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_Journal of Structural Geology 32 (2010) 1754–1767_ Geomechanical integrity of sealing faults during depressurisation of the Statfjord Field F. Cuisiat a,*, H.P. Jostad a, L. Andresen a, E. Skurtveit a, E. Skomedal b, M. Hettema b Norwegian Geotechnical Institute, P.O. Box 3930 Ullevål Stadion, N-0806 Oslo, Norway b Statoil, N-4035 Stavanger, Norway Article history: Received 5 March 2009; Received in revised form 7 January 2010; Accepted 8 January 2010; Available online 18 January 2010 Keywords: Geomechanics Production Reservoir Fault Sealing integrity Abstract In this paper the results of geomechanical analyses of fault behaviour at the Statfjord Field carried out as part of Statfjord Late Life Project are presented. The objective was to assess the sealing integrity of the horst structure between the Statfjord and Snorre fields during final depressurisation of the Statfjord Field. According to field pressure observations, the Brent Fault is still acting as a pressure seal between the Brent Field and the Statfjord Field, despite large present-day depressurisation of the Brent Field. These observations were used as a calibration and verification of the stress conditions that can be sustained without modifying the seal integrity of the fault. Based on calculated stress changes in the horst structure which are equal to or less critical than the calculated present stress changes on the Brent Fault, it is concluded that the mechanical effects associated with planned depressurisation of the Statfjord Field during its late life phase will not affect significantly the hydraulic resistance of the horst structure. A parametric study was conducted to investigate the sensitivity of the calculated stress changes to various input parameters for fault geometry and properties. The largest uncertainty relates to the peak shear strength of the fault (core) zone. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction In this paper, the stress response of a major sealing fault to planned reduction in reservoir pressure during production is analysed. The fault and the horst structure it bounds appear as sealing pressure boundaries between Statfjord and the neighbouring field, Snorre, in the Norwegian North Sea. In response to the operator’s concern, the main focus of the study is to investigate the possibility to develop zones with shear or tensile failure during production, as such zones may be subjected to significant changes in hydraulic resistance. As documented in the literature, fluid injection or fluid withdrawal can both induce active faulting in oil and gas reservoirs (Grasso, 1992). Re-activation of reservoir bounding faults can cause a loss of seal integrity (Wiprut and Zoback, 2000), and slip on active faults can also cause shearing of production wells (Maury et al., 1992) or wellbore instability during drilling (Willson et al., 1999). The analyses presented in this paper are carried out within the context of Statfjord Late Life Project, which was started by the operator to increase gas recovery by reducing reservoir pressure (Boge et al., 2005). In order to assess the stress response caused by this pressure reduction and the sealing integrity of the horst structure, geomechanical models have been constructed to predict production-induced stress changes during the final phase of field production. A calibration of the numerical approach is performed by calculating the stress changes in the Brent Fault due to present depressurisation of the Brent Field. From field observations, the Brent Fault, which acts as a hydraulic barrier between the Brent Field and the Statfjord Field has not experienced any significant change in the hydraulic communication across it, despite the present pore pressure depletion of nearly 30 MPa at the Brent Field. In our approach, the maximum shear stress and minimum principal effective stress due to the present pore pressure reduction at the Brent Field are used as an indicator of the stress condition that does not give significant change in the mechanical sealing integrity of the Brent Fault. It is then argued that the properties of the Brent Fault and horst are similar such that the above stress condition for the Brent Fault can be extrapolated to the horst structure during depressurisation phase planned for the Statfjord Field. Two dimensional (2D) plane strain, geomechanical analyses are carried out by looking at characteristic 2D cross sections through a geological model of the area. It is considered that 2D analyses are sufficient as first-order approximation, given the general geometry and uncertainties in some of the input data (see discussion in Section 2.1). The geomechanical analyses are performed using the numerical code Plaxis v8.2 (2004). Plaxis is a finite element program specially developed for geomechanical applications. In order to apply Plaxis to reservoir-type problems, a user-defined poroelastic constitutive model has been implemented into Plaxis in order to take into account the compressibility of the grains due to changes in pore pressure (i.e., the Biot effect), as well as the compressibility of the pore fluid in shales during undrained deformation. A critical input in the analyses is the mechanical properties of faults. Based on results from laboratory experiments on intact and faulted material, as well as an estimation of clay content within fault planes, fault properties are suggested for the analyses. A key feature that controls the stress changes in a fault zone is the degree of drainage of the fault zone for the timescale considered. It is shown that for actual production time histories, the fault core can be considered to be drained. In light of uncertainties related to geometries and geomechanical properties of fault zones, a parametric study is performed in order to identify main parameters controlling shear and normal stress changes and therefore, the possibility of developing shear and tensile failure of the faults. Based on results of the parametric study, a base case model of the Brent Fault is defined for geomechanical assessment of the structure. 1.1. Description of the Statfjord Field The Statfjord Field is located in the Tampen area in the northern part of the North Sea, between the Brent and Snorre fields (Fig. 1 top right). The water depth in the area is about 150 m. The Statfjord Field is located in a large, faulted block, which is tilted towards the west and comprises a number of smaller, faulted compartments along its east flank. Towards the south-west, the field is bounded by the Brent Fault, and in the north-east a horst structure marks the boundary with the Snorre field (Fig. 1 bottom). Within the Statfjord Field there are two reservoir units; the Brent Group at around 2500 m below mean sea level and the Statfjord Formation at a depth of around 2700 m below mean sea level. The two reservoirs are separated by the Dunlin Group, which consists mainly of shale. The field can be separated into a relatively undeformed main field area and an eastern flank which is heavily deformed by rotational slide blocks. The main tectonic event in the area is related to the opening of the Viking Graben which started in the Middle Jurassic. Subsequent deposition of the Draupne Formation caused gravitational instabilities along the crest of the field and the formation of rotational slide blocks in the Upper Triassic and Jurassic sections (Hesthammer et al., 1999). 1.2. Statfjord late life project The Statfjord Late Life Project started in 2005 in order to improve recovery from the Statfjord Field by converting the field from an oil field with associated gas, into a gas field with associated oil (Boge et al., 2005). This is achieved with a very extensive well programme and modifications of the platforms and associated Tampen link pipeline to export gas. Gas export began in 2007. Production from the Statfjord Field is expected to continue until 2020. The production strategy for the field was previously based on injecting gas and water for enhanced oil recovery and reservoir pressure maintenance. This resulted in over 60% crude oil recovery from the stock tank oil originally in place (STOOIP). In the late life phase of the field, the remaining non-recoverable oil is produced together with large volumes of previously-injected gas by reducing pressure in the reservoirs. A pressure reduction of about 30 MPa (300 bar) is planned, thereby reducing the pressure differences with the Brent Field and inducing a large planned pressure drop through the horst connecting it with the Snorre field. 2. Description of numerical models In this section... 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