Formulation of "Capillary Force Barriers" in Moderately-Oil Wet Systems and Its Application to Reservoir Simulation
- Yasuhiro Muneta (Zakum Development Co.) | Magdi I. Mubarak (Zakum Development Co.) | Hadi H. Alhasani (Zakum Development Co.) | Kazuyoshi Arisaka (Zakum Development Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- October 2005
- Document Type
- Journal Paper
- 388 - 396
- 2005. Society of Petroleum Engineers
- 5.5.2 Core Analysis, 5.8.7 Carbonate Reservoir, 5.2.1 Phase Behavior and PVT Measurements, 5.1 Reservoir Characterisation, 4.1.5 Processing Equipment, 5.5 Reservoir Simulation, 5.6.2 Core Analysis, 4.3.4 Scale, 1.2.3 Rock properties, 1.6.9 Coring, Fishing, 5.5.3 Scaling Methods, 5.1.5 Geologic Modeling, 4.1.2 Separation and Treating, 5.5.8 History Matching, 5.2 Reservoir Fluid Dynamics
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As a common production aspect of the Thamama formation (a carbonatereservoir) in both onshore and offshore Abu Dhabi fields, unexpectedearly-water breakthrough through specific high-permeability layers without aclearly impermeable layer underneath has been observed in severalwater-injection schemes. Observed field data such as pulsed neutron capture(PNC) logs indicate the absence of injected water slumping away from wellbores.The concept of capillary force barriers was introduced a decade ago to resolvethis issue, in which the role of capillary pressure forces on crossflow instratified layers is modeled.
This paper tries to revisit and fine-tune the concept of capillary forcebarrier and model hysteresis expected in a moderately oil-wet system. First,some measurements of special core analysis and related interpretations arepresented in which the results are analytically formulated by a publishedmethodology to generate saturation functions consistent with hysteresis usingan assumption of wettability.
An application of the formulation to numerical reservoir simulation wascarried out in a systematic manner because the reservoir-rock-type (RRT) schemeof the model was based on primary-drainage curves that can be fully linked withthe generated saturation functions. It is demonstrated on cross sections howsmall differences in imbibition capillary pressures can affect the watermovement across contrasting RRT boundaries in a moderately oil-wet system.
The proposed formulation is an effective tool for generalizing saturationfunctions related to matrix properties in a consistent manner, and itsystematically incorporates hysteresis and wettability into the numericalreservoir-simulation model.
Many giant carbonate reservoirs in the Middle East, including those of theThamama formation in both onshore and offshore Abu Dhabi, are developed withwater-injection schemes. These reservoirs typically exhibit oil-wet character;in such cases, the injected water does not slump, instead moving through thin,high-permeability layers. This has been considered as one of the key reasonsfor unexpected early water breakthrough to oil producers. To explain thephenomenon, the concept of capillary force barriers was introduced to model therole of negative imbibition capillary pressures in the water-displacementprocess for an oil-wet system.
The concept, however, is difficult to apply to actual reservoir-simulationmodeling because of the general heterogeneity of carbonate rocks and thedifficulty in characterizing them in a systematic manner with due considerationof geological features. Meanwhile, numerous papers have described detailedmeasurements of special core analysis to emphasize the importance of some ofthe specific rock properties such as capillary pressure, relative permeability,wettability, and so on. However, the literature is sparse regarding theapplication of such measurements to field-scale reservoir-simulation modelingin an integrated manner, probably because of the data unavailability and thepoor link with geological features, which is the most important guide todistributing the petrophysical parameters in numerical reservoir-simulationmodels.
This paper develops a systematic scheme of saturation functions tied torock-matrix properties for reservoir-simulation modeling. The targets of thiswork are as follows:
• Analytical formulation of specific saturation functions, maintaining theirconsistency by linking them to pore-size distribution (PSD).
• Understanding the mechanism of capillary force barriers in theformulation.
• Incorporating wettability into reservoir simulation in a consistentmanner.
It is worth mentioning that for successful formulation of the saturationfunctions on reservoir-simulation modeling, consistent RRT schemes areessential. A concept of RRT contrast, therefore, is discussed.
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