Patent Application
20180217289
The present disclosure generally relates to logging tools and, more particularly, to methods for reducing borehole signals for side-looking Nuclear Magnetic Resonance (“NMR”) logging tools.
A variety of techniques have been used in determining the presence quantities of hydrocarbons (oil and gas) in earth formations. One such technique involves the use of NMR logging tools for determining, among other things porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the fluids in the geological formations in the vicinity of the wellbore so that certain parameters such as spin density, spin-lattice relaxation time (generally referred to in the art as “T1”), and spin-spin relaxation time (generally referred to as “T2”) of the geological formations can be estimated. From such measurements, porosity, permeability, and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
A conventional “side-looking” NMR tool is sensitive to NMR excitation on one side of the tool and less sensitive to NMR excitation on the other side. The more sensitive side of the tool is typically pressed against the side wall of a borehole adjacent a formation, thereby providing minimum separation between the NMR tool's radio frequency (“RF”) field generating assembly and the formation volume of NMR investigation. The less sensitive side of the tool is thus exposed to the borehole, which gives rise to the issue of borehole signal whereby the NMR signal from the borehole tends to erroneously contribute to the received NMR signal (i.e., “total signal” as referred to herein) from the volume of interest.
A variety of conventional techniques have been utilized to combat the issue of the borehole signal. In one example, a side looking NMR tool design includes a dipole RF antenna displaced toward the formation and a single permanent magnet, having a single magnetization direction, placed toward the back of the tool. However, for such a configuration, the signal from borehole will still be unacceptably large for larger boreholes. To solve this problem, other approaches have utilized a tool having an active RF spoiler design where a spoiler in the back, possessing a dipole moment opposite to that of the front antenna, spoils the borehole signal to an acceptable level. However, a spoiler implementation introduces a high level of complexity in terms of building and testing of the tool, as well as reliability issues.
Therefore, there is a need in the art to provide alternative, reliable and less complex NMR logging tools capable of eliminating and/or reducing the borehole signal to an acceptable amount.
Illustrative embodiments and related methods of the present disclosure are described below as they might be employed for reducing the borehole signals for side-looking NMR tools. In the interest of clarity, not all features of an actual implementation or methodology are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. Further aspects and advantages of the various embodiments and related methodologies of the disclosure will become apparent from consideration of the following description and drawings.
As described herein, methods and related embodiments of the present disclosure are directed to the reduction of the borehole signal for a side-looking NMR logging tool. In a first generalized embodiment of the present disclosure, a side-looking NMR logging tool includes a magnetic assembly to generate a static magnetic field, and an RF transceiver antenna to induce an RF field into the volume of interest and to receive corresponding total signals from the volume of interest. The axial extent of the RF transceiver antenna has a length selected to reduce a borehole signal.
In a second generalized embodiment of the present disclosure, a side-looking NMR logging tool includes an RF transceiver antenna to induce an RF field into a volume of interest and thereby receive a corresponding total signal from the volume of interest. The NMR logging tool further includes a magnet assembly having at least two magnets with magnetizations in different directions. As a result, the static magnetic field produced by the magnet assembly acts to reduce the borehole signal by shifting the sensitive area in the back of the tool further away from the tool and, thus, further out of the borehole.
In a third generalized method of the present disclosure, an NMR logging tool is deployed into a wellbore to conduct logging operations. A static magnetic field is induced into a volume of interest to thereby polarize the nuclear spin within the volume of interest. An RF excitation pulse is then induced into the volume of interest to rotate the nuclear spin. The RF refocusing pulse is induced into the volume of interest to generate a spin echo. The flipping angle of the RF refocusing pulse is reduced to less than 180 degrees. Reducing the flipping angle of the refocusing pulse to less than 180 degrees has a disproportionate reduction effect on the total signal received from the volume of interest via the reduction effect on the borehole signal being much larger than the formation signal. Thus, the borehole signal as a percentage of the total signal will be reduced. In an alternate embodiment, the same effect is achieved by reducing the excitation pulse to less than 90 degrees. Accordingly, various embodiments of the present disclosure minimize borehole signals in NMR logging for side-looking tools.
Magnet assembly 104 includes a first and second magnet 104a and 104b, respectively. NMR logging tool 100 further includes RF transceiver antenna 110 that includes a soft magnetic core and antenna wire at the right of tool 100. RF transceiver antenna 110 induces an RF field, B1, into the volume of interest surrounding the borehole of wellbore 102 and receives the is corresponding total signals from the volume, as will be understood by those ordinarily skilled in the art having the benefit of this disclosure.
In this illustrative embodiment, NMR logging tool 100 is positioned against the right side of the wall of wellbore 102. Magnet assembly 104 is positioned on a second side (i.e., back side) of NMR logging tool 100, while RF transceiver antenna 110 is positioned on the first side (i.e., front side), which is opposite of the second side, of NMR logging tool 100. As indicated by the boldened arrows, first and second magnets 104a,b have different directions of magnetization which produces static magnetic field 106. Because of the differing magnetization directions of magnets 104a,b, the resulting static magnetic field 106 is shifted outside the borehole of wellbore 102, thus reducing the borehole signal. In this example, first magnet 104a is polarized in the upper-left direction, while the magnetization of second magnet 104b is tilted in upper-right direction. Alternatively, however, first magnet 104a may be polarized in the downward-right direction, while second magnet 104b may be polarized in the downward-left direction. In yet other examples, For example, first magnet 104a may be tilted 10-20 degrees in relation to second magnet 104b. Those ordinarily skilled in the art having the benefit of this disclosure realize there are a variety of tilt orientations which may be utilized without departing from the spirit and teachings of the present disclosure.
Still referring to
To further explain the embodiment of
As will be understood by those ordinarily skilled in the art having the benefit of this disclosure, in order to shift the effective center of the B0 field 106, a magnet may be positioned as far left as possible in the tool (as shown in
Although static magnetic field 106 is shown completely outside the borehole of wellbore 102, such a borehole reduction is not required. For example, in some applications, the sought borehole signal reduction may not be as stringent, and therefore it may not be necessary for static magnetic field 106 to be pushed completely outside the borehole.
The tilting of the net magnetization may be achieved in a variety of ways.
In yet another illustrative embodiment of the present disclosure, the design of a side-looking NMR logging tool takes advantage of the three-dimensional (“3D”) effect of the RF dipole. As will be understood by those ordinarily skilled in the art having the benefit of the present disclosure, the magnetic field generated by a 3D RF dipole will decay faster than that of a 2D RF dipole (α 1/r3 for 3D far field as compared to a 1/r2 for 2D far field). Here, as illustrated in
In certain embodiments, for example, the very short antenna refers to an antenna having an axial extent not much longer than the distance from the antenna to the sensitive volume of interest on the back of tool 100,200. In terms of NMR, the tipping angle for nuclei in the back will be much smaller, which in turn reduces the sensitivity of transceiver antenna 110,210 for the signal coming from that back area. Accordingly, the resulting effect is a much smaller borehole signal. In certain other embodiments, the length of RF transceiver antenna 110,210 may be selected to reduce the borehole signal to less than 1 p.u. (porosity unit) of the total signal in a borehole approximately 12.25 inches in diameter. As defined herein, porosity unit refers to the percentage of signal to the total signal when the formation is 100% water. In another embodiment, the length of RF transceiver antenna 110,210 is selected to reduce the borehole signal to less than 0.5 p.u. of the total signal in an approximately 8.5 inch borehole.
In yet another illustrative method of the present disclosure, the flipping angle of refocusing pulses (e.g., Carr-Purcell-Meiboom-Gill (“CPMG”)) is reduced in order to minimize the borehole signal. As will be understood by those ordinarily skilled persons described herein, an NMR tool generates a static magnetic field B0 in the vicinity of the wellbore, and an oscillating field B1 in a direction perpendicular to B0. This oscillating field is usually applied in the form of short duration pulses. The effect of the B0 field is the polarization of the magnetic moments of nuclei parallel to the static field and the purpose of the B1 field is to rotate the magnetic moments by an angle θ controlled by the duration, tp, and the amplitude |B1| of the oscillating pulse. For NMR logging, the most common sequence is the Carr-Purcell-Meiboom-Gill (“CPMG”) sequence that can be expressed as
P90-TE/2-(P180-TE/2-ACQ-TE/2)n, Eq. (1),
where P90 represents the 90 degree excitation pulse, P180 is the 180 degree refocusing pulse, TE/2 is the half inter-echo spacing, ACQ is the spin echo acquisition, and n is the looping n times. When the refocusing pulse is adjusted away from 180 degrees, with no consideration to the spin-spin relaxation (T2) or spin-lattice relaxation (T1), the spin echo amplitude will reach a so-called pseudo steady state after a few echoes, and the amplitude is M0 sin(θ/2), where M0 is the total spin magnetization and θ is the flipping angle of the refocusing pulse. It can also be noted that the derivative of such an amplitude with respect to θ is proportional to cos(θ/2).
Therefore, in certain illustrative methods of the present disclosure, the flipping angle for the area in front of the tool is reduced away from the regular 180 degrees. The resulting change of echo amplitude from that area will be small (cos(θ/2)≈0 when θ≈180°. For the area in the is back of the tool, however, since the spins there are severely under-rotated, the reduction in echo amplitude will be more significant when the flipping angle is proportionally reduced (cos(θ/2)≈1 when θ≈0°. Thus, in certain embodiments, by reducing the flipping angle of the refocusing pulse (or the excitation pulse), the signal received from the back of the tool is disproportionately affected and, thereby, results in a reduced borehole signal. During testing of embodiments of the present disclosure, spin dynamics simulations showed that if the flipping angle of the refocusing pulse is reduced to 135 degrees, for example, the signal in the front of the tool will be reduced by less than 5%, while the signal in the back will be reduced by more than 30%. As a result, the borehole signal is greatly reduced. Note also that in one or more alternative embodiments, this same effect may be achieved by reducing the excitation pulse to less than 90 degrees.
Therefore, in a generalized method and with reference to the flow chart 500 of
In such a deployment, NMR logging tool 600 performs logging operations as desired is while reducing the borehole signal as described herein. Once the desired logging data has been acquired, the resulting signals may be saved to a memory disk onboard NMR logging tool 600 and processed in-situ using circuitry onboard tool 600, or transmitted to the surface via cable 602 for well site processing. A logging facility 608 collects measurements from NMR logging tool 600, and may include circuitry 610 for processing and storing the measurements received from NMR logging tool 600.
As stated, in certain embodiments, the methods described above may be performed by processing circuitry onboard the NMR logging tool or located at some remote location. In either case, such processing circuitry would comprises a signal processor, communications module and other circuitry necessary to achieve the objectives of the present disclosure, as will be understood by those ordinarily skilled in the art having the benefit of this disclosure. In addition, it will also be recognized that the software instructions necessary to carry out the objectives of the present disclosure may be stored within storage located within the processing circuitry or loaded into that storage from a CD-ROM or other appropriate storage media via wired or wireless methods. If the processing circuitry is remotely located, a suitable wired or wireless communications link may provide a medium of communication between the processing circuitry and the logging tool. Alternatively, however, the communications link may be an electromagnetic device of suitable frequency, or other methods including acoustic communication and like devices.
Accordingly, the illustrative embodiments and methods described herein provide alternate ways in which to reduce the borehole signal for a side-looking NMR logging tool. As a result, more reliable and less complex NMR logging tools are realized.
Embodiments and methods described herein further relate to any one or more of the following paragraphs:
1. A method for nuclear magnetic resonance (“NMR”) logging, the method comprising inducing a static magnetic field in a volume of interest using a magnet assembly of a logging tool to thereby polarize nuclear spin within the volume of interest, the logging tool being deployed along a wellbore; selecting a radio frequency (“RF”) refocusing pulse or RF excitation pulse to reduce a borehole signal; inducing the selected RF excitation or refocusing pulse into the volume of interest using an RF antenna of the logging tool to thereby rotate the nuclear spin of the volume of interest; and receiving a total signal from the volume of interest using the RF antenna.
2. A method as defined in paragraph 1, wherein selecting the RF refocusing pule or RF excitation pulse comprises reducing a flipping angle of the RF refocusing pulse or the RF excitation pulse to an angle less than an angle in which a maximum total signal is generated.
3. A method as defined in paragraphs 1 or 2, wherein selecting the RF refocusing pule or RF excitation pulse comprises reducing a flipping angle of the RF refocusing pulse to less than 180 degrees; or reducing a flipping angle of the RF excitation pulse to less than 90 degrees.
4. A method as defined in any of paragraphs 1-3, wherein reducing the flipping angle comprises manipulating a width of the RF refocusing pulse.
5. A method as defined in any of paragraphs 1-4, wherein reducing the flipping angle comprises manipulating an amplitude of the RF refocusing pulse.
6. A nuclear magnetic resonance (“NMR”) logging tool to perform the methods defined in any of paragraphs 1-5.
Although various embodiments and methodologies have been shown and described, the disclosure is not limited to such embodiments and methodologies and will be understood to include all modifications and variations as would be apparent to one skilled in the art. For example, in yet other illustrative embodiments of the present disclosure, one or more of the inventive features described herein may be combined into a single NMR logging tool. Therefore, it should be understood that embodiments of the disclosure are not intended to be limited to the particular forms disclosed. Rather, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the disclosure as defined by the appended claims.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/065260 | 11/12/2014 | WO | 00 |
