinity, as well as the use of pozzolan additive, in which these cement samples were exposed. In addition, the CO2-brine-well cement interactions have let to a series of changes in porosity and permeability. Despite cement samples were exposed to different experimental conditions, there is a general reduction in porosity (except in some samples) caused by secondary calcium carbonate (CaCO3(S)) precipitation. However, this reduction of porosity does not always lead to a reduction of permeability because the variation of cement permeability is related with factors as the development of secondary thin micro-cracks and/or a potential better connectivity between pores of secondary porosity. An increase (by a factor of 10) in permeability is observed in those samples that suffered higher degradation.