Using Depositional and Petrographical Rock-Types to Define Hydraulic Rock-Types - A Case Study from the Permian-Carboniferous Unayzah Formation, Offshore Abu Dhabi

Abstract

Rock typing represents an important tool for classifying clastic reservoir rocks, especially in reservoirs showing low permeability and low porosity values. For this study we first approach these challenges from a geological perspective and subsequently translate the results of traditional facies and RQ analysis into rock types by integrating previously obtained results with mercury injection capillary pressure (MICP) data. The rock typing workflow proposed by Rushing et al. (2008) is applied to the low porosity-low permeability Unayzah Formation penetrated in three offshore wells, UAE. The characterisation of this type of low reservoir quality (RQ) rock confronts the geologist with some challenges. The Unayzah Formation is an important clastic reservoir unit in the Middle East, especially in the UAE. It is characterized by low porosity and low permeability values and therefore, according to Holditch (2006) can be classified as a tight gas reservoir where a tight gas reservoir is "a reservoir that cannot be produced at economic flow rates nor recover economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores". In order to characterize the Unayzah Formation we used newly acquired geological and laboratory data obtained from detailed core description, facies, petrographic- and reservoir quality analysis. In a second step we translated these geological laboratory related data into rock types by integrating these data with MICP data considering pore throat geometries for a more accurate reservoir quality characterisation of the Unayzah Formation.