Eric James Mackay

Understanding fluid displacement patterns under the combined influence of viscous, gravitational, and capillary forces is fundamental to predicting CO₂ behaviour during geological storage operations. Laboratory studies at the Darcy scale (rock core, mineral packs etc.) of CO2/water flow in porous media to support field CCS projects are of two main types: (i) conventional core analysis using rock cores to measure CO2/water relative permeabilities (Relperms) and capillary pressures (Pc); and (ii) larger pack or slab floods which attempt to model 2D or possibly 3D flows associated with CO2 migration in larger systems. In previous work, we have presented results showing the serious inadequacies of published core-scale laboratory-derived CO2/water Relperms [1]. It was shown previously that conventional Corey-type functions derived by assuming stable one-dimensional flow conditions fail to represent actual flow behaviour in multi-force environments. In particular, these Relperms do not show immiscible viscous fingering under viscous dominated conditions, even when the water/CO2 viscosity ratio is (mw/mCO2) > 50. This is because Corey type Relperm curves inherently suppress the mobility contrasts required for fingering instabilities to develop. Viscous fingering relative permeabilities (VF-Relperms), obtained through history matching of experimental pressure and recovery data, were shown to give a much better representation of the observed unstable displacement patterns [1, 2].,The second type of laboratory study of CO2/water displacement, using larger 2D packs or slabs, are far fewer in number in the literature. However, an excellent experimental study of this type has been published by Van de Ven and Mumford (VdVM) [3], who have provided sufficient detail to enable the results of this work to be modelled numerically. Our objectives in modelling this work are: (a) to test whether the published conventional Relperms or our VF-Relperms best match (qualitatively) the VdVM observed experimental results at the lab scale, and (b) if our match is qualitatively satisfactory, to extrapolate the VdVM experimental results to examine rescaled systems at the field scale using the scaling theory developed by Rapoport [4, 5].,In the scaling analysis of the VdVM results, we examine the sensitivity of the system to viscous/gravity and viscous/capillary scaling groups, and , respectively. Applying the Rapoport scaling theory, both two-force and three-force (viscous, gravitational, and capillary forces) models are successfully upscaled across systems spanning three orders of magnitude in dimension by preserving and , and the geometric shape groups. Block-by-block saturation comparisons between prototype and rescaled configurations confirm quantitative agreement in finger morphology, plume geometry, and displacement efficiency. Within certain stated assumptions, this establishes a validated framework for relating laboratory observations to field-scale CO₂ storage predictions.

In the design and optimization of an underground hydrogen (H2) storage facility in an aquifer or other reservoir system, the H2/water relative permeabilities (RP) are the most critical two-phase data for input to numerical simulation. In this paper, we present a critical analysis of the published experimental H2/water RP functions in the literature. We present fine-grid simulations of H2 displacing water (denoted H2 --> water) using three of the most widely cited steady-state RP datasets (13, 20, 38) in a mildly heterogeneous permeability field, at a field length scale of ~100m. Since the viscosity ratio between water and H2 is (µw/µH2) ≈ 70, then at some length scale above a few meters, it is inevitable that the system must show immiscible viscous fingering of the H2 into the water phase. Indeed, the emergence of viscous fingering at some length scale is a “sense check” that the input data used in the simulations are correct, especially the H2/water relative permeability functions. In fact, none of the three published H2/water RP curves leads to viscous fingering. Instead, they all show stabilized flood fronts. The reasons for this are due to shortcomings of the (conventional) gas/liquid experimental methods used to obtain the RP functions. These methods yield RP functions at the wrong force balance between the capillary, gravity and viscous forces. For fingering to emerge, it is necessary to derive the viscous dominated RP functions. An alternative, more physically appropriate set of viscous dominated “fingering RP functions” is proposed and applied. When applied at the core scale, these new RP functions show fully dispersed flow, as they do when applied in a vertical (downwards) gravity stable displacement. However, the viscous fingering emerges naturally in horizontal flow as the length scale of the system increases and viscous forces become dominant.

Carbon Capture and Storage (CCS) technologies are crucial for reducing anthropogenic carbon emissions by enabling the subsurface storage of millions of tons of CO2 annually. These activities are essential for achieving international climate agreements and contributing to short- and mid-term climate mitigation efforts.The extensive saline aquifer of the Bunter Sandstone Formation in the Silverpit Basin in the UK Southern North Sea has been identified as a high storage capacity reservoir that can contribute to assessing climate targets. This region will be the focus of multiple CO2 injection pro-jects. Therefore, understanding the regional sensitivity of the aquifer is of primary importance.This study investigates the boundary conditions and reservoir structure to assess the regional behaviour of the aquifer. Simulations of 14 wells injecting 1 Mt/yr/well over 50 years have been developed. Subsequently, the reservoir has been observed for 5000 years to analyze the impact of the available pore volume affecting reservoir pressure build-up and plume migration. Results suggest that the reservoir can safely accom-modate 750 Mt of supercritical CO2 beneath the Bunter closures, although pressure front expansion highlights the need for a thorough under-standing of the aquifer regional framework. The latter is crucial for evaluating the necessity of brine production for pressure management and addressing potential pressure interference between licenses.