Carbonate Reservoir Rock Typing A Case Study in North Oman Presented by: Majid Hasani, PDO SPWLA conference Abu Dhabi 15th Feb – 18th Feb 2010 Outline • Field introduction – General field data – Basic Reservoir Data – Rudist types • The issue • Classical approach for Carbonate Reservoirs – RFC & RRT • Integrated Modelling Workflow! – Optimising RRT • Conclusion 2 General field data • Location: North of Oman • The field is subdivided in a number of areas, based on structure and reservoir development: Main Area, Eastern Satellite, Southwest Area. • Lower Cretaceous rudist reef complex • Very heterogeneous reservoir properties; their effect on well inflow is poorly understood. 3 Basic Reservoir Data RESERVOIR PROPERTY VALUE Formation Water Resistivity 0.022 ohm.m Reservoir Thickness 10 - 50m (av. 35m) Porosity 14 - 26% (av. 20%) Permeability 0.1 - 1000mD Oil Density 38o API Oil Viscosity 1.2 cP Oil Formation Factor 1.2 Solution GOR 45 m3/m3 Bubble Point Pressure 5,960 kPa Initial Reservoir Pressure 17,200 kPa Current Reservoir Pressure 9,000 - 15,000 kPa Drive Mechanism Natural Water Drive (edge/bottom/fracture) Sorw 25% (variable) 4 Rudist types Rudists are a group of box, tube or ring shaped marine heterodont bivalves that arose during the Jurassic, and became so diverse during the Cretaceous that they were major reef-building organisms in the Tethys Ocean. Wikipedia, the free encyclopedia Schumann & Steuber 1997 5 Paleogeographic map of the Early Aptian in some of Part the Arabian Palate 6 A type log •Variable geological facies within well level 7 Well example of the large variation in permeability seen in the Field. RRT 0.5-1.5 RRT 1-1.5 8 The issue ! Permeability ranges ~ 3 decades Permeability Variation *Coloured per well High variability, No unique Poro Perm relation 10 Classical approach for Carbonate Reservoirs RFC & RRT • RFC: Rock Fabric Classification – Assigned to each different geological facies, discrete value; ex: • RFC =1  Grainstone • RFC = 2  Grain-Dominated Packstone • RFC = 3  Mud-Dominated Wackstone – Lucia used RFC to compute permeability for different geological facies (SPE 71336 by Lucia 2001) – These RFC can only be obtained in cored wells where geological facies have been defined • RRT: Reservoir Rock Type – Link geological facies to well logs variables (interparticle porosity & water saturation) – RRT is used in all logged wells to generate rock typing (instead of RFC) in uncored wells – it is a continuous variable used to mimic RFC 12 Permeability and RRT RRT Matrix Class 1 RRT 0.5 – 1.5 Carbonate Classification Field Example Grainstone RFC 1 AH-10H1: 5061ft Grain Dominate d Fabric Class 2 RRT 1.5-2.5 RFC 1 RFC 1 RFC 1 GrainDominated Packstone RFC 2 Class 3 RRT 2.5-3.5 RRT 3.5+ RFC 2 Mud Domiated Wackestone and Packstone Matrix Dominate d Fabric RFC 2 RFC 3 Mudstone RFC 3 RFC 3+ 13 RFC 3+ RFC to RRT RRT coefficients Original Lucia equation 14 Lucia equation parameters Integrated Modelling Workflow! Permeability Integration Workflow • RRT and Lucia equation – RRT computed from porosity and water Saturation – A process of iteration through history match in dynamic model and Lucia Rock fabric line are used to fix RRT coefficients. – The computed RRT is found to fit in to Lucia Rock fabric number 16 HM Integration Workflow DM upscale QA/QC. Submit HM runs in DM Update SM PVT+Pc+Kr Feedback to PP for updating RRT & Lucia perm HM: history match SM: static model Analyse Liquid Match Run sensitivities* Liq HM OK? Y N Feedback to PG for SM update Check connectivity and adjust perm N HM OK? DM: dynamic model Adjust Kr and submit runs in DM Y N HM OK? Y Forecast NFA and compare with DCA 17 Check Saturation Match Optimising RRT • A process of iteration is used to optimise RRT coefficients (dynamic model HM) Matching only with core perm After HM iteration History Production History Production Model Production Model Production Black: Total fluid Black: Total fluid Blue: Water Blue: Water Red: Oil Red: Oil 18 RFC match with RRT (after HM) RFC *Colour indicates RRT 19 Lucia permeability match with core Perm GR DEN NEU RES POR SH DEN Lucia perm Core perm 20 21 RRT 0.5 – 1.7 SH RRT 2.2 – 2.9 RES RRT 3.2 – 3.9 POR Rudist Packstone DEN Rudist Wackstone GR Algal Mudstone RRT match with geological facies Perm Geological facies RRT Conclusion • Predicting permeability in complex heterogeneous carbonate reservoir was initially carried out by the standard Lucia approach • RRT is used instead of RFC to compute permeability in uncored wells after calibrating RRT in cored wells • RRT coefficients should be optimized by history matching the total field liquid off-take 22 !|! Thanks !|!