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PORE-SCALE STUDY OF THE IMPACT OF FRACTURE AND WETTABILITY ON T H E TWO-PHASE FLOW PROPERTIES OF ROCK DMITRIY SILIN, JONATHAN AJO-FRANKLIN, JOHAN OLAV HELLAND, ESPEN JETTESTUEN, AND DIMITRIOS G. HATZIGNATIOU Abstract. Fractures and wettability are among other factors that can strongly affect the two- phase flow properties of porous media. Maximal-inscribed spheres (MIS) and finite-difference flow simulations on computer-generated structures mimicking micro-CT images of fractured rock suggest the character of the capillary pressure and relative permeability curves modification by natural or induced fracture and wettability alteration. 1. Introduction The presence of two or more different immiscible fluids (e.g., water, oil, gas) makes the pore space saturation and fluid flow multiphase. Any oil or gas recovery operation has to deal with two or three-phase flow. The same is true for subsurface injection, for example, for the purpose of CO 2 geologic sequestration. Capillary pressure and relative permeability curves are conventional concepts characterizing the capability of porous materials to store the fluids and the possibility of fluid flow and migration [16]. These curves can be determined from core laboratory experiments. Recently, this traditional approach has been complemented by digital rock methods [18, 21, 22] including pore-network modeling [2, 4, 6–8, 14, 19] and simulations on 2D and 3D micro-tomography data [9, 12, 15, 17]. The method of Maximal Inscribed Spheres (MIS) [23, 24] uses digitized micro-CT data as input information for evaluation of fluid distribution in capillary equilibrium. This method estimates two-phase fluid occupation of the pores, computes capillary pressure and relative permeability curves. The results are in agreement with experimental data [25, 27, 28]. In this study, the MIS method is employed to estimate the impact of natural fractures and wettability on capillary pressure and relative permeability curves. Micro-CT imaging of fractured rock can be difficult. In this study, we generate (via computer) a synthetic material with angular grains. We use micro CT data for samples of Bentheimer sandstone to validate the procedure. After model validation, a fracture is simulated by parting sand grains whose centers are on difference sides of the fracture plane. 2. The method of maximal inscribed spheres The idea of the MIS method of is simple [23, 27, 28]. In capillary equilibrium, the curvature of the fluid-fluid interfaces is determined by the capillary pressure, which is the pressure difference between wetting and nonwetting fluids. For a given radius of curvature, R, the nonwetting fluid saturation is estimated by evaluating the relative pore volume that can be covered by spheres that can be inscribed in the pores, and whose radii are greater than or equal to R. Such a calculation assumes a zero contact angle at the interface between the solid and the two fluids. In order to account for a non-zero contact angle, the MIS computation is modified. Namely, each inscribed Date : August 17, 2012. Key words and phrases. Pore-scale flow modeling, maximal inscribed spheres, wettability, fracture. |
