Effect of Scaleup and Aggregation on the Use of Well Tests To Identify Geological Properties
- Olubusola O. Thomas (Stanford U.) | Rajagopal S. Raghavan (ConocoPhillips Co.) | Thomas N. Dixon (ConocoPhillips Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2005
- Document Type
- Journal Paper
- 248 - 254
- 2005. Society of Petroleum Engineers
- 5.1.5 Geologic Modeling, 2.4.3 Sand/Solids Control, 5.1 Reservoir Characterisation, 5.5.3 Scaling Methods, 5.1.1 Exploration, Development, Structural Geology, 5.6.3 Pressure Transient Testing, 5.6.4 Drillstem/Well Testing, 4.1.5 Processing Equipment, 5.2 Reservoir Fluid Dynamics, 5.5.8 History Matching, 4.3.4 Scale, 5.5 Reservoir Simulation
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This paper discusses specific issues encountered when pressure tests areanalyzed in reservoirs with complex geological properties. These issues relateto questions concerning the methodology of scaleup, the degree of aggregation,and the reliability of conventional methods of analysis. The paper shows thatif we desire to use pressure-transient analysis to determine more complexgeological features such as connectivity and widths of channels, we need amodel that incorporates reservoir heterogeneity. This complexity can lead tosignificantly more computational effort in the analysis of the pressuretransient.
The paper demonstrates that scaleup criteria, based on steady-stateprocedures, are inadequate to capture transient pressure responses.Furthermore, the number of layers needed to match the transient response may besignificantly greater than the number of layers needed for areservoir-simulation study. The use of models without a sufficient number oflayers may lead to interpretations that are in significant error.
The paper compares various vertical aggregation methods to coarsen thefine-grid model. The pressure-derivative curve is used as a measure ofevaluating the adequacy of the scaleup procedure. Neither the use ofpermeability at a wellbore nor the average layer permeability as criteria forthe aggregation was adequate to reduce the number of layers significantly.
The objectives of this paper are to demonstrate the impact of the detailedand small-scale heterogeneities of a formation on the flow characteristics thatare obtained from a pressure test and how those heterogeneities affect theanalysis of the pressure test. The literature recognizes that special scaleupprocedures are required in the vicinity of wells located in heterogeneousfields. Our work demonstrates that these procedures apply only to rather smallchanges in pressure over time and are usually inadequate to meet objectives forhistory-matching well tests. Using a fine-scale geological model derived bygeological and geophysical techniques, this work systematically examines theinterpretations obtained by various aggregation and scaleup techniques. We willdemonstrate that unless care is taken, the consequences of too much aggregationmay lead to significant errors on decisions concerning the value of areservoir. Current scaleup techniques presume that spatial (location ofboundaries, location of faults, etc.) variables are maintained. In analyzing awell test, however, one of our principal objectives is to determine therelationship between the well response and geometrical variables. We show thata limited amount of aggregation will preserve the spatial and petrophysicalrelationships we wish to determine. At this time, there appears to be no methodavailable to determine the degree of scaleup a priori. Because the objective ofwell testing is to estimate reservoir properties, the scaleup process needs tobe made a part of the history-matching procedure.
By assuming a truth case, we show that too much vertical aggregation maylead to significant errors. Comparisons with traditional analyses based onanalytical techniques are made. Whenever an analytical model is used in theanalysis, unless otherwise stated, we use a single-layer-reservoirsolution.
|File Size||602 KB||Number of Pages||7|
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