Abstract

With urgent need of greenhouse gas sequestration and booming oil prices, underground oil/gas reservoirs seems the only value added choice. A great portion of current CO2 injection projects in the world is in naturally fractured reservoir. The matrix part of these reservoirs constitutes the major oil storage unit and this oil is targeted during CO2 injection. It is our intention to show that this media could also be used as permanent CO2 storage unit while recovering oil in it. These reservoirs, however, are complex in nature and the physics of the matrix- fracture interaction process during CO2 injection is still not known to great extend.

To ease the complex nature of the problems, experiments were performed on artificially fractured (single) Berea sandstone cores saturated with n-decane. CO2 was injected at constant rates into the fracture while maintaining the high pressure into the core and the system. Injection and production data were monitored and collected using continuous data logging system. After continuous injection diffusion of CO2 was allowed to occur by shutting down the system for a specific period of time and then following a blowdown period to produce oil recovered by diffusion from matrix to fracture. At different pressure steps, produced liquid was analyzed using gas chromatography while the produced gas measured using continuous flow meter. The CO2 storage capacity of the rock with change in the pressure and the amount of oil recovered during blow down period was analyzed.

The results of the continuous injection experiments were used to obtain diffusion coefficients by matching the simulation results. Using dimensionless analysis and matrix-fracture diffusion groups, a critical number for optimal recovery/sequestration was obtained. The pressure decay behavior during the shutdown was analyzed in conjunction with the gas chromatograph analysis of produced oil sample collected during blowdown after the quasi-equilibrium reached during pressure decay. This led to insights into the governing mechanism of extraction/condensation and miscibility for recovering lighter to heavier hydrocarbons during pressure depletion from fractured reservoirs.

Introduction

Although there exists considerable amount of experimental work on modeling matrix-fracture interaction using first contact miscible solvents to mimic fully miscible CO2 injection (Burger and Mohanty, 1997; Burger et al. 1996; Gabitto, 1998; Firoozabadi and Markeset, 1994; Trivedi and Babadagli, 2008a), experimental studies using CO2 as solvent are limited. Mostly Berea sandstones are used in these researches because of their readily availability, ease of cleaning and homogeneous structures. Studies using fractured carbonate rocks or low permeable matrix are rare (Karimaie et al. 2007; Darvish et al. 2006a and 2006b). In one of its kind experimental work, Chakravarthy et al. (2006) used polymer gels to show the effect of delayed breakthrough during immiscible CO2 injection into fractured Berea cores. Improved recovery was observed during huff-and-puff performance in miscible range over immiscible injection of CO2 (Asghari and Torabi 2007; Torabi and Asghari 2007). Because of the complexity and duration involved into the diffusion experiments as well as limitations of simulators to precisely predict the multiphase diffusion/mass transfer functions, the previous attempts to understand the physical mechanism are highly valuable. None of these works, however, considered the sequestration aspect during EOR. In the first part of this study, we performed dynamic diffusion experiments to show that oil recovery from the matrix can be enhanced as well as greenhouse gas storage can be accomplished by optimizing flow dynamics. In the second part, we provided a quantitative analysis showing the efficiency limits of the process.

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