Induced rough fracture in Berea sandstone core

• https://doi.org/10.1016/j.jcis.2006.10.082
• doi:10.1016/j.jcis.2006.10.082

Abstract — The purpose of this work is to study the effects of fracture morphology on the distribution and transport of immiscible fluid phases, such as oil and water, through a vertical fracture. An experimental approach, using micro-computed tomography (MCT), was selected to characterize the internal fracture structure and to monitor two immiscible phases. The experiment was performed in Berea sandstone cores with a single longitudinal fracture. The artificially created fracture was oriented parallel to the natural bedding of the rock. The sample was initially vacuum saturated with water, and oil was later injected through the longitudinal crack. Fluid occupancy in the fracture was mapped under four different flowing conditions: continuous oil injection, continuous water injection, simultaneous injection of oil and water, and a static pseudo-segregated state. Some of the mechanisms observed in this experiment include fluid trapping, preferential flow paths, snapping-off of non-wetting fluid globules, and coalescence and redistribution of globules between dynamic and static conditions. Experimental results indicate that distribution was mainly determined by fracture geometry, saturations, and wetting characteristics of the rock. A strong correspondence between fluid distribution and fracture apertures was found through direct comparison of two- and three-dimensional fracture structures.

• https://doi.org/10.1016/j.ijrmms.2010.03.015
• doi:10.1016/j.ijrmms.2010.03.015

Abstract — Understanding how fracture wall-roughness affects fluid flow is important when modeling many subsurface transport problems. Computed tomography scanning provides a unique view of rock fractures, allowing the measurement of fracture wall-roughness, without destroying the initial rock sample. For this computational fluid dynamics study, we used several different methods to obtain three-dimensional meshes of a computed tomography scanned fracture in Berea sandstone. These volumetric meshes had different wall-roughnesses, which we characterized using the Joint Roughness Coefficient and the fractal dimension of the fracture profiles. We then related these macroscopic roughness parameters to the effective flow through the fractures, as determined from Navier–Stokes numerical models. Thus, we used our fracture meshes to develop relationships between the observed roughness properties of the fracture geometries and flow parameters that are of importance for modeling flow through fractures in field scale models. Fractures with high Joint Roughness Coefficients and fractal dimensions were shown to exhibit tortuous flow paths, be poorly characterized by the mean geometric aperture, and have a fracture transmissivity 35 times smaller than the smoother modeled fracture flows.

• https://doi.org/10.1103/PhysRevE.76.016315
• DOI:10.1103/PhysRevE.76.016315

Abstract — This paper presents a two-dimensional pore-scale network model of a rough-walled fracture whose inner structure had been mapped using x-ray microtomography. The model consists of a rectangular lattice of conceptual pores and throats representing local aperture variations. It is a two-phase model that takes into account capillary, viscous, and gravity forces. Mapping of fluids and fracture topology was done at a voxel resolution of 0.027×0.027×0.032mm3, which allowed the construction of realistic fracture representations for modeling purposes. This paper describes the necessary data conditioning for network modeling, a different approach to determine advancing and receding contact angles from direct x-ray microtomography scans, and the network model formulation and methods used in the determination of saturation, absolute and relative permeabilities, capillary pressures, and fluid distributions. Direct comparison of modeled results and experimental observations, for both drainage and imbibition processes, is presented in the companion paper [M. Piri and Z. T. Karpyn, following paper, Phys. Rev. E 76, 016316 (2007)].

• https://doi.org/10.1103/PhysRevE.76.016316
• DOI:10.1103/PhysRevE.76.016316

Abstract — This paper presents the implementation of the pore-scale network model described in paper I [see preceding paper, Z. T. Karpyn and M. Piri, Phys. Rev. E 76, 016315 (2007)]. The model is used to estimate flow properties and predict fluid occupancy during two-phase flow displacements in a rough-walled fracture. The fracture’s inner structure is available from the reconstruction of x-ray microtomography images of a fractured sandstone core. The model is able to represent mechanisms such as pistonlike displacement, cooperative pore filling, and snapoff. We study the effects of aperture map scales, rate of injection, and gravity on the distribution of phases inside the fracture and present successful predictions of fluid occupancy during primary drainage, imbibition, and secondary drainage. Results were validated rigorously against x-ray microtomography scans obtained from two-phase flow experiments [see Z. T. Karpyn, A. S. Grader, and P. M. Halleck, J. Colloid Interface Sci. 307, 181 (2007)] and showed two-phase fluid structures in agreement with experimental observations.

• https://doi.org/10.2118/116110-PA
• DOI:10.2118/116110-PA

Abstract — Multiphase flow and transport phenomena within fractures are important because fractures often represent primary flow conduits in otherwise low-permeability rock. Flows within the fracture, between the fracture and the adjacent matrix, and through the pore space within the matrix typically happen on different length and time scales. Capturing these scales experimentally is difficult. It is, therefore, useful to have a computational tool that establishes the exact position and shape of fluid/fluid interfaces in realistic fracture geometries. The level set method (LSM) is such a tool. Our progressive quasistatic (PQS) algorithm based on the level set method finds detailed, pore-level fluid configurations satisfying the Young-Laplace equation at a series of prescribed capillary pressures. The fluid volumes, contact areas, and interface curvatures are readily extracted from the configurations. The method automatically handles topological changes of the fluid volumes as capillary pressure varies. It also accommodates arbitrarily complicated shapes of confining solid surfaces. Here, we apply the PQS method to analytically defined fracture faces and aperture distributions, to geometries of fractures obtained from high-resolution images of real rocks, and to idealized fractures connected to a porous matrix. We also explicitly model a fracture filled with proppant, using a cooperative rearrangement algorithm to construct the proppant bed and the surrounding matrix. We focus on interface movement between matrix and fracture, and snap-off of nonwetting phase into the fracture during imbibition in particular. The extent to which nonwetting phase is trapped in fracture/enclosed gaps is very sensitive to the direction of the displacement. Simulated drainage curves in matrix differ systematically from drainage curves in fracture and matrix with transfer between them. In a reservoir simulation, the latter might serve as an upscaled drainage curve input for a fractured medium.

• https://doi.org/10.1007/s11242-019-01243-9
• DOI:10.1007/s11242-019-01243-9

Abstract — The shear-thinning fluid flow in rough fractures is of wide interest in subsurface engineering. Inertial effects due to flow regime, fracture aperture variations as well as fluid rheology affect the macroscopic flow parameters in an interrelated way. We present a 3D microscale flow simulation for both Newtonian and Cross power-law shear-thinning fluids through a rough fracture over a range of flow regimes, thus evaluating the critical Reynolds number above which the linear Darcy’s law is no longer applicable. The flow domain is extracted from a computed microtomography image of a fractured Berea sandstone. The fracture aperture is much more variable than any of the previous numerical or experimental work involving shear-thinning fluids, and simulations are 3D for the first time. We quantify the simulated velocity fields and propose a new correlation for shift factor (parameter relating in situ porous medium viscosity with bulk viscosity). The correlation incorporates tortuosity (parameter calculated either based only on fracture image or on detailed velocity field, if available) as well as a fluid-dependent parameter obtained from the analytical/semi-analytical solutions of the same shear-thinning fluids flow in a smooth slit. Our results show that the shift factor is dependent on both the fracture aperture distribution (not only the hydraulic/equivalent aperture) and fluid rheology properties. However, both the inertial coefficient and critical Reynolds number are functions of the fracture geometry only, which is consistent with a recent experimental study.