Summary
An oil/water capillary transition zone often contains a sizable portion of a
field’s initial oil in place, especially for those carbonate reservoirs with
low matrix permeability. The field-development plan and ultimate recovery may
be influenced heavily by how much oil can be recovered from the transition
zone. This in turn depends on a number of geological and petrophysical
properties that influence the distribution of initial oil saturation
(Sor) against depth, and on the rock and fluid interactions
that control the residual oil saturation (Sor), capillary
pressure, and relative permeability characteristics as a function of initial
oil saturation.
Because of the general lack of relevant experimental data and the
insufficient physical understanding of the characteristics of the transition
zone, modeling both the static and dynamic properties of carbonate fields with
large transition zones remains an ongoing challenge. In this paper, we first
review the transition-zone definition and the current limitations in modeling
transition zones. We describe the methodology recently developed, based on
extensive experimental measurements and numerical simulation, for modeling both
static and dynamic properties in capillary transition zones. We then address
how to calculate initial-oil-saturation distribution in the carbonate fields by
reconciling log and core data and taking into account the effect of reservoir
wettability and its impact on petrophysical interpretations. The effects of
relative permeability and imbibition capillary pressure curves on oil recovery
in heterogeneous reservoirs with large transition zones are assessed. It is
shown that a proper description of relative permeability and capillary pressure
curves including hysteresis, based on experimental special-core-analysis (SCAL)
data, has a significant impact on the field-performance predictions, especially
for heterogeneous reservoirs with transition zones.
Introduction
The reservoir interval from the oil/water contact (OWC) to a level at which
water saturation reaches irreducible is referred to as the capillary transition
zone. Fig. 1 illustrates a typical capillary transition zone in a homogeneous
reservoir interval within which both the oil and water phases are mobile. The
balance of capillary and buoyancy forces controls this so-called capillary
transition zone during the primary-drainage process of oil migrating into an
initially water-filled reservoir trap.
Because the water-filled rock is originally water-wet, a certain threshold
pressure must be reached before the capillary pressure in the largest pore can
be overcome and the oil can start to enter the pore. Hence, the largest pore
throat determines the minimum capillary rise above the free-water level (FWL).
As shown schematically in Fig. 2, close to the OWC, the oil/water pressure
differential (i.e., capillary pressure) is small; therefore, only the large
pores can be filled with oil. As the distance above the OWC increases, an
increasing proportion of smaller pores are entered by oil owing to the
increasing capillary pressure with height above the FWL. The height of the
transition zone and its saturation distribution is determined by the range and
distribution of pore sizes within the rock, as well as the interfacial-force
and density difference between the two immiscible fluids.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
15 July 2005
- Meeting paper published:
21 November 2005
- Revised manuscript received:
15 November 2006
- Manuscript approved:
16 November 2006
- Version of record:
20 April 2007
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