The present invention relates to wireless telemetry for drill strings for transmitting data from downhole to the earth""s surface.
The problem of improving the data transmission from drill strings whilst drilling has been under investigation for some time, however the increasing drilling depth, especially with directional drilling for oil and gas. The aim is to increase the reliability and rate of transmission of data such as the properties of the formation, navigation and condition of the bore-tool etc.
Among presently used wireless communication techniques such as mud-pulse, acoustic and electromagnetic, the latter is the most promising in terms of its potential for measurement whilst drilling purposes. Typically, known electromagnetic drill string telemetry transmitters comprise a low-frequency radio transmitter located adjacent the drill bit. Data obtained from transducers in the drill bit are first digitised and then transmitted from the borehole to the earth""s surface through the rock formation. The signal is then detected and decoded by a receiver placed on the earth""s surface, adjacent the rig site. In the frequency range of 2.5-50 Hz, this technique is capable of receiving data from a depth of 5000 meters.
The electromagnetic signal is transmitted in either one of two ways. British Patent No. GB 2,142,804 discloses one way in which the data is modulated onto a pulse code electric signal, which is injected by a current dipole into the rock formation surrounding the borehole bottom area. At the earth""s surface the electromagnetic signal generated by the current is detected, and the data is extracted by demodulation. In the described case the rock formation itself is an electrically conductive medium for the signal from the current dipole, which comprises two electrically insulated elements acting as electrodes. It is evident that the signal amplitude will strongly depend on the electrical conductivity properties of rock formation.
We have found that a disadvantage of this current dipole arrangement is that the magnetic field radiated from the current dipole extends substantially radially from the drill string and thus little magnetic signal can be detected at the earth""s surface, when the portion drill string containing the transmitter is vertically orientated.
Another disadvantage of known current dipoles is that it is difficult to inject sufficient current into rock types, such as dolomite, which have a high resistivity value.
U.S. Pat. No. 4,800,385 discloses a second way in which the data is modulated onto a signal, which is applied to magnetic dipole in the form of a coil placed adjacent the outer surface of the drill pipe. The magnetic dipole is electrically insulated both from the pipe itself and, on the outside, from drilling mud and is connected to an electrical power source.
We have found that a disadvantage of this magnetic dipole arrangement is that the magnetic field radiated from the dipole decreases with inverse cube of the distance from the transmitter and therefore diminishes to immeasurably small values at large distances. The above-mentioned magnetic dipole transmitter offers a solution which is independent of the conductivity of formation at the location of the transmitter. In this case it is possible to achieve a larger magnetic signal at shorter drilling depths. Therefore, for the initial stages of drilling (i.e. shorter transmission distances and vertical orientation of magnetic transmitter) the magnetic dipole is preferable to the current dipole. However, for drilling depths over 2 km the signal produced by the magnetic dipole source is too small for simple magnetometric detection.
The power consumption and the mass of the current dipole is much less than those of the magnetic dipole. The current dipole source has a simpler construction and therefore is more economical.
Therefore in both said methods of magnetic field generation, the data at the earth""s surface is often undetectable or weak depending on the resistivity and/or orientation of the portion of the drill string containing the transmitter.
Magnetic telemetry transmitters operate under extreme conditions, high mechanical loads, contact with corrosive and abrasive materials, and elevated temperatures. Furthermore, the cross-sectional dimensions of the transmitter is limited by the size of the borehole.
The examination of the technical art shows a trend of improvement in transmitter construction, in the first place oriented towards improving the quality of the data transmitted from the borehole bottom. This concerns the increase in the power of the emitted signals in order to compensate for the inherent signal attenuation in the conductive media of the formation. Moreover, in deep drilling and especially in directional drilling for oil and gas, the drill traverses many layers with variable conductive and dielectric properties and the resistivity of clay layers over hydrocarbon deposits can change by more than 100%. This may alter the amplitude of the measured telemetry signal by superimposing on it a noise component thereby complicating the decoding of the telemetry signals.
We have now devised a method of transmitting an electromagnetic signal comprising data obtained from at least one transducer situated in a drill string and a drill string telemetry transmitter therefor.
In accordance with this invention there is provided a method of transmitting an electromagnetic signal containing data obtained from at least one transducer situated in a drill string, the method comprising adaptively controlling the power output by magnetic and current dipoles of a transmitter located in a portion of the drill string, according to the orientation of said portion of the drill string and according to the electrical resistivity of the medium surrounding said portion of the drill string.
The present invention is partly based on the realisation that a current dipole transmitter is unsuitable for use when drilling vertically and when drilling through high resistivity rock formations, and partly based on the realisation that the magnetic field radiated by a magnetic dipole transmitter diminishes very quickly with distance and thus is unsuitable for use when drilling at larger depths. The transmitter of the present invention thus combines a current dipole and a magnetic dipole source, wherein electrical power can be adaptively distributed between these two sources, according to the orientation of the drilling and the properties of the formation, in order to establish optimised and reliable reception of data at the earth""s surface.
The transmitter is formed by current and magnetic dipole sources, which can be operated independently or simultaneously, in order to create a distribution of combined magnetic fields with a predominance of the azimuthal component.
The magnetic telemetry transmitter forms a part of the drill string, preferably with its housing elements made of non-ferromagnetic electrically conductive material, and with at least one section of electrical insulation separating the two conductive parts of the transmitter.
The transmitter consists of combined current and magnetic dipoles, wherein the electrodes of the current dipole are capable of injecting electrical currents into the surrounding formation. The magnetic dipole unit consists of a number of coils having ferromagnetic cores preferably in the shape of elongated bars, placed in parallel to the longitudinal axis of the transmitter in a cylindrical cavity inside the transmitter housing. The electrodes of the current dipole and the coils of the magnetic dipole are connected to the power source via an adaptive power control switch.
The said method of magnetic field generation by the use of a combined source of the magnetic field leads to an improved telemetry technique, in that the magnetic signal from this transmitter can be detected at any orientation of the drilling and is much less effected by the variations in the conductivity of formation at the location of the transmitter.
Moreover, the extreme environmental conditions of the transmitter (considerable mechanical strains, vibration, high temperatures and pressures) imply strict requirements for the reliability of construction. It is a feature of the said transmitter, that there is a degree of redundancy (a) between the magnetic dipole and the current dipole, and (b) within the magnetic dipole itself in that it is formed by several emitters connected in parallel so that damage to the windings of one of them does not disrupt the operation of the device as a whole.
Preferably, the power output by the magnetic dipole is increased when the portion of the drill string containing the transmitter is at or adjacent a vertical orientation.
Preferably, the power output by the magnetic dipole is increased when the resistivity of the medium surrounding the portion of the drill string containing the transmitter is above a predetermined value.
Preferably, the power output by the current dipole is reduced or inhibited when the power output by the magnetic dipole is increased.
Preferably, the power output by the current dipole is increased when the portion of the drill string containing the transmitter is at or adjacent a horizontal orientation.
Preferably, the power output by the current dipole is increased when the resistivity of the medium surrounding the portion of the drill string containing the transmitter is below a predetermined value.
Preferably, the power output by the magnetic dipole is reduced or inhibited when the power output by the current dipole is increased.
In one embodiment, the power output by said electric dipole and magnetic dipole sources are controlled from the earth""s surface by transmitting a control current to the transmitter, according to data which is received from the transmitter concerning its orientation and the resistivity of its surrounding drilling medium.
In an alternative embodiment, the orientation of the portion of the drill string and the electrical resistivity of the medium surrounding said portion of the drill string are detected by the transmitter, which is then arranged to adaptively control the power output by said magnetic and current dipoles, according to the sensed orientation and resistivity values. Also, in accordance with this invention there is provided a drill string telemetry transmitter located in a portion of the drill string, the transmitter comprising a input for receiving data obtained from at least one transducer arranged to sense a parameter of the drill string and/or a parameter of the surrounding medium, a magnetic dipole arranged to transmit an electromagnetic signal comprising said data, a current dipole arranged to transmit an electromagnetic signal comprising said data and control means for adaptively varying the power output from said magnetic and current dipoles, so as to alter their respective output signals, according to the orientation of said portion of the drill string and according to the electrical resistivity of the medium surrounding said portion of the drill string.
In one embodiment, the transmitter preferably comprises a sensor for sensing the orientation of said portion of the drill string, said control means being arranged to adaptively vary the power applied to said magnetic and current dipoles according to an output of said orientation sensor.
Preferably the transmitter also comprises a sensor for sensing the electrical resistivity of the medium surrounding said portion of the drill string, said control means being arranged to adaptively vary the power applied to said magnetic and current dipoles according to an output of said resistivity sensor.
In an alternative embodiment, the control means is arranged to adaptively vary the power applied to said magnetic and current dipoles according to a control signal received from the earth""s surface.
In one embodiment, the transmitter is located adjacent the measurement transducer(s), with the input of the transmitter being connected to an output of the transducer(s).
In an alternative embodiment, the input of the transmitter is connected to a receiver, which receives signals from a remote transmitter having an input connected to said transducer(s). In this manner a relay link can be provided along the drill string from the transducer to the earth""s surface.
The remote transmitter may transmit signals along the drill string to the adaptive transmitter. Alternatively, the remote transmitter may itself also comprise an adaptive transmitter.
These and other objects, features and advantages of the present invention will be clearly understood through consideration of the following detailed description.