TECHNICAL FIELD

[001] This disclosure relates to monitoring a well system, and more specifically, to monitoring a velocity and location of a downhole tool in a wellbore of the well system.

BACKGROUND

[002] After a wellbore has been drilled, the wellbore is often cased by inserting lengths of steel pipe (“casing sections”) connected end-to-end into the wellbore. Threaded exterior rings called couplings or collars are typically used to connect adjacent ends of the casing sections at casing joints. The result is a “casing string”, i.e., a series of casing sections with connecting collars that extends from the surface well into the wellbore, and in many instances to a bottom of the wellbore. The casing string is then cemented in place to complete the casing operation.

[003] After a wellbore is cased, downhole tools are often lowered into the wellbore to perform various operations, such as obtaining measurements or perforating the casing. The casing is often perforated to provide access to a desired formation, e.g., to enable formation fluids to enter the well bore. Performing the operations, such as perforating, require the ability to position a tool at a particular and known position in the wellbore. One method for determining the position of downhole tools is to count the number of collars that the tool passes as it is lowered into the wellbore. As the length of each of the steel casing sections of the casing string is known, correctly counting a number of collars or joints traversed by a device as the device is lowered into a well enables an accurate determination of a depth or location of the tool in the well. Such counting can be accomplished with a casing collar locator (CCL).

BRIEF DESCRIPTION

[004] Reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: [005] FIG. 1 shows a diagram of an illustrative well system including a differential casing collar locator (DCCL) tool;

[006] FIG. 2 A shows a diagram of a section of a first illustrative DCCL tool for an auto-correlation;

[007] FIG 2B illustrates a waveform of an example of a first signal generated after a first sensor traverses a downhole casing collar;

[008] FIG. 2C illustrates a waveform of an example of a second signal generated after a second sensor traverses the downhole casing collar, wherein a polarity orientation of the second sensor is opposite a polarity orientation to the first sensor;

[009] FIG. 2D illustrates a waveform of a summed combination of the first signal and the second signal of FIG. 2B and 2C;

[010] FIG. 3A illustrates a waveform of an example of a first signal after the first sensor traverses the first casing collar and a second casing collar;

[Oil] FIG. 3B illustrates a waveform of an example of a second signal after the second sensor traverses the first casing collar and the second casing collar, wherein a polarity orientation of the second sensor is opposite a polarity orientation of the first sensor;

[012] FIG. 3C illustrates a waveform of a summed combination of the first signal and the second signal of FIG. 3 A and FIG. 3B;

[013] FIG. 3D illustrates an alternative waveform of a summed combination of the first signal generated by the first sensor of FIG. 3A, and a third signal generated by inverting the signal from the second sensor, wherein the first and third signal have the same polarity;

[014] FIG. 4A illustrates a waveform of an x, y cross-correlation of the first signal and the second signal of FIG. 3 A and FIG. 3B ;

[015] FIG. 4B illustrates a waveform of an x, y cross-correlation of the first signal of FIG. 3A and the third signal of a polarity-reversed signal of FIG. 3B; [016] FIG. 5A illustrates a waveform of an auto-correlation of the sum of the first and second signals of FIG. 3 A and FIG. 3B, as illustrated in FIG. 3C;

[017] FIG. 5B illustrates a waveform of an auto-correlation of the sum of the first and third signals of

FIG. 3A and a polarity-reversed signal of FIG. 3B, as illustrated in FIG. 3D, wherein the first and third signal have the same polarity;

[018] FIG. 6 shows a diagram of an illustrative DCCL tool that employs x, y cross-correlation;

[019] FIG. 7 shows a diagram of a section of a second illustrative DCCL tool that couples (e.g. sums) an output of the first sensor to an output of the second sensor to employ auto-correlation;

[020] FIG. 8 shows a flow diagram of an example method of using cross-correlation for finding a velocity of a downhole tool; and

[021] FIG. 9 shows a flow diagram of an example method of using auto-correlation for finding a velocity of a downhole tool.

DETAILED DESCRIPTION

[022] A CCL may be attached to a downhole tool and suspended in a wellbore with a conveyance, such as a slickline, a coiled tubing, or a wireline. A wireline is typically an armored cable having one or more electrical conductors to facilitate the transfer of power and communications signals between the surface electronics and the downhole tools. Such cables can be tens of thousands of feet long within a wellbore and subject to electrical noise interference and crosstalk. The noise and crosstalk can interfere with wellbore operations, such as detecting the position of a downhole tool and velocity of a wireline coupled thereto in relation to the casing collars of a casing string.

[023] The velocity of a wireline can be employed for improved depth correlation between the wireline and the casing collars. Use of the depth correlation between the wireline and the casing collars can be combined with the velocity of the wireline to determine a depth of a downhole item of interest, such as a downhole tool. For example, the velocity of a downhole tool over time can be integrated to determine a physical location of the downhole tool within the wellbore. This can be especially useful at low logging speeds, when a generated location signal can be weaker than at higher logging speeds.

[024] Accordingly, the disclosure provides an improved casing collar locator, and method for using the same, that removes or at least reduces the drawbacks of those that currently exist. The disclosure provides a differential casing collar locator (DCCL) tool that includes a pair of sensors, has a reduced sensitivity to magnetic disturbances caused by lateral movement of the locator within the casing, and an increased sensitivity to magnetic mass changes. Thus, the DCCL tool is substantially insensitive to magnetic disturbances caused by lateral locator movement in a well, but more sensitive to magnetic mass changes, such as is found when traversing between a casing section and a downhole collar. “Differential” of the DCCL refers to employment of a first sensor and a second sensor in the tool. Correlation processing, such as auto-correlation processing, of the signals from the pair of sensors can provide the time of flight of the DCCL tool passage in a casing string. Knowing the distance between the sensors in the DCCL tool allows estimating the velocity of the DCCL tool independently of the collar length or magnetic signature. Thus, the DCCL tool allows for an estimation of velocity but does not require a previous knowledge of the magnetic signature of a casing collar. This, in turn, renders the DCCL tool less liable to approximation errors as compared to some conventional technologies. Accordingly, the differential arrangement of the DCCL tool increases the quality of the location by removing the effects of radial movement while, at the same time, providing velocity information.

[025] Turning now to the figures, FIG. 1 provides a side elevation view of a well system 100 with an illustrative wireline tool system 110. The wireline tool system 110 includes a sonde 105 suspended in the well system 100 by a conveyance 120 (e.g., a wireline) having wires 125 for downhole telemetry. While many different types of conveyance members may be used and remain within the purview of the disclosure, for the purpose of the present discussion the conveyance 120 will be discussed with regard to a wireline. In one example, the sonde 105 transmits a signal that allows personnel on the surface to track the location of a tool string. [026] The well system 100 is cased with a casing string 115 having a first casing section 150A and a second casing section 150B connected end-to-end by a casing collar 155, which is a first casing collar. Coupled to the second section 150B is a second casing collar 180. In the illustrated embodiment, the first and second casing sections 150A, 150B of the casing string 115 and the casing collars 155, 180 are made of steel, an iron alloy, and hence exhibit a fairly high magnetic permeability and a relatively low magnetic reluctance. In other words, the material of the casing string 115 and the casing collars 155, 180 conveys magnetic field lines much more readily than air and most other materials.

[027] In the example of FIG. 1, the wireline tool system 110 includes a DCCL tool 130. In one aspect, the DCCL tool 130 includes a first sensor 160 which includes a first permanent magnet having a northern polarity orientation and a second sensor 165 which includes a second permanent magnet having a southern polarity orientation. These opposing magnetic polarities thereby generate opposing magnetic fields between the first permanent magnet and second permanent magnets of the first and second sensors 160, 165, respectively, diverting the respective magnetic fields radially, towards the casing string 115 and collars 155, 180.

[028] In another aspect, the DCCL tool 130 can include separate CCL modules, with each module having its own sensor and own respective housing. With two CCL modules, in-phase autocorrelation can be used, such as is illustrated in FIG. 5B. Alternatively, one of the CCL modules can be re-wired to create an auto-correlation, such as is illustrated in FIG. 5A, to be discussed below.

[029] Around the first permanent magnet is a first winding and around the second permanent magnet is a second winding. The first and second windings are substantially parallel to the wireline 120, and the first and second sensors 160, 165, are communicatively coupled to the wires 125. The first sensor 160 and the second sensor 165 can be within a housing 169. The housing 169 can be constructed of a non magnetic material appropriate for, e.g, downhole drilling and exploration. An example of sensors having first and second permanent magnets and their respective windings are more clearly illustrated and denoted in FIG. 2A. [030] The first sensor 160 and the second sensor 165 each produces an electrical signal in response to magnetic field changes attributable to passing collars (e.g., the first collar 155) in the casing string 115. As the DCCL tool 130 passes a collar, the resultant change in the strength of the magnetic flux passing through each of the first and second windings causes an electrical voltage to be induced between the ends of the windings in accordance with Faraday's Law of Induction. These induced electrical signals are used as electrical“location” signals for determining a location within a wellbore.

[031] Employing a known distance between DCCL sensors of a tool string, such as the first sensor 160 and the second sensor 165, allows estimating the velocity of the DCCL tool 130 independent of the collar length or particular magnetic signature of a collar. In one embodiment of the example well system 100, a x, y cross-correlation processing of a signal of the first sensor 160 with a corresponding later, second signal of the second sensor 165, both produced when approaching or leaving a selected collar, can provide the time of flight of the wireline 120 when cross-correlated. Alternatively, the first and second signals from the first and second sensors 160, 165 can also be added or be combined and auto-correlated.

[032] The first and second sensors 160, 165, can transmit the electrical signals to a well system controller 145 via the wires 125 of the wireline 120 for auto or cross correlation. In addition to the electrical signals, the wireline 120 can also convey signals (i.e., transmitted information) from a first logging tool 135 and a second logging tool 140 of the sonde 105, which can be one example of a downhole tool. The sensor signals can be processed on the surface, or within the well itself, depending on use of the sensor signals downhole or at the surface, or a combination thereof. In at least some embodiments, the well system controller 145 can also provide electrical energy for tools 135 and 140 and for electrical circuits of the DCCL tool 130. The well system controller 145 can also include a processor and storage to provide processing and storing of data from the sonde 105 as well as the ability to process the cross-correlation or the auto-correlation of the DCCL tool 130 depending on the implementation scheme chosen. [033] In the illustrative well system 100, the first and second sensors 160, 165, may be separated by an optimum distance that is close or substantially equal to the typical length of a casing collar, such as within ±10% of a casing collar length. The typical length of a collar, such as the collar 155, can be six to twelve inches. The distance between the first and second sensors 160, 165, can also be determined experimentally so that the exit signal of one casing section of the first sensor 160 does not overlap with the signal of the second sensor 165 becoming proximate to a different casing section.

[034] The first and second sensors 160, 165, have their same respective magnetic poles opposed so as to divert the magnetic field towards the casing and casing collars. The voltages caused by a uniform magnetic flux change detected by the first sensor 160 and the second sensor 165, approaching the first casing section 150A, for example, are opposite in phase and cancel, e.g., due to such factors as an opposing polarity of the first sensor 160 and the second sensor 165. Alternatively, the magnetic poles are opposite on the first and second sensors 160, 165, and the terminals can be selectively connected to be inverted to respective amplifiers so that the two windings produce signals of the same electrical polarity. The orientation of the windings can also be chosen so that the signature of entering the collar is in the same direction (e.g. positive) for the first sensor 160 and the second sensor 165 even if the magnetic polarities are opposed. Moreover, signals with opposite phase can be added instead of subtracted from each other, as can occur with signals of the same phase.

[035] For example, when approaching a collar upon being retracted from a well, such as the collar 155, the second sensor 165 will sense the collar 155 first, while the first sensor 160 is still sensing the pipe, such as the second casing section 150B. Conversely, when the tool string moves radially in a collar section 150A or 150B, both sensors will see the same magnetic disturbance, and when combining the signals with opposite polarity, either by opposing magnetic poles or winding phase, the common voltages induced will cancel. This scheme will make the assembly substantially insensitive to magnetic disturbances caused by radial movement in the well system 100, but more sensitive to magnetic mass changes, such as is found when traversing between a casing section and a downhole collar when wireline 120 pulls the tool string up or down in the well system 100. [036] Generally, the first logging tool 135 and the second logging tool 140 of the sonde 105 are designed and constructed to gather information regarding a formation property or a physical condition downhole. For example, the first logging tool 135 and the second logging tool 140 may be adapted to gather information about the casing string 115 and/or the well system 100, such as sonic properties, active and/or passive nuclear measurements, dimensional measurements, borehole fluid sampling, flow, pressure and temperature measurements, etc. The DCCL tool 130 can assist with determining the location of the logging tools 135, 140, within the wellbore. Other downhole tools, such as a perforating tool, can also be associated with the DCCL tool 130. In the illustrated embodiment, a winch 170 is used to lower and retract the sonde 105 in the wellbore via the wireline 120. The winch 170 includes a wireline slip 175 that enables a drum of the winch 170 to rotate.

[037] FIG. 2A depicts an illustration of an example of a sensor section 200 of the DCCL tool 130 in the environment of the well system 100 of FIG 1. The sensor section 200 includes the first sensor 160 with a first permanent magnet 205 and the second sensor 165 having a second permanent magnet 210. The first and second permanent magnets 205, 210, are arranged with opposite polarity orientations to one another. Moreover, the first and second permanent magnets 205, 210, have their own respective first winding 215 and second winding 220, respectively. The first winding 215 and the second winding 220 are coupled to wire 126 and wire 127 of the wires 125 of the wireline 120, wherein wire 127 is a groundline. As such, in this aspect each sensor 160, 165, is also coupled to ground and therefore coupled in parallel. The first sensor 160 and second sensor 165 can be within a housing, such as the housing 169. Alternatively, the first sensor 160 and the second sensor 165 can each be in their own respective housings and coupled together

[038] Generally, the sensor section 200 produces a first signal and a second signal generated from an output of the first sensor 160 and the second sensor 165, respectively, when the first sensor 160 and second sensor 165 each traverse the casing collar 155. The first signal and second signal are combined and sent though the wire 126 for processing. The combined first signal and second signal can then be auto-correlated to determine velocity of the DCCL tool 130 with respect to the casing collars of the wellbore, such as the first casing collar 155.

[039] The auto-correlation helps to determine a degree of similarity between different signals. When adding the signals with auto-correlation, for example, signals of opposite phase will tend to cancel correlated noise, such as the effect of the tool string moving radially in the well. With auto-correlation, this could advantageously produce a more visible collar signature when applying appropriate signal filtering.

[040] Moreover, in at least some embodiments, unlike many conventional downhole sensors, an active driver in an auto-correlation in the DCCL tool 130 is not required to generate the various electrical outputs of the first sensor 160 and the second sensor 165 to transmit the signals to the surface. In the well system 100, the collar signals are instead generated as a function of a movement of the permanent magnets of the first and second sensors 160, 165, and their respective first and second winding 215, 220 vis a vis the casing collars, such as the first collar 155. Typically, the sensor section 200 is used when processing is performed at the surface. FIG. 6 and FIG. 7 illustrate DCCL tools with examples of other sensor sections where processing of the sensed signals can be performed downhole.

[041] The employment of auto-correlation at the surface removes most, perhaps substantially all, of the electronics from downhole, allowing a DCCL tool having the example sensor section 200 to be a “passive” device, i.e., substantially without electronics. In the example sensor section 200 there are two signals to be processed, the first signal and the second signal, and it is, in at least some embodiments, inconvenient sending them separately to surface; especially when the DCCL is a passive device. Because the DCCL signals have low frequency content, they can be superimposed to the power (DC) and telemetry signals (higher frequency) that are sent uphole and recovered at the surface by filtering.

[042] FIG. 2B, FIG. 2C, and FIG. 2D correspond to signals that are generated when the first and second sensors of a DCCL tool traverse casing collars of a casing string. DCCL tool 130 and the first and second sensors 160, 165, are used as an example within the well system 100. The first signal and second signals generated by the DCCL 130 tool can be employed to determine a velocity of a downhole tool, and its location. The x axis for each of these figures is time in seconds and the y axis is voltage amplitude.

[043] FIG. 2B illustrates a waveform of an example first signal when the first sensor 160 traverses from the first casing 150A to the second casing 150B across the first collar 155. In the illustrated example, high pass filtering has been applied to remove superimposed DC power, with a sampling rate of 83.33 milliseconds (or 1 data point/inch at 60 feet/minute).

[044] The waveform of FIG. 2B includes a positive spike and a negative spike. These terms, positive spike and negative spike, refer to orientations on the waveforms, not a comparison of absolute amplitude. A positive spike can have a higher absolute amplitude than a negative spike, or a negative spike can have a higher amplitude than a positive spike.

[045] As is illustrated in FIG. 2B, a first signal, when the first sensor 160 traverses the collar 155, has a smaller initial negative spike 225, a larger positive spike 230, and a smaller negative spike 235. The first signal corresponds to the change in magnetic flux of the first sensor 160 traversing the first casing collar 155 between the casing 150A and 150B.

[046] As is illustrated in FIG. 2C, a second signal is generated when the second sensor 165 traverses the first casing collar 155. The second signal has an initial smaller positive spike 255, a larger negative spike 260, and a second smaller positive spike 265. The second signal is polarity-reversed when compared to the first signal, due to the opposite magnetic polarities of the first sensor 160 and the second sensor 165.

[047] FIG. 2D illustrates a waveform of a combined, summed first signal of the first sensor 160 and the second sensor 165. The summed signal can be processed in the well system controller 145. The time between a summed positive spike 275 and a summed negative spike 290 can be used to calculate the movement of the DCCL tool 130, which is not necessarily the same as the movement, measured at the surface, of the wireline 120 itself due to such factors as a stick-slip and the cable elasticity, e.g. a“yo-yo” effect. [048] As is illustrated in FIG. 1, the first sensor 160 and the second sensor 165 are within the DCCL tool 130. Determining the DCCL tool 130 movement allows a correction of the positioning of the DCCL tool 130 versus depth of the tool measurements. The various other smaller spikes 270, 280, 285 and 290 of FIG. 2D can also be employed to determine the speed of a tool movement of the DCCL tool 130, which can correlate between different polarities of an individual sensor traversing a casing collar. Velocities of other downhole tools, of which DCCL tool 130 is a subset, can also be compared versus the speed of the wireline 120, measured at the surface.

[049] FIG. 3 A - 3D illustrate various waveforms of example signals generated by the first sensor 160, the second sensor 165, and an alternative embodiment of the second sensor 165 having a magnet of a reversed polarity when compared to the second sensor 165. For each of these figures, the x axis is time in seconds and the y axis is voltage amplitude. The first sensor 160, the second sensor 165, and the alternative embodiment of the second sensor 165 traverse the first casing collar 155 and the second casing collar 180 of FIG. 1 to generate FIGs. 3A-3D. Hereinafter, for ease of discussion and clarity, the alternative embodiment of the second sensor 165 will be referred to as a third sensor. The third sensor is not illustrated in FIG. 1 but could be, e.g. , the second sensor 165 with its polarity reversed.

[050] In an alternative embodiment, the third sensor has the same magnetic polarity as the second sensor 165, but the winding of the third sensor is reversed when compared to the winding 220 of the second sensor. The waveforms generated by the sensors allow employing a known distance between two casing collars to determine tool movement as a function of time, such as movement of the DCCL tool 130.

[051] FIG. 3A illustrates waveforms generated by the first sensor 160 traversing sequentially both the first collar 155 and the second collar 180. A smaller negative spike 302 is generated when the first sensor 160 reaches the first collar 155, then a larger positive spike 304 is generated at the first collar 155, and another smaller negative spike 306 is generated when the first sensor 160 leaves the first collar 155. The first sensor 160 then traverses the second collar 180. A smaller negative spike 308 is generated when the first sensor 160 first reaches the second collar 180, then a larger positive spike 310 is generated, followed by another smaller negative spike 312.

[052] In the illustrated example of FIG. 3 A, the speed of the wireline 120 occurs at 60 ft/min, with collars 155 and 180 forty-one feet apart. The distance between larger spikes 304 and 310, are forty-one seconds apart. As the physical distance between the collars, such as first collar 155 and second collar 180, is known, the velocity of the DCCL tool 130 can be determined.

[053] FIG. 3B illustrates the second sensor 165 traversing the first collar 155 and the second collar 180. A smaller spike 322 is generated when the second sensor reaches the first collar 155, then a negative larger spike 324 is then generated when the second sensor 165 is at the collar 155, and then another smaller spike 326 is generated when the second sensor 165 leaves the collar 155.

[054] Regarding the second collar 180, a smaller positive spike 328 is generated when the second sensor 165 first reaches the second collar 180, then a larger negative spike 330 is generated when the second sensor 165 is at the second collar 180, followed by another smaller spike 332 when the second sensor 165 leaves the collar 180.

[055] FIG. 3C illustrates an addition of the signals of FIG. 3 A and FIG. 3B. For FIG. 3C, the background noise is partially cancelled in its correlated output due to the opposite polarities of the first sensor 160 and the second sensor 165. As is illustrated in FIG. 3C, the positive spike 344 and negative spike 350 correlate (with noise of other sensor added) to spike 304 and spike 324, respectively. The spike 356 and negative spike 362 correlate (with noise of other sensor added) to spike 310 and spike 330, respectively.

[056] The time between the larger spike 344 of the first sensor 160 and the larger spike 350 of the second sensor 165 is shown, and as the distance between sensors is known, e.g. 2 feet in the example, and the spikes occur with an interval of 2 seconds, the velocity of the DCCL tool 130 can be determined to be 60 feet/min. A similar analysis can be done on spikes 356 and 362. In contrast to employing a conventional CCL, the timing information between consecutive collars is not needed to determine tool velocity.

[057] FIG. 3D illustrates a waveform of the combination of signals generated by the first sensor 160 and a third sensor when the polarities of the magnetic polarity of the third sensor is flipped when compared with the second sensor 165, wherein the third sensor generates a third signal, and the third sensor has the same magnetic polarity as the first sensor 160. In this scenario, the noise is additive. In other words, the common correlated noise is cancelled in FIG. 3C, and is added in FIG. 3D.

[058] FIG. 4A illustrates a cross-correlation of the first signal of the first sensor 160 (FIG. 3 A) and the second signal of the second sensor 165 (FIG. 3B). Cross-correlation of the first signal and the second signal can lead to greatly sharpened signals and easier-to-read information. Generally, in signal processing, cross-correlation is a measure of similarity of two series as a function of the displacement of one relative to the other. This is also generally known as a“sliding dot product” or“sliding inner- product”.

[059] When signals are of the same polarity, such as the first signal and the third signal, a cross correlator output maximum will correspond to the delay between the signals from the two windings. If the signals are of opposite polarity, such as between the first signal and the second signal, this delay will be indicated by a minima. Knowing the delay and the distance between windings, the velocity of the tool set can be calculated.

[060] In the cross-correlation of FIG. 4A, there are three points of special interest - smaller negative spike 410, larger negative spike 420, and smaller negative spike 430. The main trough 420 indicates where the signals are the most dissimilar, e.g., having opposite polarities, when“x” (the first signal) precedes“y” (the second signal) by two seconds. In other words, when delaying the signal of the first sensor 160 by two seconds so that its peak matches the trough of the second sensor 165 - i.e., the first sensor 160 traversing the casing collar 155, and the second sensor 165 then traversing the casing collar 155. Knowing that the sensors are 2 feet apart, a sonde velocity of 1 foot/second, or in a more practical unit for well logging, 60 feet/minute, can be inferred. In some aspects, cross-correlation as employed also provides an indication of the direction of displacement, i.e. when the signal from the first sensor precedes or succeeds the signal from the second sensor passing the same collar. The first spike 410 and the third spike 430 also both indicate a secondary dissimilarity occurs with a correlation between the first and second collars 150, 180 themselves, that of forty-one seconds apart in this example.

[061] FIG. 4B illustrates a waveform that shows how the orientation of the peaks of generated electrical signals are changed if the magnetic polarity or the winding path of the second sensor 165 is reversed, such as would occur with cross-correlating the signal from the first sensor 160 with the third sensor, discussed above. The main peak 470 indicates that the first and third signals are most similar when the“x”, the first signal, precedes“y”, the third signal, by two seconds. The smaller peaks 460 and 480 correspond to the difference in seconds between signals of the first sensor 160 passing a casing collar and the second signal passing a casing collar.

[062] FIG 5 A illustrates an auto-correlation, wherein the first and second sensors 160, 165 are approximately two feet apart, and the collars 150, 180 are approximately forty-one feet apart.

[063] Generally, auto-correlation is a measure of a sameness of a signal. Autocorrelation, also known as serial correlation, is the correlation of a signal with a delayed copy of itself as a function of delay; the similarity between observations as a function of the time lag between them. The analysis of auto correlation is a mathematical tool for finding repeating patterns, such as the presence of a periodic signal obscured by noise, or identifying the missing fundamental frequency in a signal implied by its harmonic frequencies.

[064] In FIG. 5 A, a main spike 510 at zero delay is generated with an auto-correlation, when the first signal and the second signal are summed together. The negative spikes 513, 516 indicate that the combined signal is more dissimilar to itself when shifted by two seconds. This corresponds to the difference between the first sensor 160 and the second sensor 165. The smaller negative spikes 520, 530 correspond to the distance between the signals of the next collar, such as second collar 180, which are sixty feet per second are approximately forty-one seconds apart. Time-correlated common mode noise cancels in this auto-correlation, as the second sensor 165 has a reverse magnetic polarity to the first sensor 160. Qualitatively, the auto-correlation of FIG. 5 A can provide a higher signal to noise ratio than the signal to noise ratio of the auto-correlation of FIG. 5B, even though the information on the direction of movement has been lost.

[065] Generally, when the signals are of the same polarity, a correlation output maximum, different from the inherent maximum at delay zero, will correspond to the delay between the signals from the two sensors. If the signals are of opposite polarity this delay will be indicated by a minimum. Knowing the delay and the distance between windings, the velocity can be calculated.

[066] In FIG. 5B, the third sensor has the same polarity as the first sensor 160, the signals add, and the main peak 560 at zero is expected with auto-correlation. The positive peaks 563, 566, indicate that the combined signal is more similar to itself when shifted by two seconds. This corresponds to the length between the first sensor and the third sensor. The smaller positive peaks 570, 580, correspond to the distance between the signals which is approximately forty-one seconds apart. Time-correlated common mode noise adds in this auto-correlation, as the third sensor has the same polarity to the first sensor. Although the spike 560 has higher absolute magnitude than the spike 510 of FIG. 5B, the overall auto- correlated noise floor is significantly higher when comparing FIG. 5B to FIG. 5A.

[067] FIG. 6 illustrates an example of a DCCL tool system 600 which employs cross-correlation and includes a DCCL tool 610 constructed according to the principles of the disclosure. The DCCL tool system 600 is represented in the environment of the well system 100 of FIG 1 wherein the DCCL tool 610 provides an example of the DCCL tool 130. Accordingly, similar components of the DCCL tool 610 are represented by the same element numbers of the DCCL tool 130. The DCCL tool 610 includes a housing 615, such as the housing 169. The DCCL tool 610 has the first permanent magnet 205 of the first sensor 160 and the second permanent magnet 210 of the second sensor 165. Each of the first and second permanent magnets 205, 210, are oriented with opposite polarity orientations and each have their respective windings 215, 220. Generally, the magnetic poles are opposite on both windings, however, the terminals can be selectively connected inverted to the respective amplifiers so that the two windings produce signals of the same electrical polarity, or inverse of each other.

[068] The DCCL tool system 600 produces a cross-correlation signal from output of the first sensor 160 and the second sensor 165 within correlator 650. The correlation can be correlated to a velocity of the DCCL tool 130 with respect to casing collars, such as casing collar 155.

[069] Inputs of a first amplifier 630 and a second amplifier 640 of the DCCL system 600 are connected via wires 625 across the first winding 215 of the first sensor 160 to create the first signal, and across a second winding 220 of the second sensor 165 to create the second signal that is conveyed to the second amplifier 640. Specifically, regarding the first amplifier 630 and the second amplifier 640 illustrated in FIG. 6, their respective positive inputs are coupled to the north pole of their respective first and second permanent magnets 205, 210, and their respective negative inputs are coupled to the south pole of their respective first and second permanent magnets 205, 210. Signals, sent to their respective first amplifier 630 and second amplifier 640 are then sent with an unmodified polarity or phase. A first amplified signal from the first amplifier 630 and a second amplified signal from the second amplifier 640 may then be conveyed to the correlator 650. The correlator 650 is designed and constructed to, i.e., adapted to, help determine a degree of similarity between different signals. For the DCCL tool 600, the correlator 650 is located within the housing 615 and can perform cross-correlation downhole. Depending upon needs and whether a cross-correlation or auto-correlation is performed, the correlator can be on the surface. The correlator 650 is coupled to a processor 660 across a connection 670. The correlator 650 can be implemented as a processor and, in some examples, the correlator 650 can be a part of the processor 660.

[070] The processor 660 can employ the resulting cross-correlation of these differentiated signals from the correlator 650 to determine a velocity of the corresponding wireline 120, or otherwise control the wireline 120 by, for example, controlling the speed of the winch 170. The processor 660 is located within the housing 615. Alternatively, the processor 660 can be located external to the housing 615, such as at the surface. For example, the processor 660 can be implemented in the well system controller 145. If within the well system controller 145, the processor 660 can employed with a display for displaying data, a keyboard, and other user interfaces.

[071] Taking measurements across a winding by the first amplifier 630 and the second amplifier 640 can improve the common mode rejection ratio of its corresponding sensor. These signals can be of various collar signatures. The cross-correlation is employed to help determine a degree of similarity between different signals.

[072] In the DCCL tool system 600, an output of the correlator 650 will be at a maximum if the winding signals are in phase, and a minimum if they are in opposite phase, corresponding to the“time delay” for a coinciding of collar signature signals. In other words, after cross correlation of the signals has occurred, a time delay is therefore known between a signal of the first sensor 160 and a corresponding signal of the second sensor 165. From this information, and a previously-known distance/measurement on the wireline 120 between the first sensor 160 and the second sensor 165, the time of flight of the coupled wireline 120, can be calculated.

[073] Indeed, as the signal of the first sensor 160 and the second sensor 165 is acquired separately, such as in the DCCL tool system 600, and then the signals are cross-correlated by the correlator 650, the DCCL tool system 600 can estimate velocity from changes in pipe magnetic properties and/or thickness that will produce a magnetic signature. The nature of the signature does not need to be known before hand, but should be detectable by both the first and second sensors 160, 165.

[074] Lateral movement of a tool string can be a source of noise. However, this lateral movement noise is expected to be highly correlated on the two windings of the two sensors if these the two sensors are not too far apart. When different polarities are used for the two sensors, such as in the illustrated aspects of the present disclosure, this will contribute to cancellation of the lateral movement noise.

[075] An advantage of the DCCL tool system 600 is that this differential casing collar locator arrangement allows for an estimation of velocity, but does not require a previous knowledge of the magnetic signature of a casing collar, such as the first collar 155. This, in turn, renders the DCCL tool system 600 less liable to approximation errors as compared to some other technologies. Moreover, unlike many conventional downhole sensors, an active driver is not required to generate the various electrical characteristics associated with the first sensor 160 and the second sensor 165. Instead, the collar signals are generated as a function of a movement of the permanent magnets of the first and second sensors 160, 165, vis a vis the casing collars, such as collar 155 in the well system 100.

[076] Generally, regarding the collar signatures, a cross-correlation of corresponding collar signatures will be at a maxi um at a point that corresponds to the time that elapses between the first sensor 160 traversing a collar and the second sensor 165 traversing the same collar. This information, gathered and then amplified by the first amplifier 630 and the second amplifier 640 and employed by the correlator 650, will allow for velocity estimation of the wireline 120 independent of the collar length or magnetic signature of a collar.

[077] Velocity information can be extracted from unwanted road noise from the collar signatures. Moreover, there may be other signal anomalies being sensed which may also be determined through employment of cross-correlation or auto-correlation. These signal anomalies could be local changes in the magnetic properties or thickness changes, and anything that causes a change in the magnetic flux that can be detected by the windings when they pass over, e.g. centralizers attached to the casing . Moreover, the wireline 120 may be raised or lowered, but the principles of employment of the collar signatures and the first and second sensors 160, 165 are interchangeable, depending upon situation. Accordingly, any denotation as“first” or“second” sensor is done for ease of explanation with the provided examples, but should not be deemed a limitation of the named first sensor or second sensor.

[078] FIG. 7 illustrates a diagram of another example sensor section 700 for auto-correlation that can be employed in a DCCL tool as disclosed herein. For example, the sensor section 700 can be employed by the DCCL tool 130. In the sensor section 700, an output of the first sensor 160 is coupled to (summed with) an output of the second sensor 165 by a connection wire 710. This coupling of the winding 215 of the first sensor 160 and the winding 220 of the second sensor 165 provides a physically combined voltage signal derived from a first signal and a second signal generated by the first sensor 160 and the second sensor 165, respectively. The summation of the first signal and the second signal is then used in auto correlation by auto-correlator 760.

[079] The first sensor 160 and the second sensor 165 are coupled to an amplifier 750 via wires 720 and 730 and the amplifier 750 is coupled to the auto-correlator 760. The sensor section 700, the amplifier 750, and the auto-correlator 760 are all located in a single housing 715, such as the housing 169. The auto-correlator 760 can be implemented as a processor that is operable to perform the auto-correlation.

[080] The winding 215 of the first sensor 160 and the winding 220 of the second sensor 165 can be coupled together, such as by connection wire 710, so that the induced signal voltages would add in opposite phase, such as by the opposite magnetic polarity. In some examples, the induced signal voltages can then be sent to the surface for detection, employing either threshold or by auto-correlation, as illustrated in FIG. 5A.

[081] With the sensor section 700, and also the sensor section 200, the two signals derived from the first sensor 160 and the second sensor 165 are simply added to make only one signal for processing. With sensor section 700, the signal is processed by the auto-correlator 760, a local auto-correlator. If the signals are added in counter-phase, the correlated noise resulting from lateral tool movement will cancel and the peaks easily recoverable by simple threshold detection. Auto-correlation is another approach which can provide better performance. Employment of auto-correlation will produce a maximum at zero delay, but also lower peaks at the delay between the pulses, e.g., lower noise floor, which are measurements of interest. If the signals are added in opposite phase, these peaks will correspond to signal minima, otherwise maxima.

[082] FIG.8 shows a flow diagram of an example method 800 of using a cross-correlator for downhole collar location and velocity determination of a wireline. The method 800 can employ a DCCL tool, such as the DCCL tool system 600. The method begins in a start step 805.

[083] In a step 810, a first signal is generated by a first sensor when the first sensor traverses a downhole casing collar. The first sensor includes a permanent magnet that has a first winding and can be of a first polarity orientation. A first signal, for example, can be generated when the first sensor 160 traverses a downhole casing collar.

[084] In a step 820, a second signal is generated by a second sensor having a second permanent magnet. This second signal can be generated, for example, when the second sensor 165 traverses a downhole casing collar. In some embodiments, the second permanent magnet can be of a second phase orientation, and has a second winding, wherein the second polarity orientation is opposite of the first polarity orientation. A distance between a center of the first sensor and a center of the second sensor is known.

[085] In a step 830, the first signal and the second signal are cross-correlated, such as is illustrated in FIG. 4A. In a further embodiment, if the cross-correlation windows cover more than one casing section, estimates can also be made knowing the length of said casing section.

[086] In a step 840, a velocity of a downhole tool associated with at least the first and second sensors, such as the DCCL tool 130, is determined from the cross-correlation. In one embodiment, the cross correlation calculates a delay between the first and second signals, and therefore the movement of the downhole tool can be inferred as a consequence of knowing the distance between sensors, and determining the time of flight, and hence velocity and movement (both terms are used interchangeably), from the cross-correlation. This determination can occur at the surface. From this velocity information, location of the downhole tool can also be determined.

[087] In a step 850, a velocity of the downhole tool is recorded and/or adjusted as a function of the determined movement of the downhole tool. Thereafter, the method can end in a stop step 855.

[088] FIG. 9 shows a flow diagram of an example method 900 of use of an auto-correlation for downhole collar location and velocity determination of a wireline, such as the sensor section 200. The method 900 begins in a start step 905.

[089] In a step 910, a first signal is generated by a first sensor having a fist permanent magnet. This first signal can be generated, for example, by the first sensor 160, when the first sensor traverses a first downhole casing collar. In some embodiments, the first permanent magnet can be of a first polarity orientation, and has first winding.

[090] In a step 920, a second signal is generated by a second sensor having a second permanent magnet. This second signal can be generated, for example, by the second sensor 165 when the second sensor 165 traverses a second downhole casing collar. In some embodiments, the second permanent magnet can be of a second phase orientation, and has a second winding, wherein the second polarity orientation is opposite of the first polarity orientation. A distance between a center of the first sensor and the second sensor, such as the first sensor 160 and the second sensor 165, is known. In one embodiment, the first downhole casing collar and the second downhole casing collar are the same downhole casing collar.

[091] In a step 930, an auto-correlation occurs of the sum of the first signal and the second signal. FIG. 5A provides an example of the auto-correlation. In some embodiments employing the auto-correlation, the first signals and the second signals are added, with one of them producing signals of opposite phase to the other. As such, correlated common noise caused by lateral movement of a downhole tool can be rejected. The addition of the first and second signals can occur due to a consequence of the connection such as the wire 710 between the first and second sensors. In other embodiments, the summing of the signals can be performed separately by a processor, such as in the sensor section 200 or sensor section 700. Furthermore, secondary negative spikes from the auto-correlation provide the delay between the signals from the two windings. In a further embodiment, if the auto-correlation windows cover more than one casing section, estimates can also be made knowing the length of said casing section.

[092] In a step 940, a movement of a downhole tool associated with at least the first and second sensors, such as the DCCL tool 130, is determined from the auto-correlation. In one embodiment, the auto correlation calculates a delay when the combined signal repeats, due to the delay of the combined signals, and therefore the movement of the downhole tool can be inferred as a consequence of knowing the distance between sensors 160 and 165, and determining the time of flight, and hence velocity and movement, from the auto-correlation. This determination can occur at the surface. From this velocity information, location of the downhole tool within the wellbore can also be determined. In some aspects, the auto-correlation calculates a delay between the sum of the first signal and the second signal.

[093] In a step 950, a velocity of the downhole tool is adjusted as a function of the determined movement of the downhole tool. Thereafter, the method can end in a stop step 955.

[094] Those skilled in the art to which this application relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments.

[095] A portion of the above-described apparatus, systems or methods, such as the cross-correlation and the auto-correlation, can be embodied in or performed by various digital data processors or computers, wherein the computers are programmed or store executable programs of sequences of software instructions to perform one or more of the steps of the methods. The software instructions of such programs may represent algorithms and be encoded in machine-executable form on non-transitory digital data storage media, e.g., magnetic or optical disks, random-access memory (RAM), magnetic hard disks, flash memories, and/or read-only memory (ROM), to enable various types of digital data processors or computers to perform one, multiple or all of the steps of one or more of the above -described methods, or functions, systems or apparatuses described herein.

[096] Portions of disclosed embodiments may relate to computer storage products with a non-transitory computer-readable medium that have program code thereon for performing various computer- implemented operations that embody a part of an apparatus, device or carry out the steps of a method set forth herein. Non-transitory used herein refers to all computer-readable media except for transitory, propagating signals. Examples of non-transitory computer-readable media include, but are not limited to: magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks; magneto-optical media such as floptical disks; and hardware devices that are specially configured to store and execute program code, such as ROM and RAM devices. Examples of program code include machine code, such as produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. [097] Various aspects of the disclosure can be claimed including the apparatuses, systems and method as disclosed herein. Aspects disclosed herein include:

[098] A. A differential casing collar locator (DCCL) tool, including (1) a first sensor having a first permanent magnet of a first polarity orientation and a first winding around the first permanent magnet, wherein the first sensor generates a first signal in response to traversing a casing collar; (2) a second sensor having a second permanent magnet of a second polarity orientation and a second winding around the second permanent magnet, wherein the second sensor generates, in response to traversing the casing collar, a second signal in opposite polarity of the first signal; and (3) a cross-correlator coupled to the first sensor and the second sensor and that generates a correlated output employing the first signal and the second signal.

[099] B. A well system including a wellbore having a casing string with casing collars, the well system, including (1) a differential casing collar locator (DCCL) tool, having: (2) a first sensor having a first permanent magnet of a first polarity orientation and a first winding around the first permanent magnet, wherein the first sensor generates a first signal when traversing a casing collar of the casing string; (3) a second sensor having a second permanent magnet of a second polarity orientation and a second winding around the second permanent magnet, wherein the second sensor generates a second signal in opposite polarity of the first signal when traversing the casing collar; and (4) an auto-correlator that generates an auto-correlated output employing a combination of the first signal and the second signal.

[0100] C. A method of determining velocity of a downhole tool including (1) generating a first signal by a first sensor having a first permanent magnet when the first sensor traverses a first downhole casing collar within a wellbore; (2) generating, by a second sensor having a second permanent magnet, a second signal in opposite polarity of the first signal when the second sensor traverses a second downhole casing collar; (3) auto-correlating the first signal and the second signal; and (4) determining the velocity of the downhole tool in the wellbore from the auto-correlating of the first signal and the second signal. [0101] Each of aspects A, B, and C can have one or more of the following additional elements in combination:

[0102] Element 1: wherein the first polarity orientation is opposite to the second polarity orientation. Element 2: wherein at least the first sensor and the second sensor are located within a same housing. Element 3 further comprising: a first amplifier coupled between the first winding and the cross-correlator; and a second amplifier, different from the first amplifier, coupled between the second winding and the cross-correlator. Element 4: further comprising a processor coupled to the cross-correlator, wherein the processor determines a velocity of a downhole tool in a wellbore as a function of cross-correlation. Element 5, wherein the processor determines a location of the downhole tool in the wellbore as a function of the velocity of the downhole tool. Element 6: wherein the combination of the first signal and the second signal are both coupled to the auto-correlator through a single amplifier. Element 7: further comprising a downhole tool associated with the DCCL tool; and a processor that employs the auto- correlated output to determine a velocity of the downhole tool in the wellbore. Element 8: wherein the processor determines a location of the downhole tool in the wellbore as a function of the velocity of the downhole tool. Element 9: wherein the first polarity orientation is opposite to the second polarity orientation. Element 10: wherein the first winding is coupled to the second winding. Element 11: wherein at least the first sensor and the second sensor are located within a same housing. Element 12: wherein the velocity of the downhole tool is different from an estimated velocity of the downhole tool at the surface. Element 13: wherein at least the first sensor and the second sensor are located within a same housing. Element 14: further comprising summing the first signal and the second signal before the auto-correlating. Element 15: wherein the second permanent magnet is of an opposite polarity of the first permanent magnet. Element 16: further comprising determining a location of the downhole tool in the wellbore as a function of the velocity of the downhole tool. Element 17: wherein the auto-correlating occurs downhole.