Carbonate reservoirs typically contain highly variable distributions of pore sizes and pore throat geometry. These distributions are a primary control on fluid movement in the reservoir. Knowledge of the pore geometry is critical to optimize the number of wells in a carbonate field, to optimize facilities, and to estimate recoverable reserves, etc.
Recent work to define petrophysical rock types (PRT) has lead to improved characterization of pore geometry and key reservoir properties in Tengiz Field, one of the world's deepest supergiant fields. Tengiz is an isolated Paleozoic carbonate build-up in the Pricaspian Basin, Republic of Kazakhstan, containing over 26 Billion Barrels OOIP. The introduction of petrophysical rock types at Tengiz has contributed to improved estimates of volumes of oil in place and improved reservoir management.
Carbonate rock typing and pore type identification are critical parameters in the modeling and distribution of reservoir properties, such as porosity and permeability. The character and distribution of pore geometries in Tengiz buildup is determined by a variety of depositional and diagenetic processes. The latter include early and burial diagenetic modification, mechanical compaction and failure, and phases of corrosion and cementation. Petrophysical rock types (PRT) were developed to characterize various depositional and diagenetic regions of Tengiz Field. Pore types and distribution in the central region of Tengiz (the platform) have been significantly affected by early meteoric diagenesis, mechanical compaction, burial calcite cementation and late burial corrosive diagenesis as well as bitumen cementation.
The Tengiz flank areas are dominated by microbial boundstone deposition in an upper slope environment. Carbonate facies in the Tengiz flanks are generally tight with bitumen cement and are dissected by fracture systems which act as fluid conduits.
Several distinct groupings of petrophysical rock types are recognized in the Tengiz platform and flank. These facies groupings include Volcanic Dominated, Bitumen Rich, Tight/Cemented, and reservoir facies of varying quality.
Key wire line logs for the definition of PRT include Spectral Gamma, Multimin-derived bitumen volume and porosity, and the T2 distribution from nuclear magnetic resonance (NMR) logs. Petrophysical Rock Types are closely associated with pore types and pore throat distributions.
Several pore type classification schemes have been proposed in the literature, and many of these are based on the depositional texture. This paper proposes a pore type classification based on pore throat distributions using the Winland parameter. This classification includes: nano-porosity, micro-porosity, meso-porosity, macro porosity, isolated and connected vugs. Modified Winland equations were developed to allow prediction of Tengiz pore types from logs. The classification of pore types and development of algorithms to predict pore types are based on an extensive data set of more than 550 mercury injection pressure measurements on Tengiz samples. The Tengiz dual porosity general classification is scale-driven and has a direct impact on permeability and flow performance.
The proposed matrix pore type classification was used to construct an improved reservoir simulation model for Tengiz Field. The new simulation model used petrophysical rock types to improve the characterization of reservoir porosity, oil saturation, and permeability.
