Pump it up? Quantifying the capacity and risks of injecting CO2 into depleted hydrocarbon fields

In this exciting project, the student will work at the forefront of the Energy Transition. As plans accelerate towards injecting carbon dioxide into depleted gas fields around the UK, all stakeholders need to have confidence in the process. In particular, we need to ensure both the mechanical stability of the reservoir over time and the immediate safety of drilling new injection wells. You will tackle this problem with an integrated programme of data science, laboratory rock deformation experiments and numerical modelling. The results from this project will inform policy at national and local levels.

Background & Rationale

Depleted gas fields comprise some of the main targets for carbon dioxide storage projects, in the UK and overseas. We know that production of oil, gas and water over time changes the effective stress in a reservoir, and likely therefore the total stresses in the reservoir and overburden. These changes in stress may involve pore pressure-stress coupling where the minimum horizontal stress decreases as a proportion of the reservoir pressure; but equally, they may not. Either way, the present-day effective stress in these depleted fields is different to when production started, and will change again on injection of CO2. Injection of CO₂ will raise the reservoir pressure, reduce the effective stress and possibly change the total stress in the reservoir and the surrounding rocks. Several such schemes are scheduled to start around the UK in the next few years, but published data or models on the forecast response of the reservoirs are hard to find. To ensure the integrity of any proposed storage scheme, we need to be able to reliably predict how the effective and total stresses will change on injection; and to enable safe and cost-effective drilling, we need to constrain the present day fracture gradient. Pore pressure-stress coupling is well described, although not necessarily well understood. Critically, there is no consensus in terms of the governing constitutive rheological model to explain observed reservoir behaviour during depletion. Laboratory experiments, numerical modelling and observations from oil and gas fields confirm that stress path hysteresis is possible, or even likely, on injection after depletion; i.e., the stress path on injection will probably not be the simple reverse of that on depletion.

Depending on their interests, the student will aim to deliver:

·      Quantitative estimates of depleted reservoir stress paths;

·      Constraints on the rheological behaviour of depleted reservoir rocks; 

·      A tool to predict the geomechanical behaviour on CO₂ injection, including estimates of uncertainties in the current in situ and predicted future stress state; 

·      Specific policy recommendations at national and local levels targeted at improving regulatory oversight and public confidence in the injection process.

Methods & Approach

The student will integrate borehole and reservoir geomechanics, structural geology, experimental rock mechanics and data science. To quantify reservoir stress paths during depletion, we will use open data published historical sources. To provide constraints on the rheological behaviour of depleted reservoirs, we will integrate the data from the stress path estimation with targeted rock deformation experiments on selected rock samples from core and outcrop analogues. These experiments will be conducted in the Geosolutions Leeds Geomechanics Laboratory using the Sanchez triaxial rock deformation apparatus (confining pressures up to 250 MPa, and temperatures up to 200°C). The student will measure stress-strain behaviour for elastic properties, pore volume changes, and P- and S-wave velocity variations under changing loads. By integrating the findings from the borehole geomechanical analysis and the laboratory deformation experiments, the student will develop a predictive numerical model in open source Python to predict future stress paths on injection. At all stages of the work, the student will be trained to estimate and constrain errors and uncertainties in the measured and estimated variables. Impact will be generated through collaboration with the North Sea Transition Authority at the national regulatory level, and the Yorkshire & Humber Climate Commission at the local community level.

This project will benefit from the wider Geosolutions Leeds project portfolio, including ongoing research into geothermal energy, subsurface storage and critical minerals. Geosolutions Leeds is a core part of the University of Leeds Climate Plan, an ambitious £174M programme designed to achieve Net Zero on campus by 2030 and support decarbonisation of the city and wider region.