CO2 - H2 Optimisation in Rocks for Underground Storage (CHORUS)

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The UK is poised to embrace net zero carbon emission technologies to meet its Paris accord targets, by combining an increasing use of the renewables with efficient underground hydrogen storage (UHS) and long-planned Carbon Capture Usage and Storage (CCUS) schemes. This proposal will advance knowledge needed to combine UHS with CCUS (i.e., a win-win strategy), by developing a novel seismic monitoring tool to capture key geophysical properties of the stored hydrogen (H2). Specifically, UHS implies cyclic injection/depletion activities to deal with seasonal fluctuations associated with energy demands. But for the cycle to be successful, a cushion gas is needed to keep the reservoir pressurised. Carbon dioxide (CO2) being an abundant greenhouse gas, is a promising, environmentally friendly alternative. Its use as cushion gas, if successful, could significantly reduce the cost of seasonal H2 retrieval and open up novel research directions.In most storage projects, fluids injected in geological formations are seismically monitored by associating the variation of seismic velocity and amplitude with fluid content. However, if H2 is injected in a CO2-cushioned reservoir, the similar acoustic properties of both gases together with the short timescales to settle within an injection/extraction cycle obscure the H2 seismic visibility. CHORUS will test the hypothesis that a viscosity contrast is the key to seismic H2 detectability in a H2-CO2 storage scenario. We propose to test this hypothesis in three stages, using our current expertise with multi-flow laboratory tests, modelling and dispersive wave propagation. First, we will perform laboratory measurements of the elastic and transport properties of reservoir rocks saturated with the fluids present in UHS applications. We will control these experiments to emulate viscosity contrasts of H2-CO2 at reservoir conditions. Second, we will apply existing rock physics models established for CCUS to calculate the seismic velocities, attenuation and dispersion of these rocks under different saturation conditions. This will involve reservoir rocks saturated with H2-water and CO2-water below a caprock seal. We will calibrate these models using the novel dataset. Third, we will scale up our finds by calculating synthetic seismic data corresponding to a vertical seismic profile (time-lapse experiment with attenuation contrast between different fluid regimes) controlled by the viscosity contrast and informed by the experimental data. Using this synthetic dataset, we will conduct a sensitivity analysis to assess the limits of seismic detectability of H2.Outcomes of this proposal have the potential to de-risk UHS monitoring by enhancing our ability to quantify H2 through better seismic resolution of the H2-CO2 interface. A better understanding of the dispersive properties of H2-saturated rocks will enable policy-makers to identify seismic attributes associated with fracturing and quantifying leakage risk. Altogether will facilitate both the planning of efficient monitoring strategies for industrial seasonal UHS. We propose to disseminate our results in the form of two (6-monthly) reports, a collaborative scientific publication in a lead academic journal and a conference publication, and openly accessible datasets from the rock physics and the synthetic seismic experiments. Using this project as springboard proof-of-concept, we intend to consolidate its finds by pursuing a long-term UK collaboration through a NERC Pushing Frontiers funding proposal, to include the assessment of caprock integrity by incorporating geomechanical effects from fracturing on seismic signatures. Such a proposal would incorporate fundamental research, as well as detailed anisotropic modelling of fractured top-seal/reservoir and seismic data. In addition, our theoretical advancements can complement ongoing UHS studies, both in NOC (NERC MOET), and UoE (EPSRC HyStorPore) with novel rock physical knowledge.