Summary
As a common production aspect of the Thamama formation (a carbonate reservoir) in both onshore and offshore Abu Dhabi fields, unexpected early-water breakthrough through specific high-permeability layers without a clearly impermeable layer underneath has been observed in several water-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 resolve this issue, in which the role of capillary pressure forces on crossflow in stratified layers is modeled.
This paper tries to revisit and fine-tune the concept of capillary force barrier and model hysteresis expected in a moderately oil-wet system. First, some measurements of special core analysis and related interpretations are presented in which the results are analytically formulated by a published methodology to generate saturation functions consistent with hysteresis using an assumption of wettability.
An application of the formulation to numerical reservoir simulation was carried out in a systematic manner because the reservoir-rock-type (RRT) scheme of the model was based on primary-drainage curves that can be fully linked with the generated saturation functions. It is demonstrated on cross sections how small differences in imbibition capillary pressures can affect the water movement across contrasting RRT boundaries in a moderately oil-wet system.
The proposed formulation is an effective tool for generalizing saturation functions related to matrix properties in a consistent manner, and it systematically incorporates hysteresis and wettability into the numerical reservoir-simulation model.
Introduction
Many giant carbonate reservoirs in the Middle East, including those of the Thamama formation in both onshore and offshore Abu Dhabi, are developed with water-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 reasons for unexpected early water breakthrough to oil producers. To explain the phenomenon, the concept of capillary force barriers was introduced to model the role of negative imbibition capillary pressures in the water-displacement process for an oil-wet system.
The concept, however, is difficult to apply to actual reservoir-simulation modeling because of the general heterogeneity of carbonate rocks and the difficulty in characterizing them in a systematic manner with due consideration of geological features. Meanwhile, numerous papers have described detailed measurements of special core analysis to emphasize the importance of some of the specific rock properties such as capillary pressure, relative permeability, wettability, and so on. However, the literature is sparse regarding the application of such measurements to field-scale reservoir-simulation modeling in an integrated manner, probably because of the data unavailability and the poor link with geological features, which is the most important guide to distributing the petrophysical parameters in numerical reservoir-simulation models.
This paper develops a systematic scheme of saturation functions tied to rock-matrix properties for reservoir-simulation modeling. The targets of this work are as follows:• Analytical formulation of specific saturation functions, maintaining their consistency by linking them to pore-size distribution (PSD).• Understanding the mechanism of capillary force barriers in the formulation.• Incorporating wettability into reservoir simulation in a consistent manner.
It is worth mentioning that for successful formulation of the saturation functions on reservoir-simulation modeling, consistent RRT schemes are essential. A concept of RRT contrast, therefore, is discussed.
Top-Down Model Development Using Data Generated from a Complex Numerical Reservoir Simulation with Water Injection