GB2370144A - Method and apparatus for downhole command communication and data retrieval - Google Patents

Abstract

Apparatus for transmitting a command signal to a tube comprising: (a) at least one clamp (300) for coupling a housing (302) to the tube; and (b) a piezo-electric stack (318) within said housing (302); wherein a first end of the piezo-electric stack (318) contacts an upper housing portion (302a) and a second end contacts a lower housing portion (302b). Preferably two clamps are provided, one on each portion of the housing. The clamps are in the form of jaws (304, 306, Fig 3 not shown), through which the tube e.g. a drill string passes. Also disclosed is a method of transmitting data to and from the tools etc. in the well bore. The piezo-electric stack is excited at two frequencies representing binary 0 or 1. To compress the data, it is sent in a format comprising first an intial value (24 bits), 2 bits indicating how many bytes of value change follow and finally the 0/1/2/3 bytes indicated which give the magnitude of the change in the previous value.

Description

METHOD AND APPARATUS FOR DOWNHOLE COMMAND COMMUNICATION AND DATA RETRIEVAL The present invention relates to a method and apparatus for downhole command communication and data retrieval. More specifically, the invention relates to an acoustic telemetry system for use in communicating commands to a down hole device. The device produces an acoustic signal that is transmitted through the tubing that extends into the well.

The completion and production of an oil well involves running a number of down hole tools into the well on tubing. The down hole tools include packers, safety valves, perforation guns, adjustable ports and sleeves, pressure and temperature sensors, samplers, flow sensors and the like. In each case, some signal is required to actuate the device. Sometimes that signal is conveyed with a fluid pressure. Other times it is conveyed by an acoustic wave. However, it can be difficult to transmit a discernable acoustic signal into a down hole environment. Typically, the device to be actuated is located many thousands of feet down hole (1 foot = 0. 305m). Further, the device is usually immersed in a fluid under high pressures and temperatures.

One method of sending an acoustic signal down hole is the use of a piezo-electric stack coupled to the tubing. A piezo-electric material is one that vibrates when subjected to a voltage. Likewise, if it is mechanically vibrated, the piezo-electric material will produce a voltage. The intensity of the vibration is proportional to the strength of the voltage. Thus, a votage signal can be readily translated into an acoustic signal. The piezo-electric stack comprises a group of piezo-electric crystals. The use of a piezoelectric stack to actuate a down hole tool is generally discussed in US 5,293, 397.

Acoustic telemetry through tubing is inherently bi-directional. However, acoustic signals through tubing are subjected to a non-uniform comb filter generated because of the structure of the tubing. In other words, the physical shape of the tubing and the collars that connect lengths of tubing filter out certain frequencies. The bi-directional nature of the tubing means that command signals can be sent down hole while data signals can be sent up hole from down hole sensors.

A need exists for a modular, downlink acoustic telemetry device. The transmitter part of the device should generate acoustic waves of pre-defined characteristics in the body of the tubing without being affected by the operating environment. The acoustic waves need to be generated with well-defined frequency and shape characteristics, as well as with sufficient amplitudes such that they can be readily detected by the receivers at depths exceeding 2000 feet (610m). The receiver part of the device is required to receive acoustic pulses from the transmitter downhole via the piezo-electric stack or an accelerometer mounted on the device. The device should be easy to dismount once a test is complete.

A need further exists for a means for sending coded communications to a plurality of down hole devices so that only specific selected tools are activated. A need also exists for a repeater for boosting the range of a command or data signal. Further, a need exists for communicating data back to the surface from a down hole testing device.

The present invention relates to an improved piezo-electric stack device as well as to a method of using the improved device in conjunction with a repeater and a downhole transceiver to control down hole devices and to relay data back to the surface. A stack of piezo-electric discs, preferably with a hole in the center of each disc, are stacked together, preferably to form a cylinder. This assembly can be held together by a sufficiently prestressed section comprising either of tie rods or a tie collar. The assembly connects on one end to the lower connector and at the other end to the upper connector. The upper and lower connectors are integral with the upper and lower mechanical clamps. The mechanical clamps convey the vibrations developed in the assembly to the tubing. An accelerometer is located on the lower clamp.

According to one aspect of the invention there is provided apparatus for transmitting a command signal to a tube comprising: (a) at least one clamp for coupling a housing to the tube; and (b) a piezo-electric stack (PES) within said housing; wherein a first end of the PES contacts an upper housing portion and a second end contacts a lower housing portion.

According to another aspect of the invention there is provided a method of actuating a down hole device using a piezo-electric stack (PES), said method comprising:

(a) coupling a piezo-electric stack to the tubing ; (b) exciting the PES to create a vibration at a first and a second frequency, wherein said second frequencies are used to encode a signal.

With the present invention down hole tools can be controlled through the use of a signal encoded onto an acoustic wave that is transmitted through tubing to a down hole transceiver that receives the signal and is coupled to the tool. Likewise, data from down hole sensors can be conveyed back to the surface encoded onto acoustic waves sent through the tubing. In either case, the present invention can provide the apparatus required to encode the signal and to convey it to the tubing by means of a piezo-electric stack.

Reference is now made to the accompanying drawings in which: Figure 1 is a schematic showing the general system of actuating a down hole device using an acoustic signal, in accordance with the invention; Figure 2 is a more detailed schematic showing the interface between the surface transceiver and a computer used to initiate a command signal or to store retrieved data; Figures 3,4 and 5 are perspective views of an embodiment of surface transceiver unit; and Figures 6a to 6e is a sectional of an embodiment of a down hole transmitter.

The ability to actuate a down hole tool with an acoustic signal requires a transceiver mounted directly to the tubing that is suspending the tool in the well. Figure 1 illustrates one embodiment of the present invention 100, namely the use of a transceiver 110 coupled to tubing 12 in a well 10. The transceiver generates acoustic signals that are transmitted down the tubing 12. The acoustic signals can be varied in shape and frequency. Likewise, the signal can be packetized so that it contains a header and a data portion. The header might contain an address or other identifier for a specific down hole tool. The data portion might be a command signal or data from a down hole sensor. The command signal can be generated by a computer on the surface 170. Likewise, the computer 170 can store information retrieved from a down hole sensor.

The command signal propagates down the tubing 12 because the tubing acts as a wave guide. However, the amplitude of the signal diminishes in proportion to the distance

it travels. Thus, in one embodiment, a repeater 120 can be used to re-amplify the signal and thus allow it to reach tools deeper in the well. Multiple repeaters can be used to allow use of the present system in almost any depth well. A wireline probe 130 can also be used as an alternative mode of communication and/or system calibration. A second transceiver 140 is located down hole to receive the acoustic signal. The transceiver can contain a power source and be coupled to a motor that can be used to actuate a down hole device. For example, a command signal might be used to deploy a packer.

Alternatively, at least one transducer 160 can be used as a sensor of down hole conditions. For example a pressure transducer could be used to sense down hole pressures. As mentioned above, the tubing is a bi-directional waveguide. Thus, the data retrieved from the sensor 160 can be sent back up the tubing to the surface.

Referring to Figure 2, the surface transceiver 110 is shown in more detail. The transceiver 110 is clamped onto the tubing 12 with clamps 202. The transceiver 110 comprises a sealed box that contains the electronics 212, an accelerometer 206 and batteries 208. The accelerometer 206 is used to detect an acoustic signal in the tubing 12. A Nema 7 enclosure, for example, could be used. An interface cable is used to convey the signal from the accelerometer to the surface computer 170. Further, a wireline cable 214 can be used to send signals to the wireline probe 130 that is suspended beneath the rig floor 20.

Figures 3,4, and 5 provide a more detailed view of the clamp assembly 300 used to attach the transceiver 110 to the tubing 12. The transceiver comprises a sealed box 302 that contains the piezo-electric stack and accelerometer. A first clamp assembly 312 and a second clamp assembly 314 are used to couple the transceiver to the tubing. Each clamp assembly comprises a first jaw 304 and a second jaw 306. Each jaw has a contact surface 310 that makes direct contact to the tubing. The first and second jaws are held in alignment by a pair of alignment bolts 308. The first and second clamp assemblies are not rigidly fixed to each other. Instead, they can move relative to each other. This is accomplished with a two-part housing 302. The nature of the housing 302 is best shown in Figure 5. Note that the upper housing 302a is connected to the lower housing 302b by a bolt 316. A gap 336 is provided to allow a certain amount of vertical translation between

the two. An o-ring 330 can be used to seal between the upper and lower housings 302a, 302b.

The piezo-electric stack (PES) is formed by a plurality of discs 318 that are captured between the upper and lower housing. A sheet of copper 340 or other conductive material is placed between each disc 318. Also, an insulator 332 is placed between the end discs 318 and the upper and lower housing. A conductor 338 is used to carry voltage to each of the discs. A power cord 334 connects the conductor 338 to a voltage source such as battery 328. A conduit 326 can be used to protect the conductor 338. In use, a voltage is applied to the PES causing them to vibrate. The vibration is experienced by both the upper and lower housings and by the first and second clamp assemblies. The vibration is transferred to the tubing and travels down the tubing to either a repeater for retransmission or to the down hole transceiver. The voltage can be varied to include a coded command.

The bolt 316 used to connect the upper and lower housing also places a predetermined strain on the PES. As temperatures fluctuate, the length of the bolt can change, as can the size of the discs 318. For that reason, it is important that the bolt have a coefficient of thermal expansion compatible to that of the PES, thus maintaining a consistent load on the PES over a range of temperature.

In use, the operator must place the transceiver 110 onto the tubing 12 so that the tubing goes between the first and second jaws of both the upper and lower clamp assemblies. To aid in the placement of the transceiver, the transceiver can be equipped with handlebars 322. The handlebars are shown in Figure 4. Once the user has placed the tubing between the jaws, the second jaw 306 is tightened against the tubing. When a command signal is desired, a signal carrying voltage is then applied to the PES causing it to vibrate in accordance with the signal. An acoustic frequency can be used because it can carry down the tubing the greatest distance before attenuation requires the signal to be amplified by a repeater. For example, a frequency between 100 Hz and 2000 Hz can create an acoustic signal. In one embodiment, two frequencies are used with the first frequency representing a 0 and the second frequency representing a 1. Thus, a

command signal of 010011 could be created by alternating the frequency of the PES between the two frequencies.

Referring to Figure 6, an illustration of the down hole transceiver is provided. It is similar to the surface transceiver, but has an ability to read data from a plurality of sensors. A controller polls those sensors. The controller is usually a programmable digital signal processor. Once it harvests data from the sensors, the data can be stored in the individual controller's memory such as a Flash RAM. In addition, the data is transferred to the transmitter portion of the transceiver, compressed, encoded, and transmitted acoustically by the firmware running on the transmitter's DSP chip.

The transmitter is connected to each controller's built-in serial port, and utilizing a special 9 bit protocol, individual controller's may be addressed for commands and/or data transfer. The serial connection, while similar to RS-232, uses different signal levels than the RS-232 standard. While operating in this 9 bit mode, each controller monitors the serial communications port, and when (and only when) the 9th bit is set, they check the remaining 9 bits of data for their individual address. When the address in the lower 8 bits match the address programmed into a controller, that controller then accepts all data transmitted over the serial port until the 9'"bit is set again. The remaining controllers ignore all communications until the 9th bit is again set. Utilizing this protocol, the transmitter has the ability to pick and choose which controller it is sending information to, and receiving data from.

In one embodiment, the transmitter gathers a set of 12 pressure/temperature readings, based on the data acquisition rate, from a controller, and holds them in memory. Once 12 readings have been collected, the data set is then processed by the transmitter. This processing involves compressing the data to reduce the total number of bits to be transmitted; encoding the resulting bits into a'packet'of information; and transmitting that data serially to the next device utilizing acoustic energy.

The individual data sets to be transferred can be quite large when you consider the bandwidth that is available with the current acoustic transmission. With 12 readings per 'packet', and 48 bits per reading, and another 25 bits for packet information, there could be over 600 bits of information to be transferred. With typical baud rates limited to 6.3 to

10 bits per second, this could take from 1 to 11/2 minutes to transfer the information acoustically. If it takes 2 minutes to collect the data set for a packet, a 11/2-minute transmission time would not allow enough time for the data to be repeated by another device before another packet would be sent out by the transmitter. Because of these bandwidth limitations, a compression scheme can be deployed.

While numerous compression schemes are available for compressing numerous bytes of information into a smaller package, a novel technique is used in present invention. The compression technique is referred to as a delta data technique. In the delta data technique, the 12 reading data set is comprised of 1 full initial pressure and temperature reading, followed by 0,1, 2, or 3 bytes that represent how much the data has changed from the previous reading. To identify how many delta bytes are being transmitted prior to transmitting a delta data set (the 0,1, 2, or 3 bytes of changed data), there are 2 bits to identify the number of bytes being transmitted (2 bits are all that are needed to represent the numbers 0 through 3). This results in a compressed data set of 24 bits of Pressure Data, 24 bits of Temperature Data, and 11 sets of 2 bits of Pressure Byte Count, followed by 0/1/2/3 bytes of Delta Pressure, and 2 bits of Temperature Byte Count, followed by 0/1/2/3 bytes of Delta Temperature. Using this technique, the 576 bits of data information (12 readings with 24 bits per pressure and 24 bits per temperature reading) can be reduced to a theoretical minimum of 92 bits (if no delta bytes are required). The theoretical maximum number of bits needed to represent the data in a worst-case scenario (all of the delta's need all 3 bytes to represent the change) is 620 bits. Typical compressions with static pressures and temperatures result in a size of the data portion of the packet of around 220 bits.

When sending data, there is always a possibility that a data packet might not be received. Thus, in one embodiment, each packet starts with a full pressure and temperature reading. Alternatively, each packet can start with deltas from the data in the previous packet, thereby allowing for greater compression. The delta data technique is modified slightly for history data. History data is data that the transmitter reads from the controller's flash memory, and represents data that has been previously collected and stored by the controller. This data can be retrieved by commands from the transceiver or

probe to replace data that may have been lost due to equipment problems, or to retrieve every sample point in instances where the controller was programmed to gather information faster than the transmitter was programmed to collect it. This data needs to have a time associated with the data so that it can be correctly integrated in with the other real-time data.

The history data compression starts with the full 72 bits for the initial reading (24 bits each for elapsed time, pressure, and temperature). The delta data is then repeated for 11 sets of 2 bits of time byte counts followed by 0/1/2/3 bytes of delta time, 2 bits of pressure byte count, followed by 0/1/2/3 bytes of delta pressure, and 2 bits of temperature byte count, followed by 0/1/2/3 bytes of delta temperature. After the data has been compressed, the actual packet can be built by prefixing a packet header, and appending a checksum. The packet header consists of a standard 15-bit pattern that when received by any other device, is recognized as the start of a data packet.

In real-world situations, there is often a lot of extraneous noise that the system can hear. Most of this noise is filtered out by the receiving device because it is at the wrong frequency. Noise often does occur at the frequencies that are being monitored, so some method of discriminating noise from data was required. By prefacing the data packet with the packet header, the receiving device can identify the start of a data packet and start saving it for later decoding.

To take care of noise that may hit at the receiving frequency during the transmission of a packet of data, there is a 10 bit negative checksum at the end of the packet. During encoding, this checksum starts at 0, and each number placed in the packet is subtracted from the checksum. When the data is then decoded by the receiving software, the checksum is initialized to 0 and each number is added to it. By the time the checksum that is transmitted in the data packet is summed in, the checksum calculated by the receiving software should be 0. If the sum is not zero, then there is an error in one or more bits in the packet received.

There is some additional information that is added to the packet after the packet header, and before the actual compressed data. Immediately following the header, there are two bits that are used to identify the controller from which the data was collected.

Following this is one bit that identifies the packet as a'normal'data packet or a'history' data packet. One more bit follows which is reserved for future use. After all this encoding, the final packet will look like this: 15 bits Packet Header (Ox4D78) 2 bits Controller Number 1 bit Data/History packet flag 1 bit Reserved 24 bits Complete Elapsed Time (only if this is a History packet) 24 bits Complete Pressure reading 24 bits Complete Temperature Reading Repeat the following 11 times: 2 bits for a count of Delta Elapsed Time bytes (only if this is a History packet) 0/1/2/3 bytes of Delta Elapsed Time (only if this is a History packet) 2 bits for a count of Delta Pressure bytes 0/1/2/3 bytes of Delta Pressure 2 bits for a count of Delta Temperature bytes 0/1/2/3 bytes of Delta Temperature 10 bits Negative Checksum of the data from the Controller Number down to the last Delta Temperature byte.

After the data is collected and encoded into a packet, the software in the Transmitter sends the data acoustically to the next receiving device via a piezoelectric stack. During programming of the Transmitter, parameters are entered that control the frequencies and timing for converting the packet data to an acoustic packet. These parameters are the Stack On Time, Stack Off Time, FskO Frequency, and Fsk1 Frequency.

Once the acoustic packet is transmitted to the pipe, it travels up the steel body where it is picked up as vibrational energy by an accelerometer attached to the next device (each device has an accelerometer for reception, and a stack for transmission).

The accelerometer is basically a very sensitive microphone that picks up vibrations and converts them to electrical energy for processing.

The receiving portion of each device operates by constantly'listening'to all sounds that are moving over the pipe. By using a software'filter', only the frequencies that were defined for the device (the 0 and 1 frequencies) are scanned, and the data header is searched for (Ox4D78). When the data header is found in a stream of sounds, the software starts decoding and saving the data until a quiet period occurs where there are no 0 or 1 bit frequencies coming in. At that time, the software can either check the packet integrity and pass it back out it's own stack (Repeater); or decode the packet back into a numeric stream and pass it on to the surface computer (Probe/Transceiver).

Referring to Figures 6a to 6e, the down hole transceiver assembly 140 is illustrated.

Unlike the surface transceiver, the down hole transceiver must be compact and sufficiently sealed to protect the electronics involved. Likewise, the down hole transceiver must be capable of integration into a string of tubing. Thus, the down hole transceiver 140 can have an upper coupling element 600 that is connected to a lower outer mandrel 602. A piezo-electric stack 606 is captured between the coupling element 600 and a lower coupling element 604. Further, the PES 606 is captured radially between the lower outer mandrel 602 and an inner sleeve 608. A conductor 610 connects the PES to a voltage source contained in cavity 616. The voltage source can be at least one battery. An electrical connector 614 with a slip ring is used to hold a battery in place. The voltage source is controlled by a control means 618 that comprises a circuit board having a memory for storing control commands and data. Sensor assembly 624 can send data to the control means by conductor 622.

The description of the present invention has been presented for purposes of illustration and description, but is not limited to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims (27)

CLAIMS :

1. Apparatus for transmitting a command signal to a tube comprising : (a) at least one clamp for coupling a housing to the tube; and (b) a piezo-electric stack (PES) within said housing; wherein a first end of the PES contacts an upper housing portion and a second end contacts a lower housing portion.

2. Apparatus according to Claim 1, wherein said upper housing portion and said lower housing portion are loosely coupled so as to allow relative translational movement between the portions.

3. Apparatus according to Claim 1 or 2, wherein said PES is coupled to a voltage source.

4. Apparatus according to Claim 3, wherein said voltage source is coupled to a computer and a means for varying the frequency of a voltage supplied by the voltage source.

5. Apparatus according to any preceding Claim, wherein said clamp has a first and second opposing jaw.

6. Apparatus according to any preceding Claim 1, further comprising: (c) a means for detecting an acoustic signal.

7. Apparatus according to Claim 6, wherein said means for detecting comprises an accelerometer.

8. Apparatus according to any preceding Claim, wherein said PES comprises a plurality of piezo-electric discs, wherein each disc is separated by a sheet of conductive material.

9. Apparatus according to Claim 8, wherein said conductive material is copper.

10. Apparatus according to any preceding Claim, further comprising a means for encoding the command signal.

11. Apparatus according to any preceding Claim, further comprising a means for compressing the command signal.

12. Apparatus according to any preceding Claim, further comprising a repeater coupled to the tubing.

13. Apparatus according to any preceding Claim, further comprising a down hole transceiver coupled to the tubing.

14. Apparatus according to Claim 13, wherein said down hole transceiver comprises at least one sensor coupled to a transmitter.

15. Apparatus according to Claim 14, wherein said sensor comprises a pressure sensor.

16. Apparatus according to Claim 14, wherein said sensor comprises a temperature sensor.

17. Apparatus according to Claim 14,15 or 16, wherein said at least one sensor is coupled to a controller having a memory.

18. Apparatus according to Claim 14,15, 16 or 17, further comprising a means for encoding a data measured by said at least one sensor.

19. Apparatus according to any one of Claims 13 to 18, wherein said down hole transceiver comprises an actuator coupled to a down hole tool.

20. A method of actuating a down hole device using a piezo-electric stack (PES), said method comprising : (a) coupling a piezo-electric stack to the tubing ; (b) exciting the PES to create a vibration at a first and a second frequency, wherein said second frequencies are used to encode a signal.

21. A method according to Claim 20, wherein step (a) further comprises coupling a housing to the tubing wherein said housing includes the PES.

22. A method according to Claim 20 or 21, wherein step (b) further comprises transmitting the vibration to the tubing.

23. A method according to Claim 22, further comprising detecting the vibration with a repeater located down hole.

24. A method according to Claim 22, further comprising detecting the vibration with a transceiver located down hole.

25. A method according to Claim 24 further comprises detecting a down hole device in response to the detection of said vibration.

26. Apparatus for transmitting a command signal to a tube substantially as herein described with reference to and as shown in the accompanying drawings.

27. A method of actuating a down hole device using a piezo-electric stack (PES) substantially as herein described with reference to and as shown in the accompanying drawings.