An aspect of the invention relates to an apparatus used for the electrical investigation of a borehole penetrating geological formations. The apparatus and method enables lateral measurement of the resistivity of the geological formations surrounding the borehole. Another aspect of the invention relates to a method used for the electrical investigation of a borehole penetrating geological formations. The invention finds a particular application in the oilfield industry.
The electronic module 3 or electronic and software arrangement PA of the surface unit SU may derive an indication of the conductivity of the geological formations as being proportional to:
(R1×M02+R2×M01)/M21,
where:
R1 designates the first electrical signal measured when the transmitter T1 is energized,
R2 designates the second electrical signal measured when the transmitter T2 is energized,
M02 designates the axial current measured by sensor M0 when transmitter T2 is energized,
M01 designates the axial current measured by sensor M0 when transmitter T1 is energized, and
M21 designates the axial current measured by sensor M2 when transmitter T1 is energized.
With the hereinbefore configurations, the hereinbefore formula gives accurate results with the lateral current sensor R or R3 positioned closely to the axial current sensor M0, but less accurate results with the lateral current sensor R1 or R2. Thus, it is necessary that each lateral current sensor is positioned closely to an axial current sensor when an apparatus is used to measure the geological formations at a different radial depth of investigation.
Thus, the prior art apparatus and method have difficulty in precisely focusing the survey current in a selected zone of the geological formations. The prior art apparatuses and methods are complex because each axial current sensor must be associated with a close lateral current sensor for measuring the resistivity at different radial depth of investigation with sufficient accuracy. Otherwise, the calculation of the resistivity results in a lack of accuracy. Further, it may not be mechanically or economically possible to position an axial current sensor closely to each lateral current sensor, particularly in configuration where there are various lateral sensors at different axial position.
It is an object of the invention to propose an apparatus and a method that overcomes at least one of the drawbacks of the prior art apparatus and method.
According to a first aspect, the invention relates to an apparatus used in electrical investigation of geological formations surrounding a borehole, comprising:
The apparatus further comprises:
The at least one lateral current sensor may be formed by the first and second axial current sensors and may determine a lateral current based on a difference of the first axial current measured by the first axial current sensor and the second axial current measured by the second axial current sensor.
One of the axial current sensors may be positioned adjacent to the transmitter. A common antenna may selectively form an axial current sensor or a transmitter. At least one of the axial current sensors may be positioned adjacent to a lateral current sensor.
The transmitter may be a toroidal antenna or an electrode.
The axial current sensor may be a toroidal antenna.
The lateral current sensor may be a ring electrode or a button electrode.
According to a further aspect, the apparatus used in electrical investigation of geological formations surrounding a borehole may comprise:
According to another aspect, the invention relates to a method of electrical investigation of geological formations surrounding a borehole, comprising the steps of:
The method further comprises the steps of:
The step of calculating a lateral current may be based on a difference of the first axial current measured by the first axial current sensor and the second axial current measured by the second axial current sensor.
According to still a further aspect, the invention relates to a method of electrical investigation of geological formations surrounding a borehole, comprising the steps of:
The virtual axial current sensor of the invention provides improved focusing for the lateral current sensor. Thus, the invention enables focusing the resistivity measurements to a well defined selected zone of the geological formation than prior art apparatus and method. Consequently, with the invention, the vertical resolution is improved and the shoulder bed effect is reduced while a satisfactory radial depth of investigation is maintained. The corresponding resistivity can be calculated with a greater accuracy than prior art apparatus and method.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.
The present invention is illustrated by way of example and not limited to the accompanying figures, in which like references indicate similar elements:
In the following description, the terminology “radial depth of investigation” defines a dimension around the borehole along the circumference whatever the orientation of the borehole, namely horizontal, vertical or inclined.
Further, the terminology “electronic module” defines an entity made of electronic circuit, software or a combination of both that can performed a plurality of functions that is known by those versed in the art. For example, the electronic module may comprise a processing module for calculation purpose, a power amplifier module for energizing the transmitters, a control module for switching the function of the antenna from transmitter to sensor and vice-versa, a filtering module, a AND and D/A module, a memory for storing untreated measurements or calculation results, etc. . . .
Furthermore, in the following the indication of the conductivity is indicated as being equivalent to the inversed resistivity and proportional to the current. However, the skilled person knows that this is correct in the direct current case, while this is an approximation in the alternative current case because of the existence of a skin effect correction in particular in the high conductivity range. The skin effect correction is neglected in the following description.
The first transmitter T1 can induce a first current that travels from the first transmitter position in a path that includes a first portion of the body and the selected zone of the geological formations. The second transmitter T2 can induce a second current that travels from the second transmitter position in a path that includes a second portion of the body and the selected zone of the geological formations.
The first M0, second M1 and third M2 axial current sensor measures the axial current flowing along the body at the first, second and third axial current sensor position, respectively.
Each of the first R1, second R2 and third R3 lateral current sensor measures a first electrical signal resulting from the first current and a second electrical signal resulting from the second current induced by the transmitter. Each lateral current sensor being positioned at a different distance from the transmitter, it measures the electrical properties of the selected zone at a different radial depth relatively to the borehole axis.
The electronic module 103 derives an indication of the resistivity and/or conductivity of the formations based on said measured electrical signals and currents.
According to the invention, a virtual axial current sensor is provided. The virtual axial current sensor provides a virtual axial current measurement by interpolating or extrapolating two axial current measurements made at different locations which are not adjacent to the lateral current sensor. More precisely, the lateral current sensor R2 is focused with a virtual axial current sensor derived by interpolating the axial current measured by the first M0 and second M1 axial current sensor.
In the example of
(R21×VC2+R22×VC1)/M21,
where:
R21 designates the first electrical signal (current measured by lateral current sensor R2 when the first transmitter T1 is energized),
R22 designates the second electrical signal (current measured by lateral current sensor R2 when the second transmitter T2 is energized),
VC1 and VC2 designates the first and second virtual current, respectively,
M02 designates the axial current measured by axial current sensor M0 when transmitter T2 is energized,
M12 designates the axial current measured by axial current sensor M1 when transmitter T2 is energized,
M01 designates the axial current measured by axial current sensor M0 when transmitter T1 is energized,
M11 designates the axial current measured by axial current sensor M1 when transmitter T1 is energized, and
M21 designates the axial current measured by axial current sensor M2 when transmitter T1 is energized.
The above formula can be generalized such that an indication of the conductivity (or inversed resistivity) of the geological formations is approximately proportional to:
where:
a designates the distance between the lateral current sensor R2 and the axial current sensor M1, and
b designates the distance between the lateral current sensor R2 and the first axial current sensor M0.
In the particular example of
(R11×VC2′+R12×VC1′)/M21,
which is equal to:
[R11×(0.9×M02+0.1×M12)+R12×(0.9×M01+0.1×M11)]/M21
where:
R11 designates the first electrical signal (current measured by lateral current sensor R1 when the first transmitter T1 is energized), and
R12 designates the first electrical signal (current measured by lateral current sensor R1 when the first transmitter T1 is energized).
Similarly to the first embodiment, the first T1 and second T2 transmitter can induce a first and a second current, respectively, that travels from the first and second transmitter position, respectively, in a path that includes a first and second portion of the body and the selected zone of the geological formations, respectively.
The first M0, second M1 and third M2 axial current sensor measures the axial current flowing along the body at the first, second and third axial current sensor position, respectively. The first M0 and second M1 axial current sensors are positioned between the first T1 and second T2 transmitters. The third axial current sensor M2 is positioned dosed to the second transmitter T2.
The electronic module 103 derives an indication of the resistivity and/or conductivity of the formations based on said measured electrical signals and currents.
In the example of
(M12×M01−M02×M11)/M21.
Similarly to the first embodiment, the first transmitter T1 and the common antenna used either as transmitter T2 can induce a first and a second current, respectively, that travels from the first and second transmitter position, respectively, in a path that includes a first and second portion of the body and the selected zone of the geological formations, respectively.
The first M0 and second M1 axial current sensors and the common antenna used as a third axial current sensor M2 measures the axial current flowing along the body at the first, second and third axial current sensor position, respectively. The first M0 and second M1 axial current sensors are positioned between the first T1 and second T2 transmitters. The position of the third axial current sensor M2 is identical to the position of the second transmitter T2.
In this embodiment, the same toroidal antenna is alternatively a transmitter T2 and an axial current sensor M2 when the first transmitter T1 is energized. For example, the antenna is automatically switched from one function to the other by a control and switch circuit (not shown) of the electronic module 103.
The electronic module 103 derives an indication of the resistivity and/or conductivity of the formations based on said measured electrical signals and currents.
In the example of
The lateral current sensor E1 is a current transformer recessed in the body 102. The lateral current sensor E2 is an electrode insulated from the body 102. The lateral current sensor E3 is a button electrode, i.e an array of current measuring electrodes and voltage sensing electrodes (such a button electrode is described in details in U.S. Pat. No. 6,373,254). Advantageously, all these lateral current sensors have an azimuthal sensitivity.
The lateral current measurements made by the lateral current sensor E1 can be focused with the virtual axial current sensor derived from interpolating the measurements of the first M0 and second M1 axial current sensors.
The lateral current measurement made by the lateral current sensor E2 or E3 can be focused with the virtual axial current sensor derived from extrapolating the measurements of the first M0 and second M1 axial current sensors.
The electronic module 103 derives an indication of the conductivity (or inversed resistivity) of the geological formations in a way similar to the one described in relation with
In this embodiment all the common antennas can be used alternatively as a transmitter and as an axial current sensor. Each common antenna when acting as a transmitter T1, T2, T3, T4, T5 can induce a current that travels from the transmitter position in a path that includes a portion of the body and the selected zone of the geological formations. The common antennas are toroidal antenna.
Each common antenna when acting as an axial current sensor M1, M2, M3, M4, M5 measures the axial current flowing along the body at the axial current sensor position. As an example, the common antenna may be positioned all along the body 102 with each common antenna at an equal distance from a directly adjacent common antenna. As an example, the lateral current sensor B may be positioned between the first common antenna T1, M1 and the second common antenna T2, M2. The lateral current sensor B may be a button electrode which is described in details in U.S. Pat. No. 6,373,254.
The five common antennas which are alternatively used as transmitter and as axial current sensor enables obtaining focused measurements at four different radial depths of investigation from the single lateral current sensor B. More precisely, in turn, each common antenna is used as a transmitter, while the four other common antennas can be used as axial current sensors. Alternatively, time multiplexing and/or frequency multiplexing on subsets of the five common antennas can be implemented.
The automatic switching of the common antenna from one function to the other, or the time multiplexing and/or frequency multiplexing may be implemented by a control and switch module (not shown) of the electronic module 103. Such an electronic module is known in the art and will not be further described.
The lateral current measurements made by the lateral current sensor B can be focused with a virtual axial current sensor. The virtual axial current sensor is derived from interpolating the measurements of two common antennas, both antennas being operated as axial current sensors.
With the fourth embodiment of
With increasing radial depth of investigation, a first focused conductivity measurement CB3 or CM3 can be determined by energizing the third T3 and fourth T4 transmitters, and a second focused conductivity measurement CB4 or CM4 can be determined by energizing the fourth T4 and fifth T5 transmitters.
As an example related to the second measurement CB4 or CM4, the electronic module 103 derives an indication of the conductivity (or inversed resistivity) of the geological formations as being approximately proportional to:
or with the lateral current sensor comprising the space between the axial current sensors M1 and M2:
CM4=(M24×M15−M14×M25)/M54
where:
B4 designates the current measured by lateral current sensor B when the fourth transmitter T4 is energized,
B5 designates the current measured by lateral current sensor B when the fifth transmitter T5 is energized,
b designates the distance between the lateral current sensor B and the first common antenna T1, M1,
a designates the distance between the lateral current sensor B and the second common antenna T2, M2,
M15 designates the axial current measured by axial current sensor M1 when transmitter T5 is energized,
M25 designates the axial current measured by axial current sensor M2 when transmitter T5 is energized,
M14 designates the axial current measured by axial current sensor M1 when transmitter T4 is energized,
M24 designates the axial current measured by axial current sensor M2 when transmitter T4 is energized, and
M54 designates the axial current measured by axial current sensor M5 when transmitter T4 is energized.
Similar formulae can be determined for the third measurements CB3 or CM3.
In the general case using as transmitters the antenna Ti (i>2) and the common antenna Tj, Mj (j>i), the electronic module 103 derives an indication of the conductivity (or inversed resistivity) of the geological formations as being approximately proportional to:
or with the lateral current sensor comprising the space between the axial current sensors M1 and M2:
CMi=|M2i×M1j−M1i×M2|/Mji
where:
Bi designates the current measured by lateral current sensor B when the transmitter Ti is energized,
Bj designates the current measured by lateral current sensor B when the transmitter Tj is energized,
b designates the distance between the lateral current sensor B and the first common antenna T1, M1,
a designates the distance between the lateral current sensor B and the second common antenna T2, M2,
M1j designates the axial current measured by axial current sensor M1 when transmitter Tj is energized,
M2j designates the axial current measured by axial current sensor M2 when transmitter Tj is energized,
M1i designates the axial current measured by axial current sensor M1 when transmitter T1 is energized,
M2i designates the axial current measured by axial current sensor M2 when transmitter T1 is energized, and
Mji designates the axial current measured by axial current sensor Mj when transmitter T1 is energized.
In the fourth embodiment, at least four antennas may be required, namely one transmitting antenna Ti, two receiving antennas M1, M2, and at least one common antenna Tj, Mj. Advantageously, the antennas Ti, M1 and M2 may also be common antenna in order to enable others measurements at a different radial depth of investigation.
In the above general case presented hereinbefore, it will be apparent to those versed in the art that, by reciprocity, the transmitters and current sensors can be inverted without departing from the scope of the present invention. In particular, a reciprocal sensor arrangement can be designed by replacing the antennas Ti, M1, M2 and (Tj, Mj) by the antennas Mi, T1, T2, and (Mj, Tj), respectively. In this case, T1, T2, Tj are transmitters, and M1 and M2 are axial current sensors.
The above formula becomes:
CMi=|Mi2×Mj1−Mi1×Mj2|/Mij
where:
Mi2 designates the axial current measured by axial current sensor Mi when transmitter T2 is energized,
Mj2 designates the axial current measured by axial current sensor Mj when transmitter T2 is energized,
Mi1 designates the axial current measured by axial current sensor Mi when transmitter T1 is energized,
Mj1 designates the axial current measured by axial current sensor Mj when transmitter T1 is energized, and
Mij designates the axial current measured by axial current sensor Mi when transmitter Tj is energized. Thus, the invention is an improvement over the prior art because in the prior art, the difference of two large numbers (M2i−M1i) is considered. The difference of two large numbers is subject to a large error if either one of the two current sensors has an incorrect gain or scale factor. In contradistinction, with the invention, if one of the sensors has an incorrect gain or scale factor, the same error in percentage is made on both terms of the subtraction. As a consequence, the relative error on the focused measurement is not amplified.
Final Remarks
It will be apparent for a person skilled in the art that the invention is applicable to onshore and offshore hydrocarbon well locations.
Further, those skilled in the art understand that the invention is not limited to vertical borehole as depicted in the drawings: the invention is also applicable to inclined borehole or horizontal borehole.
Furthermore, it will also be apparent to those skilled in the art that the calculation of the conductivity or resistivity according to the invention can be performed elsewhere than in an electronics module within the instrument; for example, the calculation can be performed at the surface.
Finally, it is also apparent for a person skilled in the art that application of the invention is not limited to the oilfield industry as the invention can also be applied in others types of geological surveys.
The drawings and their description hereinbefore illustrate rather than limit the invention.
Any reference sign in a claim should not be construed as limiting the claim. The word “comprising” does not exclude the presence of other elements than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such element.
Number | Date | Country | Kind |
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07290316.4 | Mar 2007 | EP | regional |