Patent Application
20080217057
This invention relates to oil and gas drilling, and more particularly to apparatus and methods for recording downhole seismic measurements. The introduction of the wired pipe networks into the oil and gas drilling industry allows downhole clocks to be continuously synchronized with top hole clock sources. U.S. Pat. No. 7,142,129 and U.S. Patent Publications 2005/0284645 and 2006/0221768, which are herein incorporated by reference for all that they disclose, deal with various aspects of taking downhole seismic measurements using wire pipe. However, there are instances where the tool string may be disconnected from the surface such as adding or removing pipe during drilling or tripping and other instances which breaks communication between the top hole and downhole clocks. During these breaks timing accuracy of any recordings taken downhole may be inaccurate since the downhole clocks may drift.
In one aspect of the present invention a method has steps for taking downhole measurements. A network integrated into a downhole tool string in a well bore may be in communication with a downhole sensor, which has a downhole clock that is in communication over the network with and synchronized with a top-hole clock. A signal source may be within a transmitting distance of the downhole sensor and may be activated while the network is connected or disconnected from the top-hole clock. The downhole sensor may record a portion of the signals from the signal source at a time recorded by the downhole clock when the network is disconnected. When the network is reconnected, the downhole clock and the top-hole clock are re-synchronized and the clock drift that occurred during the disconnection may be calculated. The recorded time may then be adjusted to reflect the actual time according to the top-hole clock.
Clock synchronization pulses may be generated by the top hole master clock source which are received by the downhole clocks. A divisor number may be determined after comparing the clock synchronization pulses with oscillation from the downhole clock which adjusts the downhole clock. Electronic time stamps may be used to measure transmission latency between processing elements. The network may also have hardware that fixes computational latency to known constants.
The signal source may be a top-hole source, a cross-well source or a source located within the well bore. The signal source may be a seismic source, a sonic source, induction sources, an explosive, a compressed air gun or array, a vibrator, a sparker, a speaker, or combinations thereof. The top-hole clock source may be disposed within a GPS, a network server, surface equipment, a satellite, or combinations thereof. The downhole sensor may be a geophone, a 3 component geophone, an accelerometer (1c or 3c) an induction receiver, an electrode, a nuclear sensor, a hydrophone an array thereof or a combination thereof. Also, the tool string may be deployed in a drill string, a production string, an injection string, a casing string, or combinations thereof.
A stabilizer may be attached to the tool string and may have at least two stabilizer blades with a pocket adapted to receive at least three geophones oriented at three different orthogonal axes. In some embodiments there are three to five stabilizer blades and there is a set of geophones in each. Typically at least one of the stabilizer blades may contact the formation thereby improving the coupling that the set of geophones may have with the formation. Since there are several sets of geophones more recordings may be taken per each seismic shot produced at the surface; thus increasing the acquisition efficiency. The recordings from the several sets of the geophones may also be averaged or otherwise treated mathematically to reduce downhole and or system noise, allowing for more accurate data per surface seismic shot The acquisition sequence may be completely controlled by the engineer at surface and complex down hole state determining algorithms are not required. The downhole sensors and electronics may be adapted to take measurements according to programmable acquisition parameters which may be controlled at the surface. Such parameters may include gain, sample rate, filtering, and combinations thereof.
In the preferred embodiment, a stabilizer 108 may be attached to the tool string 100 and may have at least two stabilizer blades 109, but preferably have at least three to five blades. The downhole sensor 105 may be disposed within one of the stabilizer blades 109. At least one stabilizer blade 109 may contact a formation 110 so as to help keep the tool string 100 centralized in the well bore 101. The stabilizer blades 109 may have a pocket adapted to receive vibration measuring devices such as geophones or accelerometers. It is believed that most of the time at least one of the stabilizer blades will be in contact with the formation thereby allowing at least one of the sensor pockets to at least almost always be in contact with the formation thereby improving the coupling.
The network enables downhole tool to acquire seismic data during the entire drilling process. While drilling is taking place, drill bit energy may be recorded by the various sensors (geophone, hydrophone and accelerometers) and this data used to build a look ahead seismic image. Such data may also be used to determine accelerations, shocks and vibrations that impact the drill string while drilling is taking place. Drilling may be stopped intentionally at any time or may stop due to the drilling process and conventional surface source seismic may be recorded. It is not necessary to wait for pauses in the drilling process, nor is it necessary to detect the downhole condition remotely by the tool, since the instruction to start or stop data acquisition is sent from surface through the network. While drilling is taking place, and for a number of different reasons however, the top-hole equipment may be disconnected from the rest of the tool string, causing the down hole clock to lose communication with the top-hole clock source. Through various physical phenomena (including temperature variations) the down-hole clock will drift during the disconnected time and may not exactly reflect the time as maintained by the surface clock. When the network is reconnected to the top-hole clock source, the downhole clock and the top-hole clock may be re-synchronized. Clock drift that occurred is then measured, sent to surface and each record corrected accordingly.
In some embodiments, the seismic signal source may be located within the well bore 101. The seismic sources may be drilling hammers, drill bit energy, jars, perforating guns, piezoelectric stacks, or combinations thereof.
The downhole sensor 105 may have a downhole clock that is in communication over the network 103 with a top-hole clock source 104 and be synchronized. Cross well seismic shots may be performed while one of the tool string comprising a sensor package 105 is disconnected and its associated downhole clock is drifting. When the network 103 and the top-hole clock source 104 are reconnected, the downhole and top-hole clock sources may be resynchronized. Any clock drift that may have occurred during the disconnection may be calculated so that the recorded time may be adjusted to reflect what it would have been if the downhole clock had been synchronized with the top-hole clock source 103.
When the seismic tool 100 is disposed in a well bore, at least one stabilizer blade 109 may contact the formation which may allow better coupling of geophones to the borehole. More accurate readings may be received because the signal is not altered by the characteristics of the borehole or those of the tool before the seismic signal is recorded.
A downhole node 601 may communicate with an intermediate node 604 located at an intermediate point along the tool string 100. The intermediate node 604 may also provide an interface to sensors 605 communicating through the network. Likewise, other nodes, such as a second intermediate node 606, may be bcated along a tool string to communicate with other sensors 607. Any number of intermediate nodes 604, 606 may be used along the network between the top-hole node 600 and downhole node 601.
In some embodiments, a physical interface 608 may be provided to connect network components to the tool string. For example, since data may be transmitted directly up the tool string on cables or other transmission media integrated directly into the tool string components, the physical interface 608 may provide a physical connection to the tool string so data may be routed off of the tool string to network components, such as the top-hole node 600, or personal computer 609.
For example, a top-hole node 600 may be connected to the physical interface 608. The top-hole node 600 may also be connected to an analysis or logging device such as a personal computer 609. The personal computer 609 may be used to analyze or log data gathered from various downhole tools or sensors.
In this embodiment, it is common that clock drift may occur as data is received and transmitted between nodes and/or sensors. Electronic time stamps may be used to measure transmission latency between these processing elements. Also, clock drift may occur as the various components analyze or log data. Thus, the network may have hardware that fixes computational latency to a known constant.
In some embodiments, a downhole clock 700 may include an oscillator 702 that creates a series of pulses at a rated frequency. Most oscillators 702 exhibit some frequency instability that can cause drift over a period measured in seconds, minutes, hours, etc. Many crystal oscillators 702 experience drift caused by a shift in frequency as a function of temperature. The amount of clock drift may be exacerbated by temperatures encountered in a downhole environment. Thus, in downhole environments, the frequency fluctuation of oscillators 702 may be significantly more pronounced than it would be above the surface. Other common downhole causes of clock drift may include crystal instability, vibration, pressure, crystal aging, shocks, mounting structure, bending loads on the crystal or combinations thereof.
An oscillator 702 may be operably connected to a prescaler 703. Generally, a prescaler 703 is configured to generate a clock pulse after it has received a certain number of input pulses from the oscillator 702. For example, a “Divide-by-N” prescaler may generate a clock pulse after it has received N input pulses. The basic objective of a prescaler 703 is to provide a series of clock pulses to a larger, slower counter by dividing a higher incoming pulse frequency. The output from the prescaler may form the basis for a downhole clock 700. The time-base logic module 701 may be responsible for keeping time based on the output from the prescaler 703.
During the drilling process, the network is occasionally disconnected from the top-hole clock source for a short time for addition or removal of pipe, testing, or other reasons. Often this time is used to activate seismic shots to record measurements downhole since the tool string is not rotating or otherwise moving during this period. While a network is disconnected from a top-hole clock source, the time-base logic module 701 may not be able to compare the top-hole clock to the downhole clock allowing clock drift to occur without correction. Once the network and the top-hole clock source are reconnected the downhole clock time may be compared to the top-hole clock source obtained through the network. By comparing these two times, the drift of the downhole clock 700 may be calculated. If clock drift occurs during the disconnection, amount of drift may be calculated by subtracting the downhole clock time from the top-hole clock time. Further, it may be desired to calculate the actual time, relative to the top-hole clock source, of a certain event that may have occurred downhole during the disconnection. In some embodiments, the actual time of an event may be calculated by subtracting the time the seismic shot was received times the clock drift divided by the total time of the disconnection from the time the seismic shot was received. In its simplest form, an example of how the real time of an event may be calculated is described in the following expression:
Received Time−(Received Time)(clock drift/total time of disconnect)=actual time of event
In some embodiments, the drift may be measured in parts per million (ppm) or parts per billion (ppb) which expresses the drift in terms of a number of errant pulses for every million pulses. For example, if the drift is measured at 200 ppm, then the clock pulse has drifted from the reference time by 200 pulses for every million pulses output. Once this error rate, or drift, is calculated, this number may be used to take appropriate corrective or compensative action.
Other methods of clock synchronization and recalculation may be used. Methods described in U.S. Pat. Nos. 5,689,688; 7,180,332; 7,167,031; 7,134,033; 4,602,375 and U.S. Patent Applications 20070033294; 20070025483; 20070009075; all of which are herein incorporated by reference for all that they disclose, may be compatible with the present invention.
The time-base logic module 701 may be operably connected to a trigger module 704. The prescaler 703 may be connected to a compensator module 705 configured to adjust a preloaded number. The compensator module 705 may be controlled by the trigger module. The trigger module 704 may be programmed to send a trigger signal 706 at calculated intervals to modify settings of the prescaler 703. The trigger module 704 may also be configured to trigger the compensator module 705 to adjust the preloaded number. A series of electrical clock pulses may be produced after counting a preloaded number of electrical pulses from the oscillator 701. Time may be measured based on the electrical clock pulses. Thus, clock drift may be calculated and compensated for by adjusting the preloaded number. For example, depending on clock drift, it may be determined that a correction needs to be made after every 80,000 pulses from the oscillator 701. In some embodiments, the trigger module 704 is simply a counting circuit that counts 80,000 pulses before sending a trigger signal 706.
The trigger signal 706 may be received by the compensator module 705. The compensator module 705 may be configured to load a number into the prescaler 703. For example, if the prescaler 703 normally waits for 16 pulses from the oscillator 701 before outputting a clock pulse, the compensator module 705 may increase or reduce this number. This may have the effect of advancing or retarding the timing of the output 707. In some embodiments, the compensator module 705 simply changes the number loaded into the prescaler 703 for a single cycle. For example, if the prescaler 703 normally waits for 16 oscillator pulses before outputting a clock pulse, the compensator module 705 may reprogram the prescaler 703 to wait 17 oscillator pulses before outputting a clock pulse for one cycle, and then return to the normal operating mode of waiting for 16 pulses. Thus, the compensation module 705 may temporarily change the number that is loaded into the prescaler 703 to either advance or retard the timing of the output 707.
In this embodiment, the downhole clock 700 has developed a drift from the top-hole clock 104, causing the period 800 of a cycle in the downhole clock to be slightly more than the period 801 of a cycle in the top-hole clock. Although at one moment 802 in time the clocks 104, 700 are synchronized, at another moment 803 the top-hole clock 104 has completed five pulses and the downhole clock 700 has not yet completed five pulses, creating a situation of oscillator drift. The present invention provides a way to correct such clock drift that occurs when the network is disconnected from the top-hole clock in order to record accurate downhole measurements.
The baud rate generator apparatus 900 may comprise an oscillator 702 configured to output electrical pulses at a certain frequency. Many different oscillators 702 may be configured to operate in this type of apparatus 900. For example, an oscillator 702 may comprise a crystal, a transistor-based circuit, an RC circuit, an LC circuit, or an RLC circuit. Oscillators 702 have some amount of inherent clock drift due to natural properties. This drift may be measured over a period of time. Most oscillators 702 are also affected in some way by changes in temperature, pressure, vibration, pressure, crystal aging, shocks, mounting structure, bending loads on the crystal or combinations thereof.
The oscillator 702 may be configured to output electrical pulses to an accumulator module 901, to which it is operably connected. The accumulator module 901 is configured to receive pulses from the oscillator 702, output an accumulated value, and store a new accumulated value. The accumulator module 901 is operably connected to a digital adder 902, and may output its stored accumulated value to the adder 902. The accumulator module 901 may also receive a sum value from the digital adder 902 and store it. The digital adder 902 is configured to receive values from the accumulator module 901 and an adjuster module 903 and add them together. Digital adders 902 may have a sum output 904 and a carry output 905. In this embodiment, the sum output 904 of the digital adder 902 is operably connected to the accumulator module 901, and the carry output 905 is operably connected to a logic module 906.
The adjuster module 903 is configured to store an adjuster value and output it to the digital adder 902. In some embodiments, the adjuster module 903 may be a memory register. The adjuster value may be modified periodically in order to compensate for clock drift from the oscillator 702 or to change the baud rate frequency. Clock drift may occur during a disconnection between the top-hole clock source and the downhole network 103.
The logic module 906 is configured to receive an electrical pulse from the digital adder 902 whenever an addition is performed that produces a carry value. The logic module 906 is further configured to change the adjuster value stored in the adjuster module 903. If the apparatus 900 is functioning solely as a baud rate generator, it is not required to keep time based on the electrical pulses received in order to function properly. However, even if the apparatus 900 is functioning primarily as a baud rate generator, the logic module 906 may still be configured to keep time based on the output pulses it receives. This particular characteristic proves useful in calculating and correcting clock drift. The logic module 906 may also comprise a connection to the network 103.
A network communications device such as a universal asynchronous receiver/transmitter (UART) 907 may also be connected to the apparatus 900 and configured to receive electric pulses corresponding to carry values from the digital adder 902.
The present invention may also apply to other LWD measurements include resistivity measurements, nuclear measurements, acoustic measurements, caliper measurements, pressure measurements, torque measurements, WOB measurements, strain measurements, and combinations thereof.
Whereas the present invention has been described in particular relation to drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.
