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1 Stage 5 Forward Modelling
Published: 21 Mar 2019, 2:59 p.m. Revised: 30 Sep 2019, 11:22 a.m.During appraisal, the absence of data from which facies architecture and diagenetic modification can be mapped means multiple scenarios can be envisaged to both explain platform evolution through time, and also the resultant reservoir properties. Stratigraphic forward models and reactive transport models therefore offer an opportunity by which the sensitivity of the platform to particular environmental parameters – and its resultant response – can be analysed.
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2 Stage 1 Structural Evaluation
Published: 21 Mar 2019, 2:32 p.m. Revised: 2 Apr 2019, 8:07 p.m.The rationale behind the workflow for this phase of reservoir evaluation is to refine estimates of in-place volumes and determine the reservoir recovery factor based on a range of potential recovery mechanisms. The workflow largely follows that defined for basin scale/frontier exploration, but assumes-
a) Prior analysis of basin formation mechanism, tectonostratigraphy and palaeo-plate reconstruction
b) A richer dataset, for example reprocessed seismic data, well data and/or more evolved analogue studies
c) A larger, more multi-disciplinary team, including petrophysicists, reservoir engineers and well & production engineers
3 Stage 3 Sedimentological and diagenetic interpretation of well data
Published: 2 Apr 2019, 8:05 p.m. Revised: 2 Apr 2019, 8:05 p.m.If core is available during appraisal, then it should be optimised for geological interpretation. It is expected that this will be conducted partly in parallel with core petrophysical analysis, although there is value in ensuring the routine core petrophysical data is used to select samples for petrographical anlaysis.
4 Stage 4 Petrophysical Analysis
Published: 21 Mar 2019, 2:37 p.m. Revised: 21 Mar 2019, 2:37 p.m.Petrophysical analysis involves the integration of core, log and seismic data to determine the volume and distribution of porosity, saturation, net reservoir and permeability. During appraisal, it is assumed there are only 1-2 wells available with data, and possibly no core data.
5 Stage 5 Interpretations
Published: 21 Mar 2019, 2:38 p.m. Revised: 21 Mar 2019, 2:38 p.m.Once core description, wireline log analysis and seismic interpretation has been conducted, the spatial distribution of facies and diagenetic overprint can be assessed and their impact on petrophysical properties interpreted.
6 Stage 6 Forward Modelling
Published: 21 Mar 2019, 2:11 p.m. Revised: 21 Mar 2019, 2:41 p.m.During appraisal, the absence of data from which facies architecture and diagenetic modification can be mapped means multiple scenarios can be envisaged to both explain platform evolution through time, and also the resultant reservoir properties. Stratigraphic forward models and reactive transport models therefore offer an opportunity by which the sensitivity of the platform to particular environmental parameters – and its resultant response – can be analysed.
7 Stage 7 Reservoir modelling
Published: 21 Mar 2019, 2:21 p.m. Revised: 21 Mar 2019, 2:42 p.m.Reservoir models are critical to concept testing and selection for field development, including optimisation of well placement and analysis of full-field economics. In order that the most economically and technically feasible development option is selected, it is imperative that reservoir models are
a) constructed using geologically robust rules sets, providing confidence in interwell permeability prediction
b) have appropriately upscaled petrophysical properties, so that flow controlling layers are not obscured by averaging
c) have assigned dynamic data (e.g. relative permeability) that reflects, rather than obscures, reservoir heterogeneity
d) able to incorporate past and present reservoir performance within the geological interpretation, so that flow-controlling layers (e.g. baffles, barriers and high permeability streaks) are represented in the model, and
e) fundamentally linked to a fracture model
A range of geostatistical methods can be used for modelling carbonate reservoir architecture. None are used consistently by the industry, and the modelling workflow is often driven by corporate workflows (perhaps based on clastic reservoirs), reservoir architectural elements, data type, quality and volume, the timeframe available for model construction and prior experience of the reservoir modeller.
Once faults have been picked, surfaces mapped and the model grid constructed, most carbonate reservoirs should be modelled by a workflow that includes facies modelling, diagenetic modelling, petrophysical modelling and fracture modelling. However, during appraisal, it is unlikely that there will be sufficient data for this workflow to be developed in full. At this point it is more important to undertake a robust risk and uncertainty analysis and thereby assess the impact of these uncertainties by running multiple scenarios. One approach for ensuring that the most appropriate range of scenarios is modelled, taking account of the combined uncertainty of different parameters, is through experimental design (e.g. Hollis et al., 2011).