It is common practice in modeling of naturally fractured reservoirs, NFRs, to neglect capillary pressure, Pc, and utilize straight line, zero to one, relative permeability, Kr, curves for the fracture network. More often than not, however, reservoir stresses and partial mineralization of fractures render fracture conductivity to remain relatively low for simultaneous ease of flow of two phases. Furthermore, fracture Kr is often found to be lowered by fractures’ tortuosity, and wall roughness. The objective of this study is to expand on the characterization of fractures Pc and Kr data and to determine its effect on the performance of heavy oil NFRs produced under thermal recovery processes.
According to the percolation model1, 2 , fracture Kr curves approaches straight line form when gravity segregation forces dominate the capillary forces, producing larger fracture height dimensionless parameters. In heavy oil NFRs, however, smaller water-bitumen density differences coupled with smaller fractures apertures may lead to the need of using non-straight line, physical, Kr curves. This may be associated with non-zero Pc curves. The differences in heavy oil recovery performance, due to physical Pc and Kr curves, was established through Dual Porosity/Dual Permeability modeling of Steam Assisted Gravity Drainage, SAGD, thermal recovery process of a heavy oil NFR.
Fractures' heights and apertures, gas- and water-bitumen density differences, and interfacial tensions would either support the use of non-straight line, physical Kr curves or renders the use of straight line Kr curves to be acceptable. Large density differences between steam and heavy oil supports the use straight-line Kr curves for the gas-heavy oil system, while smaller water-heavy oil density differences supported the use of physical Kr curves for the water-bitumen (heavy oil) system.
Utilization of physical Kr curves revealed appreciable differences in the performance of SAGD process for smaller fracture apertures and relatively minor differences for larger fracture apertures. Straight-line Kr curves gives hot water more mobility in the fractures quickly expanding the steam chamber to contact more matrix rock and recovering more oil in the process. Physical Kr reduces the size of the steam chamber and in turn recovering less oil in the process. This paper presents the details of these results and tries to determine the effect of other reservoir parameters such as the fracture aperture, level fracture-matrix rock permeability contrast, and wettability of matrix rock on the process.
This paper adds value to the effort of proper modeling of thermal recovery processes of heavy oil NFRs through the use of more theoretically and experimentally backed fracture Pc and Kr input data. For some reservoirs, this may make or break certain thermal recovery processes, which may help to avoid wrong economic decisions for some projects.