Dynamic Rock Typing Study of a Complex Heterogeneous Carbonate Reservoir in Oil Field, Iraq

Abstract

Abstract The study covers a giant oil field situated in Iraq. M formation is the major producing reservoir in the field, and is characterized by a complex heterogeneous carbonate system. Reservoir rock typing in carbonates is a critical process by which geological lithofacies are characterized by their dynamic behavior, which is needed for better reservoir characterization required for reservoir modelling and numerical simulation to optimize development plans and maximize oil recovery. The operator company has conducted a comprehensive reservoir characterization study for M reservoir involving, routine core analysis, SCAL, Static and Dynamic rock typing studies in M formation, alongwith routine geological and petrophysical analysis. Thomeer methodology was used for Static Rock Typing analysis, utilizing the extensive available Mercury Injection Capillary Pressure Data (MICP) to capture the complexity of the M carbonate reservoir, and the Thomeer parameters (G – shape factor, Bv – Bulk Volume and Pd – entry pressure) were derived for every pore mode of these selected MICP plugs. Five pore geometry groups were proposed based on modes identified in the PTR frequency distribution, then created five equivalent Static ‘rock-types’. These were given further reservoir context through integration with Petrography, Sedimentology and NMR data available from core and logs. Due to the complex nature of heterogeneous carbonate formation of M, the five major static rock types were further characterized by their dynamic behavior to reflect the interaction between the rock lithofacies and reservoir fluids. While porosity, permeability and pore size distribution characterize the rock texture, SCAL parameters, capillary pressure, relative permeability and wettability will capture rock-fluid interaction and will impact oil recovery from such complex heterogeneous system. Different methods were tried to correlate dynamic rock types in M formation including Classical rock typing methods based on logarithmic regression of permeability versus porosity cross plots, as well as Flow Zone Index (FZI) and Reservoir Quality Index (RQI) methods. This was followed by applying Winland method for dynamic rock typing characterization. This paper describes the successful workflow adopted in the study to define the Dynamic rock types for M formation based on Winland methodology, where a total of 16 core plug samples were selected using CT scans screening (2 plug samples selected from each of the major rock types RT) for Reservoir Conditions relative permeability testing as well as 4 samples were earmarked for ambient condition relative permeability measurements and further 4 plug samples were selected for wettability and capillary pressure testing covering all the major types. The Winland R35 methodology [1] was applied on the selected samples in order to verify if any of the core samples shifted from the previously defined static rock types. The new porosity and permeability and SCAL relative permeability and capillary pressure measurements for the selected samples were used as part of the workflow to calculate Winland R35 algorithm, then plotted to derive and characterize the new dynamic rock types for M formation in the field