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PETROMAKS2-Stort program petroleum

Forecasting of architecture, seismic characteristics and flow behaviour in paleokarst reservoirs.

Alternative title: Prognostisering av arkitektur, seismiske egenskaper og væskestrømsoppførsel i paleokarstreservoarer.

Awarded: NOK 9.6 mill.

Karstification (dissolution) introduces extreme, but highly localized secondary porosity in soluble rocks. Fluids moving through fractures, faults and porous beds dissolve limestone and form connected cavities, sinkholes and cave systems in the subsurface. The shape and geometries of these systems is linked to rock properties, base-level changes, orientation and spatial distribution of fault- and fracture networks, climate, and in some cases the presence of aggressive hydrothermal fluids moving up through the earth?s crust. Over time and during burial these cavities experience degradation by sediment infill, precipitation of cements, breakdown and collapse. Although some cavities remain intact down to several kilometers depth, most cavities during burial fill with sediments transported into the system from without or with breccia formed as cavities collapse. The products of this degradation of karst is termed ?paleokarst?, and is encountered in various forms in most carbonate reservoirs, Paleokarst reservoirs can host significant hydrocarbon volume and exhibit excellent production properties, but commonly exhibit highly complex spatial property distributions. Modelling and forecasting the properties of such reservoirs in the subsurface is therefore highly challenging, even when seismic and well data is available. Paleokarst reservoirs area not well known in Norway and were only quite recently identified as viable prospects in the Barents Sea. The aim of the FOPAK project has been to improve our basic understanding of paleokarst reservoirs by addressing the link between petrophysical properties (such as porosity and permeability), seismic signatures and production behavior, and the initial karst configuration and processes active during the transformation from karst to paleokarst. We utilize a series of 3D models of mapped caves representing different geometric scenarios. Infill and changes in cave configuration and shape due to collapse are captured using established concepts for rock mechanics and sedimentology. The project emphasized the use of industry standard software where possible in order to facilitate practical application of results. For the same reasons, implementation of karst and paleokarst features in geo-cellular models received particular attention in order to allow using recent cave systems as input to the modelling process, Sediment fill commonly is overlooked or ignored during surveying. This has significantly impacts the handling of these data for reservoir modelling purposes, as the original dimensions of the cavity are underestimated and substantial parts of the paleokarst complex may consist of sediment infill. To account for this and to ensure the quality of the input data to our models, the project carried out resistivity surveys in several caves in order to map spatial distribution of sediment thickness. The use of resistivity proved to be an easy-to-use tool, providing a highly useful addition to standard cave surveying methods. The amount and spatial distribution of sediments in the initial karst system will affect the spatial distribution of collapse-related breccia in paleokarst. This will also influence seismic response and production behavior. The project has developed a procedure for seismic forward modelling of paleokarst reservoirs and tested it on a series of 3D reservoir models and outcrops with mapped paleokarst features. The methods is simple and computationally inexpensive, Our results show that it is possible to identify characteristic paleokarst features in seismic data, The modelling results highlight the impact of acquisition methods and modelling parameters on how paleokarst is rendered in seismic data. Fluid flow simulations were used to investigate and forecast the production behavior of the models. The aim was to identify if specific model scenarios exhibit characteristic simulation responses. Fluid flow simulations show that volume and composition of sediment infill and breccia can significantly affect production behavior and drainage of the reservoir, and that simulation responses are closely connected to relative permeability-contrasts between the different components of the paleokarst system. The project also addressed upscaling issues i.e. how property variations at fine scale in high-resolution models can be captured and reproduced using a coarser model rendering with lower computational costs and increase simulation speed. Our results show that models are very sensitive to upscaling and that stepwise up-scaling causes increasing overestimation of producible volumes. The effect increases with increasing complexity of the initial model and merits further studies to find suitable up-scaling methods for these reservoirs.

Prosjektet har etablert et velfungerende tverrfaglig samarbeid mellom forskere ved NORCE og UiB, som kan danne kjernen i en norsk kompetansegruppe innen paleokarstreservoarer. Behovet for en slik forskningsgruppe i Norge har vært signalisert fra flere industriaktører. Framgangsmåten prosjektet har benyttet er nyskapende, og har vakt interesse fra flere hold. Prosjektet har etablert samarbeidsrelasjoner med Bristol University, UK, innen karbonatstudier, University of Texas, Austin innen geofysiske studier av paleokarst og Aristotle University, Thessaloniki, Hellas innen speleologi og bruk av geofysiske metoder i hulekartlegging. Prosjektet har siden oppstarten hatt en løpende dialog med flere industriselskaper rundt anvendelse av resultater og samarbeidsprosjekter knyttet til utforsking og produksjon i Barentshavet og Brasil.

The proposed study is a cross-disciplinary effort involving spelology, geophysics, reservoir geology and -engineering and geo-modelling. It will employ a suite of forward modelling techniques to investigate the link between a) Different karst systems and resulting paleokarst reservoir configurations and properties b) Fluid flow behaviour and paleokarst reservoirs configurations c) Seismic characteristics and paleokarst reservoirs configurations Work will be framed around the construction of a suite of paleokarst resevoir models based on a combination of modern karst/cave types/geometries, representing different tectonostratigraphic settings, and process-based forward modelling of infill and collapse of these systems. Model input will be based on empirical data from present day karst systems as extant in speleological databases and geomechanical and physical principles known to control collapse and infill. Standard industrial reservoir modelling tools will be employed. We aim to build a suite of models representing a range of tectono-stratigraphic settings and populated with a selection of representative petrophsical properties derived from published sources. These models will serve as input to fluid flow simulation studies which will map and identify contrasts in reservoir responses to different production strategies and identify their geological causes. Seismic forward modelling will be carried out on the reservoir geo-models in order to investigate if the different types of paleokarst reservoirs can be idenified based on their seismic characteristics.

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Funding scheme:

PETROMAKS2-Stort program petroleum