History Matched Full Field Geomechanics Model of the Valhall Field Including Water Weakening and Re-Pressurisation

Abstract

Valhall is a large chalk field in the Norwegian sector of the North Sea. The reservoirs consist of highly overpressured (0.84 psi/ft) chalk. The high overpressure and early oil migration has resulted in very well preserved porosity, exceeding 50% in parts of the field. This highly porous chalk is extremely weak, which results in liquefaction of the chalk at certain conditions during production. The weak nature of the chalk does also result in significant reservoir compaction, exceeding 10 meters in some locations in the reservoir. The compaction is providing an excellent source of reservoir energy, accounting for 50-60% in certain areas of the field. The compaction is transferred to the seafloor in the form of subsidence and is currently exceeding 6 meters below the central platform complex.The seafloor subsidence has resulted in a diminishing air gap for the platforms at Valhall, resulting in demobilisation of personnel during very rough winter storms. Water injection started in the field in 2004 with a gradual step up in rate. An offset chalk field in the area have experienced an increasing subsidence rate due to water weakening of the chalk. The micro-scale physical and chemical processes involved in water weakening of chalk have not been quantitatively determined, but a strategy to handle this had to be developed. This paper documents the development of a prediction model for the subsidence at Valhall between 2002 and 2008. The paper covers the development of a suitable and effective constitutive model to account for strain rate dependent reservoir compaction during depletion, re-pressurisation and waterflooding. The constitutive deformation model is driven by pore pressure and watersaturation calculated in a reservoir flow model and imported into the geomechanics model.It will be described how the constitutive model for the reservoir was populated. The overburden, underburden and sideburden were also modelled and required properties. The initialisation requires an initial stress state and it will be presented how this was defined. We also included faults in the model and check for re-activation and amount of slip on them. The Valhall model can also be constrained by a relative large amount of field and surveillance data. It will be presented how this data was used in the history matching process. The data used for the history matching process includes vertical and horizontal platform movements from GPS, time lapse seafloor bathymetry maps, radioactive markers in the reservoir and overburden as well as 4D seismic. It will be presented how the model matched the observed data from the field, showing a gradual decrease in subsidence rate from around 25 cm/year with the risk to increase due to water weakening, to the current reduced rate at 11 cm/year. This response is different to the one reported from the offset field. We will also present other high impact business applications of the model prediction, besides seafloor subsidence forecasting.

Radial anisotropy in Valhall: ambient noise-based studies of Scholte and Love waves

“…It is very well related to the oil production, there is a compaction in the reservoir that leads to the subsidence the whole overburden. The subsidence varies along the rock column and at the reservoir level, it is stronger (∼10 m) than at the surface (∼6 m; Kristiansen & Plischke 2010). This subsidence differential stretches the rocks in the overburden from ∼180 m down to the reservoir and the resulting volumetric strain decreases the seismic velocities (Barkved et al 2005).…”

Integration of the life of field seismic data with the reservoir model at the Valhall Field‡

“…They observed a linear relationship between the observed time‐shifts maps and the modelled compaction maps and proposed an R‐factor to link the two. By cross‐plotting the observations from the time‐shift extractions and the modelled compaction from geomechanical modelling (Kristiansen and Plischke 2010), an R‐factor of 5.7 was found to be applicable for the Valhall Field. We have now generated the main time‐lapse products that can be used for comparisons to the reservoir model.…”

Reservoir stress path and induced seismic anisotropy: results from linking coupled fluid-flow/geomechanical simulation with seismic modelling

“…In particular, the depth dependence of the rock physics model is poorly constrained, which can lead to biases in predicted magnitude of seismic anisotropy. However, in full field simulations the models can be calibrated via history matching (e.g., Kristiansen and Plischke 2010). Certainly a parametric study of seismic attributes to the stress sensitivity of nonlinear rock physics models would be useful to determine the most influential model input parameters.…”