Use of Capillary Pressure Data and Log Calculated Water Saturation for the Characterization of Dual Porosity, Dual Permeability Systems
This paper describes a simple method to divide the effective porosity values that can be calculated from log analysis into two flow media (fracture and matrix) pore volumes for use in numerical simulation models. The division of the porosity is based on the assumption that the total porosity of the system is made up from two contributing rock types or lithofacies that have distinct pore size and/or pore size distribution such that they can be represented by different capillary pressure functions. This assumption is applicable to the non-fractured as well as to the fractured systems.
When the method is used for the fractured systems, the fracture is typically represented by a zero capillary pressure value (gravity segregation). The matrix has its own laboratory measured capillary pressure curve. Based on a predetermined water-hydrocarbon contact and using the difference between the density gradients of the fluids, the capillary pressures of the two media are converted to theoretical water saturation profiles as a function of TVD. Next, at each depth point, the two extreme theoretical water saturation values are compared to the log calculated water saturation measurements. Based on a simple weighting algorithm, the contributions of the fracture and matrix media to the total porosity are calculated. The results can be upscaled to the numerical simulation grid block size or be used in geostatistical approaches.
Using the developed technique, we have built simulation models for two different reservoirs. One of the models represents a fractured system, while the other represents a non-fractured system with two distinct rock types. For the non-fractured reservoir, the dual porosity formulation enabled us to accurately upscale the capillary pressure functions by default. The approach and the results of the numerical models are presented in the body of this paper.