EP3114317B1 - Downhole low rate linear repeater relay network timing system and method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention
• The present invention relates generally to telemetry apparatuses and methods, and more particularly to acoustic telemetry relay network timing for exploration, completion and production wells for hydrocarbons and other resources, and for other telemetry applications.
2. Description of the Related Art
• Acoustic telemetry is a method of communication used in the well drilling, completion and production industries. In a typical drilling environment, acoustic extensional carrier waves from an acoustic telemetry device are modulated in order to carry information via the drillpipe as the transmission medium to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data. In production wells, downhole information can similarly be transmitted via the well casings. Acoustic telemetry transmits data to the surface in real-time and is independent of fluid flow, depth, well trajectory and other drilling parameters.
• The theory of acoustic telemetry as applied to communication along drillstrings and well casings has a long history, and a comprehensive theoretical understanding has generally been backed up by accurate measurements. It is now generally recognized that the nearly regular periodic structure of drillpipe and well casings imposes a passband/stopband structure on the frequency response, similar to that of a comb filter. Dispersion, phase non-linearity and frequency-dependent attenuation make drillpipe a challenging medium for telemetry, the situation being made even more challenging by the significant surface and downhole noise generally experienced.
• When exploring for oil or gas, in coal mine drilling and in other drilling applications, an acoustic transmitter is preferentially placed near the BHA, typically near the drill bit where the transmitter can gather certain drilling and geological formation data, process this data, and then convert the data into a signal to be transmitted up-hole to an appropriate receiving and decoding node. In some systems, the transmitter is designed to produce elastic extensional stress waves that propagate through the drillstring to the surface, where the waves are detected by sensors, such as accelerometers, attached to the drill string or associated drilling rig equipment. These waves carry information of value to the drillers and others who are responsible for steering the well. Examples of such systems and their components are shown in: Drumheller U.S. Patent No. 5,128,901 for Acoustic Data Transmission through a Drillstring; Drumheller U.S. Patent No. 6,791,470 for Reducing Injection Loss in Drill Strings; Camwell et al. U.S. Patent No. 7,928,861 for Telemetry Wave Detection Apparatus and Method; and Camwell et al. U.S. Patent No. 8,115,651 for Drill String Telemetry Methods and Apparatus.
• Acoustic communication through drilling and production strings (collectively "drillstrings") is generally limited by available frequency spectra and signal attenuation. Consequently, transmission data rates tend to be relatively low, e.g., in the range of tens of bits per second, and multiple repeater nodes have previously been used to boost the telemetry signals and overcome the problem of acoustic signal attenuation and associated range limitations. The inclusion of multiple acoustic transceiver nodes within a drillstring forms a low rate linear repeater data network. As used herein "nodes" are defined as receivers (Rx), transmitters or transceivers (Tx) for telemetry signals traveling between adjacent pairs of nodes. Alternatively, the nodes could be associated with and referred to as "stations" (e.g., ST0, ST1, ... STn) located along the drillstring. The low data rate linear repeater networks suffer from high latency (time for data to propagate through the network) due to the time it takes for each node to receive data packets and relay data onward. An objective of repeater networks is to relay data as quickly as possible after initial receipt, in order to minimize latency of data delivered to the surface (or other destination) and to maximize data throughput.
• The latency of delivered measurement data translates into a potentially large time difference between the time at which a downhole sensor measurement is made and when that value is delivered to the surface, obscuring potentially valuable correlation between downhole and uphole events. Additionally, as sensor acquisition at each node within the network occurs at different points in time, the accuracy of inter-node differential measurements is limited, impairing the ability to discern transient events traversing the string.
• A possible solution to drillstring acoustic communication latency-associated problems is to include time-of-measurement information with transmitted information from each node. However, bandwidth limitations make the inclusion of time-of-measurement (e.g., sensor acquisition time) information overhead in the acoustic packets undesirable, and require all downhole clocks to be very accurately aligned, which can be problematic given the significant temperature differentials across the networks (e.g., 150° C or more) and the long periods of continuous network operation.
• Related art includes US 2010/0097890 A1 which discloses methods and apparatuses for data collection and communication in drill string components, US 2010/0313646 A1 which discloses a system and method for associating time stamped measurement data with a corresponding wellbore depth, and US 2012/274477 A1 which discloses a reliable downhole data transmission system.
SUMMARY OF THE INVENTION
• The invention is set out in the appended set of claims.
• The advantages of the repeater network timing control may include, without limitation:
• ▪ No costly overhead associated with time-of-measurement within acoustic packets.
• ▪ No network synchronization signal is required.
• ▪ Variable inter-node propagation delays do not impact.
• ▪ Agility, timing can change from frame-to-frame (packet type-to-packet type).
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 is a diagram of a typical drilling rig, including an acoustic telemetry system, which can be provided with a downhole linear repeater relay network timing system embodying an aspect of the present invention.
• FIG. 2 is a fragmentary, side-elevational and cross-sectional view of a typical drillstring, which can provide the medium for acoustic telemetry transmissions for relaying, repeating and timing with the present invention.
• FIG. 3 is a schematic diagram of the repeater relay network timing system of the present invention, particularly showing accurate surface time-of-measurement.
• FIG. 4 is another schematic diagram of the repeater relay network timing system, particularly showing how a surface decode time-of-receipt of telemetry signal can be related back to the sensor acquisition time of any network node.
• FIG. 5 is another schematic diagram of the repeater relay network timing system, particularly showing how a surface decode time-of-receipt of telemetry signal of a packet containing synchronized data is related to synchronized sensor acquisition across the network.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• In the following description, reference is made to "up" and "down" waves, but this is merely for convenience and clarity. It is to be understood that the present invention is not to be limited in this manner to conceptually simple applications in acoustic communication from the downhole end of the drillstring to the surface. It will be readily apparent to one skilled in the art that the present invention applies equally, for example, to subsurface nodes, such as would be found in telemetry repeaters.
I. Drilling Rig, Drillstring and Well Environment
• Referring to the drawings in more detail, the reference numeral 2 generally designates a downhole low rate linear repeater relay network timing or control system embodying an aspect of the present invention. Without limitation on the generality of useful applications of the system 2, an exemplary application is in a drilling rig 4 (FIG. 1). For example, the rig 4 can include a derrick 6 suspending a traveling block 8 mounting a kelly swivel 10, which receives drilling mud via a kelly hose 11 for pumping downhole into a drillstring 12. The drillstring 12 is rotated by a kelly spinner 14 connected to a kelly pipe 16, which in turn connects to multiple drill pipe sections 18, which are interconnected by tool joints 19, thus forming a drillstring of considerable length, e.g., several kilometers, which can be guided downwardly and/or laterally using well-known techniques.
• The drillstring 12 terminates at a bottom-hole assembly (BHA) 20 at acoustic transceiver node (ST0). Other rig configurations can likewise employ the present invention, including top-drive, coiled tubing, etc. Moreover, additional applications include completion rigs, completion strings, casing strings, gravel packs, frac packs and other applications. Without limitation on the generality of useful applications of the present invention, acoustic telemetry systems in general can utilize the repeater network timing control system and method of the present invention. FIG. 1 also shows the components of the drillstring 12 just above the BHA 20, which can include, without limitation, a repeater transceiver node 26 ST1 and an additional repeater transceiver node 22, ST2. An upper, adjacent drillpipe section 18a is connected to the repeater 22 and the transmitter 26. A downhole adjacent drillpipe section 18b is connected to the transmitter 26 and the BHA 20. A surface receiver (node) 21 can be provided at or near the upper end of the drill string 12.
• FIG. 2 shows the internal construction of the drillstring 12, e.g., an inner drillpipe 30 within an outer casing 32. Interfaces 28a, 28b are provided for connecting drillpipe sections to each other and to the other drillpipe components, as described above. W.1 illustrates an acoustic, electromagnetic or other energy waveform transmitted along the drillstring 12, either upwardly or downwardly. The drillstring 12 can include multiple additional repeaters 22 at intervals determined by operating parameters such as optimizing signal transmissions with minimal delays and errors. The drillstring 12 can also include multiple sensors along its length for producing output signals corresponding to various downhole conditions.
II. Acoustic Network: Operation
• FIG. 3 shows the operation of a downhole low rate linear repeater acoustic network timing control system. Other applications of the present invention include electromagnetic signal telemetry systems and systems transmitting signals through other media, such as drilling mud, ground, water, air, etc.
• Telemetry data packets contain sensor or tool status data and are transmitted from the primary node (ST0, typically the deepest node) and relayed from node-to-node to the surface receiver 21 (Surface Rx), which is generally located at or near the wellhead. The telemetry data packets include sensor measurements from the BHA 20 and other sensors along the drillstring 12. Such data packet sensor measurements can include, without limitation, wellbore conditions (e.g., annular/bore/differential pressure, fluid flow, vibration, rotation, etc.). Local sensor data can be added to the data packet being relayed at each sensor node, thus providing along-string-measurements (ASMs).
• A single node functions as the master node (e.g., ST0) and is typically an edge node at the top or bottom of the drillstring 12. The master node monitors well conditions and sends data packets of varying types and intervals accordingly. In addition to the long transmission/reception times associated with low data rate links, the asynchronous nature of wellbore variation tends to cause latency in an ASM operating mode because data-receiving nodes must await incoming packets before determining what sensor measurements must be acquired for inclusion in the packets being relayed. Such latency in a low-throughput repeater network translates into a potentially large time difference between the point when a downhole sensor measurement is made and when that value is delivered to the surface. Although including time-of-measurement (i.e., telemetry signal receive time) information within each acoustic packet with measurement data delivered to the surface can partly address this problem, additional problems can arise based on prohibitively large bandwidth requirements necessitated by the network low data rates, and the necessity of highly accurate alignment (synchronization) of downhole and surface clocks, which can be problematic due to relatively wide temperature differentials across the network (e.g., 150° C +), and long periods of network operation.
III. Acoustic Network: Relay Timing and Control
• According to the system and method of the present invention, all time constraints are controlled based on pre-configured constants, which are input to all nodes. The pre-configured constants can include:
• Guard Times: time allocated between receipt and transmission (relay) to allow for processing time, acquisition of sensor data and channel delay spread (echoes) subsiding. Typically about 0.5 to 5.0 seconds.
• Packet Transmission Time: a function of the internode data rate and packet bit length. For example, transmitting 100 bits @ 20 bps link rate = 5 seconds.
• Sensor Acquisition Time: time between the acquisition and measurement from a sensor to transmission of data through the telemetry network. Typically negligible, e.g. about 5-100 ms.
• With all time controlled within such a frame, the surface receiver can calculate the relative timing offsets of all relay transmissions within the network based on the telemetry signal received time (e.g., time-of-measurement) of any packet and its type. With the additional information of sensor acquisition time, an exact time of sensor measurement can be calculated from the received time and used as an accurate time-of-measurement as follows:
• N: Decoded Packet STID
• o : Originating Station ID
• Packet Time : # payload bits / (link bit rate)
$$\begin{array}{l}\mathrm{Time}\mathrm{of}\mathrm{measurement}=\\ \mathrm{Surface}\mathrm{DecodeTime}\left[\mathrm{N}\right]-{\displaystyle \sum _{\mathrm{n}=\mathrm{o}}^{\mathrm{N}}\mathrm{Packet}\mathrm{Time}\left[\mathrm{n}\right]}-\mathrm{Guard}\mathrm{Time}\times \left(\mathrm{N}-\mathrm{o}\right)-\mathrm{Ac}\mathrm{Time}\left[\mathrm{o}\right]\end{array}$$

• Wave propagation delays tend to minor relative to the above delays, and can be neglected, or can be easily accounted for with an additional subtraction based on originating node separation from the surface and group velocity of the packet signal (i.e. propagation delay = node depth x group velocity). In this way, a surface decode time-of-measurement can be related back to the signal receive time of any network node, as shown in FIGS. 4 and 5.
• In cases requiring quality differential measurements between nodes, all nodes must acquire sensor measurement data at the same point in time, and add the data to the appropriate relay packet such that the packet delivered to the surface contains time-synchronized sensor data acquisition. This can be accomplished with controlled network timing, if, based upon receipt time and type of a packet, all nodes can calculate the relative point in time at which the primary node (e.g. ST0, deepest node) acquired its measurement data, and acquire sensor data at that same point in time.
• From the perspective of the receiver node(s), the primary node sensor acquisition point occurred in the past. Sensor acquisition must therefore occur regularly and be buffered such that past measurement values are accessible. Buffer capacity and sampling rate are determined by the greatest possible frame length of all configurable modes, and the required alignment accuracy in the data of the network synchronized measurement. At the surface, the packets that are configured with network synchronized payload data will have their times-of-measurement adjusted according to that of the primary node.
• In the practice of the method of the present invention, all nodes acquire sensor measurement value at the same point in time as the primary node. All nodes have the same acquisition time. A surface decode time-of-receipt of telemetry signal can be related back to the sensor acquisition time of ST0, as shown in FIG. 5.
IV. Extensions and Additional Applications
• Without limitation on the generality of useful applications of the present invention, the network timing control system and method described above can be extended and applied to a wide range of additional applications, including:
• Applicable to electromagnetic pulse systems as well as acoustic.
• Applicable to downlink, uplink and bi-directional networks.
• The network synchronized sensor acquisition could be aligned with any node within the network, or any point in time within a frame.

Claims (11)

1. A relay timing control method for a downhole low rate linear repeater network for a drilling rig (4) comprising a drillstring (12) extending subsurface downwardly from a surface wellhead, a node (STO) located near a drillstring end and including a sensor adapted for providing a signal data set output corresponding to downhole conditions, and multiple nodes (ST) located downhole between said node (STO) and said wellhead and associated with said drillstring, said nodes (ST) configured to receive and re-transmit said signals, the method comprising the steps of:

   controlling and specifying network timing constants including:

   packet transmission times as a function of internode data rate and packet bit length;

   guard times allocated between receipt and transmission to allow for processing time, acquisition of sensor data and subsiding of channel delay spread;

   signal propagation times between nodes as a function of node separation; and intra-node sensor acquisition times;

   determining node timing offsets relative to network nodes (ST, STO) based on a receipt time of a telemetry data packet originating from another node (ST, STO),

   type of packet received, and the network timing constants; and using said node timing offsets to set node (ST, STO) timing.
2. The timing control method according to claim 1, which includes the additional steps of:
   determining point in time for sensor measurement acquisition based on said timing offsets and network timing constants.
3. The timing control method of claim 2, which includes the additional steps of:
   deriving point in time of acquisition of sensor measurement data contained within a received packet, based on said data packet receipt time and network timing constants.
4. The timing control method of claim 1, which includes the additional steps of:

   said nodes acquiring sensor measurement data corresponding to a point in time aligned with the measurement data acquisition time of another node;

   buffering sensor measurements sensor measurements to enable access to past sensor data; and

   configuring said network for containing measurement data from multiple nodes corresponding to said same point of acquisition time within a single data packet.
5. The timing control method of claim 1, which includes the additional step of:
   supporting multiple modes of network timing, corresponding to different sets of network timing constants.
6. The timing control method of claim 1, which includes the additional step of selecting network mode telemetry transmission method from among the group comprising acoustic, electromagnetic pulse, mud-pulse and wired networks.
7. A downhole low rate linear repeater relay network timing system (2) for a drilling rig (4) including a drill string (12) extending subsurface downwardly from a surface wellhead, which system includes

   a node (STO) located near a drillstring end and including a sensor adapted for providing a signal data set output corresponding to downhole conditions; multiple nodes (ST) located downhole between said node (STO) and said wellhead and associated with said drillstring, said nodes (ST) receiving and retransmitting said signals; and

   said relay network timing system being adapted for controlling all times within a frame according to pre-configured constants known to all nodes (ST, STO) said pre-configured constants including: guard times allocated between receipt and transmission to allow for processing time, acquisition of sensor data and subsiding of channel delay spread; packet transmission time as a function of internode data rate and packet bit length; signal propagation time between nodes as a function of node separation; and intra-node sensor acquisition times;

   wherein the relay network timing system is configured to derive timing offsets for said nodes relative to each other based on a receipt time of a telemetry data packet originating from another node (ST, STO), type of packet received, and the pre-configured constants.
8. The relay network timing system according to claim 7, which includes:
   the relay network timing system being applied at surface calculating the relative timing offsets of all relay transmissions within the network based on the point of time of telemetry signal reception of any packet and its type.
9. The relay network timing system according to claim 7 which includes node sensor measurements being buffered in time.
10. The relay network timing system according to claim 7 wherein each said node selects a buffered past measurement for aligning with a predefined reference node point in time of measurement.
11. The relay network timing system according to claim 7 wherein a surface node calculates relative timing offsets of all relay transmissions within the network based on the telemetry signal received point in time of a telemetry data packet and type from any one node.

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