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.