FR2656896A1 - METHOD FOR DECODING MWD SIGNALS USING ANNULAR PRESSURE SIGNALS. - Google Patents

Abstract

A method is provided for obtaining measurements at the bottom of the well using a borehole meter in which pulse telemetry signals in the mud are decoded using the pressure signals in the annular area (and not the pressure signals in the drilling area). The signals carrying the data are improved by combining the pressure signals in the annular zone and the pressure signals in the drilling zone, using the techniques of addition, multiplication or correlation.

Description

i

MWD SIGNAL DECODING PROCESS

  USING RING PRESSURE SIGNALS

  1 The present invention relates, in general, to the field of measurements at the bottom of the well. More particularly, this invention relates to a new and improved method for obtaining measurements at the bottom of the well using a measuring device during drilling ( MWD) in which pulsed mud telemetry (MPT) signals are decoded using the pressure signals in the annular area (and not

  pressure signals in the drill string).

  When drilling oil and gas wells, the efficient operation of the drilling rig, especially when the wells are drilled deeper and offshore activities become important, requires that the data of interest to the driller be obtained at the bottom of the well and are detected and transferred to the surface continuously, that is to say without the prolonged delays that would require the stopping of the drilling and the descent of measuring instruments to the bottom of the drilling well Au In recent years, significant progress has been made in the measurement technique during drilling (MWD). As examples of MWD systems to be used to measure the directional parameters of the wellbore one can mention the

  U.S. Patents 3,982,431; 4,013,945 and 4,021,774.

  The measurement systems of the aforementioned patents 1 use pulse telemetry in the mud to transmit information coming from the vicinity of the drilling tool to the surface drilling platform. Pulse telemetry in the mud consists in transmitting information by a column of moving drilling fluid, that is to say of mud, the information corresponding to the parameters detected at the bottom of the well being transformed into a binary code of pressure pulses in the drilling fluid at the inside the drill string or the tube column, these pulses then being detected at the surface These pressure pulses are produced by periodically modulating the column of moving mud at a point located at the bottom of the well using mechanical means, while that the resulting periodic pressure pulses appear at the end of the column of surface mud and are detected by a tra Pressure transducer suitably arranged in the drill string The drilling mud is pumped down through the drill string (tube line) and then returned to the surface by the annular area between the drill string and the wall from the well in order to cool the drilling tool, to evacuate the drilling waste produced by the operation of the drilling tool and coming from the vicinity of this tool and so

  to resist geostatic pressure.

  Since using Pulse Mud Telemetry (MPT), the MPT signal has been

  detected on or near the drill string

  this using drilling column pressure signals (SPP) The MPT process can be positive or negative but all suppliers of MWD devices use this technique This has proven to be adequate and effective for directional information (for example azimuth, inclination, etc.) but is not as effective when it is now a question of transmitting information concerning the formation (resistivity, gamma, porosity, etc.) while the drilling column participates

  has an intense and often difficult drilling action.

  During certain MWD transmissions and, in particular, under difficult drilling conditions, noise caused by drilling hinders the correct decoding of signals in the drill string In fact, under certain drilling conditions, MPT signals in the drill string do not cannot be decoded at all and drilling information at the bottom of the well cannot

  be provided in real time (i.e. MWD) to the driller.

  This inability to decode the pressure signals in the drill string is due to the presence of interference pressure pulses or noises that decrease the signal to noise ratio (SNR) in the drill string to a level below the detection threshold of the MWD decoder located on the surface We analyzed the causes of the interference pressure pulses arriving at the drill string pressure transducer (SPP) with the useful MPT pulses It has been found that the annoying pulses resemble signals but are actually background noise This noise decreases the signal-to-noise ratio (SNR) which is a measure of the efficiency of signal decoding It is not known exactly how all the noise from the drilling operation enters the pressure wave measured by the SPP transducer, but some of the causes have been identified or have been the object of an identification and would be longitudinal vibrations d e the drill string, axial vibration, vibration of the drill bit, accumulation resonance, hydraulic resonance, hydraulic resonance in the drill string, pressure waves produced by the drilling fluid pumps

1 and vibrations of the derrick.

  As already mentioned, the results of these disturbances of the SPP decrease the SNR and therefore the possibility of decoding the MPT signal. This results in a highly undesirable consequence that the driller is unable to use the measurement techniques during drilling to obtain information. directional and training related and must use more costly and time-consuming processes to obtain

  information required at the bottom of the well.

  The object of the present invention is to overcome or alleviate the aforementioned difficulties and shortcomings specific to the prior art by means of a new method for decoding telemetry signals.

of pulses in the mud (MPT).

  In accordance with the present invention, there is provided a method for decoding information at the bottom of the hole in a well being drilled, the drilling operation comprising the use of a tubular drill pipe having a diameter less than the diameter of the borehole obtained, while a generally annular space is defined between the drill string and the well, said decoding being carried out during drilling of the well while the drilling fluid is pumped down inside the column drilling fluid, the drilling fluid emerging approximately at the base of the drill string and returning to the surface through the generally annular space between the drill string and the wall of the well, while the information at the bottom of the well is acquired using MWD detectors in the drill string and that the information at the bottom of the tzonu is encoded as signals carrying the data, which are pressure pulses in the drill string, which are transmitted to the surface in the drilling fluid by the operation of pulse pulse telemetry means 1 in the drill string, the method comprising the following steps: detecting in said annular space the pulses pressure reflected from the pressure pulses in the drill string in the drilling fluid in the drill string, said reflected pressure pulses defining annular return pressure pulses; and decode the detected annular return pressure pulses to obtain information at the

  bottom of the well acquired thanks to the MWD detector.

  In accordance with the present invention it has been discovered that, under certain drilling conditions, the annular pressure signal or the annular return pressure (ARP), although weak, contains an MPT signal whose SNR is better than the SNR of the MPT signal in the drill string In other words and under certain drilling conditions, the different noise generators mentioned above (for example, vibration of the tool and the drill string, noise of the pump, etc.) have

  less influence on the ARP signal than on the SPP signal.

  Therefore, critical MWD information from the ARP can be obtained and provided to the driller under conditions for which the driller has so far been unable to obtain this information because the MPT signal in the drill string (SPP) does not was not decodable. The discovery proper to the present invention (i.e., ARP signals can be precisely decoded to provide MWD information) is both unexpected and surprising Although techniques of pulsed telemetry in mud are known for more than fifteen (15) years, the MPT signals have only been obtained by the drill string in order to decode the information at the bottom of the well This is 1 due to the fact that it has been admitted until now that any signal MPT in the annular area would be so low that it

  would be masked by noise in this annular area.

  Furthermore, it was thought that if the SNR in the drill string was not satisfactory, the SNR in the annular zone would be just as unsatisfactory and therefore unusable for the driller However, a significant aspect of the present invention is the finding that noise in the annular area is not necessarily related to noise in the drill string (and may in fact be much lower) so that, under certain drilling conditions, the SNR in the annular area is better than the SNR in the drill string although the MPT signal is much weaker in the annular zone than in the

drilling column.

  The present invention includes various embodiments for improving the decoding of MPT signals. In a first embodiment, the ARP signal is used to successfully decode the

  information at the bottom of the well from MWD detectors.

  Preferably, in this first embodiment, means are provided to allow the MWD operator to examine the MPT signals both in the drill string (SPP) and in the annular zone (ARP), so that the signals with the best SNR can be used to obtain the lowest drilling error In additional embodiments of the present invention, the SPP and ARP signals are combined to obtain an improved overall MPT signal and, therefore, to obtain an improved SNR This combination can be achieved using addition, multiplication, or correlation. The summing, multiplication, and correlation methods associated with the alternative embodiments can be implemented using known techniques for processing digital signals. The characteristics and advantages mentioned above and others still specific to the present invention will be understood and appreciated by all specialists in this technique after examining the

  detailed description below and attached figures.

  These figures, in which similar elements are numbered in the same way, are respectively: FIG. 1, a general schematic view of a well drilling apparatus according to the present invention; FIG. 2, a schematic representation of a device in the mud using pulse telemetry in the mud; FIG. 3A, a block diagram representing the MPT diagram according to the prior art; FIG. 3B, a block diagram representing the MPT diagram in accordance with a first embodiment of the present invention; FIGS. 4 a to 4 D, graphical representations showing the MPT diagram for the additional embodiments of the present invention; FIG. 5, a block diagram representing the MPT diagram for the embodiments of FIG. 4; and

  Figures 6 A and 6 B, graphs depicting

  both raw MWD data from an operation of

  actual drilling and representing respectively SPP and ARP.

  If we first look at Figure 1, we can see a drilling rig having a derrick 10 supporting a drill string or a drill pipe generally designated by 12, which ends in a drilling tool 14 As as is known in this technique, 1 the entire drilling column can be in rotation, or else the drilling column can be kept fixed while only the drilling tool is in rotation The drilling column 12 is made up of '' a series of interconnected tube segments, new segments

  being added as the depth of the well increases.

  The drill string is suspended from a movable block 16 of a winch 18 and a cross member 19, and the assembly of the drill string of the apparatus described is rotated by a square rod 20 which slides through it the rotary table 22 while being rotated by it which is located at the foot of the derrick A motor assembly 24 is connected so as to actuate the

  both the winch 18 and the rotary table 22.

  The lower part of the drill string may contain one or more segments 26 of larger

  diameter than the other segments of the drill string.

  As is known in this technique, these larger diameter segments may contain detectors and electronic circuits for preprocessing the signals supplied by the detectors. The segments 26 of the drill string may also contain current sources such as mud-driven turbines, which control generators which in turn provide electrical power to operate the sensing elements and data processing circuits An example of a system in which a mud turbine, generator and detection elements are incorporated in a lower segment of the column

  drilling is described in patent U S N O 3,693,428.

  The drilling waste produced by the operation of the drilling tool 14 is evacuated by a current of mud rising through the free annular space 28 between the drilling column and the wall 30 of the well. This mud is brought in by a pipe. 32 to a filtration and decantation system 1 represented diagrammatically by the tank 34 The filtered sludge is then entralée by a pump 36 equipped with a pulsation absorber 38 and is brought by the pipe 40 under pressure to the rotating injection head 42, then inside the drilling column 12 to be brought to the drilling tool 14 and to the turbine

  slurry in the drill string segment 26.

  In an MWD system as shown in FIG. 2, the mud column in the drilling column 12 serves as a transmission medium for transporting the signals of the drilling parameters from the bottom of the hole to the surface. This signal transmission is achieved by the well-known technique of producing pulses in mud or pulsed telemetry in mud (MPT), in which the pressure pulses, represented schematically by 11, are produced in the mud column in the drill string 12 to represent the parameters detected at the bottom of the well, The drill parameters can be detected in a detection unit 44 in the segment of the drill string 26, as shown in Figure 1, and which is located near the drilling tool In accordance with well known techniques and as shown in FIG. 3 A, the pressure pulses 11 created in the mud column in the drilling column 12 are received at the surface by a pressure transducer 46 and the resulting electrical signals are then transmitted to a signal receiving and processing device 48 which can record, display and / or perform signal calculations to provide information about

  different conditions at the bottom of the hole.

  If we still look at Figure 2, we can see that the mud descending in the drill string 12 must pass through a variable flow orifice 50, then is 1 brought to the turbine 52 which it drives The turbine 52 is mechanically coupled to the rotor of a generator 54 and drives it, which supplies electric current to actuate the detectors in the detection unit 44 The output comprising information from the detector unit 44, generally in the form of an electric signal , controls a valve actuator 58 which in turn acts on a plunger 56 which changes the size of the variable orifice 50 The plunger 56 can be controlled electrically or hydraulically Variations in the size of the orifice 50 create pressure pulses 11 in the drilling mud stream and these pressure pulses are detected at the surface by the aforementioned transducer 46 to give indications of the various conditions which are monitored by r unit condition detectors 44 The direction of the drilling mud flow is indicated by arrows in Figure 2 The pressure pulses 11 rise through the drilling mud descending column and at

  inside the drill string 12.

  The detection unit 44 normally comprises a means for transforming the signals proportional to the various parameters which are monitored and putting them in binary form, the encoded information of this

  way being used to control the diver 56.

  The detector 46 on the surface will detect pressure pulses in the drilling mud stream and these pressure pulses will be related to a binary code. In actual practice, the binary code will be manifested by a series of pulses in the bearing mud. information having different durations, with pulse amplitudes generally in the range of 2 105 to 24 105 PA The transmission of information to the surface by the modulated drilling mud current will normally involve the creation of a 1 1 1 preamble, followed by serial transmission of encoded signals corresponding to each of the well parameters

drilling to watch.

  As mentioned above, the drill string, after having descended through segment 26 of the drill string, flows over the drill tool 14 and then returns to the surface through the annular zone 28 between the drill string and the wall 30 of the well As mentioned in US Pat. Nos. 4,733,232 and 4,733,233, it was previously discovered that the pressure pulses resulting from the movements imparted to the plunger 56 also descend into the drill string and are reflected by the bottom of the well (albeit in a clearly attenuated form) with production of pulses generally designated by 55 in FIG. 2 in the annular zone 28 and which can be detected on the surface In order to measure this second pressure pulse in the annular zone or the pulse annular return (ARP) as shown in Figure 1, a second pressure transducer 60 is located on the surface and upstream in the direction of the return mud stream pr from pipe 32 Normally, the magnitude of the pressure pulses detected by the transducer 60 is at least an order of magnitude smaller than the corresponding or related pressure pulses detected by the transducer 46 However, if filtering is used appropriate, these small pressure pulses can be

detected in the annular area.

  In accordance with the present invention, it has unexpectedly been discovered that the ARP signal can be decoded to obtain valid information at the bottom of the well from the MWD detectors (see FIG. 3B). This discovery is both surprising and unexpected because it was generally believed that the ARP signal was so weak that it would be masked by the noise produced 1 at the bottom of the well by the various noise sources described above. An important characteristic of the present invention is the discovery that the SNR in the annular zone is not simply proportional to the SNR in the drill string On the contrary, it has been found that the SNR in the annular zone can be much better than the SNR in the drill string, under certain drilling conditions The possibility of decode a pulse telemetry signal in mud under unfavorable conditions for the detection of conventional pressure signals in the column is of significant importance in the field of MWD, since it allows MWD measurements in situations where it was previously impossible to carry out such measurements. In the example below, the importance of

  the present invention is clearly demonstrated.

EXAMPLE

  Experimental MWD measurements were made on an oil exploration platform in the North Sea Significant noise was recorded when an attempt was made to decode the MPT signals in the

  drill string (SPP signals) in the conventional manner.

  The noise was so strong that it completely masked the SPP signal, so that the drill string signal was undecodable Figure 6 A shows a graph showing the undecodable raw data from the SPP After trying to manipulate the drilling parameters for some time in an attempt to minimize noise, a pressure pulse detector in the annular area was connected to the signals input from the drill string of the decoding device The result was unexpected and surprising , with immediate decoding of the ARP signal into useful MWD information at the bottom of the well (see Figure 6B) The ARP signal was decoded from 3,170 m from 1 depth to around 3,318 m Throughout this drilling section, both the SPP signals that ARP were monitored, but there were few periods during which the SPP would have been decodable Therefore, MWD information could not be provided to the driller

  using only the ARP signal.

  Looking again at Figure 1 it can be seen that, in a preferred embodiment, both the SPP and ARP signals are monitored so that the detected signal having the lowest SNR can be used to decode the information at bottom of the well On the surface, therefore, the pressure variations in the drill string will be detected by the transducer 46 to produce the SPP signal. In an analogous manner, the pressure variations (reflected pulses) in the annular zone will be detected. by transducer 60 and the resulting ARP signal will be processed in the circuit which

  may include an amplifier 62 and a filter 64.

  The computer 68 can be used to compare the SNR of SPP and ARP and to choose the signal having the most favorable SNR for decoding. Preferably, the ARP transducer 60 must be located as far as possible at the bottom of the hole in order to create sufficient hydrostatic pressure (since the ARP signal is so weak) In practice, however, the ARP transducer will be located just

  above the breakout device (BOP).

  As a variant, the signals SPP and ARP can be monitored to compare still other criteria or parameters (different from SNR and the deformation, in order to choose the "best" signal to use. These other criteria include the comparison of each signal in order to choose the lowest bit error rate or compare each signal to choose the signal with the most likely decoding An example of a signal with the most likely decoding is given by

  1 the case where the MWD information encoded includes parity.

  Therefore, if the ARP signal includes a correct parity signal and the SPP signal contains an incorrect parity signal, then the ARP signal is detected. If we now examine FIGS. 4 A to 4 D, we can see there another embodiment of the invention in which the two MWD signals (ARP signal and SPP signal) are used together to produce more decoded data. reliable It will be understood that the signal

  SPP and the ARP signal have in common the MWD pulse.

  On the other hand, the noise does not have the same correlation For example, the pump noise is perceived without attenuation by the SPP transducer and very shortly after it has left the noise source The pump noise detected in the annular zone by the ARP transducer is however (1) attenuated by the effects of the double displacement descending in the drill string and rising in the annular zone and (2) displaced in time by the duration of the course and according to the speed of the impulse (approximately the speed of sound in this fluid) This example implies that, in general, the noise detected by the SPP and the ARP coming from any source can present amplitude differences compared to the signal and a displacement in time In addition, the noise coupled to the SPP signal can be coupled differently or not at all to the ARP signal. Based on this principle, an improved signal can be obtained by combining the common signal information. to the two signals A graphical example is given in FIGS. 4 to 4 D and a means of implementation is shown

by figure 5.

  Figures 4 A 4 D represent a noise pulse created in the drill string near the SPP transducer This pulse descends to the bottom of the 1 hole and returns to the surface with a delay due to the travel time The time during which the SPP transducer detects the noise pulse and the time when the ARP transducer detects the noise pulse are different Consequently, when the SPP and ARP signals are added as shown in the figure (or multiplied or correlated), the signal present in the two forms waves will be reinforced while the uncorrelated noise at all times will keep the same amplitude or will be reduced In the example of figure 4, the noise after summation is located in two places equal to the initial dimension but the signal is doubled Consequently, the SNR is doubled As the possibility of decoding the signal is directly related to the SNR, this is improved by this technique It will be understood that the ARP signal will be weaker than the SPP signal

  depending on where and how to enter it.

  There will however be a constant gain relationship between

  these two signals, which can be measured and adjusted.

  Similarly, there may be a slight time lag, which is generally unimportant This effect will vary with depth and flow but, if significant, can be calculated and eliminated by technique The signal processing techniques necessary to perform the addition, multiplication and correlation phases are steps in processing electronic signals.

well known.

  FIG. 5 represents a block diagram showing how the technique of FIG. 4 can be implemented The amplification of the ARP signal 100 is adjusted so as to be similar to the dimension of the signal SPP 102 This is essentially a function of the transducers chosen, which are commercially available The time offset adjustment can be 1 carried out, if necessary, with an ordinary "all-pass" filter, in order to align the SPP and ARP signals. This process can be automated by calculation correlation or visually measured in a fairly simple way by simultaneously comparing the graphs of the two signals, while the measured delay is introduced into the all-pass filter As already mentioned, the summation step 104 may comprise an algebraic sum of the two signals,

  a multiplication or a correlation.

  In an alternative embodiment of the present invention, the return current in the annular region is monitored with respect to the return pressure pulses. As is known, the slurry impeller causes instantaneous changes in the fluid flow rate. drilling These flow changes are related to the pressure pulses produced by the impeller in the mud Therefore, a flow meter can be used at 60 in Figure 1 (and not a pressure transducer), and the flow changes can be measured and decoded to obtain the information at the bottom of the well acquired using the MWD detector. As already mentioned, US Pat. Nos. 4,733,232 and 4,733,233 report that the mud pulse transmitter located in the drill string also provides a detectable pressure pulse in the annular zone. However, neither of these two patents had neither mentioned nor suggested that the pressure pulse signals in the annular zone contain information which can be decoded with precision to obtain the information acquired at the bottom of the well by the MWD detectors. Similarly, none of these patents suggests or only indicates signal SNR

  ARP can be improved compared to the SNR of the SPP signal.

  The two aforementioned patents relate to 1 methods for detecting an indication of the flow of fluid in a wellbore. The patent 4,733,233 uses the pulse generator in the mud to carry out this detection of fluid flow. In the patent U S .4,733,233, the pressure in the annular zone between the drill string (rod or drill pipe) and the wall

  of the hole is monitored at the surface Amplitude modulation

  study or frequency of the mud flow in the drill string obtained by actuating a valve or a plunger can produce, for example, MWD directional signals in accordance with the principle of the aforementioned US patents no. 3,942,431, 4,013,945 and 4,021,774, so that the flow of mud in the annular zone contains reflections of the MWD signals in the drill string. Monitoring the pressure in the flow of mud from the annular zone to the surface therefore makes it possible to detect the reflected signals originating from the modulation. of the drilling mud column in the drilling column (drill pipe) In an application of the method described by patent US Pat. 4,733,233, the pressure variations detected at the annular zone are compared with the pressure variations detected in the drill string A significant variation in the phase and / or amplitude ratio constituting a significant deviation from the previously obtained data will indicate that it xiste a fluid entry in the annular zone seen that the fluid, for example a gas, passing in the drilling mud will cause an attenuation of the modulated information and / or will affect the speed of transmission According to a second application of the patent US 4 733 233 , the pressure variations in the drilling mud going up in the annular zone are calculated taking into account the very recent history of these pressure variations in the annular zone and, after appropriate compensation for the modifications which took place in the drilling operation, 1 the results of the comparison are used to detect the introduction of a fluid When the signal of the annular zone is lost or greatly altered, either in amplitude, in arrival time or for both factors, an alarm can be created to indicate that a

  fluid has entered the wellbore.

  As mentioned above, U.S. Patent 4,733,233 does not suggest that the pressure pulse signal in the annular area can be decoded to provide bottom-of-the-well information acquired by the MWD detector and encoded in the serial pulses. It also does not suggest that the SNR in the annular zone is improved compared to the SNR in the drill string. US Patents 4,733,233 (and 4,733,232) use only the ARP to measure its phase and / or amplitude or for self-comparison over time; they do not decode the ARP to obtain measurements at the bottom of the well obtained with the MWD detectors.

Claims (7)

1 R CLAIMS   1 Method for decoding information at the bottom of the well in a well drilling operation, the drilling operation comprising the use of a tubular drilling pipe having a diameter less than the diameter of a well bore dug in which a generally annular space is defined between the drill string and the wellbore, said decoding being carried out during drilling of the wellbore and into which the drilling fluid is pumped down inside the drill string, the drilling fluid exiting near the base of the drill string and returning to the surface through the generally annular space between the drill string and the wall of the wellbore, and in which the information at the bottom of the well is obtained by using an MWD detector in the drill string and in which the information at the bottom of the well is encoded in the form of signals carrying the data in the form of pressure pulses which are transmitted its at the surface in the drilling fluid by the use of an induction telemetry means in the mud in the drilling column and detected at the surface, the method comprising the following steps: detecting, in said annular space, the pressure pulses propagated from the encoded MWD signal pulses applied to the drilling fluid in the drill string, said pressure pulses propagated in the annular zone defining return pressure pulses in the annular zone; and decoding the return pressure pulses in the annular zone thus detected to obtain the information at the bottom of the well acquired thanks to the MWD detector. 1 2 A method of decoding information at the bottom of the well in a well drilling operation, the drilling operation comprising the use of a tubular drill pipe having a diameter less than the diameter of a dug well bore, in wherein a generally annular space is defined between the drill string and the wellbore, said decoding being effected during drilling of the well and into which the drilling fluid is pumped downwardly inside the drill string, the drilling fluid emerging approximately at the base of the drill string and returning to the surface through the generally annular space between the drill string and the well wall and in which information at the bottom of the well is acquired using a MWD detector in the drill string and in which the information at the bottom of the well is encoded in the form of signals carrying data in the form of pressure pulses which are transmitted to the on facing in the drilling fluid using means of telemetry of pulses in the mud in the drilling column and which are detected on the surface, the method comprising the following steps: detecting in said annular space the pressure pulses propagated from the pulses an encoded MWD signal applied to the drilling fluid in the drill string, said pressure pulses propagated in the annular zone defining return pressure pulses in the annular zone; detecting pressure pulses in the drill string at or near the surface; calculating the signal-to-noise ratio (SNR) and / or the distortion measurement of the signals in the pressure pulses in the drill string to obtain a first SNR and / or a distortion measurement; calculating the SNR and / or the strain measurement 1 in the signals in the return pressure pulses of the annular zone to obtain a second SNR and / or a strain measurement; comparing the first and the second SNR and / or the first and the second strain measurement; and decoding the return pressure pulses in the detected annular area or the pressure pulses in the drill string in response to the lower value of the first or second SNR and / or   the first or second measurement of deformation.   3 A method of decoding information at the bottom of the well in a well drilling operation, the drilling operation comprising the use of a tubular drilling pipe having a diameter less than the diameter of a wellbore dug in which a generally annular space is defined between the drill string and the wellbore, said decoding being carried out during drilling of the borehole and in which the drilling fluid is pumped down inside the drill string, the drilling fluid leaving approximately at the base of the drill string and returning to the surface through the generally open space between the drill string and the wall of the wellbore and in which the information at the bottom of the well is acquired using an MWD detector in the drill string and in which the information at the bottom of the well is encoded in the form of signals carrying data in the form of pressure pulses transmitted to the surface in the drilling fluid using a range of pulses in the mud in the drill string which are detected at the surface, the method comprising the steps of: detecting in said annular space the pressure pulses propagated from the pulses encoded MWD signal applied to the drilling fluid in the drill string, said pressure pulses 1 propagated in the annular zone defining the return pressure pulses in the annular zone; detecting pressure pulses in, on or near the drill string; combining the signals from the pressure pulses in the drill string and the return pressure pulses in the annular zone; and decode the combined signals to obtain signals carrying improved data representative of the information at the bottom of the hole acquired thanks to the MWD detector.   4 The method of claim 3, wherein said combining step comprises summing the signals. 5 The method of claim 3, wherein said combining step comprises multiplying signals.   6 The method of claim 3, wherein said combining step comprises correlating the signals.   7 A method of decoding information at the bottom of the well in a well drilling operation, the drilling operation comprising the use of a tubular drilling pipe having a diameter less than the diameter of a borehole dug in which a generally annular space is defined between the drill string and the wellbore, said decoding being carried out during drilling of the well, and into which the drilling fluid is pumped down inside the drilling string, the fluid drill pipe emerging approximately at the base of the drill string and returning to the surface through the generally annular space between the drill string and the wall of the wellbore and in which information at the bottom of the well is acquired using an MWD detector in the drill string and in which the 1 information at the bottom of the well is encoded in the form of signals carrying data in the form of pressure pulses which are transmitted s at the surface in the drilling fluid using a means of telemetry of pulses in the mud in the drilling column and which are detected at the surface, the method comprising the following steps: detecting in said annular space the changes in flow rate caused by the use of pulse telemetry means in the mud, said rate changes defining changes in return rate in the annular area; and decoding the detected return flow changes in the annular zone to obtain the information at the bottom of the well acquired by the MWD detector. 8 A method of decoding information at the bottom of the well in a well drilling operation, the drilling operation comprising the use of a tubular drilling pipe having a diameter less than the diameter of a borehole dug in which a generally annular space is defined between the drill string and the wellbore, said decoding being carried out during drilling of the well, and into which the drilling fluid is pumped down inside the drilling string, the fluid drill pipe emerging approximately at the base of the drill string and returning to the surface through the generally annular space between the drill string and the wall of the wellbore and in which information at the bottom of the well is acquired using an MWD detector in the drill string and in which the information at the bottom of the hole is encoded in the form of signals carrying data in the form of pressure pulses which are transmitted the surface in the drilling fluid by the use of telemetry means given in the form of pressure pulses which are transmitted to the surface in the drilling fluid by the use of pulse telemetry means in the mud in the drill string and which are detected at the surface, the method comprising the following steps detecting in said annular space the pressure pulses propagated from the MWD signal pulses to be encoded applied to the drilling fluid in the drill string, said pressure pulses propagated in the annular region defining return pressure pulses in the annular region; detecting pressure pulses in the drill string at or near the surface; monitoring a parameter selected from the pressure pulse signals in the drill string to obtain a first monitored parameter; compare the first and second monitored parameters; and decoding the detected back pressure pulses in the annular area or the pressure pulses in the drill string in response to the first   or the second parameter monitored.   9 Claim according to claim 8, in   wherein said first and second monitored parameters are selected from the group consisting of SNR, strain measurements, bit error rates or signals of parity.