Evaluation of Steam Flooding and Cyclic Steam stimulation (CSS) for a Fractured Carbonate Heavy Oil Reservoir
Authors: Mousavi Mirkalaei Seyed Mousa , Masoudi Rahim , Ataei Abdolrahim 
Published: March 2015

Abstract
Although Steam injection is a potentially effective and efficient improved heavy oil recovery method, it can be extremely challenging process in naturally fractured heavy oil reservoirs. Communication between matrix and fracture provides a coupling of the two distinct and contrasting systems which results in a highly non-linear problem, if added complexity changes in fluid properties due to the thermal effects of steam then it gets even more complicated. Accurate reservoir and fluid characterization as well as steam injection operation parameters and dynamics are must for reliable modeling of such complex process.
It this study the steam injection including steam flooding and Cyclic Steam Stimulation (CSS) are studied and compared in detail as potential development scenario of a highly fracture heavy oil reservoir in Iran. The field is giant structure like a symmetrical anticline with 90 km length and 16 km width in the surface and also 60 km length and 10 km width on 1000 mss depth with about 3.6 billion barrels of initial oil in place. Initial pressure is about 927 psi in 1700 ft below the sea level. The oil gravity is 8–12 API with about 2700 cp viscosity at surface condition. In this work, a thermal dual porosity model for the reservoir structural, stratigraphic, petrologic and petrophysical description of the reservoir, geometry, rock types/ properties, layering, porosity types, fracture orientation and density, faults, etc are used. The steam interaction and the subsequent property changes with the reservoir fluid and rock are studied and incorporated in the 3D reservoir model.
The result shows that steam flooding could improve the recovery factor from almost zero to about 12 % while CSS will give about 37 % of recovery factor in the studied sector model which makes it more attractive method as development scenario. Furthermore the results illustrated that the injection strategies, well spacing, well type, pattern type and size are among the important parameters for designing the steam injection. Additionally, the effect of other parameters such as injected steam quality, oil-water capillary pressure in matrix blocks, steam injection and oil production rate have been studied for both steam flooding and CSS process.

Introduction
Fractured reservoirs are estimated to contain 25–30% of the world supply of oil. Many of these reservoirs contain heavy oil or tar sand which can only be recovered by thermal recovery techniques(Saidi 1987). For viscous crudes and tars, steam is the most widely used and the most successful technique. Field test results indicate that steam injection has good potential for fractured reservoirs(Briggs 1989). Fractures can also be created artificially during steam injection into reservoirs containing viscous crudes and tars. Physical processes taking place during steam injection should be understood thoroughly, and reliable models should be developed, for effective and economic recovery of oil from fractured systems(Pruess 1985).
Most of the theoretical and experimental work done on fractured reservoirs has been on isothermal processes. The numerical models developed for thermal processes are generally extensions of models for isothermal processes(Chen, Wasserman et al. 1987). The developed models have generally not been validated against experimental data. The limited experimental studies lack detailed measurements, especially temperature and saturation distributions in the fracture and matrix(Coats 1989).