The Development and Demonstration of Best Practice Guidelines for the Safe Start-up Injection of CO₂ into Depleted Gas Fields

Background

Highly-depleted gas fields represent prime potential targets for large-scale storage of captured CO₂ emitted from industrial sources and fossil-fuel power plants. Given the potentially low reservoir pressures as well as the unique thermodynamic properties of CO₂, especially in the presence of the various stream impurities, the injection process presents significant safety and operational challenges.

In particular, the start-up injection leads to the following risks:

• blockage due to hydrate and ice formation following the contact of the cold CO₂ with the interstitial water around the wellbore;
• thermal stress shocking of the wellbore casing steel, leading to its fracture and ultimately escape of CO₂;
• over-pressurisation accompanied by CO₂ backflow into the injection system due to the violent evaporation of the superheated liquid CO₂ upon entry into the wellbore.

Aims and objectives 

The aim of our work is two-fold:

• to develop a homogeneous relaxation flow model for the numerical simulation of the highly-transient phenomena taking place in wells accounting for variable cross-sectional area during the injection of CO₂. Mass, momentum, and energy conservation equations are considered in the tubing. Wall friction, gravitational force, and heat transfer between the fluid and the surrounding formation are also taken into account.
• to perform a sensitivity analysis using various injection rates and temperatures, in order to propose optimal transient-operation design and develop best-practice guidelines for the minimisation of the risk associated with start-up injection of CO₂ into highly-depleted gas fields.

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