Uncertainties in Standard Analyses of Boundary Effects in Buildup Data
- Leif Larsen (Statoil ASA)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2005
- Document Type
- Journal Paper
- 437 - 444
- 2005. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.6.4 Drillstem/Well Testing, 2.4.3 Sand/Solids Control, 4.1.2 Separation and Treating, 5.1.2 Faults and Fracture Characterisation
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Boundary effects are often observed in buildup data—or at least that is theconclusion frequently drawn from an observed increase in derivative on alog-log plot or an increase in slope on a semilog plot. Furthermore, if (forinstance) it is concluded that the effects of a sealing fault are seen in agiven data set, then simple line methods or direct analytical-modeling effortsare normally used to determine the distance to the boundary. A sealing fault isthe normal choice of boundary model if a doubling is observed in derivative orsemilog slope. If a four-fold increase in derivative is observed, then a modelwith the well placed somewhere between two sealing faults forming a right anglewould be a normal choice. But what if the two faults are not sealing? If theflow capacity on the other side of the faults is only one-third of the value onthe well side, what will be the derivative characteristics?
Problems like these are addressed in detail in this paper, with a series ofsimple rules given for possible combinations that will generate buildup data ofa specific type (i.e., with specific "familiar characteristics"). The rules canbe used to list alternative interpretations without running separate analyses.For instance, it is shown that the derivative characteristics of any sectormodel bounded by sealing faults correspond to an infinite number of two-zonesector models with an angle between the boundaries and permeability contrastsatisfying a single equation. Other pairs of models with similarcharacteristics are models with partially sealing faults and specificthree-zone sector models, and either of these types of models and radialcomposite models. This clearly complicates analyses.
Also addressed are problems related to possible differences in the boundaryeffects observed in drawdown and buildup data for certain models. As oneexample, U-shaped and sector models can have identical buildup characteristicsover a wide time range, although drawdown data from the models have distinctlydifferent boundary characteristics. Radial composite and composite sectormodels are also of this type, with potentially significant differences betweendrawdown and buildup data. The reason for bringing up such cases is toemphasize the importance of attempting to collect high-quality drawdown data inaddition to buildup data to limit the range of possible interpretationmodels.
For completeness, effects of uncertainties in basic input parameters on thefinal analyses are also covered in the paper.
It is well known that buildup data from a well near a sealing fault mightexhibit a doubling of derivatives on a log-log diagnostic plot, as illustratedin Fig. 1. This doubling of derivatives corresponds to a doubling of slope on asemilog plot, as shown by Horner, and refers to a change between early and latedata requiring storage effects to become negligible and radial flow to bereached before the onset of boundary effects. For this behavior to occur, it isalso necessary for the flow period before shut-in to be long enough to be fullyor almost fully affected by the boundary effect.
|File Size||670 KB||Number of Pages||8|
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