Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1998-08-11
2001-04-03
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
Reexamination Certificate
active
06211679
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to wellbore resistivity measurements. More particularly, the present invention relates to correcting erroneous downhole resistivity measurements.
2. Description of the Related Art
Modem petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions down hole. Such information typically includes characteristics of the earth formations traversed by the wellbore, in addition to data relating to the size and configuration of the bore hole itself. The collection of information relating to conditions down hole commonly is referred to as “logging.” Logging has been known in the industry for many years as a technique for providing information regarding the particular earth formation being drilled and can be performed by several methods. In conventional oil well wire line logging, a probe or “sonde” is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. A wireline sonde may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. The sonde typically is constructed as a hermetically sealed cylinder for housing the sensors, which hangs at the end of a long cable or “wireline.” The cable or wireline provides mechanical support to the sonde and also provides an electrical connection between the sensors and associated instrumentation within the sonde, and electrical equipment located at the surface of the well. Normally, the cable supplies operating power to the sonde and is used as an electrical conductor to transmit information signals from the sonde to the surface and to control signals from the surface to the sonde. In accordance with conventional techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole, as the sonde is pulled uphole.
One concern for every downhole tool is the accuracy of its measurements. For example, in the prior art, real world constraints have limited the accuracy, and hence the reliability, of downhole resistivity tools. Referring now to
FIG. 1
, a wellbore
100
in formation
105
surrounds downhole current supply electrodes on resistivity tool
110
. Formation
105
may contain high resistivity portion
150
and low resistivity portion
155
. Also shown are return B-electrode
120
, reference N-electrode
125
, and comparator
130
. Tool
110
provides electrical current
140
to formation
105
. Current
140
flows to return B-electrode
120
. Comparator
130
, attached to tool
110
and N-electrode
125
, measures the potential drop between the tool
110
and the N-electrode
125
. The resistivity of the formation
105
may then be calculated based upon this measured voltage differential at comparator
130
.
Nonetheless, a calculated resistivity based upon the assembly of
FIG. 1
may be inaccurate, particularly when it occupies a formation with low and high resistivity strata. More particularly, measurements between resistivity tool
110
and the N-electrode
125
should ideally approximate the measurements between a resistivity tool
110
and infinity. However, when the N-electrode
125
and B-electrode
120
are spaced relatively near to one another, they interact and affect the voltage measurement at logging device
110
. This interaction is particularly pronounced when the formation
150
surrounding the N-electrode
125
and B-electrode
120
has a high resistivity, whereas the formation
155
surrounding logging tool
110
has a low resistivity. The problem under these conditions is that the measured survey voltage from the tool is relatively low. However, there is a very high potential drop to the B-electrode from infinity due to its location in a high resistivity bed. Because the N-electrode is also surrounded by the high resistivity bed, the potential at the N-electrode approaches the potential at the B-electrode and thus a highly erroneous tool reading results. This effect often occurs in the Delaware basin in West Texas and as such is known as the Delaware effect. A similar phenomenon is called the Groningen effect so named after the Groningen formation in Holland.
One attempt to solve this problem in the prior art involved placing the B-electrode
120
at the surface (not shown). By placing the B-electrode
120
at the surface, it was thought that resistivity measurement problems would be solved because the B-electrode
120
would not be proximate to the very high resistive bed surrounding the N-electrode
125
. However, this solution was not as effective as had been hoped, with substantial measurement error still present. Schlumberger attempted to correct these errors in their ARI-type laterolog tools. Such corrections are complicated, and are based on mathematical modeling. The correction factors often are dependent upon knowledge that is not known “a priori.” For example, the bore hole diameter, and formation and mud resistivity upon the tool (which also must be measured as they are not known beforehand). Further, a system placing the B-electrode at the surface is complicated because placement of the B-electrode
120
on the surface requires control of the current supply at the surface.
An alternate and more successful approach to solving the Delaware effect problem was placement of the N-electrode
125
on the surface in a mud pit or some other location that gives a good electrical connection to the ground. This approach also separates the B-electrode from the N-electrode, and thus was expected to improve the downhole measurements of resistivity. It was also thought that such an approach would not require much of the complexity involved when placing the B-electrode on the surface. While this solution yields improved results over placing the B-electrode
120
on the surface, it still has certain drawbacks. These problems arise from the conductive cable armor that extends downhole and that supports and connects the down hole resistivity tool
110
and B-electrode
120
to the surface. Thus, interaction still occurs between the B-electrode
120
and N-electrode
125
.
These and other problems exist in the prior art, and thus there is a need for a device or method to solve these problems.
SUMMARY OF THE INVENTION
The present invention features a method and apparatus to correct for severe resistivity measurement errors present in prior wellbore resistivity tools. In particular, one embodiment of the present invention features a downhole laterolog array and return electrode, in addition to two measurement amplifiers. The measurement amplifiers detect the voltage potential at the laterolog array with respect to a reference voltage, and the voltage potential at a point on a cable armor to which this embodiment is attached. These measurements can be used to determine more accurately the downhole resistivity based on a disclosed resistivity formula.
Another embodiment of the invention comprises a system including a downhole resistivity tool and a processor to compute the resistivity of a formation.
Another embodiment of the invention is a method for operating the disclosed wellbore resistivity tool and determining each tool's individual characteristic. This characteristic may then be used to estimate more accurately the resistivity of a downhole formation.
Thus, the present invention comprises a combination of features and advantages which enable it to overcome various problems of prior devices. The various characteristics described above will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention, and by referring to the accompanying drawings. Additional features and advantages will be set forth
Moore Robert A.
Zea Horacio A.
Conley & Rose & Tayon P.C.
Halliburton Energy Service,s Inc.
Patidar Jay
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