Patent Application
20120176862
The present application generally relates to measurement of perforation tunnels in oil wells, and more specifically to measurement of depth and other dimensions of perforation tunnels with an ultrasonic pulse and reflection of such.
The productivity of oil and gas fluids from subterranean formations is typically controlled by casing and perforating the wellbore. To maximize the return of the well, perforation properties are optimized through their vertical placement, phasing and internal morphology. If observations of perforation properties may be taken in situ, then stimulation services may be optimally designed to increase productivity or injectivity. Specifically, for old and new wells, to optimize the performance it is desirable to know the open perforation tunnel length.
Accordingly, the present application provides a number of preferred embodiments that address many of those and related issues.
The following description is a brief synopsis of a combination of features according to a preferred embodiment of the present application.
A method of logging a perforation tunnel and associated features of the perforation tunnel can include the following features. A logging device including an ultrasonic transducer is located downhole into a well. The well has a casing. The ultrasonic transducer has a focal point that is a distance from the ultrasonic transducer so as to be behind the inner face of the casing. An ultrasonic signal is projected from the ultrasonic transducer. A reflection of the ultrasonic signal is reflected from an internal portion of the perforation tunnel, the perforation tunnel extending through the casing and into formation. A transit time is measured between transmission and reception of the ultrasonic signal. A position of the ultrasonic transducer corresponding to the ultrasonic transmission and reception of the reflected signal is determined.
This summary is not meant in any way to unduly limit any claims related to this application and is merely meant to present a summary of some preferred combinations of features according to preferred embodiments in the present application. Many preferred embodiments can include different combinations including other features.
a and 5b are plots of ultrasonic signals received by an ultrasonic transducer according to preferred embodiments.
a and 6b are plots of ultrasonic signals received in open hole perforation according to preferred embodiments.
In the following description, numerous details are set forth to provide an understanding of the preferred embodiments. However, it will be understood by those skilled in the art that embodiments according to the present application may be practiced without many of these details and that numerous variations or modifications from the described embodiments are possible.
As used here, the terms “above” and “below”; “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate.
To extract hydrocarbons or other valuable fluids from subterranean formations, wells are created that extend into the ground. To support these wells, provide isolation of reservoir zones, and deter cave-ins, among other things, casings are often provided. These casings are cemented in place and line the wellbore. In order to extract fluids from the formation into the wellbore, holes (perforations), which are often generally circular in cross section and tubular or “carrot” in shape, are created through and beyond the casing into the formation. Perforations are the starting point for natural completions, acidizing, gravel packing and fracturing. Each application has different requirements of the perforation morphology: from short and fat to narrow and long. In open-hole perforating, where there is no casing lining the wellbore similar procedures take place.
Measurement of the open perforation tunnel length is most desirable in determining which perforating stimulation services may be applied to a completion to increase the well's performance.
To create holes or perforations, perforating guns are lowered into the wellbore. The perforating guns contain a plurality of shaped charges that fire projectiles through the casing and into the earth formation thereby creating holes in the casing and perforation tunnels in the formation. Where debris material has been removed, or otherwise ejected into the wellbore, an open perforation tunnel results. Hereinafter, we may refer to the open perforation tunnel as the tunnel. Where material from the shaped charge, the formation or the casing is deposited in the perforation, debris is present.
The entrance hole in the casing may fall in the range of 0.17 to 0.45 inches for natural completions, and be larger for other applications. Behind the casing: typically, the tunnel depth (Lpen in
The small casing hole and the larger internal void will represent a premier challenge for anyone attempting an acoustic measurement of the depth of a perforation tunnel.
Firstly, the casing has very high acoustic impedance, ensuring that nearly all of the energy striking it is reflected back towards the transmitting device. This insures that the back-scattered glare is very intense relative to the weak signal resulting from a perforation hole. An analogy of this would be using a flashlight, in a dark room, to find a small pit in a mirror. Essentially all the investigator would see is the reflected energy of the light source.
Secondly, the small perforation hole through the casing makes it even more challenging to get energy directly into the perforation. Since the exact location of these perforation holes is difficult to determine, the acoustic device may be scanned in azimuth and depth, meaning that the preferred embodiment is to deploy the device on the measurement tool. However, the acoustic device could be stationary. This ensures some amount of beam spreading will occur as the acoustic beam travels from the transmitter to the casing inner surface.
A challenge for an ultrasonic measurement becomes detecting a reflection from the end of the perforation tunnel, over the large backscatter signal of the casing reflection, having such a small entry hole to work with. Since that backscatter signal contains components that reverberate between the casing and the transducer face, and since the wellbore fluid is typically of a low acoustic speed, these reverberations can extend a great deal in time.
According to preferred embodiments, at least two techniques can be used simultaneously to reduce the amplitude of this backscatter noise relative to the amplitude of the tunnel end reflection:
1) orient the transmitting/receiving ultrasonic transducer system at a standoff distance, relative to the casing, that is reduced to the point that these multiple reverberations decay faster in time so as to allow for differentiation from signals reverberating from inside a perforation tunnel.
2) choose transducer parameters that produce a beam profile, or shape, that provides a beneficial signal to noise ratio. In this case, the signal is the portion of the transmitted energy that enters the perforation through the small hole in the casing, reflects from the end of the open tunnel section, and returns back to the receiving transducer to be received. The noise is the transmitted energy that reflects back to the receiving transducer from the steel casing and the layers behind it, or other structures contained in the wellbore. By choosing transducer parameters along specific well-defined guidelines, one can produce a strong reflected signal relative to the amplitudes of the reflected noise events, in the time window of the returned signal. This idea is illustrated in
As the stand-off distance between the transducer and the casing inner surface is decreased, the time spacing of the reverberation echoes occurring between the two also decreases. Since each multiple echo involves yet another partial reflection from the transducer face or from the casing surface, each multiple echo contains less energy. Thus, decreases in transducer standoff relative to the casing lead to the amplitudes of the multiple echoes decaying to a very low level rapidly.
Since the attributes of the fluid of propagation are the same or similar in both the travel of the energy to the casing and to the open tunnel end, the speed is essentially always the same. Thus, the timing can become a geometry situation. Empirically, we have shown that by using a focused transducer that is focused beyond the casing, an acceptable amplitude level of the casing reverberations is achieved after about 3 reverberation cycles. Thus, according to a preferred embodiment, it follows that we can measure an open tunnel that is as short as 3 times the standoff distance. Using this relationship, it is possible to select a standoff based on the minimum perforation depths expected to be measured.
The plot of
According to preferred embodiments, a second technique is utilized to increase this signal-to-noise ratio. The first technique served to reduce the time extent of the casing reflected noise. This second technique serves to increase the ratio of the amplitude of the signal returned by the end of the open perforation tunnel to the casing noise level.
On exciting the transducer, a beam (signal) of a finite shape is propagated in the fluid, toward the casing. For circular, planar transducers, the shape and size of that beam depends on the transducer diameter, the operating frequency, and the acoustic speed of the propagation media.
For the problem of maximizing the amount of energy of a transducer that enters the small hole of a casing perforation, one solution is to use a narrow, confined beam. If a transducer could emit a perfectly collinear beam, having a smaller diameter than the hole in the casing, once the beam was aligned centered on the hole, the returned signal would have none of the casing reflection noise described above, and only contain the tunnel end reflection. However, getting such a highly collimated beam, at transducer diameters below 0.2″ would require extremely high frequency, which is problematic due to attenuation in the wellbore fluid.
As the beam propagates away from a circular, planar transducer, it starts as a rather confined beam, having a beam width that is roughly the same as the transducer diameter. This is known as the near-field region of the beam, also known as the Fresnel length. As it propagates further, the beam begins to spread more rapidly in circular width. This region is known as the far-field. “Acoustic Waves: Devices, Imaging, and Analog Signal Processing”, Kino, Gordon. S., Prentice-Hall, Inc., 1987, which is incorporated herein by reference in its entirety, explains that the end of the near-field is given as when S=1, for:
S=Z2/a2, where
Z=the distance of propagation from the transducer,
λ=the wavelength, and
a=the transducer radius;
or, rewritten for S=1, and expressing as speed over frequency, the near-field end point is roughly when:
a
2
=Zc/f, where
f=frequency, and
c=speed of media.
To use this relationship for specifying a transducer that will provide the collinear beam desired, we start with the given knowledge of the desired standoff distance determined above for reduced reverberation time. Since it is preferable to have the near-field range extend beyond the 20 mm standoff chosen, we choose a near-field range of triple the standoff, or Z=0.060M. This margin of safety ensures a collinear beam well past the casing interface, deep into the perforation tunnel. Next, we choose a frequency. From the above equation, it is clear that if frequency is too low (i.e., below 300 kHz.) the transducer diameter will be too large for a borehole configuration. If frequency is too high, (i.e., above 5.0 MHz.) the absorption and scattering losses of the wellbore fluids will diminish the signal strength. 1.0 MHz is a preferred frequency. Taking the wellbore fluid to be water, having an acoustic speed of 1500M/Sec., the equation can now be solved for the transducer radius, ‘a’, as:
a=SQRT(0.060M*1500M/Sec/1.0 MHz), or
a=0.0095M, or 9.5 mm.
Thus, the diameter of the transducer is 9.5 mm. The Panametrics V303-SU immersion, circular, planar transducer is preferable for this application and is commercially available and known as Olympus-Panametrics Transducer circular planar transducer (from Olympus-NDT of Waltham, Mass.).
An alternate method of increasing the amplitude of the signal returned by the end of the open perforation tunnel is to shape the beam in a focused manner. By having an emitted beam profile that focuses to a small spot-size behind the inside casing surface, all, or most of the energy from the transducer will enter the perforation. It will then travel along various paths towards the tunnel end, and reflect back to the transducer. The amount of back-scatter noise from the inner surface of the casing will be acceptably low. The result is a very favorable signal-to-noise ratio.
In designing the parameters for a focused transducer, again first consideration is given to the preferred casing standoff, chosen above, 20 mm. The optimal improvement in signal-to-noise ratio will occur if the focal spot diameter is less than the casing hole size, and the focal point occurs at a point just beyond the casing, or at a standoff distance of at least equal to the casing standoff plus at least a portion of the casing thickness (i.e., behind the inner face of the casing). Taking 10 mm as a typical casing thickness number, we get a focal length of 30 mm. To keep the degree of focus of the transducer, or F number, to a reasonable level, we choose a transducer diameter that is on the order of half the focal length. Since diameters of 0.5 inches (13 mm) are a preferred transducer size, this will be preferable. Also, the significant focusing used here allows reduction of the operating frequency somewhat, to lessen sensitivities to attenuation.
From Kino (noted earlier) we also get a description of the calculation of this spot size, in SI units, which are rewritten slightly in form as:
Beam Diameter(−6 dB)=(1.02*Fc)/fD, where
F is the focal length of the transducer
c is the sound speed in the wellbore fluid
f is the frequency of the transducer
D is the diameter of the transducer element.
Thus, we calculate a −6 dB beam diameter of 7.22 mm, for a 0.5″ diameter transducer, focused at 30 mm, operating at 500 kHz, in water. A transducer that fits this criterion can be acquired from Ultran Laboratories, Inc. of State College, Pa., as a customized version of part number LS100-0.5-P76.
According to preferred embodiments, a method of determining the length of an open perforation tunnel includes lowering an ultrasonic transducer device (configured as noted above) downhole. An ultrasonic pulse is transmitted into the perforation tunnel and the time for the pulse to return to the transducer from reflection off the casing and from reflection from the interior of a perforation tunnel is measured. Based on the transit time and the speed of the ultrasonic pulse in the wellbore fluid, the length of the open perforation tunnel can be calculated.
There are a number of methods according to preferred embodiments that can be used to measure the perforation tunnel's depth and dimensions. For example, one method includes locating the ultrasonic transducer directly adjacent to and facing into the perforation tunnel. The ultrasonic transmission into the perforation tunnel is measured and used to determine the length of the open perforation tunnel. Also, the returning signal can be used to determine the presence and dimensions of debris at the end of the perforation tunnel.
Another method involves movement of the transducer over the interior of the casing (or wellbore in open hole), and a repeated transmission of an ultrasonic pulse and reception of the reflection. Essentially, the wellbore and perforation tunnels are mapped. From the return signals, the data can be gathered and used to determine the location of perforation tunnels, the depth and dimensions of the perforation tunnels, and the dimensions of debris at the end of the perforation tunnels, can be determined. See this idea illustrated in
In connection with the ultrasonic transducer device being moved while transmitting pulses of an ultrasonic signal, the speed of the transducer should be taken into account so that the speed is slow enough to enable reception of the reflected pulse by the transducer. If the transducer moves at a high speed, a separate transducer can be positioned adjacent to the transducer that transmits the pulse so that one transducer can be used to transmit ultrasonic pulses and another transducer can be used to receive the return signals.
The location of the ultrasonic transducer can be recorded and correlated to the recorded data. The detected data (transducer location, time between transmission and reflection, and amplitude of reflection) can be plotted to create a representation of perforation tunnels (e.g., a map of the wellbore), showing the location, depth and width of the open perforation tunnels (i.e., the portion of a perforation tunnel that is open and free from debris). Also, the amplitude of the response can be plotted (e.g., grayscale pixel(s) or color pixels corresponding to the amplitude of the response ultrasonic signal) to depict the shape and location of debris in the perforation tunnel.
Looking visually at some preferred embodiments,
The ultrasonic transducer 104 can transmit an ultrasonic signal with a frequency at least as high as 300 kHz and up to 5000 kHz, but preferably only as high as around 3000 kHz. The PTMD 100 can measure both the amplitude of the reflected ultrasonic signal and the transit time from when the signal leaves the PTMD 100 to when the signal reflects and returns.
The transducer 104 can be a focused beam ultrasonic transducer, which creates a converging waveform with a focal point hence allowing a greater portion of the ultrasonic energy created by 104 to enter the casing entrance hole and perforation tunnel. Alternatively, the transducer 104 can be a planar transducer.
The PTMD 100 is lowered in the wellbore and wellbore fluid 404. Over the course of movement, the ultrasonic transducer 104 is located adjacent to a perforation in the casing 402 and a corresponding perforation tunnel 400. The width of the entrance hole in the casing (beginning of perforation tunnel) is Ceh. The distance of the ultrasonic transducer 104 from the inside of the casing is Ls. The length of the open perforation tunnel from the inside of the casing is Lop.
To measure the depth of the open perforation tunnel, the ultrasonic transducer 104 transmits an ultrasonic signal into the perforation tunnel 400. The ultrasonic signal travels to the end of the open perforation tunnel 415 and is reflected back to the ultrasonic transducer 104 by the end of the open perforation tunnel 415, often formed by the beginning of tunnel debris 408. The time for the signal to travel from the ultrasonic transducer 104 and to reflect back to the ultrasonic transducer 104 is measured. This can be done either while the ultrasonic transducer 104 is stationary in front of a perforation tunnel, or moving past the perforation tunnel slowly.
The following formula can be used to calculate the length of the open perforation tunnel 400. If the speed of sound in the wellbore fluid is Cf and Top is the time taken for the signal in
Lop=Top*Cf/2.0−Ls
The value for the speed of the wellbore fluid, Cf, is approximated as in most cases it is largely a brine solution, the velocities of which are very close to water. Alternatively, the wellbore fluid can be accurately measured at the surface. Preferably, it can be measured downhole using a separate ultrasonic device.
The value of the standoff distance, Ls, also should be known. This value can be approximated by knowing the borehole, or casing, inner radius and the distance of the sensor face from the center of the logging tool. The difference of these values would be the mean standoff value. However, in practice this value varies significantly with tool rotation, especially in horizontal or highly deviated wells, where the tool is often at an eccenter. It is preferred to derive the standoff at each measurement location.
Each reflected pulse contains some amount of transducer-to-casing reverberation signal, discussed as noise above. Each of those reverberations is separated in time by an interval that is based on the fluid speed, Cf, and the standoff distance, Ls. By measuring the time between any two of these events, and using the known value of Cf, one can determine the standoff, Ls.
An alternate method of determining Ls is to use these same reverberations but to process them in the frequency domain. Performing a Fast Fourier Transform on a selected number of these reverberations will yield their characteristic frequency. The time between the events is the inverse of this frequency.
Then, by whichever of the above methods is used to derive the time between these reverberation signals, one calculates Ls by:
Ls=(Interval Time/2)*Cf
The same principal applies for open hole perforations (no wellbore casing).
One way to detect the width of a perforation tunnel is to move the PTMD 100 across a perforation tunnel opening and transmit an ultrasonic signal (or intermittent signals) over the course of travel. The path of travel for the PTMD 100 can be for example, circumferential, axial, or helical. The detected data over the course of travel of the PTMD 100 can then be charted where the Y-axis represents the distance traveled (position) of the PTMD 100 and the X-axis is the time for the signal to reflect and travel back to the PTMD 100. As shown in
The dimensions and location of debris 408 which can include liner material 410 and crushed zone 412 at the end of the perforation 400 can be determined by way of representations (plotted data) as shown in
The preferred range of ultrasonic frequencies is from 300 kHz to up to 3000 kHz. The upper end of this range is bounded by two main factors. First, is the loss in the fluid. Whether it be scattering loss due to particulate or true absorption loss, at some point the signal may be attenuated too much to traverse an approximate 12″ to 24″ round trip. Second is grain scatter effect of the rock formation needed to reflect the signal. As the frequency gets very high, large-grained rock begins to be interpreted like a sponge to the incoming wave, and the reflection pulse can be scattered into “pieces”, thus looking quite undefined. This upper frequency limit could begin to develop around 3 MHz.
The lower end of this range is dictated more by the geometric spreading of the beam coming from the transducer. Regardless of whether the front face of the transducer is focused (concave curved) or planar, as frequency goes down, the beam-width goes up. With transducers that can be mounted on borehole tools being limited to around 1.5″ diameter, a low-end frequency limit of about 300 kHz is the minimum frequency one could expect to use and still have a beam that can interrogate a small diameter perforation.
A preferred frequency is in the range of 1.0 MHz to 3.0 MHz, and most preferably 1.0 MHz with, for example, a 0.5″ diameter transducer (available planar or focused). Such a transducer is available from Panametrics Corporation, as P/N V303-SU.
A second path is B1-B2-B3-B4. Here we have multiple reverberations between the transducer 104 and casing 404. These beam paths are the most likely source of noise and are detrimental to detecting the end of the open perforation tunnel 415.
Based on the various ultrasonic paths noted herein, varying degrees of noise can be produced depending on the configuration implemented. For example,
In contrast,
When the standoff and beam diameter are chosen properly, as discussed above, and focused beyond the casing, as seen in
As discussed above, a circular planar transducer can also be used to control this signal-to-noise ratio, by shaping the beam to be collinear and having a small diameter, as shown in
The description herein is meant to provide one skilled in the art an understanding of the various embodiments and features and is no way intended to unduly limit the scope of any claims related to this application.
