Electricity: measuring and testing – Of geophysical surface or subsurface in situ – Using electrode arrays – circuits – structure – or supports
Reexamination Certificate
1999-11-18
2002-04-09
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
Using electrode arrays, circuits, structure, or supports
C324S324000
Reexamination Certificate
active
06369575
ABSTRACT:
BACKGROUND OF THE INVENTION
Technical Field and Prior Art
The present invention relates to the field of measurement tools, e.g., suitable for use in equipment for oil prospecting and production.
More specifically, after a well has been bored, that type of activity requires sondes or sensors, in particular electrical or electromagnetic sondes or sensors to be inserted into the hole to enable measurements to be performed serving to characterize, amongst others, which fluids are present in the terrain and layers around the borehole, and also the dip of said layers. The term “logging” is used to designate any continuous recording as a function of depth of variations in a given characteristic of the formations around a borehole.
One of the characteristics that it is important to know in a borehole is the resistivity of the drilling mud used. The resistivity of the mud is a parameter that is used, in particular, to correct measurements relating to other characteristics of the surrounding formations. In order to discover this mud resistivity, various approaches are already known.
In a first approach, mud resistivity is measured by a device that requires additional equipment on the tool already used for measuring the characteristics of the formation, which additional equipment may be, for example, of the AMS type (described in document EP013 224). That technique gives rise to additional costs and to apparatus that is of greater bulk.
In another technique, the resistivity of the mud is measured at the surface from a fluid sample. Extrapolation then makes it possible to take account of temperature dependence relative to downhole conditions by measuring the temperature down hole. The accuracy obtained is often unsatisfactory, essentially for the following two reasons:
difficultly in obtaining an accurate measurement of the temperature downhole; and
the characteristics of the fluid in the borehole can change with depth, in which case the sample available on the surface is no longer representative.
SUMMARY OF THE INVENTION
An object of the invention is to provide a novel method and novel apparatus enabling a measurement to be obtained of the resistivity of the mud in a borehole, without requiring additional specific apparatus to be implemented, but capable of making use of electrode structures that already exist. In addition, the new method and the new apparatus must be capable of measuring the resistivity of the mud in situ, without it being necessary to take samples for subsequent analysis on the surface. Finally, it is desirable to find a method and an apparatus that enable measurements to be made on the mud without requiring any prior measurement of the azimuth resistivity of the surrounding formations, and which is relatively insensitive to the influence of the diameter of the borehole.
In a first aspect of the invention, the invention provides a method of measuring the resistivity R
m
of a drilling mud inside a borehole passing through a terrestrial formation, the method comprising:
inserting a sonde into the borehole, the sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
emitting at least one current I
0
into the surrounding formation from the annular current electrode;
focusing the current I
0
in the formation by emitting two currents I
1
and I′
1
from the annular guard electrodes situated on either side of the annular current electrode; and
producing a signal in response to the emitted current I
0
, which signal is representative of the resistivity R
m
of the drilling mud.
This method is a method of measuring the resistivity of the mud in situ. It does not require any prior knowledge of the azimuth resistivity of the surrounding formations. In addition, it is relatively insensitive to the effects due to variations in the dimensions of the borehole, particularly when the borehole diameter is relatively large. Finally, it should be observed that mud resistivity is measured by emitting current into the surrounding formation, and not by emitting surface current into the mud.
A signal may be produced that is representative of a voltage induced through the borehole mud by the current I
0
circulating through said mud and the formation.
The sonde may include a single annular current electrode, first and second pairs of annular electrodes referred to as electrodes for measuring voltage in the borehole mud, each pair being disposed on either side of the annular current electrode, the resistivity R
m
being deduced from the ratio (V
1
−V
3
)/I
0
in which V
1
and V
3
are the mean potentials of the two pairs of electrodes for measuring voltage in the drilling mud.
In another embodiment, the sonde may include:
two annular current electrodes respectively emitting a current I
0
and a current I′
0
into the surrounding formation; and
an annular potential-measuring electrode situated between the two current electrodes or else an array of azimuth electrodes situated between the two annular current electrodes.
This embodiment is particularly well adapted to enabling the method to be implemented using electrode structures that already exist.
The invention also provides an apparatus for measuring the resistivity of drilling mud in a borehole passing through a terrestrial formation, the apparatus comprising:
a sonde having an elongate body provided with at least one annular current electrode and at least two annular guard electrodes situated on either side of the annular current electrode;
means for emitting at least one current I
0
into the surrounding formation from the annular current electrode;
means for focusing the current I
0
in the formation by emitting two currents I
1
and I′1 from the two annular guard electrodes situated on either side of the annular current electrode; and
means for producing a signal in response to the emission of the current I
0
, said signal being representative of the resistivity R
m
of the drilling mud.
This apparatus is associated with the same advantages as those specified above with reference to the first method of measurement of the invention: it enables measurements to be performed in situ, and it does not require prior knowledge of azimuth resistivities.
The apparatus may include means for producing a signal representative of a voltage induced through a drilling mud by the current I
0
, because of the current flowing through the mud and through the formation.
Thus, the sonde may include a single annular current electrode, first and second pairs of annular electrodes for measuring voltage in the drilling mud, each pair being disposed on either side of the annular current electrode, the means for producing a signal representative of the resistivity R
m
enabling R
m
to be deduced from the ratio (V
1
−V
3
)/I
0
, where V
1
and V
3
are the mean potentials of the two pairs of electrodes for measuring voltage in the drilling mud.
In another aspect, the same apparatus may be such that the sonde includes:
two annular current electrodes;
means for emitting into a surrounding formation a current I
0
via one of the annular electrodes, and a current I′
0
via the other annular electrode;
an annular electrode for measuring potential, situated between the two current electrodes, or else an array of azimuth electrodes situated between the two annular current electrodes.
The tools of the prior art, and those described above, require the current I
0
or the current I
0
and I′
0
as emitted from the annular current electrode(s) into the terrestrial formation to be focused. Means must therefore be implemented for providing such focusing. In general, this requires a feedback loop to enable the focusing current(s) to be adjusted as a function, for example, of a signal representative of a focusing potential. In theory this implies amplification with infinite gain, but in practice gain must be limited in order to ensure stability. In particular, when using focusing potential measurement electrod
Eisenmann Pierre
Faivre Ollivier
Smits Jan W.
Trouiller Jean Claude
Batzer William B.
Schlumberger Technology Corporation
Snow Walter E.
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