Proceedings of the World Tunnel Congress 2014 – Tunnels for a better Life. Foz do Iguaçu, Brazil. 1 1 INTRODUCTION The principle of hydrocarbon storage in URC, called hydraulic confinement, is based on the applying the groundwater pressure of the surrounding rock to contain the stored product inside the cavern (Froise, 1987; Lindblom, 1989; Hamberger, 1991). The technique of hydraulic confinement is to establish water continuously flowing toward the cavern from outside rock to prevent hydrocarbon migration into the rock mass (Bérest, 1990). To reach the hydraulic confinement, the actual groundwater pressure acting at all points on the cavern periphery should be exceeded the pressure of the hydrocarbon compounds by a certain amount. The necessitate groundwater pressure can be achieved by locating the caverns far enough below the water table or, in most of the cases, artificially supplied by installing so- called “water curtain” outside the cavern periphery, which is arrays of boreholes drilled above the cavern and fed with water with desire pressure. With the hydraulic confinement and water curtain, the water hydraulic pressure in the rock mass around the cavern should be higher than the vapor pressure of the hydrocarbon and should be strong enough to prevent any leakage. Using such water curtain poses the key questions (Liang and Lindblom, 1994; Lindblom, U., 1989; Chung et al., 2003; Bérest, 1990; Heath et al., 1998); How large of pressure difference between groundwater and stored hydrocarbon in the cavern should be maintained to prevent outward leakage of hydrocarbon compounds from the cavern? The exact criterion for preventing hydrocarbon leakage from the unlined cavern has been a matter of researches for several years. During the past decades, several gas- containment, no gas leakage, criteria have been proposed based on groundwater gradient or pressure (Åberg, 1977; Goodall et al., 1988; Kjørholt and Broch, 1992; Liang and Lindbiom, 1994; Lindblom, 1997). The gas-containment criterion suggested by Åberg (1977) is commonly used in the design of unlined cavern storage which is demonstrated that a vertical hydraulic gradient greater than 1.0 should be maintained through the rock fractures surrounding a storage cavern during operations. Goodall et al. (1988) proposed a simple criterion for practical design of hydraulic confinement around URCs, based on the simple Numerical Modeling of Hydraulic Confinement around Crude Oil Storage Cavern in Fractured Rocks: Direct Application of DFN Concept M. Javadi, M. Sharifzadeh, K. Shahriar, S. Sayadi Department of Mining & Metallurgical Engineering, Amirkabir University of Technology, 15875- 4413 Tehran, Iran. ABSTRACT: Unlined rock caverns for hydrocarbon storage are mainly excavated in strong and stable rock masses with very low-permeable matrix, where hydrocarbon migration is only possible along fractures connected to the cavern boundaries and their networks. In this paper, the hydraulic confinement around a URC cavern was simulated by directly applying distinct fracture network (DFN) model. First, a computational code, so-called “FNETF”, was developed to generate DFN and solving fluid flow equation along fractures. Proper internal hydraulic boundary condition of water- hydrocarbon interface at cavern boundary was defined based on the fluids properties and applied in the FNETF code. Fluid flow in fracture network was numerically simulated for different outer boundary hydraulic conditions assigned by different groundwater pressures of water curtain. Finally, the assessment achieved to expect the occurrence of hydrocarbon.