Device for transmitting to the surface the signal from a transmitter located at a great depth

A device for transmitting signals from a transmitter at the bottom of a drill well to the earth's surface including a drill collar which extends in an axial direction, a transmitter disposed in the drill collar, a cylindrical metal sleeve from forming part of an antenna for electromagnetic wave transmission, the cylindrical metal sleeve being disposed around the drill collar and connected electrically to the transmitter, the cylindrical metal sleeve having a diameter greater than the diameter of the drill collar and having an axial length shorter than the axial length of the drill collar, and an insulating sheath disposed between the cylindrical metal sleeve and the drill collar, the insulating sheath having an axial length shorter than the axial length of the drill collar.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for transmitting to the surface 
of the earth signals from a transmitter located at a great depth below the 
earth's surface. 
There is often a problem in practice, especially in drilling carried out in 
search of petroleum deposits, in transmitting to the site control cabin at 
the earth's surface information supplied by sensors located at the bottom 
of the drilling well and relating to the pressure at the bottom, the 
density of the mud, temperature or other useful parameters. 
PRIOR ART 
To solve this problem, there has already been a proposal to arrange in the 
drill collar of the drill-pipe string, that is to say in the collar 
supporting the drill bit, a transmitter connected to the sensors and 
intended for transmitting to the earth's surface the information gathered 
by these sensors. 
In some devices which are already known, the transmitting antenna includes 
a metal element formed by a portion of an actual drill collar, insulated 
from the latter and connected to it by a mechanical connection element 
made of insulating material. 
Thus, for example, in a thesis submitted to the University of Lille in 1969 
by Mr. Clarisse, it was proposed to divide the drill-pipe string, at the 
level of the last drill collar, into two portions which are separated by a 
bridge made of insulating material and between which an alternating 
electrical potential difference of specific frequency is established; the 
lower part of the drill collar then forming part of an antenna, and the 
upper portion, connected to the upper drill-pipes by means of screwing, 
constitutes an axis which radiates and guides toward the surface of the 
ground an electromagnetic wave capable of remote transmission of signals 
supplied by telemetering sensors located at the bottom of the drilling 
well. 
A system of this type is also found in the article entitled 
"Second-generation MWD tool" published on Feb. 21, 1983 in the journal 
entitled: Oil & Gas Journal, especially in the last paragraph on page 86 
of this publication. 
The serious disadvantage of this system is that, to make an insulating 
connection between the two metal portions of the drill collar, it is 
necessary to use a hoop or insulating bridge which reduces the mechanical 
strength of the drill collar which, as is known, is subjected to 
considerable torsional and shearing forces. 
Moreover, such a system is absolutely useless when it is necessary to 
transmit information from a drill collar immersed in an oil-drilling mud 
which is essentially insulating per se. 
SUMMARY OF THE INVENTION 
The present invention overcomes these disadvantages, and at the same time 
also makes it possible to supply information from the bottom of the 
drilling well by means for radiating electromagnetic waves of a special 
type carried by the drill collar, this being achieved without causing any 
mechanical weakening of the latter, while allowing the signals to be 
transmitted from the bottom of the drilling well, even when the drill 
collar is immersed in an oil mud which is insulating per se. 
Another subject of the invention is the production of an antenna which also 
makes it possible to measure the resistivity of the terrain at the bottom 
of the well. 
According to the invention, the device for transmitting signals to the 
earth's surface includes a cylindrical metal sleeve for forming part of an 
antenna for electromagnetic wave transmission connected electrically to 
the transmitter arranged in a known way within the drill collar together 
with the sensors supplying it with the parameters to be transmitted, 
comprising a cylindrical metal sleeve of greater diameter and shorter 
length than the drill collar and which is strung coaxially on the latter, 
being separated from it by an insulating sheathing covering the drill 
collar up to a certain distance from its ends. 
According to a characteristic of the invention, the insulating sheathing 
which covers; the drill collar is longer than the cylindrical sleeve 
forming part of the transmitting antenna. 
According to another characteristic of the invention, the cylindrical 
sleeve is subdivided on its side adjacent to the bit, to form in the lower 
part of the drill collar a metal ring of small height which is connected 
electrically to the transmitter only when the latter is itself decoupled 
electrically from the other part of the sleeve. 
This ring of small height makes it possible to measure the resistivity of 
the terrain substantially at the bottom of the well, whereas such a 
measurement has been practically impossible hitherto. 
In the first embodiment mentioned above, there may be the disadvantage 
that, as a result of the wear which the cylindrical sleeve undergoes in 
the well because of the circulation of fragments of rock and friction on 
the walls of the said well, the cylindrical sleeve may be damaged to the 
extent of fracturing, thus giving rise to drilling problems which are 
themselves more important than the destruction of the cylindrical sleeve, 
because in some cases this can cause a reduction or even stopping of the 
circulation of the drilling mud and, in an extreme case, jamming of the 
drill-pipe string. 
It is true that it could be possible to increase the thickness of the 
cylindrical sleeve serving as part of an antenna and make it in the form 
of a hollow cylinder of considerable thickness which can range up to a 
centimeter, but, to maintain the same outside diameter for the said 
cylindrical sleeve it would be necessary to reduce accordingly the 
thickness of the drill collar supporting it, and this will result for the 
latter in a reduced resistance to buckling and to compression, thus giving 
rise to other risks and disadvantages. 
If the cylindrical sleeve is to be produced by means of netting, it is 
important that this should have the strength and electrical qualities 
required, equivalent to those of a simple sheet-metal sleeve connected 
electrically to the transmitter, contained in the drill collar, by means 
of a conductor insulated from the latter at the point where it passes 
through its wall. 
The present invention also provides a satisfactory solution to this 
problem, and it is also aimed at an advantageous alternative embodiment of 
part of the antenna of the transmitter device, which is easy and cheap to 
produce, without entailing expenses involved in costly and complicated 
assembly to connect it to the insulating sleeve forming a dielectric 
between it and the drill collar, and which furthermore, if it happens to 
wear out, breaks down into small fragments which, far from preventing the 
elimination of fragments of rock resulting from drilling, are, on the 
contrary, easily carried along with them. 
According to an essential characteristic of this embodiment of the 
electromagnetic wave radiating metal sleeve part of the antenna according 
to the invention, the said metal sleeve is accommodated in an annular 
groove made in the drill collar, with the outer cylindrical surface of 
which it is made flush when its production is completed. 
The two ends of the drill color form as it were protective stops for the 
outer surface of the metal sleeve. 
According to another characteristic of the invention, the metal sleeve 
consists of a cylindrical sheet of netting made of helical springs 
interlaced parallel to one another by means of their adjacent turns, the 
said netting being embedded in an electrically insulating material, for 
example in a hardenable plastic or in an epoxy resin of the type 
distributed by Messrs. CIBA-GEIGY of Basle under the protected brand name 
"ARALDITE", which is advantageously applied over the bottom of the annular 
groove made in the drill collar. 
To ensure that the springs are maintained at a sufficient distance from the 
bottom of the annular groove made in the drill collar according to another 
characteristic of the invention the insulating layer is applied in two 
stages, the first bedding layer insulating the netting from the drill 
collar and serving, before it has set completely, to retain and secure the 
netting pressed on it. 
To make it possible to ensure good electromagnetic wave transmission of the 
metal sleeve formed by the netting consisting of overlapping helical 
springs, the cylindrical sheet of helical springs has such a thickness 
that, after it has been laid on the first layer and incorporated in a 
second layer covering it completely, it projects, over approximately a 
third of its thickness in the radial direction, from the shell surface of 
the drill collar which borders on either side of the annular groove made 
in the said drill collar for the purpose of accommodating the metal sleeve 
and its dielectric. 
To ensure a good connection between the first layer and the drill collar, 
the epoxy resin used for this purpose to constitute the first layer has 
advantageously incorporated in it a network of glass fibers which are 
advantageously twisted. 
This results in reinforcement and a better adhesion of this first layer on 
the bottom surface of the annular groove made in the drill collar. 
It is also advantageous to proceed in an identical way for securing the 
netting on the first layer which is not yet completely solidified. 
Consequently, according to another characteristic of the invention, before 
being completely embedded in the second insulating layer of epoxy resin, 
the netting consisting of helical springs is gripped in the first still 
soft layer of insulating material by means of ties which are made of glass 
fibers advantageously twisted helically or of other sufficiently resistant 
synthetic fibers. 
This results in a reinforcement making it easier to attach the second layer 
which surrounds the assembly consisting of the drill collar and the 
netting in the form of helical springs. 
To put in contact with the outside the embedded metal part of the netting 
which is to form the metal sleeve according to another characteristic of 
the invention the second resin layer, after hardening, is machined flush 
with the shell surface of the drill collar which borders the annular 
groove in which the netting is arranged on the first layer of insulating 
material, separating it from the metal part of the drill collar. 
This results in an insulating collar, at the surface of which appears a 
plurality of sections of spring turns which perform the same function as 
the metal sleeve. 
To complete the electrical continuity of the netting embedded in the 
insulating resin, according to another characteristic of the invention the 
said netting is provided with weld points connecting the turns of two 
adjacent helical springs to one another at their point of intersection. 
Moreover, to reinforce the electrical conductivity of the entire netting of 
the antenna, an oblique metal connecting wire is welded, preferably with 
tin, approximately every ten meshes of the netting, the said connecting 
wire keeping electrically connected to one another all the rows of 
C-shaped wire fragments, the tips of which appear at the outer surface of 
the second resin layer when the latter has been machined so as to be 
either flush with the drill collar or set back in the groove of the 
latter. 
According to another characteristic of the invention, the metal wire 
connecting the metal sleeve to the transmitter located within the drill 
collar is either welded to one or more of the metal connecting wires or 
itself forms the connecting wire nearest to the electrically insulated 
passage orifice which is made in the drill collar to connect the metal 
sleeve to the transmitter.

DETAILED DESCRIPTION OF THE INVENTION 
In the known MWD (Measurement-While-Drilling), which is illustrated in FIG. 
1, the drill collar of the drill-pipe string, which is designated by the 
general reference numeral 1, is screwed in a known way in its upper part 
to the following drill-pipe (not shown) of the drill-pipe string and 
carries, in its lower part, the bit 2 shown diagrammatically with its 
drilling rollers 3. 
In this device, the drill collar 1 is subdivided into an upper portion 4 
and a lower portion 5 coupled mechanically to one another by means of an 
annular assembly element 6 made of an insulating material. In this 
embodiment, the sensors, indicated as a whole by reference numerals 7 and 
8, are retained in place, in the lower part 5 of the drill collar, by 
means of a metal element 9 which makes the electrical connection between 
the transmitter and the part 5 forming part of an antenna. 
The modulator transmitter is also connected electrically to the upper part 
of the drill collar which is itself connected conductively to the rest of 
the drill-pipe string by means of a metal ring 10 arranged according to 
the axis of the ring 9, the said modulator transmitter E likewise being 
connected, in turn, to the supply batteries indicated at 11. 
Such an embodiment is of great fragility because the assembly element 6, 
which must be electrically insulating, can be made only of materials which 
do not ensure satisfactory strength of the drill collar, and also because 
the lower part of the latter, serving as part of an antenna, can be 
exchanged, as a result of the considerably wear which it undergoes, only 
at the expense of complete replacement of the drill collar, and this 
details substantial dismantling costs and a considerable loss of time. 
This disadvantage does not exist in the device according to the invention, 
which behaves in a uniform way as regards mechanical stresses and part of 
the antenna of which can be exchanged easily at low cost. 
In a first embodiment of the device according to the invention, illustrated 
by way of a simple example in FIG. 2 of the attached drawings, it is 
possible to see, again shown diagrammatically in the form of blocks, the 
various elements of the transmitter E which are accommodated in the drill 
collar 12 within leak-proof cylindrical sheaths which are wedged by rings, 
such as 17, and round which the drilling mud can circulate. The actual 
electromagnetic wave transmitting part of the antenna of the device 
consists of a cylindrical metal sleeve for radiating electromagnetic waves 
which comprises a cylindrical metal sleeve 13 possessing the physical, 
chemical and mechanical qualities required to withstand the various 
stresses exerted at the bottom of the drilling well (temperature, 
pressure, chemical corrosion, mechanical abrasion, etc.), the said sleeve 
being mounted coaxially relative to the drill collar 12, and, interposed 
between them, there is an insulating sleeve or liner 14 laid and 
immobilized by any means on the outer surface of the drill collar 12, the 
resistance of which is thus in no way affected. 
The cylindrical metal sleeve 13 can be produced in one or more pieces from 
solid, perforated or slit sheet metal, gauze with metal links or the like, 
and it can be embedded partially or completely in the insulating liner 14 
surrounding the drill collar 12. 
The insulating liner 14 is advantageously made of a suitable, sufficiently 
resistant material, for example silicone rubber, elastomers or synthetic 
resins, for example reinforced with glass fibers. 
This insulating liner is fixed mechanically to the outer surface of the 
drill collar 12 which is itself appropriately prepared for this purpose by 
any suitable means enabling perfect adhesion to be obtained. 
According to a characteristic of the invention, the drill collar 12 and the 
insulating liner 14 each have at least one perforation intended for the 
passage of a conductor connecting the cylindrical metal sleeve 13 serving 
as part of an antenna to the output of the transmitter E which is 
incorporated within the cavity of the drill collar 12. 
At the location of this perforation, the insulating liner 14 is 
advantageously reinforced, according to the thickness of the drill collar 
12, by an insulating element 15 penetrating within the drill collar 12, as 
illustrated on a large scale in FIG. 3, and the connection 16 with the 
transmitter E, transmitting signals from the sensors grouped in the 
enclosures C.sub.1 and C.sub.2, is symbolized by the arrow F.sub.1. 
As regards the insulating element 15 serving to accommodate the conductor 
16 connecting the output of the transmitter E electrically to the metal 
sleeve 13 serving as part of an antenna, it can be in one piece with the 
insulating liner 14 or can consist of a separate attached element in the 
form of a bush which has the shape of a rivet and which incorporates a 
receptacle allowing a welded connection to be made between the conductor 
16 and the metal sleeve 13 forming part of the transmitting antenna. 
This attached element 15 can, for example, be made of a ceramic material 
resistant to shocks, vibrations and wear. 
The insulating layer 14 must be made sufficiently thin so as not to 
increase too much, together with the cylindrical metal sleeve 13 also made 
with a relatively small thickness, the outside diameter of the drill 
collar 12 and so as not to disturb the conditions of circulation of the 
drilling mud, whether it is oily or not. 
As already mentioned, the length of the insulating liner 14 must be less 
than that of the drill collar 12, in order to provide at the ends of the 
latter zones which are accessible to gripping and clamping tools. 
Since the standard length of the drill collar is 9 meters, the difference 
in length between it and the insulating liner can vary between 
approximately 4 and 10% of the total length of drill collar 12, although 
these values are merely indicative and are in no way limiting and can vary 
as a function of the conditions envisaged for operating the drill-pipe 
string and as a function of the dielectric properties of the insulating 
layer 14. 
As mentioned, the cylindrical metal sleeve serving as part of an antenna is 
made shorter than the insulating layer 14, to avoid leakage at the end of 
the cylindrical sleeve 13 forming part of an antenna and to prevent 
interference. 
An advantageous embodiment is obtained by means of a cylindrical metal 
sleeve which is, for example, 5 meters long and one to 2 mm thick and 
which is centered on an insulating liner 14 which is 8 meters long. This 
leaves, particularly in the lower part, an uncovered zone of insulating 
material 14 over a length of 150 cm between the edge of the insulating 
material and the edge of the cylindrical metal sleeve. 
This cylindrical metal sleeve is advantageously made of a metal resistant 
to shocks and to abrasion, which may or may not be identical to the metal 
constituting the drill collar 12. The uncovered part of the insulating 
material, which precisely must have a high resistance to abrasion, is 
indicated by the distance d in FIG. 2 showing the electrical diagram of 
the transmitter which will be discussed later. 
To make the insulating liner resistant to abrasion, it is advantageous to 
produce it from elastomers or resins reinforced, for example, by glass 
fibers. 
FIG. 4 shows an alternative embodiment of the device illustrated in FIG. 2, 
in which the cylindrical metal sleeve 13 is subdivided mechanically and 
electrically into two cylindrical portions 13' and 13" of unequal length, 
the sleeve 13" forming a resistivity measuring means for measuring 
resistivity of the terrain at the bottom of the drill well in the vicinity 
of th transmitter comprising a ring of small width located as low as 
possible on the insulating liner 14 in the vicinity of the bit, that is to 
say substantially at the drilling level reached by the latter. 
As will be seen, the transmitter E, which is not shown in detail, but is 
merely indicated in FIG. 4 for the sake of greater clarity, can be 
alternatively coupled electrically either to the sleeve 13' forming 
electromagnetic wave a transmission part of antenna (arrow F.sub.1) or to 
the lower insulated metal ring 13", the special function of which will be 
described later. 
In FIG. 5, which illustrates an equivalent electrical diagram of the 
system, the transmitter E delivers a low-frequency alternating current, 
via its internal resistance R.sub.g, on the one hand into the line of the 
drill-pipe string of impedance Z.sub.c and on the other hand into the 
cylindrical metal sleeve 13, of which the impedance relative to the 
terrain at the bottom of the well is designated by R.sub.a, and, as 
already emphasized, the resistance R.sub.s is the leakage resistance of 
the currents passing directly through the drilling mud between the drill 
collar 12 and the cylindrical metal sleeve 13 over the part of the 
insulating liner 14 bared along the length d. 
It is important to minimize these losses by giving the distance d the 
highest value possible, and to reduce the impedance R.sub.a of the 
cylindrical metal sleeve, to obtain the best injection of current, that is 
to say to maximize the height of the cylindrical metal sleeve, the best 
possible solution being found between the maximum size of the cylindrical 
metal sleeve and a minimum of leakage currents. 
The transmitter device described makes it possible to operate in oil muds, 
the mud circulating at the bottom of the well forming the dielectric of a 
cylindrical capacitor, the inner armament of which is the cylindrical 
metal sleeve 13 and the outer armament of which is the surrounding 
terrain. 
This embodiment makes it possible, especially by means of the ring 13" 
(FIG. 4), to measure the resistivity of the terrain at the level reached 
by the bit and to obtain from this important information for carrying out 
the drilling work. 
This resistivity can be measured from the cylindrical metal sleeve 
impedance given by the formula: 
##EQU1## 
in which .rho. denotes the resistivity of the terrain, R the radius of the 
cylinder forming the cylindrical metal sleeve and L the length of the 
cylindrical metal sleeve. 
It will therefore be seen that, to measure the resistivity accurately over 
a small thickness of terrain, it is useful to "focus" measurement over a 
narrow band; this is the reason for the embodiment shown in FIG. 4, where 
the ring 13" is made as short as possible and located as low as possible 
on the insulating liner covering the drill collar 12. Since this ring 13" 
is insulated electrically from the cylindrical sleeve 13' forming part of 
the antenna, its impedance R.sub.a relative to the ground can be measured 
very easily. 
It was mentioned above that the metal sleeve 13 could be divided in its 
lower part to form a ring 13" which is used specially, when the upper 
cylindrical part 13' is decoupled electrically, to measure the resistivity 
of the zone of terrain reached by the bit at the bottom of the drilling 
well. 
However, it is perfectly possible, according to another characteristic of 
the invention, to produce the cylindrical metal sleeve 13 forming the 
resisting measuring means, secured to the insulating casing 14 covering 
the drill collar 12, in the form of a plurality of cylindrical rings or 
collars placed side by side at a short distance from one another, and the 
transmitter output can be connected, by means of a programmable switching 
device, to one or more of the said rings so as to provide the capacitive 
measurement desired. 
It is thus possible, by connecting only one of these rings, each time, to 
the device measuring the resistivity of the terrain, to carry out such 
measurement at several levels over practically the height of the drill 
collar. 
In the embodiment illustrated in FIG. 6 of the drawings, there is a partial 
plan view of the cylindrical metal sleeve comprised of netting 13 intended 
to constitute part of the antenna which can be seen in FIGS. 9 and 10. As 
can be seen more clearly in FIG. 7, the netting 13 forming the cylindrical 
metal sleeve consists of a plurality of overlapping helical springs 18 
with interlaced turns 19. 
As can be seen on a larger scale in FIG. 10, these turns 19 are welded, for 
example with tin, at their points of intersection, such as 20. To ensure 
good electrical conductivity of the assembly as a whole and reinforce the 
stability of the cylindrical metal sheet 13 of springs 18, the turns 19 
are connected to one another, for example over ten rows, by means of a 
metal wire 21 welded to them (see FIGS. 6 and 7). 
The netting 13 incorporated in an electrically insulating material, for 
example consisting of synthetic material or more particularly an epoxy 
resin, the hardening time of which can advantageously be adjusted, is 
accommodated (FIG. 10) in an annular groove 22 of suitable length which is 
made in the drill collar 23. A first layer 24 of epoxy resin, 
advantageously reinforced with a network or sheet of glass fibers 25 which 
are, for example, wound helically to form a hoop, is first placed in this 
annular groove. 
Although, in FIG. 10 of the drawings, the sheet of helical springs has been 
placed relatively near to the edges of the annular groove 22 made in the 
drill collar 23, it is advantageous if the insulating layer filling the 
said groove projects considerably on either side of the incorporated 
netting. 
The sheet of helical springs 18 is laid on the first layer 24 of epoxy 
resin, which is not yet hardened completely, by means of a sheet or ribbon 
of glass fibers 26 which can be twisted and knotted to reinforce the 
penetration of the netting 13 forming the cylindrical metal sleeve into 
this still soft first layer 24, before it is advantageously covered 
completely with a second layer 27 of epoxy resin which then projects on 
the outside of the drill collar 23. 
When this second layer 27 covering the netting 13 has hardened completely 
and is integral with the first layer 24 of epoxy resin, the outer surface 
28 of the collar produced in this way is machined, the turns 19 of the 
netting 13 being flush with the outer surface of this collar. 
By leveling off up to the unbroken line 29 visible in FIG. 10, that is to 
say up to the level of the shell surface of the drill collar 23, the part 
represented by broken lines in the drawing is thus eliminated. 
This results in a cylindrical metal sleeve surface which has the appearance 
shown in FIG. 11, in which the small circles designated by reference 
numeral 30 represent the ends of the C-shaped parts or fractions of turns 
31 which are embedded in the two layers 24 and 27 of epoxy resin. 
Before being assembled, the cylindrical metal sleeve is, of course, 
connected electrically to the transmitter (not shown), which is 
accommodated in the drill collar 23, by means of a metal connecting wire 
designated by the reference numeral 16 in FIGS. 3 and 4. 
It goes without saying that the device has been described and illustrated 
only in a purely explanatory and in no manner limiting way and that 
various detailed modifications could be made to the embodiments described, 
without thereby departing from the scope of the invention. 
Thus, in particular, it would also be possible to place in the annular 
groove 22 of the drill collar 23 not a netting, but a smooth, perforated, 
slit or suchlike metal sheet, as illustrated diagrammatically in FIG. 3, 
and also to provide several successive grooves to receive either a 
cylindrical metal sleeve in the form of several rings or cylindrical metal 
sleeve and a ring for measuring the resistivity at the drilling level.