Abstract
The integrity of wells, which are key components for CO2 sequestration, depends mainly on the seal between the wellbore cement and the geologic formation. To identify the reaction products that may alter the cement/caprock interface, batch experiments and computer modelling were conducted and analysed. Over time, the dissolution and precipitation of minerals alters the physical properties of the interface, including its tightness. One main objective of the simulation was thus to analyse the evolution of the porosity of cement and caprock over time. The alteration of the cement/caprock interface was identified as a complex problem and differentiated depending on rock type. The characteristic feature of a cement/shale contact zone is the occurrence of a highly carbonated, compacted layer within the shale, which in turn causes cement/shale detachment. In the case of a cement/anhydrite interface, the most important reaction is severe anhydrite dissolution. Secondary calcite precipitation takes place in deeper parts of the rock. The cement/rock contact zone is prone to rapid mineral dissolution, which contributes to increased porosity and may alter the well integrity. Comparison of computer simulations with autoclave experiments enabled the adjustment of unknown parameters. This enhances the knowledge of these particular assemblages. Overall, a good match was obtained between experiments and simulations, which enhances confidence in using models to predict longer-term evolution.
Highlights
The principal function of well cementing is to support the casing as well as to restrict fluid movement between the well and formation rocks
The experiments performed by Jung and Um (2013) demonstrate that preferential cement alteration can occur along the cement/steel and cement/rock interfaces and highlight the importance of further investigation of cement degradation along the interfaces to ensure permanent geologic carbon storage
Geochemical changes upon CO2 injection are susceptible to reservoir conditions (Czernichowski-Lauriol et al 2004); the important issue is to study the potential geochemical reactions that are possible during the process of sequestration
Summary
The principal function of well cementing is to support the casing as well as to restrict fluid movement between the well and formation rocks. The experiments performed by Jung and Um (2013) demonstrate that preferential cement alteration can occur along the cement/steel and cement/rock interfaces and highlight the importance of further investigation of cement degradation along the interfaces to ensure permanent geologic carbon storage. The injected CO2 reacts with the formation rocks and wellbore cement, changing their mineral composition and porosity and the integrity of the materials. Similar reactions are possible within caprocks, which are an important element of the wellbore system sealing capacity, preventing fluids from escaping the sequestration reservoir (Carey et al 2007). Geochemical changes upon CO2 injection are susceptible to reservoir conditions (Czernichowski-Lauriol et al 2004); the important issue is to study the potential geochemical reactions that are possible during the process of sequestration. The assessment of the sealing efficiency of a storage site is a key factor in evaluating the storage site performance (Gherardi and Audigane 2013)
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