(724b) The Development and Demonstration of Best-Practice Guidelines for the Start-up Injection of CO2 into Highly-Depleted Gas Fields

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 pressure as well as the unique thermodynamic properties of CO₂, especially in the presence of stream impurities, the injection process presents significant safety and operational challenges.

The most effective way of transporting the captured CO₂ for subsequent sequestration using high-pressure pipelines is in dense phase. The CO₂ arriving at the injection well will typically be at pressure greater than 70 bar and at temperature between 4 and 8 ºC. Given the substantially lower pressure at the wellhead, the injection of CO₂ will result in its rapid, quasi-adiabatic Joule-Thomson expansion leading to significant temperature drops. This could pose several risks, including hydrate and ice formation around the wellbore and thermal shocking of the wellbore casing steel, leading to its fracture and ultimately escape of CO₂.

Despite its importance, the modelling of the flow dynamics taking place during the start-up injection of CO₂ has been devoted mainly to either steady-state operations¹ or flows where only pure CO₂ is injected². The steady-state flow model is inappropriate as it ignores important phenomena, such as expansion wave propagation and bubble nucleation, occurring during the rapid depressurisation process. In addition, there is little work in the literature regarding the impact of the injection rate and the upstream inlet conditions (e.g. temperature and pressure) on the degree of cooling along the well³. Moreover, all previous work has been confined to pure CO₂. In practice the injected CO₂ will have a range of different impurities which will have a profound impact on the CO₂ fluid phase behaviour and hence its injectivity⁴.

The aim of our work is twofold. First, we present in detail a homogeneous relaxation flow model for the numerical simulation of the highly transient phenomena taking place in the wellbore. 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. The phase equilibrium and thermodynamic properties for CO₂ are determined using the Peng-Robinson equation of state, which guarantees both accurate and computationally efficient predictions of VLE data for CO₂⁵. Operational envelopes for the injection well are obtained from data already available in the literature³. Second, by performing a sensitivity analysis on various injection rates and temperatures, we 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.

1. Hughes D., Carbon storage in depleted gas fields: Key challenges, Energy Procedia, 1, 2009, 3007-3014.

2. Galic H., Cawley S. and Bishop S., CO₂ injection into depleted gas reservoirs, SPE 123788, 2009, 1-8.

3. Li X., Xu R., Wei L. and Jiang P., Modeling of wellbore dynamics of CO₂ injector during transient well shut-in and start-up operations, International Journal of Greenhouse Gas Control, 42, 2015, 602-614.

4. Brown S., Beck J., Mahgerefteh H. and Fraga E. S., Global sensitivity analysis of the impact of impurities on CO₂ pipeline failure, Reliability Engineering & System Safety, 115, 2013, 43-54.

5. Oosterkamp A. and Ramsen J., State-of-the-art overview of CO₂ pipeline transport with relevance to offshore pipelines, Technical Report, PolyTec.