A Modified Archie Relationship - A Novel Approach to Evaluate Carbonate Reservoirs with Regard to Fracture and Isolated Vuggy Porosityfor Sw=1

Abstract

Abstract Carbonate rocks hosting major proportions of the world's hydrocarbons are characterized by intricate pore systems encompassing different pore scales, shapes and interconnections resulting both from their organic origin as well as different diagenetic processes (e.g. dolomitization, fracturing, leaching, cementation or recrystallization). Apart from interparticle pore space composed of micro-, meso- and macropores, there are also separate and connected vugs (e.g. fractures), influencing important reservoir parameters such as total and effective porosity or permeability. Therefore profound knowledge of the impact of pore types and interconnections on log data has to be acquired in order to ensure accurate evaluation with respect to porosity. Particularly regarding electrical properties and Archie's relationship - having been established for clean rocks and connecting formation resistivity factor F and total porosity - a special modification in carbonates with regard to their complex pore systems is required. Thus a petrophysical database including resistivity and porosity data of a range of carbonate rocks with various pore types (interparticle and vuggy, both separate and connected) needs to be established. The analysis of the database leads to a modified Archie relationship taking account of the effect of separate vugs and fractures on electrical current flow. This novel modification allows a first step towards porosity partitioning in a water-saturated carbonate formation into proportions that are isolated or accessible for electrical current flow and thus also for hydraulic fluid flow - one of the most important features with regard to producibility and recovery. A set of log data from a sample well is presented to apply and test this new approach on carbonates with challenging pore space characteristics. This novel modification of Archie's equation may prove beneficial to fields of research related to reservoir properties in complex carbonate reservoirs.