Wellbore cement alteration during decades of abandonment and following CO₂ attack – A geochemical modelling study in the area of potential CO₂ reservoirs in the Pannonian Basin

Highlights

• CO₂ leakage risk through abandoned wellbores in the Pannonian Basin is studied.

• PHREEQC models are used for predicting geochemical interactions.

• The present compositions of 60 years old cements in different depths are estimated.

• The interactions of hydrated cements and potentially injected CO2 are accessed.

• Porosity changes in the 5 cm width cement casing are calculated at different depths.

Abstract

In East-Central Europe, the sedimentary rocks and saline reservoirs of the Pannonian Basin provide the greatest potential for geological sequestration of CO₂. However, there is no knowledge about the integrity of cement casing and plugs in abandoned wellbores drilled in the last century, which surround the potential CO₂ geological storage sites. Thermodynamic and kinetic batch, and 1D kinetic reactive transport models have been built up in PHREEQC to estimate the present composition of hydrated cement in different depths (106, 1478, 2136 and 2718 m) of these abandoned wellbores and, to access cement reactivity for the effect of potentially injected CO₂. The wellbore cements are all predicted to presently consist of mainly calcium silicate hydrate (CSH) and portlandite but differences occur due to the stability of ettringite connected to the temperature of the geological environment. With the depth, the amount of potentially dissolved CO₂ increases which induces the breakdown of portlandite and CSH in the cement, and the major precipitation of calcite and amorphous silica. The transformation affects the whole width of the cement casing after about 2–3 years in most of the depths. However, the calculated mass transfer among minerals indicate a 2–6% porosity drop, which raises the attention to potential pore clogging. This process significantly reduces the risks of abandoned wellbores when implementing CO₂ geological storage.

Introduction

The cement casing in wellbores has multiple functions. It blocks the fluid flow at depths where no connection is preferred plus, it stabilizes the wall and provides a support for the steel casing. Cement, furthermore, has been and is applied also for plugging the wellbores when they are not in use anymore.

In the 20th century, an extensive oil and gas exploration went on worldwide, as well as in the Pannonian Basin (Dank, 1997). As a result, numerous, today abandoned wellbores have been drilled. By today, the cement casing and plugs have gone through decades of hydration and alteration at different geological conditions. There is only limited knowledge of the present state of the cement in these wellbores and the differences caused by the different depths (pressure and temperature) and formation water compositions.

The same areas, where the drilling activity peaked, are frequently potential sites for CO₂ geological storage today. The abandoned boreholes of these areas, however, are considered potential CO₂ leakage pathways (e.g. Bagheri et al., 2018) which may limit the deployment of the technology. The supercritical CO₂ may migrate upwards along different interfaces of cement, or through deformed casing, cement and its pores or micro-fractures (Bagheri et al., 2018 and references therein; Gasda et al., 2004; Omosebi et al., 2017). Therefore, it is of significant interest if, and at what level, the aged cement degrades due to the effect of CO₂.

Numerous studies have been taken on cement-CO₂ interactions which were reviewed by Bagheri et al. (2018). They report that, in most studies, the carbonation degrades the outer layers of cement. However, the same process leads to a self-healing behavior of the material through the clogging of pores and fractures by CaCO₃. The self-healing is, though, negatively affected by high acidity degree or flow velocity of pore water. The findings of these studies are mainly based on laboratory experiments or geochemical modelling. Geochemical modelling, also applied in this study, provides a tool for studying times much longer than experiment durations (Bagheri et al., 2018) and the sensitivity of reactions on pore water chemistry and transport parameters (e.g. Xiao et al., 2017). However, the effect of pressure and temperature have not been previously modeled in details.

In East-Central Europe, the sedimentary rocks and saline aquifers of the Pannonian Basin provide the greatest potential for geological sequestration of CO₂ (Poulsen et al., 2015). At the Great Hungarian Plain (GHP), SE-Hungary, several potential future CO₂ reservoirs have been identified and studied in recent years (Falus et al., 2011; Hódossyné Hauszmann et al., 2011; Sendula et al., 2014; Szabó et al., 2019, 2018; 2017, 2016; Szamosfalvi et al., 2011), as well as, their natural analogue at the Mihályi-Répcelak area, W-Hungary (Cseresznyés et al., 2017; Király et al., 2017b, 2017a, 2016b, 2016a). However, so far, there is no knowledge about the integrity of cement casing and plugs in abandoned wellbores drilled in the last century at the GHP (Dank, 1997) which surround the potential CO₂ geological storage sites.

This work, by the application of geochemical modelling, aims to estimate the present mineral composition of aged cement in different depths of wellbores in the Pannonian Basin and to predict the effect of potentially injected CO₂. The differences caused by variable pore water compositions, pressures and temperatures are analyzed. Present study, overall, contributes to the better understanding of cement alteration due to aging and CO₂ attack in abandoned wellbores of sedimentary basins and to the implementation of CO₂ geological storage in East-Central Europe.