TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe estimated recoverable reserves from the Valhall field at start of production in 1982 were 250 MMSTB. Now, after twenty years on primary depletion, a total of 500 MMSTB has been produced and additional 500 MMSTB remains to be produced of the original 2.6 BSTB in place. This corresponds to a recovery factor of 40%. Possibilities to increase ultimate recovery have been defined. These will require further insights into the detailed architecture of the field and improved understanding of the reservoir energy and displacement efficiency. Approximately 50% of the drive to date has been from rock compaction. The field is a North Sea Chalk field producing from Tor and Lower Hod Formations, porosity is exceeding 50% in places, some fractured permeability is present, but matrix permeability is generally low, less than 10 mD.

Permanent ocean-bottom seismic cables have been installed in the Valhall Field, offshore Norway.Five seismic surveys were acquired from the permanent system between October 2003 and April 2005, in addition to two towed-streamer surveys acquired in 1992 and 2002.The various 3D seismic data sets show strong time-lapse or 4D effects resulting from primary production.During the same time period an extensive drilling program has provided high-quality data on reservoir pressure, pressure gradients, and reservoir fluid distribution in the field. This paper outlines how changes in seismic attributes were generated synthetically from a reservoir simulation model and compared against observed 4D seismic data in one area of the field.An objective function describing the misfit between the simulated and measured data was defined using both seismic and conventional production data.BP's Top-Down Reservoir Modelling (TDRM) technology[1] was used to generate multiple reservoir descriptions that would minimize the objective function.This involved computer-assisted history matching with a genetic algorithm that varied more than 60 parameters in the reservoir simulator to improve the overall history match. Introduction The Valhall field is located in the North Sea approximately 290 km offshore Norway in Blocks 2/8 and 2/11 (Fig. 1).Valhall is a double-plunging NNW-SSE trending anticline (Fig. 2), covering approximately 65 km[2].The reservoir is a high porosity, overpressured, undersaturated Upper Cretaceous chalk located at depths of 2400–2650 m.The main reservoir is the Tor Formation, deposited during Danian, Maastrichtian and Campanian periods.A secondary reservoir is the Hod Formation (Santonian, Coniacian and Turonian).A stratigrahpic section is shown in Fig. 3.Seismic imaging is challenging in the crestal part of the field due to gas charge in the overburden. Valhall was discovered in 1975 and began producing in 1982.The field was highly overpressured with an initial pressure of approximately 6500 psi.The reservoir chalk is extremely soft with significant rock compaction, which has resulted in high intrinsic reservoir energy.This has allowed the field to be produced on primary depletion since 1982, with approximately 500 MMstb produced to date.The oil density is light (36 API) and viscosity is roughly 0.4 cp.A waterflood program has recently begun, which is expected to extend the production plateau significantly. The high rock compaction at Valhall has resulted in significant seabed and platform subsidence and presented many challenges associated with wellbore failures.Compaction has also influenced the dynamic reservoir properties, closing natural fractures with depletion and causing normally "static" parameters such as reservoir thickness and porosity to vary significantly over time.This introduces several challenges in modeling the reservoir properties and simulating field behaviour. More than 100 wells have been drilled at Valhall, with a standard logging suite providing data on porosity, thickness, water saturation, and static reservoir pressures.Since 1990, most wells have been horizontal, providing excellent data on lateral pressure gradients across the field.Twenty-three years of production history provide good quality data on production performance (oil rate, GOR, water cut). In 2003 a wellhead platform was installed in the southern part of the field, and a corresponding infill drilling campaign targeting flank areas was carried out.During this infill campaign, several seismic surveys were acquired with the permanent seismic array.

The combination of in situ effective stress and reservoir chalk mechanical weakness in the Valhall field has resulted in compaction and associated subsidence at the mudline. This paper summarizes recent fiite element modeling of this phenomenon, emphasizing items of concern in a numerical sinmlation of this nature. The paper also presents new results from a simulation allowing inelastic overburden response. Reservoir Model InterfaceThe driving force for compaction in the reservoir is effective stress increase associated with pore pressure depletion. Spatial and temporal pore pressure distributions used in this study were retrieved directly from executions of an in house resrvoir model. In particular, elements of reservoir simulator inputioutput data used by the finite element model include the fouowing 1. Spatial positions of the model grid points. 2. The porosity at each grid point. 3. Initial and subsequent pore pressure distributions used, 3n

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