Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1999-06-23
2003-08-05
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S371000
Reexamination Certificate
active
06603314
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to electrical resistivity logging of wellbores. More specifically, the present invention relates to logging operations to determine the electrical resistivity of earth formations from within a metallic wellbore casing.
Electrical resistivity measurements of earth formations indicate the presence of oil and gas in the earth formations. Numerous devices are known in the art for measuring earth formation resistivity. Known devices such as sondes and other logging tools for measuring resistivity are typically lowered by one end of an armored electrical cable into the wellbore. Equipment at the wellbore surface is electrically connected to the other end of the cable, and electrical signals corresponding to formation resistivity are transmitted from the logging tool to surface equipment.
Logging tools originally required an “open hole” wellbore that does not have a steel pipe such as tubing or casing in the wellbore. Casing is inserted into wellbores to maintain the mechanical and hydraulic integrity of the wellbore. However, formation resistivity logging tools are adversely affected by the presence of wellbore casing since the casing resistivity can be smaller than 10
−7
to 10
−10
times the earth formation resistivity. The large resistivity contrast between casing and formation inhibits measurements made by the typical resistivity measuring devices.
Many efforts have been proposed to determine formation resistivity through a casing. A representative system for measuring resistivity in a cased wellbore was described in U.S. Pat. No. 5,075,626 issued to Vail (1991). A logging sonde having multiple electrodes was lowered by one end of an armored cable into the wellbore so that sonde electrodes were activated to make electrical contact with the casing. An electric current source was connected to one of the tool electrodes and to a current return electrode at the wellbore surface. The source comprised a low-frequency alternating current source having a frequency less than 10 Hz. Electrical current was injected into the casing and traveled both upwardly and downwardly through the casing. Current “leaks” outwardly into the earth formations decreased the voltage measured along the casing as a function of the distance from the electrode. By measuring the current leakage (⊕I) within a particular interval, the resistivity of the earth formation contacting the casing could be determined. If V
o
represents the voltage on the casing with respect to infinity, then the resistivity of the formation near the wellbore within the axial boundaries of the measured interval is calculable by the expression: V
o
&Dgr;I. Apparent resistivity within the interval was defined by the expression:
ρ
a
=
k
·
Δ
z
·
V
o
Δ
I
(
1
)
wherein k is a dimensionless constant providing equality of apparent resistivity to the formation resistivity, assuming that the casing and formation are each homogeneous, and &Dgr;Z represents the interval length along the casing.
To determine resistivity of the formation through the casing, the characteristic impedance (Q) was determined by energizing an emitter electrode and by connecting the other terminal to a surface electrode. In addition to voltage drops measured by first, second and third voltage measuring circuits, voltage drop was measured by a fourth voltage measuring circuit between a surface potential electrode and a voltage sensing electrode within the sonde. The characteristic impedance was calculated from the voltage V
o
measured by a fourth measuring circuit according to the formula:
Q
=
V
o
I
o
(
2
)
wherein I
o
represents the amount of current imparted by the source. The resistance of the particular casing section located between electrode pairs was determined, and the current from the source was returned to a current return electrode on the sonde rather than to the surface electrode. Substantially all of the electrical current flowed along the casing between the emitter electrode and the current return electrode on the sonde. The current flow in such electrical configuration is referred to by I
n
, and the current amount leaking from the casing in such electrical configuration is negligible.
The first voltage measuring circuit measured a voltage drop, represented by V′
1
, between an electrode pair for detecting the casing resistance between the electrodes. Similarly, the second voltage measuring circuit measured a voltage drop, V′
2
, between another electrode pair. The resistances of the casing between such electrode pair, were determined by the expression:
R
1
=
V
1
′
I
n
;
R
2
=
V
2
′
I
n
(
3
)
A third voltage measuring circuit determined a second difference referred to as &Dgr;V between voltage measurements made by the first and second measuring circuits. The current source was then reconnected to return the current at the surface electrode, and the current flow from the source in such electrical configuration was referred to as I
m
. Voltage drop V
1
was again measured by the first measuring circuit between a first electrode pair. Voltage drop V
2
was also again measured by the second measuring circuit between the second electrode pair. Another second difference, referred to as &Dgr;V′, was also measured by the third measuring circuit. The average current flowing along the casing between the first electrode pair was related to V
1
/R
1
, and the average current flowing along the casing between the second electrode pair was related to V2/R
2
. The average current flowing between the first electrode pair was slightly different from the average current flowing between the second electrode pair because of current leaks out of the casing into the formation. The amount of leakage current, &Dgr;I, was determined according to the expression:
Δ
I
=
V
1
R
1
-
V
2
R
2
(
4
)
The voltage present on the casing, with respect to infinity, was determined as Q·I
m
. By substitution of Equations (3) and (4) into equation (1), the apparent resistivity of the formation was determined by the expression:
ρ
a
=
K
·
Q
·
I
m
I
n
·
[
V
1
V
1
′
-
V
2
V
2
′
]
-
1
(
5
)
where K is a constant of proportionality, called a “tool factor”, related by the expression:
K
=k·&Dgr;z (6)
&Dgr;z is equal to the spacing between the first electrode pair.
The difference of the current flow between the first electrode pair and the current flow between second electrode pair is very small as previously explained. Substitution of the second difference measurements into Equation 5 results in the following expressions for apparent resistivity of the formation 6:
ρ
a
=
K
·
Q
·
A
·
[
Δ
V
V
1
-
&AutoLeftMatch;
Δ
V
&AutoRightMatch;
′
V
1
′
]
-
1
(
7
)
where A in Equation (7) is equal to:
A
=
V
1
′
/
I
n
V
1
/
I
m
[
1
-
&AutoLeftMatch;
Δ
V
&AutoRightMatch;
′
V
1
′
]
(
8
)
As described by U.S. Pat. No. 5,075,626 the combination of these three electrical configurations provide information to determine resistivity of the formation measured from inside a conductive casing. However, this approach is significantly limited because the three measurements are performed sequentially. Due to the very small signals and low frequency, up to several minutes measurement time can be required per measurement. The measurements require two separate surface electrodes to be installed several hundred feet away from the well head, and require at least three sequential, extremely accurate measurements. The cumulative time necessary to survey each wellbore section disrupts other operations and limits overall well productivity.
Other systems have been developed for determining formation resistivity through casing. In U.S. Pat. No. 4,837,518 to Gard et al. (1989), a low frequency bipolar voltage was applied to casing and a
Kostelnicek Richard J.
Maurer Hans-Martin
Baker Hughes Incorporated
Madan Mossman & Sriram P.C.
Patidar Jay
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