Summary
In a probe-type formation test, because of the geometry of the wellbore and
the sealing effect of mudcake, the flow pattern is not perfectly spherical. To
account for the deviation from spherical flow, several geometric correction
factors were proposed for different analysis techniques (Steward and Wittmann
1979; Wilkinson and Hammond 1990; Dussan and Sharma 1992; Goode and
Thambynayagam 1992; Proett and Chin 1996). A geometric factor is used in
formation rate analysis (FRA) (Kasap et al. 1999), a technique used in
analyzing a probe test to estimate formation pressure and permeability. Like
other geometric correction factors, the geometric factor is a strong function
of permeability anisotropy that is generally unknown before a test. When
analyzing the test, we would logically assume an isotropic formation and use
the corresponding isotropic geometric factor. Consequently, the FRA-estimated
permeability does not represent the true spherical permeability. In contrast,
the spherical permeability can be estimated from buildup analysis without prior
knowledge of permeability anisotropy. Therefore, there is a discrepancy between
the permeability estimates from the two analysis methods. In addition, if
considered separately, neither FRA nor buildup analysis can decompose the
estimated permeability into its horizontal and vertical components.
This paper presents the derived numerical values of several geometric
factors. Using these factors, we show that the discrepancy between the
permeabilities estimated from FRA and from the conventional buildup analysis is
attributable to the permeability-anisotropy effect. A correct geometric-factor
value must be used to estimate permeability correctly. On the basis of the
permeability-anisotropy effect, we present the procedures to estimate
horizontal and vertical permeabilities by combining FRA permeability and
buildup permeability or by history matching. These procedures are verified with
a simulated probe test. Analysis of three actual tests is presented.
Introduction
A formation test is initiated when a probe from a formation tester is set
and sealed against the formation. A measured volume of fluid is withdrawn from
the formation through the probe. The test continues with a pressure buildup
when fluid withdrawal is halted. Pressure in the tool is continuously monitored
throughout the test. The flow pattern during the formation test using a probe
is believed to be close to spherical flow. Because of the geometry of the
wellbore and the sealing effect of mudcake, however, such spherical flow is
imperfect. To account for deviations from spherical flow, several correction
factors have been proposed. These correction factors include flow shape factor
(Steward and Wittmann 1979; Wilkinson and Hammond 1990; Dussan and Sharma 1992;
Goode and Thambynayagam 1992), geometric shape factor (Proett and Chin 1996),
and geometric factor (Kasap et al. 1999). Although they have different forms or
names, their physical meaning is the same. We use a general term, geometric
factor, to represent these correction factors in this paper. The physical
meaning of these factors is that because the actual relationship between
pressure and flow rate in a probe-type test cannot be described by a
spherical-flow equation, these factors are added in the spherical-flow equation
to correct for the actual flow relationship. These correction factors are a
function of permeability anisotropy, among other parameters. Similarly, the
concept of geometric factor is applied to estimate permeability with
mini-permeameters (Goggin et al. 1988; Georgi and Jones 1992; Jones 1994,
1997).
© 2006. Society of Petroleum Engineers
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History
- Original manuscript received:
16 February 2005
- Revised manuscript received:
1 August 2006
- Manuscript approved:
10 October 2006
- Version of record:
20 December 2006
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