The Stratigraphic Method: A Structured Approach to History Matching Complex Simulation Models
- M.A. Williams (Chevron Petroleum Technology Co.) | J.F. Keating (Chevron Petroleum Technology Co.) | M.F. Barghouty (Saudi Aramco)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- April 1998
- Document Type
- Journal Paper
- 169 - 176
- 1998. Society of Petroleum Engineers
- 5.5.11 Formation Testing (e.g., Wireline, LWD), 5.1 Reservoir Characterisation, 2.2.2 Perforating, 5.6.4 Drillstem/Well Testing, 3.3.1 Production Logging, 5.5.8 History Matching, 5.6.1 Open hole/cased hole log analysis, 5.2.1 Phase Behavior and PVT Measurements, 5.5 Reservoir Simulation, 4.3.4 Scale, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 3 Production and Well Operations, 5.1.5 Geologic Modeling, 5.1.1 Exploration, Development, Structural Geology
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This paper offers a structured approach to performing a history match on a complex, multilayered reservoir model. We recommend tools for interpreting the data and simulation results, and discuss procedures for improving the quality and efficiency of the history-matching process. This approach, the Stratigraphic Method, was developed over ten years on various reservoir studies. It has been used successfully on highly complex reservoirs, some containing more than 1,700 wells and over 50 years of production history. We also report on which reservoir parameters most affect matching observed reservoir performance. This may be helpful to lesser experienced persons working on a history-match project.
Advances in simulation hardware and software have created opportunities to model more complex reservoirs. The underlying assumption is that modeling reservoirs with 100,000 or more cells will better capture the heterogeneity that was blurred with earlier coarsely gridded and layered models. The industry uses supercomputers and advanced solution techniques to reduce model run times and computational error. Yet, the largest source of error may be unintentionally overlooked - that being the geologic description with the engineering control to properly validate it. Geostatistics is oriented toward reducing the geologic error, but these probabalistic representations of the reservoir still must be validated with dynamic data such as pressures, production, and saturations.
The validation of a reservoir model is the most challenging phase; one where the integrated application of geologic and engineering principles are exercised. Large models require massive data management efforts for better understanding of reservoir performance. Although generating dozens of maps and well plots is necessary, there is no structured approach on how these maps and plots are to be used or interpreted. Three-dimensional displays of simulator results are no substitute for understanding reservoir behavior and knowing which parameters to adjust for a reasonable history match. Furthermore, few publications offer the reservoir engineer guidance on how to approach such a complex task. Without a history-match plan, validating a reservoir model can be an inefficient, haphazard procedure. These inefficiencies waste time and also impugn the credibility of reservoir simulation as an effective tool; moreover, they delay the model results which are a critical component to making sound reservoir management decisions.
Today's models primarily employ layering based on reservoir stratigraphy. Studies devote a large effort to the reservoir characterization phase and often include geostatistics. This is to ensure that the geologic model accurately portrays the reservoir flow units and their heterogeneity. Flow units are discrete portions of the reservoir that act as separate or nearly separate reservoirs. Fig. 1 shows a type log from a deltaic sandstone which has five major flow units, Zones A to E. The zones in this reservoir are usually separated by a shaley section, but not in all locations. The Stratigraphic Method assumes these correlative flow units can be treated independently while history matching. The method also recognizes that the flow units may be in vertical communication in parts of the field where the shaley sections are thin or eroded.
The type log in Fig. 1 shows that the five zones are subdivided into smaller layers. Currently fine-scale geologic models employ hundreds or thousands of layers, averaging 1 to 3 feet thick. The engineer still has to apply some level of scale-up to keep the simulation model within a reasonable size. In some cases the simulation layers may be stratigraphic, but often they are either (1) structural (equal thickness) layers to improve vertical resolution or (2) layers with similar flow characteristics that have been combined during the scale-up procedure. The Stratigraphic Method will ultimately focus history-matching on these individual layers.
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