The identification of remaining reserves in mature fields is of importance to extend the field life and production life of existing wells. The Alwyn field in Quad 3 of the North Sea has been producing from the Statfjord Formation reservoirs since 1987. The initial production targeted discrete sand bodies that are separated by laterally extensive shales. The initial field pressure measurements indicated the presence of vertical barriers that have led to depleted pressures in produced intervals. Identification of present-day reservoir pressure in the gas condensate reservoirs was required to determine missed production potential within the Statfjord Formation sand bodies. A candidate well was selected to estimate pressure behind casing by using an advanced analysis method. The method used pulsed neutron data and Monte Carlo stochastic simulation-based forward modelling to evaluate gas condensate reservoir depletion. Conventional thermal neutron capture cross section (Sigma) has low sensitivity to gas density variations, making it unable to detect reservoir pressure changes.
Drilling of Laggan development wells commenced in summer 2013 and finalization of well locations required updated reservoir models, fully incorporating the 2009 baseline seismic. Consequently, geomodelling workflows have been developed based on the 3D seismic, petrophysical and production test data. The reservoir model building methodology for Laggan consists of the following steps: (1) construction of a classical structural framework using seismic interpreted fault sticks, top reservoir depth surface and the overall reservoir thickness map estimated from seismic data; (2) building of zones model integrating well data, seismic and sedimentological concepts; and (3) generation of a 3D facies and petrophysical model constrained by seismic and well data, including well test results to calibrate the permeability.A reasonable match of the well test data was achieved without any major adjustments to the reservoir model. This methodology has led to the construction of a robust reservoir description, capturing the geological knowledge of the turbidite system and constrained by well and seismic data.The new Laggan model is now considered ‘fit for purpose’ for well locations update, well type and well number confirmation and development of the Reservoir Management Plan.
Laggan, located in the west of Shetland, was discovered in 1986. There is now an improved understanding of Laggan, thanks to innovative and fully integrated geoscience studies and a successful appraisal campaign. Development studies are well advanced, with the discovery of Tormore in 2007 providing the potential for a combined development project. Laggan and Tormore are Paleocene gas condensate discoveries in approximately 600 m water depth. The traps are both mixed, stratigraphic with updip closure against bounding faults. The reservoir comprises sand-rich turbidite channelized lobes and lobes. Reservoir properties are good (permeability range 30-300 mD) due to the presence of chlorite and pre-sorting on the shelf. The geoscience evaluation of Laggan has matured over the last four years with the help of fully integrated studies using 3D seismic and well data. The depositional model has been defined on the basis of an evaluation of cores and seismic and supported by analogue studies. Seismic inversion studies have also helped constrain the reservoir architecture. Of particular value has been the application of AVO to quantify net gas sand, recognized as the principal static uncertainty. The main dynamic uncertainty is the risk of compartmentalization. This risk has been reduced through an improved definition of the fault configuration by re-processing the seismic and detailed seismic attribute analysis. The potential of Tormore was recognized by applying the geoscience understanding of Laggan to help de-risk the prospect. In particular, it was recognized that Laggan could be used as an analogue for the Tormore trapping configuration and reservoir potential and that AVO could be used to help define the Gas -Water Contact. The exploration well, drilled in 2007, was successful in finding a similar reservoir to that encountered in Laggan. The fluid encountered was a gas condensate, approximately three times richer than Laggan.
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