Modelling acid attack of oilwell cement exposed to carbonated brine

Abstract

A diffusion-reaction model for the carbonation process of oilwell cement exposed to carbonated brine under CO2 geological storage conditions is presented. The formulation consists of two main diffusion/reaction field equations for the concentrations of aqueous calcium and carbon species in the pore solution of the hardened cement paste, complemented by two diffusion-only field equations for chloride and alkalis concentrations, and by a number of chemical kinetics and chemical equilibrium equations. The volume fraction distribution of the solid constituents of the hardened cement paste and the reaction products evolve with the progress of the reaction, determining the diffusivity properties of the material. The model is used to simulate experimental tests performed by Duguid and Scherer (2010), leading to promising results indicating that the fundamental aspects of the phenomenon are captured.Peer ReviewedPostprint (author's final draft

A review of oil well cement alteration in CO2-rich environments

“…This process gradually begins from the calcium carbonate precipitation zone and increases towards the cement-brine interface. By decreasing the pH below 12, C-S-H dissolves incongruently [32], which leads to decline in the ratio of Ca/Si from around 2 to approximately 0.8 [23,33,34]. To simplify the modelling of the alteration in the cement, the C-S-H dissolution can be constrained to the silica gel zone due to the higher portlandite dissolution rate than that of C-S-H. Chasing any alterations in C-S-H during the reactions is difficult, because composing ions of C-S-H are not dissolved proportional to the stoichiometric coefficient.…”

“…To simplify this problem, C-S-H is considered as a solid solution of hydrated silica gel and two fractions of portlandite. It is supposed that one of those portlandite fractions dissolves at the same rate of the free portlandite [33].…”

“…The direction of diffusion and flow are the same for transverse permeability and the flow direction is vertical to the diffusion direction for parallel permeability. amorphous silica gel zone(V) depleted from calcium content to a great extent immediate to the brine-cement interface [33].…”

“…A diffusion-reaction model was developed by Liaudat et al [33] to regenerate experimental data of Duguid and Scherer [27]. To simplify modelling, reactions corresponding to aluminium, iron, and sulphur (solid and aqueous) species were removed from consideration.…”

Durability of oilwell cement in CO2-rich environments

“…This process results in the formation of calcium carbonate (hereafter referred to as calcite) within the cement pores which reduces the porosity and increases the cement stiffness (Kutchko et al, 2009;Mason et al, 2013;Nakano et al, 2014;Walsh et al, 2014;Ashraf, 2016;Hangx et al, 2016). The invasion of more quantity of CO2-bearing fluids into the cement matrix re-dissolve calcite and calcium silicate hydrate (C-S-H) which leads to the stiffness reduction and an increase in the porosity (Kutchko et al, 2007;Huerta, Bryant and Conrad, 2008;Rimmelé et al, 2008;Fabbri et al, 2009;Duguid and Scherer, 2010;Lecampion et al, 2011;Brunet et al, 2013;Zhang et al, 2013;Walsh et al, 2014;Liaudat et al, 2018). This process is referred to as degradation occurring in the case of invading extra CO2-bearing fluids or decreasing pH.…”