It is well known that the Valhall field in the Norwegian North Sea has experienced significant seafloor subsidence due to compaction in the chalk reservoir. This paper documents the development of a new coupled reservoir geomechanical model to help understand the dynamic changes in the reservoir and the overburden, and to improve history-matching, reserve estimation, and production forecast. With 1.3 million grid cells and 56 layers in the geomechanical model, different coupling schemes have been investigated and compared to recommend the most effective approach. The model is calibrated using multiple sources: seafloor subsidence from GPS and bathymetry, reservoir compaction data from 4D seismic, pore pressure and stress measurement over time, individual well geomechanical models, history of bottom hole flowing pressure, and production of oil, water, and gas from wells and the field. While it confirms that the depletion has significantly reduced horizontal and vertical stresses in the reservoir and some of the overburden, local stress arching and increased horizontal stress anisotropy are observed as well. The majority of plastic deformation occurs in the reservoir, while fault reactivation is widespread in the overburden. Compaction in the reservoir can lead to major reductions in vertical and horizontal permeability while localized injection may only recover a portion of the original permeability. History-matching therefore needs be performed in a manner consistent with the rock deformation in each reservoir layer, emphasizing the importance of integrating reservoir simulation with a geomechanical model for the Valhall field.
Development and Calibrations of a Coupled Reservoir Geomechanic Model for Valhall Field
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Han, Gang, Vaughn, Ben, Davids, Andrew, Spokes, John, Newman, Robert, and Jose Adachi. "Development and Calibrations of a Coupled Reservoir Geomechanic Model for Valhall Field." Paper presented at the 47th U.S. Rock Mechanics/Geomechanics Symposium, San Francisco, California, June 2013.
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