The mechanisms, dynamics, and implications of self-sealing and CO2 resistance in wellbore cements. (August 2018)

• Record Type: Journal Article
• Authors: Guthrie, George D.; Pawar, Rajesh J.; Carey, J. William; Karra, Satish; Harp, Dylan R.; Viswanathan, Hari S.
• Is Part Of: International journal of greenhouse gas control. Volume 75(2018)
• Page Start: 162
• Page End: 179
• Publication Date: 2018-08
• Subjects: Wellbore integrity -- Portland cement -- Self-sealing -- CO2 compatibility; Greenhouse gases -- Environmental aspects -- Periodicals; Air -- Purification -- Technological innovations -- Periodicals; Gaz à effet de serre -- Périodiques; Gaz à effet de serre -- Réduction -- Périodiques; Air -- Purification -- Technological innovations; Greenhouse gases -- Environmental aspects; Periodicals; 363.73874605
• DOI: 10.1016/j.ijggc.2018.04.006
• Languages: English
• ISSNs: 1750-5836

Abstract

Highlights: Self-sealing of flowpaths in a cemented wellbore is an important aspect of long-term wellbore integrity for CO2 storage. Self-sealing conditions occur in a reaction zone that exists over a wide range of cement properties and reservoir conditions. Self-sealing reaction zone moves proportional to Darcy flux, and its width is proportional to fluid velocity and time. Self-sealing conditions can be maintained in a specific section of a wellbore for favorable hydrogeochemical conditions. Hydrated-Portland-cement is a carbonic cement: it reacts with carbonic acid to form stable, compatible phases. Abstract: This study analyzes the dynamics and mechanisms of the interactions of carbonated brine with hydrated-Portland-cement; in particular, the study focuses on self-sealing, a process whereby hydrated-Portland cement reacts with carbonated brine to for silica and calcium carbonate in sufficient quantities to seal the flow pathway. The analysis is based on a comprehensive set of reactive-transport simulations that explore the complex coupled dynamics between the fluid flow and mineral reactions that underlie self-sealing, and it relies heavily on the synthesis of the extensive body of work on wellbore integrity that has been conducted over the past decade. The analysis explores a large chemical and mineralogical diversity and a wide range in physical conditions and flow regimes, attempting to assess the robustness of the analysis. Self-sealing conditions arise over a wide range in cement properties and reservoir conditions. Although some properties and conditions promote a stronger self-sealing response, self-sealing occurs for a wide range of Ca:Si ratios in cement and for various reservoir fluid compositions. Self-sealing conditions move along a wellbore proportional to the flux of the leaking carbonated brine, and the reaction zone spreads out proportional to the fluid velocity, where volumetric flux and velocity are related by porosity (flux = velocity * porosity). However, self-sealing conditions can be maintained in a specific section of a wellbore by controlling the pressure drive and/or effective wellbore permeability, which in turn can limit the flux and velocity of any leaking fluid. Finally, the phases produced by hydrating Portland cement represent a carbonic cement that will react with a carbonated brine to produce end products (calcium carbonate and silica) that can maintain integrity in the presence of carbonic acid. Moreover, the attributes that make hydrated Portland cement phases a carbonic cement are required for self-sealing.