AN EXPERIMENTAL AND MODELING STUDY OF WELLBORE INTEGRITY

In this study we explore the important geochemical reactions between common cements used in the construction of wellbores, cap rock, and supercritical CO₂ stored in deep saline reservoirs. We have reacted the end member components the heterolithic sandstone and shale unit that forms the upper section of the carbon storage reservoir at the Krechba Field, In Salah, Algeria with supercritical CO₂ and class G cement in a synthetic brine. One-dimensional reactive transport simulations of cement in contact with shale and sandstone suggest that diffusive transport of CO₂ in the cement is limited by the reaction of calcium silica hydrates and portlandite to aragonite and amorphous silica. These reactions effectively buffer the CO₂-rich brine and limit enhanced porosity to a localized area in lower reaches of a hypothetical cement – cap rock fracture.

The experimental results are consistent with preliminary simulations, but suggest that illite in the shale may be more reactive. Analysis of the solution chemistry over time and the solid products at the experiment end show that the well bore environment is dominated by reactions between cement and CO₂-rich fluids. Anhydrous calcium silicate cement phases fully reacted to amorphous silica, calcite, and aragonite in experiments with and without sandstone or shale. Sandstone integrity is retained, because quartz solubility and dissolution are minimal at acid pH in the CO₂-rich brines. Reaction of the shale is distinct from the reaction of the sandstone when directly exposed to the CO₂-rich brine and the cement. Although both surfaces show an accumulation of CaCO₃ precipitates, there was no mass precipitation of the amorphous silica phase as was observed for the sandstone. Instead we see dissolution etch pits as well as the formation of a fibrous precipitate at the illite surface. Additionally, the evidence of brine in the shale interlayers suggests that permeability between shale layers should be included to appropriately model well-bore integrity between the cement and shale-rich layers at the Krechba field.