Deliverables
The principal purposes of the SAIGUP were to quantify objectively the sensitivity of geological complexity on production forecasts, as a function of generic aspects of both the sedimentological architecture and faulted structure of shallow marine hydrocarbon reservoirs, and to validate these results using real-case reservoir and production data. Because of the case-specific nature of existing production forecasting sensitivity studies, links between geological and production uncertainty cannot be made at present. These links are a prerequisite for early recognition of the most significant geological parameters influencing production forecasting uncertainty and are a necessary basis for establishing optimal methods for including geological uncertainty in reservoir modelling studies.
The SAIGUP project has realistic ranges of small and large scale sedimentological variability, structural variability and different upscaling approaches applied to both grid-blocks and faults, using a variety of different field production plans. Results have permitted an assessment of the sources and levels of uncertainty in production forecasts in a generic suite of model reservoirs. A number of mainly structural heterogeneity measures have been identified which approximate relevant aspects of the reservoir geology as transportable dimensionless parameters with predictive capabilities. The secondary objective of validating these results using the natural field examples has been hampered by data formatting issues and the quality of pre-existing reservoir models, but some encouraging results backing conclusions from the synthetic modelling were obtained.
The sensitivity analyses, which have included models generated with the more permeable fault property predictors, indicate that variability in sedimentary variables at both the small and large scale dominate uncertainty in reserves, but that structural aspects of the reservoirs are as important as sedimentological ones in terms of recovery factors. Of the large-scale sedimentological parameters the aggradation angle is particularly significant. This variable is seldom taken into account when planning production. The relative orientations of the progradation and waterflood directions are also important. Both the barrier strength and the shoreline curvature have proven less influential than anticipated.
The choice of lamina-scale properties used in the upscaling is a very important control on reserves, and there is therefore a large potential for reducing the uncertainty in production forecasts by placing greater efforts into acquisition of special core analysis data, and into the generation of the relative permeability and capillary pressure curves used. The actual algorithms used to perform the upscaling do not appear to introduce uncertainty, and nor does the inclusion or omission of anisotropic curves. The flow rate assumed during pseudoisation is moderately important.
The analyses indicate that the principal way of reducing production uncertainty relating to structural issues is to place greater efforts into 3D fault mapping, since fault juxtapositions can have a significant effect on production irrespective of fault rock permeabilities. If a fault system is disconnected, fault permeabilities are influential on production uncertainty only over a two order of magnitude range. Small-scale fault segmentation can be particularly significant in compartmentalised systems of low permeability faults.
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