ITMI941637A1 - SEISMIC SIGNALS DETECTION PROCEDURE FOR OBTAINING VERTICAL SEISM PROFILES DURING DRILLING OPERATIONS WITH A TOP-DRIVE EQUIPMENT - Google Patents

Description

Description

The present invention relates to a process for collecting well seismic data, including vertical seismic profiles (VSP), which provide information on the geological formations of the subsoil affected by the drilling.

More specifically, the present invention relates to a process for collecting the aforementioned data during the drilling of wells with plants equipped with a "top drive" system.

The collection of vertical seismic profiles is essentially based on the recording of the waveforms of direct and reflected signals corresponding to a signal generated by a seismic source, consisting of vibrations or pulses of elastic energy, after its crossing of geological layers at various depth.

The position of the various subsoil layers is reconstructed on the basis of the delay times with which the signals return to the receivers.

The conventional technique of VSP (Vertical Seismic Profile) prospecting involves interrupting the drilling operation, extracting the string of rods and the drilling bit, introducing one or more detector geophones into the already drilled well and generating on the surface, or at shallow depths, and in the vicinity of the well, one or more acoustic pulses (for example by shining explosive charges) and detect the signals coming from the geophones placed inside the well. This mode does not give uncertainties about the "primary" signal generated on the surface and the detected return signals do not present great difficulties in interpretation, but equally presents considerable drawbacks. In fact, it is very expensive, also because it requires the interruption of drilling operations for a long period of time, and therefore can be applied only very few times during drilling. The operation of extracting and reintroducing the drill string is very difficult and not without risks, and therefore requires special safety procedures and precautions.

More recently it has been proposed to carry out such surveys using the same signal generated by the boring bit during drilling as a seismic source.

For example, the detection systems described in US-A-4,965,774, US-A-4,862,423, US-A-4,954,998 and US-A-4,718,048 can be mentioned.

The use of the same boring bit as a seismic source has the advantage of allowing surface measurements during drilling without interfering with site activities and of collecting a lot of data at low cost and with few risks, with frequent or even inexpensive survey campaigns. continuous. However, the signal generated by the bit has the drawback of being continuous over time, and it is therefore very difficult to establish which is the reliable signal generated by the bit in order to be able to usefully compare it with the return signals detected by the sensors positioned on the ground in the area surrounding the well being drilled.

The return signal that is measured is generally related to the signal emitted by the bit via a transfer function and is generally noisy. This seismic source therefore offers great advantages if it is possible to identify the signal of the bit which is distributed over time and disturbed by strong environmental noises, for example the noise generated by other construction machinery and pumps, separating it from this disturbance.

The Italian patent application in the name of the applicant ΙΤ-Α-ΜI 93A00196 dated 05/02/93 solves the problem of obtaining a sufficiently high Signal / Noise ratio to obtain significant information on the nature and configuration of the lithological formations interesting from drilling. The above application solves the problem of defining a pilot signal representative of the bit signal, using a plurality of joint measurements of the bit "noise" by means of a plurality of sensors placed on the drilling rig.

It has now been found that using the so-called "top drive" drilling technique it is possible to obtain equally effective results by using one or more vibration sensors, placed on the top-drive or on the slide that slides on the rails, and / or intensity sensors of current placed on the motor or its power cable.

In accordance with this and with reference to Figure 1, the present invention relates to a process for obtaining vertical seismic profiles (VSP) during the drilling of wells with plants equipped with top-drives, using the vibrations produced by the bit as a seismic source. drilling, by cross-correlating the signals (i) collected by a plurality of receivers arranged to form one or more seismic lines for receiving on the surface of the seismic waves that have been generated by the working bit, with a pilot signal (ii) representative of the signal generated by the bit, propagated along the drill string and registered by one or more sensors,

characterized by the fact that the aforementioned sensors are chosen from:

a) vibration sensors (13),

b) current intensity sensors (14), preferably an amperometric clamp,

with the further constraint that:

1) the sensors (a) are positioned on the top-drive (4) or on the slide, integral with it, which slides on the rails (5),

2) the sensors (b) are positioned on the motor or are connected to it, preferably they are positioned on its power supply cable (11) which reaches the motor from the ground through the sludge delivery.

The so-called top-drive is a mobile system for transmitting motion to the string of drill rods. It consists of an electric motor, supported by cables (10), operated continuously which runs on guides (rails) supported in turn by the structure of the tower and whose rotor is keyed to the shaft that transmits, through the toothed wheels , rotation at the rods.

It is equipped with air brakes and oil cooling; being an electric motor operating in the area of potential deflagrations, it is continuously ventilated in order to eliminate any accumulation of gas in its cavities.

Figure 1 shows the top-drive configuration equipped with the seismic data collection system, where:

- (1) indicates the structure of the drilling tower,

- (2) indicates the string of drill rods which carries the drill bit at its end,

- (3) indicates the drill bit,

- (4) indicates the top-drive which activates the battery of rods (2),

- (5) indicates the guides (rails) on which the top-drive (4) slides,

- (6) indicates the pit dug by the chisel (3) in its descent into the ground,

- (7) indicates a line of detection sensors (8) which receive both direct and reflected seismic signals originating from the seismic bit source that cross the ground and are collected and recorded with a recorder (9),

- (10) indicates the motor support cables, - (11) indicates the motor power cables, - (12) is the sludge delivery,

(13) represents the vibration sensors positioned on the motor or its slide,

- (14) indicates the current intensity sensors.

The line or lines of the detection sensors are currently indicated simply as a seismic line and are generally placed at a certain distance from the drilling site, according to optimization criteria that make it possible to acquire seismic data from an area of a certain extension around the borehole. This seismic line is generally made up of geophones, in the case of onshore operations, or of hydrophones, in the case of off-shore drilling.

Fig. 2 represents an example of the results obtained using an amperemeter clamp as the pilot sensor # according to one of the preferred embodiments of the present invention. In particular, figure 2a represents the cross-correlations of a single surface receiver (8) of the seismic line (7) with the signal of the amperameter clamp (14) collected at an increasing drilling depth from left to right. The vertical scale of the figure represents the correlation time in seconds, while the zero of the correlation times corresponds to 2.0 seconds in the scale of the figure itself. Fig. 2b shows the cross-correlation of all the receivers (8) of a seismic line (7) with the pilot signal of the amperometric clamp (14) recorded at a single depth of the bit (3). The technical problem which the present invention intends to solve is that of having a method suitable for obtaining measurements with a sufficiently high Signal / Noise ratio to obtain significant information on the nature and configuration of the lithological formations involved in top-drive drilling. .

The amount of noise contaminating the primary signal (-driver signal-) causes a deterioration in the detection quality, therefore the signal must be measured under the most favorable conditions in order to identify the signal component and reduce the noise component.

According to what is reported in the Italian patent application in the name of the applicant IT-A-MI 93 A 00196 dated 5/02/93, the procedure is based on the technique of adding the contribution of many data collected by the detection sensors in a discrete period of time , in order to strengthen the aforementioned signal / noise ratio. The duration of said period in which the data can be added is limited by the fact that over time the bit sinks in. This technique has practical application limits, for example the drilling of softer or less compact layers generates weaker signals and the measurements must be continued for longer periods to be meaningful. In this case, however, the drilling is faster and there are limitations in the duration of the period in order not to lose spatial resolution, due to the excessive sinking of the bit during the measurement.

The disturbance prevents clear listening to the chisel signal and can only be partially eliminated with the technique of adding the contribution of many repeated measurements in discrete periods of time: the achievable result is a better signal / noise ratio but not yet sufficiently high if the acquisition takes place in an unfavorable position.

The present detection procedure is based on the collection of the return signal (or better the signals) with the detectors (8) and on its correlation with a measurement of the signal emitted by the bit after having measured it (on the top-drive or along its cable power supply) and possibly preliminarily processed, in order to obtain a significant measurement and made under extremely favorable conditions (pilot signal).

The correlation operation, already described in the aforementioned Italian patent application in the name of the Applicant, qualitatively consists in making two signals to be compared flow temporally with respect to each other until they are aligned with strongly similar trends. If the operation does not give a useful result, there is an incorrelation between the signals compared. The autocorrelation operation consists in comparing the signal with itself to identify any echoes or reverberations.

Cross-correlation is the correlation operation between the pilot signal and the signals collected with the seismic line.

According to the procedure described above, the signal consisting of a temporal sequence of pulses (amplitude peaks with positive or negative sign) is identified: it is concentrated in a single pulse signal for each measured geophysical event, for example the direct signal of the bit followed by the his reflections. This signal is usually made up of a series of well identified relative amplitude and time coordinate pulses.

It is also necessary to take into account the fact that the pilot signal is not in fact an ideal signal because:

the pilot signal has its own delay due to the propagation of the signal from the bit to the sensor that measures it. Since the correlation time is a relative delay between two signals, in order to obtain the absolute delay of the geophysical data measured on the ground, it is necessary to compensate for the delay already contained in the pilot signal;

the pilot signal is not only the sequence of impulses generated by the bit but also contains its repetitions due to reverberation phenomena inside the drilling rig, which has its own characteristic of elastic vibration; the signal collected by the sensors must therefore be reverberated during the processing phase, before or after the cross-correlation, in order to be able to reconstruct a significant signal due to the bit.

Each measurement that is used for the determination of the pilot signal contains environmental and spurious disturbances which overlap the bit signal; the collected signals can be processed to reduce the presence of such noise and strengthen the signal.

On the basis of the terms of the technical problem set out up to now and of the general lines of the seismic detection technique using the bit as a seismic source, the detection system according to the present invention is described in its characteristic contents, with reference to the diagram of figure 1 which illustrates a good typical realization for illustrative but not limitative purposes.

Already in the aforementioned patent US-A-4,954,998 a solution of active subtraction of the noise of the pilot signal is proposed, according to which the noise is collected with a sensor placed on the battery of the rods and considered as substantially insensitive to the signal of the bit, selecting the band frequency of this noise, amplifying and then subtracting it from the pilot signal. This technical solution considers the horizontal vibrations measured on the head of the rod battery (swivel) in a system equipped with a rotary table, as noise to be subtracted from the vertical component, measured on the head itself. In fact, in a system equipped with a rotary table, the upper external part of the rods is constrained to the height of the probe plane of the rotary table, but is relatively free to oscillate laterally at the height of the head with consequent disturbance for the measurements of the bit signal. . On the contrary, in a drilling rig equipped with a top-drive, on the other hand, the upper end of the battery of the rods is solidly connected to the engine which in turn is constrained by the rails.

The present invention, limited to drilling rigs equipped with top-drives, essentially consists in using the engine to obtain at least one pilot signal which must then be correlated to the signals collected by the seismic sensors (8).

In the processes described in the prior art, the sensors intended to detect the pilot signal generated by the bit are typically placed on the battery of the rods.

The choice of sensors is important using a single pilot sensor, with which it is important to identify a signal from the bit that is not only strong (in the sense that its significant peaks have significant amplitudes) but above all good (in the sense that these significant peaks are not polluted. from noises that make them unidentifiable).

In the case of the top-drive, the motor satisfies these characteristics since the measurement of the vibrations of the bit, carried out directly on the top-drive or on its slide or by connection to the motor itself, allows to detect both longitudinal vibrations and torsional vibrations that are transmitted along the rods up to the engine block and the slide that supports it, thereby providing good quality pilot signals. The measurement of the current absorbed by the motor carried out with an amperometric clamp also serves this purpose. Torsion measurements are useful for identifying the signal of bits that emit torsional vibrations, such as those of the PDC type (Polycrystalline Diamond Compacts), as well as axial in the rods.

The above solution for the top-drive system is not satisfactory in the case of a system with a rotary table. In fact, in this hypothesis, the use of sensors placed above the rotary table (such as accelerometers on the head of the battery of the rods or even sensors in the battery itself) allows to measure the longitudinal (axial) vibrations in the rods, but it is not suitable for detecting the torsional load, which is discharged on the rotary table itself. The sensors for measuring the torsion, for example accelerometers or strain gauges, must in this case be located below the rotary table, therefore in the rods, modifying the battery and using a radio link.

According to the present invention, the pilot signal is obtained and processed on the basis of the detections of one or more sensors, even of different types, selected from:

a) vibration sensors (13) placed on the top-drive (4) or on the slide that slides on the rails (5), for example geophones, accelerometers or mono- or tri-axial sensors that give information on the impulses generated by the motion of the chisel, to detect the signal transmitted to the motor or its slide through the battery of rods for direct coupling between the top-drive (4) and the battery (2); b) current intensity sensors (14), for example an amperometric clamp positioned on the motor or connected to it, for example on the power cable that reaches the motor from the ground along the sludge delivery; said sensors have the purpose of measuring the instantaneous absorption of electric current by the motor (4). The torsional forces of the rods are transmitted through the battery to the motor. These sensors provide the measurement of a variable quantity but of a different type from that measured by the vibration sensors (13), to which however it is strictly correlated.

Each of these different types of sensors, which can be used alone or in combination with each other, detects a signal that has traveled through a different transmission medium, such as the steel of the probe and the electrical connection of the motor.

Each of these sensors provides a signal, or rather a trace, to be used for cross-correlation. A sensor thus provides a sequence of measurements, expressed in digital or analogue values, currently called trace.

Each trace consists of a series of measurements in digital or analog values, characterized by two essential data, an intensity value and a time coordinate value, which can be reduced to an ordinal number of the measurement during the determination.

The intensity value can be expressed, depending on the sensors considered, as the speed of the particles due to the elastic waves due to the propagation of sound (measured by the geophones), as pressure (measured by the pressurometers placed in hydraulic circuits), as acceleration (measured by accelerometers) or finally by the current absorbed by the electric motors (measured with the amperometric clamp). Another measurable parameter can be strain, using precision extensometers.

The traces obtained by the aforementioned sensors make it possible to identify, by means of correlation, the peaks determined by the pulses of the bit. According to a preferred embodiment of the invention, the detection and recording of the data are carried out with digital values. The measurements, possibly processed in advance with filtering or deconvolution operations, are subsequently used to prepare a very significant pilot signal, to be used for its correlation with the signals detected with the seismic line.

For example, the trace obtained from the sensor (13) can be adopted as the reference trace. Since the length of the drill string and the speed of sound in the steel are known, it is possible to obtain a reliable value of the real delay between a blow of the bit and the arrival at the sensor (13) of the corresponding impulse.

As already illustrated, the sensor (13) for listening to the signal intended to produce the pilot signal receives the trace P (). This trace is used as a transform in the frequency domain (indicates frequency).

By transform in the frequency domain we generally mean the decomposition of the signal into the elementary sinusoidal oscillations that are present in it, each with its phase and its amplitude. Two real numbers are then necessary to describe each component and therefore complex numbers are used (cf. Prodi G., Mathematical Analysis, 1976 p. 91/97). The transform is well known as the Fourier transform (see Bracewell R., The Fourier transform and its application, 1985).

The frequency domain is a very useful parameter for the determination of signals due to the simplicity of the correlation operation between two signals, which consists in making the frequency product by frequency of the first signal by the complex conjugate of the second.

And again, the filtering operation is performed simply by multiplying frequency by frequency the spectrum of the filter by the spectrum of the signal to be filtered.

The measured signals are eventually deconvoluted (by deconvolution we currently mean an inverse filtering operation that effectively removes the reverberations and "multiples" of the chisel signal) to eliminate the reverberations present in them and possibly filtered to eliminate frequency components due to the disturbance or to modify the impulse responses of individual channels. If an instantaneous event generates a signal that is measured in a channel, it produces not really a pulse but a signal with a waveform, generally very short in time, which depends on a multiplicity of factors such as, for example, the characteristics of the sensor, the quantity that is measured, the coupling between the sensor and the medium to which it is applied, etc. You can even create filtering operators that modify these responses, both from the knowledge of the characteristics of the instrumentation and with direct measurements, such as the estimation of matching filters between signals.

The pilot signal measured and possibly processed with the methods described up to now is then used in the cross-correlation between the pilot signal representative of the primary signal and the signals collected by the geophones (8) of the seismic line (7).

The result obtained with the elaborations described up to now is a good quality seismic signal representative of the pulses of the bit.

Claims (4)

CLAIMS 1. Procedure for obtaining vertical seismic profiles (VSP) during the drilling of wells with plants equipped with top-drives, using the vibrations produced by the drilling bit as a seismic source, cross-correlating the signals (i), collected from a plurality of receivers arranged to constitute one or more seismic lines for receiving on the surface the seismic waves that have been generated by the bit in operation, with a pilot signal (ii) representative of the signal generated by the bit, propagated along the drill string and recorded by one or more sensors, characterized by the fact that the aforementioned sensors are chosen from: a) vibration sensors (13), b) current intensity sensors (14), with the further constraint that: 1) the sensors (a) are positioned on the top-drive (4) or on the slide, integral with it, which slides on the rails (5), 2) the sensors (b) are positioned on the motor or are connected to it, preferably they are positioned on its power supply cable (11) which reaches the motor from the ground through the sludge delivery. Method according to claim 1, characterized in that the pilot signal is sufficiently representable by the recording of a single sensor. Method according to claim 1, characterized in that the current intensity sensor is an amperometric clamp. 4. Process according to claim 1, characterized by the fact that the current intensity sensors are placed on the power supply cable which reaches from the ground to the motor along the sludge delivery.