The macroscopic movement of subsurface fluids involved in CO2 storage, groundwater, and petroleum engineering applications is controlled by interfacial forces in the pores of rocks, micrometre to millimetre in length scale. Recent advances in physics based models of these systems has arisen from approaches simulating flow through a digital representation of the complex pore structure. X-ray microcomputed tomography (X-ray micro-CT) has emerged as the leading technology for creating these digital models and for uncovering the fundamental mechanisms governing multiphase transport in porous media. However, further progress in predicting multiphase flow in porous rocks is limited by a lack of approaches to characterising the spatial distribution of the wetting state within the pore structure, particularly when this wetting state is heterogeneous. This thesis focuses on wettability characterisation by means of X-ray micro-CT imaging. In particular, our work addresses the challenge of describing the wetting state of porous rocks, when it is non-uniform in space.

The characterisation of the wettability of such systems constitutes a challenge for the approaches currently available in literature, either from a theoretical or a practical point of view. The first part of this thesis presents a sensitivity analysis of the estimates of some multiphase fluid and solid properties of interest to image processing. This is a required step to better understanding the factors to take into account for developing a new approach to wettability characterisation which is both flexible in application and reliable. We assess the sensitivity of porosity, permeability, specific surface area, in situ contact angle measurements, fluid-fluid interfacial curvature measurements and mineral composition to processing choices. We compare the results obtained upon the employment of two processing pipelines: non-local means filtering followed by watershed segmentation; segmentation by a manually trained random forest classifier. Single-phase flow permeability, in situ contact angle measurements and mineral-to-pore total surface area are the most sensitive properties, as a result of the sensitivity to processing of the phase boundary identification task. Porosity, interfacial fluid-fluid curvature and specific (intensive) mineral descriptors are robust to processing. The sensitivity of the property estimates increases with the complexity of its definition and its relationship to boundary shape.

Following these results, we propose a novel approach to wettability characterisation rooted in the analysis of the relative fluid coverage of rock surfaces. We present a thermodynamic model explaining why changes in wettability determine a change in the extent of fluid-solid interfacial area. We thus develop a first workflow to wettability characterisation by analysing relative fluid coverage of rock surfaces as a function of fluid saturation. A first test of the approach on a water-wet and an altered wettability single-mineralogy sandstone (Bentheimer) rocks confirms its viability. In two multi-mineralogical (Berea) samples, one water-wet and the other altered with crude oil, the analysis of fluid surface coverage after imbibition revealed mineral specific wetting preferences only in the altered system. Clays and calcite preferentially alter to an oil-wet state, leading to mixed wettability in the rock.

In the final part of the thesis, an improved workflow to wettability characterisation consisting in the analysis of relative fluid coverage on a pore-by-pore basis is introduced. We demonstrate the approach with brine-oil fluid pairs on rocks with distinct lithologies (sandstone and carbonate) and wetting states (water-wet, intermediate-wet and mixed-wet). The distinct characteristics of the different wetting states encountered are identified. Finally, a quantitative measure of wetting at a pore level is proposed. This wetting index is then employed to build 3D wetting maps. These maps may be used as a deterministic input to pore scale modelling workflows and applied to all multiphase flow problems in porous media ranging from soil science to fuel cells.