Automatic in situ characterization of pore morphology and wettability

Abstract

In many important processes, that control CO2 storage in aquifers, oil recovery, and gas exchange in leaves, for instance, flow is controlled by the interaction of immiscible fluids with a rough surface. In this thesis, we present new automated methods for measuring \textit{in situ} contact angle (𝛳), fluid/fluid interface curvature, rock surface roughness and pore morphology, applied to segmented pore-scale X-ray images. We first identify and mesh the fluid/fluid and fluid/solid interfaces. A Gaussian smoothing is applied to this mesh to eliminate artefacts associated with the voxelized nature of the image, while preserving large-scale features of the rock surface. Then, for the fluid/fluid interface we apply an additional local uniform curvature smoothing and adjustment of the mesh. We then track the three-phase contact line, and the two vectors that have a direction perpendicular to both surfaces: the contact angle is found from the dot product of these vectors where they meet at the contact line. This calculation can be applied at every point on the mesh at the contact line. We automatically generate contact angle values representing each invaded pore-element in the image with high accuracy.

We validate the developed approach using synthetic three-dimensional images of a spherical droplet of oil residing on a tilted flat solid surface surrounded by brine with different resolutions of known curvature and contact angle. We show that we are able to estimate contact angle to within 3 degrees and curvature with error less than 9% when the sphere is 2 or more voxels across, which indicates that with a 2 µm voxel size we can accurately capture curvatures as high as 0.5 µm^{-1} and contact angles on pores 4 µm across.

We then apply the developed methods to study the in situ distributions of contact angle and oil/brine interface curvature measured within mm-size rock samples from a producing hydrocarbon carbonate reservoir imaged after wateflooding at elevated temperature 60-80 celcius degrees and reservoir pressure (10MPa) using X-ray micro-tomography [Alhammadi et al., 2017b]. We analyse their spatial correlation on a pore-by-pore basis using a novel approach combining the automated methods for measuring contact angles and oil/brine interfacial curvature, with a recently developed method for pore network extraction [Raeini et al.,2017]. Also, we studied the contact angle and interfacial curvature correlation on a ganglion-by-ganglion basis using a ganglia labelled images. The automated methods allow us to study image volumes of diameter approximately 1.92 mm and 1.2 mm long, obtaining hundreds of thousands of values from a dataset with 435 million voxels. We calculate the capillary pressure based on the mode oil/brine interface curvature value, and associate this value with a nearby throat in the pore space. Then, we quantify rock surface roughness and assess its impact on the wettability of the rock. Rougher surfaces are associated with a wider range of local contact angle. Finally, we establish a methodology to characterise pore morphology (wall curvature) in complex porous materials to determine their potential for developing fluid layer flow in mixed-wet systems. We apply this on mm-sized three-dimensional images of a beadpack, a sandpack, two sandstones (Doddington and Bentheimer) and six carbonates (Portland, Ketton, Estaillades and the aforementioned 3 reservoir samples from the Middle East [Alhammadi et al.,2017b]), representing porous media with an increasing degree of pore-scale complexity.

In this thesis, we demonstrate the capability of our methods to distinguish different wettability states in the samples studied: water-wet, mixed-wet and oil-wet. The measured contact angle and oil/brine interface curvature in the Middle Eastern reservoir samples are spatially correlated over approximately the scale of an average pore. There is a wide distribution of contact angles within single pores. A range of local oil/brine interface curvature is found with both positive and negative values. There is a correlation between interfacial curvature and contact angle in trapped ganglia, with ganglia in water-wet patches tending to have a positive curvature, and oil-wet regions seeing negative curvature. We observed a weak correlation between average contact angle and pore size, with the larger pores tending to be more oil-wet. Also, we identify a distinct pore-morphology signature where unconsolidated media have a large majority of pores with positive curvature, while consolidated media tend to be composed of more pores with negative curvature. In unconsolidated media there is no impact of relative pore size on pore curvature, in contrast to consolidated media for which we observe a tendency for the small pores to have negative pore curvature, while the large pores have positive ones. Both pore morphology and wettability have a large impact on the potential for layer flow. The signature of consolidated media having a wide range of positive and negative curvatures promotes layer flow in mixed-wet systems. Importantly, this allows us to understand the tendency for the large pores in mixed-wet systems to have the positive fluid interfacial curvature, while small pores show a broader range of both positive and negative fluid interfacial curvature.