A New Workflow for 3D Geological Modeling of Karstified Carbonate Petroleum Reservoirs

Yeda Backheuser and Mathieu Moriss and Marcelo Blauth. ( 2019 )
in: 2019 Ring Meeting, ASGA

Abstract

The accurate representation of karst features in 3D geological models is essential for obtaining reliable oil production curves from fluid flow simulation. In order to achieve that, it is mandatory to define first a conceptual model for the karst system in study by the integration of multiple data, such as seismic, well logs, core, etc. The multiple tasks required for the characterization of karstified reservoirs, and the relatively small number of experts in this complex area of knowledge, makes the 3D geological models incorporating karst features mostly developed by non-experts. Therefore, Petrobras developed in partnership with Emerson, a software plugin named PetroKarst, which aims not only the 3D geological modeling of karstified petroleum reservoirs, but also the leading of its users to define karst conceptual models for the modelled reservoirs. This plugin is an evolution and improvement from the original KarstMod plugin developed by Gocad Consortium. PetroKarst is not capable of modeling the chemical process responsible for the dissolution of carbonate rocks, but it is aimed to create realistic 3D cave systems conditioned to the main factors that de ne the topology and the geometry of the conduits after the simulation of the karst trajectory following an initial rock-fracture system. Cave geometry is strongly influenced by climate, and more directly, by the seasonal variation of fluid flow in subsurface. The classification of karst systems developed by Jouves et al. (2017) considers these factors, and it was used in the development of PetroKarst. The definition of the ancient phreatic water level allows the modeler to discriminate the mostly vertical dissolution that occurs in the vadose zone from the mostly horizontal caves that are developed in the epiphreatic zone. The de nition of the paleoclimate is essential to discriminate planar and elongate karst systems (humid weather) from anastomosed karst systems (arid weather). The occurrence of aquitards promotes the origin of a karst zone with maze geometry, due to the action of a continuous fluid flow of meteoric water or hypogenic fluids along a fracture system. The karstifying fluid route is a function of the ancient relief, the porosity and permeability of the rock-fluid system and the spatial position of inlet/outlet points offluids in the karst system (karst springs). Karst geometry definition is done in two phases: (1) building of the karst skeleton based on the definition of the most probable trajectory of the karstifying fluid, and (2) building of the cave envelope surface around the karst skeleton, through the ODSIM technique (Object Distance Simulation). Once PetroKarst generates the envelope surface it can be used in any modeling software to define which cells in the geological model are karstified or to estimate what percentage of the model cells are karstified and present petrophysical properties different from the rock matrix and, consequently, would show different arrival time of injection water in petroleum reservoirs. Although the models obtained through PetroKarst cannot be used directly as reservoir models due to the limitations of the grid size, they can be qualitatively used as images that represent reservoir conceptual models, or they can help to delimit the portion of the reservoir affected by the presence of paleocaves. A possible indirect use of the models built with PetroKarst would be as training images for the definitive 3D reservoir modeling.

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BibTeX Reference

@INPROCEEDINGS{BackheuserRM2019,
    author = { Backheuser, Yeda and Moriss, Mathieu and Blauth, Marcelo },
     title = { A New Workflow for 3D Geological Modeling of Karstified Carbonate Petroleum Reservoirs },
 booktitle = { 2019 Ring Meeting },
      year = { 2019 },
 publisher = { ASGA },
  abstract = { The accurate representation of karst features in 3D geological models is essential for obtaining reliable oil production curves from fluid flow simulation. In order to achieve that, it is mandatory to define first a conceptual model for the karst system in study by the integration of multiple data, such as seismic, well logs, core, etc. The multiple tasks required for the characterization of karstified reservoirs, and the relatively small number of experts in this complex area of knowledge, makes the 3D geological models incorporating karst features mostly developed by non-experts. Therefore, Petrobras developed in partnership with Emerson, a software plugin named PetroKarst, which aims not only the 3D geological modeling of karstified petroleum reservoirs, but also the leading of its users to define karst conceptual models for the modelled reservoirs. This plugin is an evolution and improvement from the original KarstMod plugin developed by Gocad Consortium. PetroKarst is not capable of modeling the chemical process responsible for the dissolution of carbonate rocks, but it is aimed to create realistic 3D cave systems conditioned to the main factors that dene the topology and the geometry of the conduits after the simulation of the karst trajectory following an initial rock-fracture system. Cave geometry is strongly influenced by climate, and more directly, by the seasonal variation of fluid flow in subsurface. The classification of karst systems developed by Jouves et al. (2017) considers these factors, and it was used in the development of PetroKarst. The definition of the ancient phreatic water level allows the modeler to discriminate the mostly vertical dissolution that occurs in the vadose zone from the mostly horizontal caves that are developed in the epiphreatic zone. The denition of the paleoclimate is essential to discriminate planar and elongate karst systems (humid weather) from anastomosed karst systems (arid weather). The occurrence of aquitards promotes the origin of a karst zone with maze geometry, due to the action of a continuous fluid flow of meteoric water or hypogenic fluids along a fracture system. The karstifying fluid route is a function of the ancient relief, the porosity and permeability of the rock-fluid system and the spatial position of inlet/outlet points offluids in the karst system (karst springs). Karst geometry definition is done in two phases: (1) building of the karst skeleton based on the definition of the most probable trajectory of the karstifying fluid, and (2) building of the cave envelope surface around the karst skeleton, through the ODSIM technique (Object Distance Simulation). Once PetroKarst generates the envelope surface it can be used in any modeling software to define which cells in the geological model are karstified or to estimate what percentage of the model cells are karstified and present petrophysical properties different from the rock matrix and, consequently, would show different arrival time of injection water in petroleum reservoirs. Although the models obtained through PetroKarst cannot be used directly as reservoir models due to the limitations of the grid size, they can be qualitatively used as images that represent reservoir conceptual models, or they can help to delimit the portion of the reservoir affected by the presence of paleocaves. A possible indirect use of the models built with PetroKarst would be as training images for the definitive 3D reservoir modeling. }
}