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.
© 2005. Society of Petroleum Engineers
View full textPDF
(
2,448 KB
)
History
- Original manuscript received:
7 December 2004
- Revised manuscript received:
27 June 2005
- Manuscript approved:
4 July 2005
- Version of record:
15 October 2005
View Cart
