Cement Core Experiments With A Conductive Leakage Pathway, Under Confining Stress And Alteration Of Cement's Mechanical Properties Via A Reactive Fluid, As An Analog For CO2 Leakage Scenario

Abstract

AbstractUnsuccessful zonal isolation and occurrence of sustained casing pressure indicate transport of fluids along a conductive pathway in a well's cemented annulus. Conductive pathways in cement can be grouped into three general classes: separation of the cement and formation or casing to form a micro annulus; micro fractures in the cement; and gas/mud channels in the cement matrix. In geologic storage of CO2 similar conductive paths are most likely to be responsible for CO2 leakage out of the reservoir along a wellbore. Moreover, cement is reactive with both CO2 gas and acidified brine (a product of CO2 dissolution into formation water) and the resulting cement product is of lower mechanical strength than the intact cement.Recent research indicates that in neat cement the reaction rate is slow and long term degradation of intact cement is not a risk. Reaction at surfaces of conductive pathways raises a new set of issues, however. In any field situation a cemented annulus, and conductive pathways within it, support the effective stress of surrounding rock and pore fluid. If the mechanical properties of the opposing faces of the conductive pathway are altered by chemical reaction, or if the effective stress changes, then relative displacement (opening or closing) of the faces will occur.Our lab has developed a system to i) create a fracture in a cement core that is representative of what one would expect in a conductive pathway within a cemented annulus and ii) measure the hydraulic conductivity of the fracture under confining stress. We use this apparatus to characterize how increasing the confining pressure controls the size of the aperture. We reacted the surfaces of the fractured cores and repeated the stress/aperture measurements. The results are consistent with the points of contacts between the core halves becoming mechanically weaker after reaction.Our observations support the possibility that conductive pathways can be self-sealing. Degradation of the cement's mechanical properties by reaction with acidic aqueous phase at the surface of the fracture would lead to a narrower pathway, if effective stress is unchanged. Further work is underway to measure the extent of irreversible (plastic) deformation in these experiments. Our ultimate goal is to project the core scale relationship onto the well scale to determine the conditions (earth stresses, cement exposure to reactive fluids, and leak source pressure) under which a conduit tends to seal.