An Improved Approach for Generating Saturation Functions for Simulation Models Using Dynamic Rock Types

Abstract

Rock typing is a key factor in reservoir characterization studies. It is often assumed that Static Reservoir Rock Types (SRRTs) are capable of assigning multi-phase flow characteristics, such as capillary pressure and relative permeability curves to the cells of dynamic simulation models. However, SRRTs fail to capture the actual reservoir variability, due to lack of representation of wettability difference at different elevations above Free Water Level (FWL), especially in highly heterogeneous thick carbonate reservoirs. These shortcomings of SRRTs can be resolved through Dynamic Reservoir Rock Types (DRRTs), in which wettability effect is imposed on SRRTs to generate saturation functions for simulation models.This research proposes a modified DRRT approach by integrating the data from geological models and SCAL tests. First, the defined static rock types are sub-divided into sub-static rock types using either porosity or permeability frequency distribution. Second, a modified correlation equation is proposed and applied to more accurately estimate the initial water saturation versus height above FWL from well logs. Third, each sub-static rock type is further divided into a number of DRRTs by determining the capillary pressure and relative permeability curves in the oil zone from the Gas-Oil Contact (GOC) to the Dry-Oil Limit (DOL). The DRRTs are extended to the zone from DOL to the FWL by including wettability effect which would affect the curvature of the relative permeability curves but not its saturation end points, through changing the Corey exponents in the modified Brooks-Corey model. This modified DRRT approach is applied in terms of the dynamic rock typing plug-ins to generate sub-rock types from static rock types, and build a comprehensive and automatic approach to generate saturation tables for dynamic rock types that can be prospectively loaded into commercial simulators for reliable reservoir initialization, history match and prediction processes. SPE 160683The main rock typing approaches can be categorized into rock-fabric number, flow zone indicator and pore-throat radius 7, 8 . The rock-fabric number links rock-types petrophysical properties to a rock fabric classification that groups carbonate rocks into three categories 9 . Al-Aruri 10 applied the Carman-Kozeny model to define nine rock types principally by the similarity of the geological texture, litholgoy, porosity and permeability (Figure 1). The pore-throat radius approach 11 links rock type's petrophysical properties to the pore-throat radius7.Carbonate reservoirs are very heterogeneous with obvious wettability variations with respect to depth. The complexity of carbonates and the importance of defining rock types have been the subjects of many papers1 ,3, 12, 13 . Most of current practices are either based on petrophysical properties, such as porosity, permeability and drainage capillary pressure, or geological description, such as facies and sedimentary environment, diagenetic overprints, or a combination of them, which are commonly con...