An Experimental and Numerical Study on Steam Injection in Fractured Systems

Abstract

Abstract Fractured reservoirs contain a large fraction of the world supply of oil. For viscous crudes, steam is the most successful technique and field tests indicate that steam has the best potential to recover significant amounts of oil from fractured reservoirs. Unfortunately, there has been little laboratory work done on steam injection in such systems. The experimental system discussed here was designed to understand the mechanisms involved in the transfer of fluids between the matrix rock and the fracture as a result of steam injection. Both continuous and cyclic steam injection experiments were performed on a fractured laboratory system. Saturations were measured in-situ both in the fracture and the consolidated matrix by a CT scanner. The results indicated that there was no steam saturation in the matrix, and that conduction was the dominant heat transfer mechanism. Numerical simulations were used to model both continuous and cyclic steam injection experiments. To model heat losses, heat loss models in the simulator had to be adjusted based on the analysis of the heat losses from the laboratory system with analytical models. After this adjustment, the results from the simulations agreed well with the experiments. When pressure cycling was applied in the simulations with no external heat losses, a considerable amount of steam saturation was observed in the matrix. While the experiments were done with water and steam, simulation runs were also performed for the laboratory system with oil present. Again, steam only flowed in the fracture. Oil recovery was found to be mainly caused by water imbibition into the matrix and heat conduction. Results of this work should be useful in modeling matrix/fracture transfer in dual porosity thermal models. Introduction Fractured reservoirs are estimated to contain 25-30 % of the world supply of oil. Steam injection is required for most of the reservoirs containing heavy oils and tars. There have been quite a few field studies on steam injection for fractured systems (Sahuquet and Ferrier (1982), Britton et al. (1982), Stang and Soni (1987), Closmann and Smith (1983), Duerksen et al. (1984), Couderc et al. (1990) and Hartemink et al. (1995)). Most of these applications were successful or promising. Thomas (1964), Lesser et al. (1966), Abdus Satter (1967) and Satman (1988) presented theoretical models for conduction heating of formations by injecting a fluid through a high permeability streak or fracture. Van Wunnik and Wit (1992) developed a detailed analytical model to study the improvement of gravity drainage by steam injection in a fractured reservoir containing heavy oil. Pooladi-Darvish et al. (1994) developed analytical solutions for heat flow and nonisothermal gravity drainage from a block surrounded by fractures filled with steam. In addition to these analytical models, there have been some numerical simulation studies for steam injection in fractured systems (Pruess and Narasimhan (1985), Lee and Tan (1987), Chen et al. (1987), Pruess and Wu (1989), Nolan et al. (1980), Abad and Hensley (1984), Lin (1988), Briggs (1989), Jensen et al. (1992) and Oballa et al. (1993)). P. 591