Effects of Wettability and Minerals on Residual Oil Distributions Based on Digital Rock and Machine Learning

AbstractThe wettability of mineral surfaces has significant impacts on transport mechanisms of two-phase flow, distribution characteristics of fluids, and the formation mechanisms of residual oil during water flooding. However, few studies have investigated such effects of mineral type and its surface wettability on rock properties in the literature. To unravel the dependence of hydrodynamics on wettability and minerals distribution, we designed a new experimental procedure that combined the multiphase flow experiments with a CT scan and QEMSCAN to obtain 3D digital models with multiple minerals and fluids. With the aid of QEMSCAN, six mineral components and two fluids in sandstones were segmented from the CT data based on the histogram threshold and watershed methods. Then, a mineral surface analysis algorithm was proposed to extract the mineral surface and classify its mineral categories. The in situ contact angle and pore occupancy were calculated to reveal the wettability variation of mineral surface and distribution characteristics of fluids. According to the shape features of the oil phase, the self-organizing map (SOM) method, one of the machine learning methods, was used to classify the residual oil into five types, namely, network, cluster, film, isolated, and droplet oil. The results indicate that each mineral’s contribution to the mineral surface is not proportional to its relative content. Feldspar, quartz, and clay are the main minerals in the studied sandstones and play a controlling role in the wettability variation. Different wettability samples show various characteristics of pore occupancy. The water flooding front of the weakly water-wet to intermediate-wet sample is uniform, and oil is effectively displaced in all pores with a long oil production period. The water-wet sample demonstrates severe fingering, with a high pore occupancy change rate in large pores and a short oil production period. The residual oil patterns gradually evolve from networks to clusters, isolated, and films due to the effects of snap-off and wettability inversion. This paper reveals the effects of wettability of mineral surface on the distribution characteristics and formation mechanisms of residual oil, which offers us an in-deep understanding of the impacts of wettability and minerals on multiphase flow and helps us make good schemes to improve oil recovery

Research on stress sensitivity of fractured carbonate reservoirs based on CT technology

Fracture aperture change under stress has long been considered as one of primary causes of stress sensitivity of fractured gas reservoirs. However, little is known about the evolution of the morphology of fracture apertures on flow property in loading and unloading cycles. This paper reports a stress sensitivity experiment on carbonate core plugs in which Computed Tomography (CT) technology is applied to visualize and quantitatively evaluate morphological changes to the fracture aperture with respect to confining pressure. Fracture models were obtained at selected confining pressures on which pore-scale flow simulations were performed to estimate the equivalent absolute permeability. The results showed that with the increase of confining pressure from 0 to 0.6 MPa, the fracture aperture and equivalent permeability decreased at a greater gradient than their counterparts after 0.6 MPa. This meant that the rock sample is more stress-sensitive at low effective stress than at high effective stress. On the loading path, an exponential fitting was found to fit well between the effective confining pressure and the calculated permeability. On the unloading path, the relationship is found partially reversible, which can evidently be attributed to plastic deformation of the fracture as observed in CT images

Process-Dependent Solute Transport in Porous Media

From Springer Nature via Jisc Publications RouterHistory: received 2021-01-18, accepted 2021-07-09, registration 2021-07-13, pub-print 2021-10, online 2021-10-01, pub-electronic 2021-10-01Publication status: PublishedFunder: ISF; Grant(s): 485/16Funder: University of Manchester; Grant(s): NAFunder: British Council; Grant(s): NAAbstract: Solute transport under single-phase flow conditions in porous micromodels was studied using high-resolution optical imaging. Experiments examined loading (injection of ink-water solution into a clear water-filled micromodel) and unloading (injection of clear water into an ink-water filled micromodel). Statistically homogeneous and fine-coarse porous micromodels patterns were used. It is shown that the transport time scale during unloading is larger than that under loading, even in a micromodel with a homogeneous structure, so that larger values of the dispersion coefficient were obtained for transport during unloading. The difference between the dispersion values for unloading and loading cases decreased with an increase in the flow rate. This implies that diffusion is the key factor controlling the degree of difference between loading and unloading transport time scales, in the cases considered here. Moreover, the patterned heterogeneity micromodel, containing distinct sections of fine and coarse porous media, increased the difference between the transport time scales during loading and unloading processes. These results raise the question of whether this discrepancy in transport time scales for the same hydrodynamic conditions is observable at larger length and time scales

Interplay of Pore Geometry and Wettability in Immiscible Displacement Dynamics and Entry Capillary Pressure

Recent studies highlight the significant role of pore geometry and wettability in determining fluid-fluid interface dynamics in two-phase flow in porous media. However, current entry capillary pressure equations, rooted in the Young-Laplace equation, consider only cross-sectional details and apply wettability data measured on flat surfaces to complex three-dimensional (3D) pore structures, overlooking the coupled effect of contact angle and pore morphologies along the flow direction. This study employs the Volume-of-Fluid method to investigate: (a) How do combined effects of pore geometry and wettability control capillary pressure change, displacement efficiency, and residual saturations? (b) Can continuous two-phase flow be achieved at the pore scale? Through direct numerical simulations in constricted idealised-geometry capillary tubes and real pore structures, we vary the contact angle to characterise its impact on fluid-fluid interface morphology, entry capillary pressure (pec), and displacement efficiency. Our results show that during the drainage, pec temporarily decreases/turns negative under intermediate wettability conditions due to forced curvature rearrangement/reversal in the converging section. Local orientation angles along the flow direction are important in controlling the interface morphology and pec evolution. Moreover, intermediate contact angles enhance displacement efficiency due to curvature reversal, while insufficient corner flow during imbibition causes pore snap-off of the receding fluid, leading to higher residual saturation. The results challenge conventional methods in predicting entry capillary pressure, highlighting the need for incorporating 3D geometry in predictive models. Eventually, the insights underscore the importance of considering corner flow in controlling displacement efficiency within constricted geometries in pore network modelling studies.<br/