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.

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.

For North Sea chalk fields, finite element models are used to evaluate the reservoir compaction and the associated seafloor subsidence. The present paper addresses aspects of the calibration of the stress strain laws to be used for the chalk and the overburden. Results of laboratory tests do not reflect the large scale deformation behaviour. The calibration is therefore mainly based on the back analysis of measured formation movements. Another topic is the coupling with reservoir simulation models, which provide the reservoir pressure. This coupling should be improved, if a local compaction cannot be derived from a simultaneous change in reservoir pressure at that location. RESUME: La methode des elements finis est utilisee pour evaluer la compaction et la subsidence du fond de la mer dans Ie cas de champs petroliers de mer du Nord. Aspects de la calibration de modeles de comportement utilises dans l'etude numerique de reservoir en craie et des couches de toit sont detaillees. Les resultats de mesures au laboratoire ne pas represent les deformations a grande echelle. Le calibration est donc essentiellement fondee sur Ie calcul a posteriori des deplacements mesures. Le couplage de calculs de geotechnique et de calcul de modeles de reservoir doit etre evalue avec des methodes ameliorees puisque la compaction en un point n'est pas fonction uniquement de la pression des pores en ce point. ZUSAMMENFASSUNG: Finite-Element-Modelle dienen der Berechnung der Kompaktion von Kreidereservoirs in der Nordsee und der daraus resultierenden Setzungen des Meeresbodens. Der Beitrag Q.ehandelt Aspekte der Kalibrierung der Spannungsdehnungsbeziehungen fUr Kreide und die Uberdeckung. Ergebnisse von Laborversuchen geben nicht das groBraumige Verformungsverhalten wieder. Die Kalibrierung erfolgt daher durch die RUckrechnung von Verformungsmessungen. Ferner wird die Kopplung mit Reservoirsimulationsmodellen diskutiert, die verbessert werden sollte, falls eine ortliche Volumenanderung nicht aus Druckanderungen zu gleicher Zeit und an gleicher Stelle resultiert.

Pressure decline associated with production induces compaction of high Porosity reservoirs and overburden subsidence in fields of the North Sea chalk Basin. More than ninety casing failures have been observed in this area which are related to the increase of axial and radial loads on the wellbore. Anumerical technique was developed to describe the mechanism of casing failurein compacted reservoirs as well as in associated overburdens. A 3D finite element modelling procedure was set up to simulate and predict the onset of casing failures. Important characteristics of this numerical model are the introduction of the concept of sliding interfaces and the transfer of deformation parameters from a large scale displacement (field)to a more refined scale analysis (casing). This 3D finite element simulation identified the failure modes and allowed an appropriate revision of casing design throughout the field area Furthermore, it has been demonstrated that the revised design solutions extend the casing lives for a large number of years. Introduction In the fields of the North Sea Chalk Basin more than 90 casing failures have been observed. They are caused by the compaction of the high porosity chalk during reservoir depletion and the associated overburden subsidence. In the Ekofisk field, up to 58 casing failures have been reported. At 38 wells, the failure is located in the reservoir, while for 20 wells the failure occurred in the overburden. Fig. 1 shows the location of wells for which casing failures were observed up to the end of 1988. The tremendous impact of these failures on the productivity of the field prompted the development of the numerical model which is presented in this paper. This model may serve: - to understand the phenomena of casing failure to evaluate casing design improvements - to predict future performance. The uncertainty of the stress conditions causing the casing failures and the necessarily limited knowledge of the large scale stress strain behaviour of the associated reservoir and overburden implies several assumptions and idealizations for such a model. Only by close cooperation of the involved engineers and geologists can a realistic model be defines in addition, numerical sensitivity studies have proven to be very useful to identify the effect of the individual assumptions. Nevertheless, it proved essential to qualify and calibrate the model. For the analysis of casing failures in the reservoir this was achieved by the back analysis of four observed casing failures in the Danian reservoir of the Ekofisk field- One of these calculations will be described in more detail in the following section- Only itis alter the location and the date of these failures were successfully simulated with the numerical model that pammetric studies were performed to evaluate the influence of casing weight, steel quality, wellbore inclination, etc. on the casing stability. Finally, the results of all calculations performed were combined in order to derive global recommendations for the field. A similar concept of qualification and calibration was used for the analysis of casing failures in the overburdened.. Horizontal movements in the overburden rock have been identified to cause most of the failures. In order to evaluate the large scale displacement field, a numerical model was developed which serves for the calculation of compaction and associated overburden subsidence in the Ekofisk field. This finite element model, which is closely coupled with a reservoir model, was calibrated by the back analysis of the observed compaction and subsidence measurements. Afterwards, in a very original manner, the results of the large scale displacement field analysis were transferred to a numerical model which served for the detailed analysis of casing stability of individual wells.

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