Quantitative Diagenesis for the Characterization of CCUS Storage in Carbonates

Recent years have seen the growth of new techniques that combine conventional stratigraphic and observational approaches to characterizing the type, scope, extent, timing, and effects of diagenetic processes with petrophysical measurements of their rock microstructure. These new Quantitative Diagenesis (QD) techniques can be used to predict post- and predolomitization porosities and permeabilities as well as track petrodiagenetic pathways. The objective of this paper is to use QD to calculate changes to the CO2 storage of a CCUS target for the first time. These QD approaches include porosity and permeability prediction resulting from varying degrees of dolomitization, calculation of porosity and permeability of the host rock before dolomitization, using petrodiagenetic pathways to track quantitatively the type, extent, and timing of diagenetic processes, and methods for determining the impact of fractures (the Fracture Effect Index, FEI). This paper reports the impact of dolomitization and fracturing on CO2 storage by considering the Butmah and Shiranish formations (NE Iraq). The Butmah Formation data show that the CO2 storage of the formation increased significantly 154.23 Mt (78%) due to dolomitization. The Shiranish Formation showed an increase in CO2 storage of 144.23 Mt (70%) from the almost unfractured rocks of its U.1(A) lithofacies (FEI = 0.31) to the highly fractured rocks of its U.4 lithofacies (FEI = 15.55). The main scientific contribution of this paper is that it shows for the first time that QD techniques can be used to calculate very significant changes in CO2 storage capacity concomitant with fracturing, dolomitization, and precipitation. Such techniques should therefore be employed when judging any legacy reservoir or aquifer in carbonates the potential CCUS use.