Drill string enabling information to be transmitted

The present invention relates to a drill string fitted with information transmission means. The hollow rods of conductive material have their inside faces covered in an insulating material. The transmission means comprise a first electrical unit having at least a first annular electrode secured to the insulating inside face of the drill string, said electrode being in electrical contact with the mud contained inside the drill string so as to establish a flow of current in the conductive mud; and a second electrical unit disposed in said borehole, close to the top end of the drill string in order at least to receive the information contained in said electrical current.

BACKGROUND OF THE INVENTION

A drilling installation constituted by drill strings enabling information to be transmitted between the bottom of a borehole being drilled and the surface of the ground is already described in French patent 2 777 594 in the name of the Applicant. As explained in that document, while a borehole is being drilled, it is very important to be able to transmit to the surface information that is collected by sensors mounted in the vicinity of the drilling tool, which tool is fixed to the bottom end of the drill string.

One of the solutions proposed in the above-mentioned French patent is described below with reference to accompanying FIG.1.

FIG. 1shows a drill string made up of a rod10aconstituting the bottom rod, a rod10bconstituting the top rod, and intermediate rods. A drilling tool12is fixed to the bottom end of the rod10aand has measurement sensors14. The inside face16of the rod10is coated in a layer of insulating material18over the entire length of the drill string. Furthermore, the drill string is disposed inside a borehole20that is being drilled, which borehole is filled with drilling mud22that conducts electricity. To enable information to be transmitted, there is a first induction coil24disposed close to the bottom end of the rod10aand connected to the measurement sensor14. There is also an inductive coupler coil26mounted inside the top rod10band connected to conductors such as28for transmitting electrical signals picked up by the coil26to a processor device.

Because of the presence of the insulating layer16, a closed current loop is established constituted firstly through the drilling mud30filling the inside of the rod10and secondly through the assembly constituted by the wall of the rod10and by the mud32outside the rod10.

Using the bottom coil24, alternating current (AC) representing information is induced in the current loop, with this AC being picked up by the receiver coil26.

Such a installation for transmitting information via a drill string gives satisfactory results in some situations. Nevertheless, it will be understood that information is transmitted via two electromagnetic couplings corresponding to the two coils or toruses which are disposed respectively close to the top end of the drill string and close to its bottom end carrying the tool. These two couplings, constituting current transformers, present the particular characteristic of comprising a very large number of turns wound on toruses. In contrast, the other element of the coupling is constituted by a single turn and consists in the single current loop established essentially via the mud contained in the drill string. Electromagnetic coupling of that type is not optimal and its efficiency or effectiveness can turn out to be insufficient, particularly when the electrical conductivity of the mud is low.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the present invention is to provide a drill string of the above-mentioned type which is fitted with means for transmitting information between the bottom of the borehole in which the equipment is to be found and the ground surface, which drill string enables information transmission to be improved, particularly at the interface between the transmitter or receiver and the mud used for conveying the information.

According to the invention, this object is achieved by a drill string fitted with information transmission means comprising:

a plurality of hollow rods made of conductive material, the inside faces of said rods being covered in insulating material;

a drilling tool secured to the bottom rod of the drill string, said drill string being disposed in part in a borehole filled with an electrically conductive mud, said mud inside the drill string and the mud outside the drill string together with the drill string itself forming a closed loop that conducts electricity;

a first electrical unit placed close to the bottom end of the drill string, in order at least to create in said loop an electrical current representative of information to be transmitted; and

a second electrical unit placed in said borehole, close to the top end of the drill string, in order at least to receive the information contained in said electrical current.

The drill string is characterized in that said first electrical unit comprises at least a first annular electrode secured to the insulating inside face of the drill string, said electrode being in electrical contact with the mud contained inside the drill string; and

means for applying to said electrode a voltage that is representative of said information to be transmitted.

It will be understood that according to the invention the information for transmission is applied in the form of a voltage to the annular transmitter electrode disposed at the bottom end of the drill string. Relative to ground, this voltage induces current in the above-defined closed current loop, said current being modulated as a function of the information to be transmitted. Reception at the top end of the drill string can be performed as in the above-mentioned state of the art by means of an electromagnetic coupling coil placed inside the drill string, said coil surrounding the mud contained in the drill string, which mud conveys the current.

A second solution of the invention consists in placing two conductive electrodes that are axially offset on the insulating inside face of a rod located close to the top end of the drill string. The potential difference which appears between the two electrodes is created by the current flowing in the current loop. This potential difference is thus itself modulated in the same manner as the current, as a function of the received information.

In a more complete version, the electrical unit placed at the bottom end of the drill string and the electrical unit placed at the top end of the drill string can both be constituted by two conductive electrodes mounted inside the insulating inside face of the rods. Alternatively, one of the electrodes can be used as a transmitter for transmitting information and both electrodes can serve in alternation as receivers of the information that is to be transmitted.

MORE DETAILED DESCRIPTION

With reference initially toFIG. 2, there follows a description of a first embodiment of the drill string that enables information to be transmitted.

FIG. 2is a simplified diagram of a borehole40while it is being bored. This borehole contains a drill string and, more particularly, the bottom rod Tihaving the drilling tool mounted thereon (not shown in the figure) and the top rod Ts. Intermediate rods are naturally interposed between these two end rods. Each rod is constituted by a conductive metal wall42having its inside face covered in an insulating layer44.

As already explained, the borehole40is filled with a mud that conducts electricity more or less well and that fills not only the inside46of the drill string, but also the annular space48between the drill string and the wall of the borehole40. This establishes a closed conductive loop constituted firstly by the mud on the inside46and secondly by the conductive wall42together with the mud in the outer annular space48. These two branches of the electrically conductive circuit are separated by the insulating layer44and they are interconnected at the bottom by the tool itself and at the top by appropriate means. In this figure, reference40F designates the borehole and40S the top end of the borehole close to the surface of the ground S.

According to an essential characteristic of the invention, the bottom electrical unit is constituted by an electrode, preferably an annular electrode50, which is fixed by any suitable means to the insulating wall44of the rod Ti. This electrode is in direct electrical contact with the mud46contained inside the drill string. The electrode is preferably disposed at a sufficient distance from the drilling tool. Similarly, at the top end of the drill string, there can be seen an electromagnetic coupling coil52. It will be understood that in order to transmit information from the bottom of the borehole close to the tool towards the top portion of the borehole, i.e. towards the surface of the ground, a modulated electrical voltage is applied to the electrode50, with the modulation corresponding to the information that is to be transmitted. This electrical voltage sets up a modulated electrical current in the zone46which flows around the above-defined current loop. This current passes in particular through the zone surrounded by the electromagnetic coil52. The flow of this modulated current causes an electrical voltage to appear at the terminals of the coil52, which voltage is representative of the information transmitted by the electrode50. It will be understood that this avoids the above-mentioned problems concerning transmission and corresponding to poor electromagnetic coupling between the current loop and the coil as provided in the prior art for sending the information. However, such poor quality coupling is less unfavorable when it comes to reception by the electromagnetic coil52.

In some cases, it is useful to transmit information not only from the bottom of the borehole to the surface of the ground, but also from the surface of the ground down to the bottom of the borehole, e.g. to send information to actuators of the drilling tool. To achieve this result, a second annular electrode54is provided in the bottom rod Tiwhere it is fixed to the insulating wall44of the rod. The electrode54is offset axially by a distance d relative to the transmitting electrode50. When AC is applied to the coil52, this induces current in the mud46which is modulated in the same manner and which flows around the above-defined current loop. This circulating current establishes a potential difference between the electrodes50and54, which potential difference can be picked up and presents the same modulation as the current initially applied to the coil52. By measuring the modulation of the potential difference between the electrodes50and54, it is possible at the bottom of the borehole to pick up information transmitted by the coil52.

Reference is now made toFIG. 3in order to describe in greater detail the bottom electrical unit that is used for transmitting and receiving information. This figure shows the electrodes50and54fixed to the insulating wall44of the conductive wall42of the drilling rod. A cavity56or the like substantially in register with the electrode can be defined by any suitable means in the wall42of the rod or in a piece fitted to the rod. Various electrical or electronic components given overall reference58are mounted in the cavity56. These components are connected by electrical conductors60and62to the electrodes50and54. These components58are also connected via a set of electrodes64and66both to the sensors disposed in the tool and also to the actuators mounted in said tool, if any.

During a transmission stage, i.e. when measurements made by the tool sensors are to be transmitted to the surface of the ground, the circuit58receives encoded information for transmission and they generate a corresponding electrical voltage for application to the transmission electrode50, thereby injecting modulated current into the closed conductive loop.

During a reception stage, the circuits58are connected simultaneously to both electrodes50and54. The circuits58have means for measuring the potential difference between the voltages picked up by the electrodes50and54. This potential difference carries modulation corresponding to the received information which can be encoded or forwarded in suitable manner by the circuits58for application to actuators of the tool via the electrical connection66.

FIG. 4shows the top portion of the drill string including the rod Tsfitted with its coil52, a standard rod T which is engaged in the drilling head80for controlling rotation of the drill string and downward drive thereof. In order to connect the receiver coil52to the processor unit82disposed on the surface of the ground, a rotary collector84is provided surrounding the rod T and electrically connected to the processor unit84.

In the embodiment ofFIG. 5, the bottom electrical unit referenced70is constituted essentially by the electrodes50and54as already described with reference toFIGS. 2 and 3. The top electrical unit, referenced72and disposed close to the ground surface is likewise constituted by two annular electrodes74and76substantially identical to the electrodes50and54and fixed to the insulating lining44of the top drilling rods Ts. The electrode74is used solely for sending information from the surface in the same manner as the transmission electrode50is used. In contrast, in order to receive information transmitted from down the borehole, both electrodes74and76are used. The modulated potential difference picked up between these two electrodes as created by the flow of the modulated current as produced by the bottom electrical unit70constitutes received information which is representative of the information transmitted by the bottom electrical unit70.

In the figures, the annular electrodes are represented as projecting relative to the insulating layer44. Preferably, the insulating layer44is of sufficient thickness to make it possible for the inside faces a of the electrodes to be flush with the inside face of the insulating layer. Nevertheless, it is necessary for the thickness of the portion of the insulating layer between the electrode and the wall of the rod to be sufficient to provide the desired degree of electrical insulation. This avoids creating head losses in the rod.

Also preferably, the annular electrode is made out of a material that is a good conductor of electricity, e.g. brass, having its inside surface treated, e.g. by nitriding, so as to give this surface sufficient hardness to enable it to withstand the abrasive effect of the liquid circulating in the rod.

The width l of the electrodes in the axial direction of the rod is preferably greater than or equal to 2D, where D is the inside diameter of the duct.

When the system is designed to operate as a receiver also, it is advantageous for the distance d between the two electrodes to be as great as possible. However, this distance is limited since, for manifest practical reasons, it is necessary for the two electrodes to be mounted on the same rod of the drill string. This distance therefore depends on the length of the bottom rod Tiand on the length of the top rod Ts.

Finally, tests have been performed which show that good signal transmission is obtained even for a drill string that is 80 meters long, when using an alternating electrical signal at 10 volts delivering about 50 milliamps.