CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No. 10/837,084 filed on Apr. 30, 2004 which claims priority of U.S. Provisional Patent Application No. 60/467,568 filed on May 2, 2003 and U.S. Provisional Patent Application No. 60/508,778 filed Oct. 4, 2003, all which are incorporated herein by reference.

FIELD OF INVENTION

This patent application is directed to nuclear magnetic resonance (NMR) measurements for wireline and logging while drilling (LWD) applications. More specifically, in different aspects the application is directed to NMR systems, methods and applications for deep-looking NMR logging and directional measurement sensitivity.

BACKGROUND OF THE INVENTION

Well logging is a common practice in the oil and gas industry to evaluate underground formations for the presence and producibility of hydrocarbon reservoirs. Among the most important parameters determined in the process are the depth and thickness of formation layers containing a potential hydrocarbon reservoir, the formation porosity (i.e., the relative amount of void space in the formation), the hydrocarbon saturation (i.e., the relative percentage of hydrocarbons versus water in the pore space), and the permeability (i.e., the ability of the oil, gas, or water to flow out of the formation, into the well and eventually to the surface for recovery).

Presently, nuclear magnetic resonance (NMR) well logging is considered to be one of the most effective technique for determining these geologic parameters. NMR technology has many advantages over other logging techniques (such as gamma ray logging, sonic logging, electric logging, and others), one of the most significant being the independence of NMR measurements from formation lithology. In particular, NMR data relates in a simple manner to formation pore sizes. This relationship facilitates detection of formation fluids (i.e., gas, oil, and water) independent of the matrix mineralogy. To this end, in addition to estimation of formation porosity, hydrocarbon saturation and permeability, NMR logging enables computation of clay-bound water, capillary-bound water, and free fluid volumes, which are essential to comprehensive formation evaluation.

Generally, NMR measurements are performed as follows. A downhole static magnetic field B₀is used to align the magnetic moment of spinning hydrogen (H) protons in the formation in the direction of the B₀magnetic field. In order to establish thermal equilibrium, the hydrogen protons must be exposed to the polarizing field for a multiple of the characteristic relaxation time T₁. Then, the magnetic component of a radio frequency (RF) electromagnetic pulse polarized in a second direction orthogonal to the static field B₀is used to tip the protons to align them in a third direction that is orthogonal to both the first and the second direction. This initial RF pulse is known as a 90Â° pulse. Following the 90Â° pulse the protons in the formation begin to precess about the axis of the first direction. As a result, the protons produce an oscillating magnetic field, having a frequency directly proportional to the B₀field intensity at the proton's location. Due to inhomogeneities in the static magnetic field and irreversible molecular processes, the protons quickly begin to de-phase, which causes the induced signal to decay. Nevertheless, the dephasing process is partially reversible. In particular, by applying an 180Â° RF pulse, the instantaneous phases are reversed such that the protons gradually come back into phase, thus rebuilding the induced signal. After the signal peaks at the time when the protons are back in phase, the signal will begin to decay again due to dephasing in the opposite direction. Another 180Â° RF pulse can be used to again reverse the instantaneous phases and thereby rebuild the signal.

By using a series of 180Â° RF pulses, the signal is periodically rebuilt after each dephasing, although each rebuilding is to a slightly lesser peak amplitude due to the irreversible molecular processes so eventually it dies out completely. Each rebuilding of the signal in this manner is called a spin echo, and the time constant associated with the decay of the spin echo amplitudes is known as the transverse relaxation time T₂. A particular sequence of pulses, known in the art as the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, is most frequently used. For a more comprehensive overview of the NMR technology including logging methods and various tool designs, the interested reader is directed, for example, to the book by Coates et al. entitled â€œNMR Logging: Principles and Applicationsâ€ distributed by Gulf Publishing Company (2000), and incorporated in its entirety herein by reference for background. Additional description of NMR logging techniques is provided, for example, in U.S. Pat. Nos. 4,710,713; 4,717,876; 4,717,877; 4,717,878; 4,939,648; 5,055,787; 5,055,788; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,051,973; 6,107,796; 6,111,408; 6,242,913; 6,255,819; 6,268,726; 6,362,619; 6,512,371; 6,525,534; 6,531,868; 6,541,969; 6,577,125; 6,583,621, 6,646,437 and 6,717,404, which are incorporated herein by reference.

NMR logging is typically performed using wireline tool or logging-while-drilling (LWD) tools. In the conventional wireline-logging technology, NMR logging is performed as the logging tool is being lowered into a drilled borehole. In the emerging LWD technology, the logging tools are generally rigged up as a part of the drilling string and follow a drill bit during actual well drilling. Each tool type has its own advantages. The wireline-tools enable high logging speeds and high-quality measurements. The LWD tools, on the other hand, provide real-time data during drilling operations that may be used to prevent loss of circulation, blowouts, stuck pipes, hole instability and other disastrous consequences of borehole drilling.

Yet another significant benefit of LWD technology is that it facilitates directional drilling of the borehole. Specifically, directional drilling involves the drilling of a well bore along a deviated course in order to reach a target region at a particular vertical and horizontal distance from the original surface location. This form of drilling is particularly useful for pay zone steering: a procedure in which directional drilling is used to obtain an appropriate wellbore trajectory into an oil producing formation bed (or â€œpay zoneâ€ ) based on real-time formation evaluation data and then drill substantially within pay zone boundaries. Directional drilling may be used to penetrate multiple pay zones by using fewer wells, as well as increase the borehole volume and flow rates in the pay zone.

Notwithstanding the numerous advantages of current NMR technology, present generation of NMR tools have one key weaknessâ€”shallow depth of investigationâ€”which is typically about 10-20 cm from the tool. This is a problem because producible formation fluids (e.g., gas, oil, and water) are often displaced in the formation surrounding the borehole by invading borehole fluids (i.e., drilling mud) driven by high borehole pressure. Such invasion may occur as far as one meter into the formation with wide variations due to fluid composition, formation permeability, and applied pressure difference. As a result, conventional NMR tools having shallow depths of investigation receive signals only from the invaded section of the formation. Measurements at such shallow depths are useful to replicate porosity, T₁and T₂relaxation measurements, type and volume of bound fluid, and volume available for producible fluids. Because of displacement of the formation fluids, the NMR LWD systems, however, cannot accurately quantify the amounts of producible hydrocarbons (i.e., oil and gas) present in the formation surrounding the boreholeâ€”a factor of great significance in predicting producibility of a hydrocarbon reservoir.

The main obstacle to conducting deep NMR measurements is the high gradient G₀of the static magnetic field B₀. In other words, the strength of the magnetic field B₀falls off very rapidly with increasing distance from the tool. Such decrease in the magnetic field strength is primarily attributed to the magnetic configuration of the NMR tool. For instance, U.S. Pat. No. 4,350,955 to Jackson et al. (â€œJackson et al.â€ ) discloses a NMR apparatus comprising a pair of cylindrical permanent magnets placed co-axially with like poles facing each other and a loop antenna placed between the magnets for transmitting and receiving radio signals. The opposing magnetic fields combine to form a toroidal region of relatively homogeneous radial static magnetic field B₀. The distance of the homogeneous field region from the axis of the magnets depends on the magnet dimensions and their separation. The closer are the magnets the stronger is the combined magnetic field. The magnetic field lines in the Jackson et al. design, however, disperse very rapidly in the relative proximity of the tool and therefore provide a low magnetic field gradient G₀only at a distance of about 10 cm from the tool.

An improvement of the Jackson's et al. tool design is disclosed in U.S. Pat. No. 4,629,986 to Clow et al. (hereinafter â€œClow et al.â€ ). Clow et al. placed a highly permeable ferromagnetic material between two permanent magnets, positioned as in Jackson's layout. This ferromagnetic material shunts more magnetic flux into the center of the tool and produces radial magnetic field lines radiating in vertical planes. Contrary to Jackson's et al. design, this configuration keeps the magnetic field lines more focused and parallel at a greater distance, which results in a stronger field B₀and low gradient G₀further away from the tool (i.e., about 20 cm). However, such investigation depths are still too shallow to enable adequate NMR measurements in regions of the formation unaffected by the invaded formation fluids.

Another prior art design is described, for example, in U.S. Pat. No. 6,246,236 to Poitzsch et al. (hereinafter â€œPoitzsch et al.â€ ), claiming priority to U.S. Pat. No. 5,977,768 to Sezginer et al., which discloses NMR tool having a low-gradient sonde and a high-gradient sonde positioned in tandem along the longitudinal axis of a tool. The '236 and '768 patents are incorporated herein by reference. The low-gradient sonde comprises two permanent magnets having separation of about 65 cm and an interposed magnetically permeable member. The configuration provides a relatively weak magnetic field B₀â€², which has low (approximately 3 G/cm) gradient G₀â€² that is measured at a distance of approximately 20 cm radially from the tool. The second sonde comprises two permanent magnets about 20 cm apart with an interposed magnetically permeable member. This configuration provides a stronger magnetic field B₀â€³ at approximately the same distance from the tool as the low-gradient sonde, but with greater gradient G₀â€³ (approximately 10-20 G/cm).

Each magnetic configuration in Poitzsch et al. has its own advantages. NMR measurements performed in the low gradient region, for instance, are less sensitive to the lateral motion of the tool than the measurements in the high gradient regionâ€”a characteristic useful in LWD applications, in which drill string typically undergoes severe vibrations. High field strength in high gradient region, on the other hand, provides better signal-to-noise ratio (SNR), which is very important in both wireline and LWD applications. The Poitzsch et al. tool, however, conducts NMR measurements in shallow volumes (about 20 cm deep), which are typically invaded by borehole fluids. As a result, information gathered by the tool is limited and the quality of its measurements may be compromised. Moreover, none of the above tools provide directionally sensitive data about the formation, which would facilitate directional drilling capabilities.

Sezginer et al. use a single transmitter that powers all antennas in transmit mode. That means that (a) all antennas have to be tuned to the same frequency, and (b) the interaction between antennas (mutual detuning) has to be negligible. It is not possible to suppress the mutual interaction while at the same time maintaining good azimuthal coverage. Thus, Sezginer et al. require either very narrow antennas, which have low SNR and poor azimuthal coverage or do not disclose a workable system due to the fact that antennas in close proximity are electrically equivalent to coupled tank circuits, which exhibit split resonances. The prior art does not disclose or suggest a system that obtains directional information from a formation and also is capable of operating at multiple frequencies.

Accordingly, it is an object of the present invention to provide a NMR tool suitable for comprehensive evaluation of underground formations during wireline or LWD operations. In particular, it is an object of the invention to enable NMR measurements in deep regions of the formation that are substantially free of borehole fluids invasion. Another object is to enable both shallow and deep measurements using a single magnetic assembly. Yet another object of the invention is to provide NMR tool having directional sensitivity and suitable for directional drilling based on directionally sensitive NMR measurements. A further object of the invention is to provide an NMR pulse sequence that minimizes the tool's power consumption, while maximizing the SNR of deep NMR measurements.

SUMMARY OF THE INVENTION

This application is directed in one aspect to NMR tools capable of conducting NMR measurements in deep sensitive volumes, where borehole fluid effect can be ignored for practical purposes. In another aspect, the application is directed to NMR tools having directional sensitivity. The tool is capable of producing azimuthally symmetric static magnetic field B₀with a low gradient G₀, which facilitates greater depths of investigation. In a specific embodiment, NMR measurements can be conducted in a plurality of sensitive volumes ranging from about 20 cm to about 80 cm and beyond from the tool. Shallow volumes may be used to replicate the conventional NMR logging measurements, such as porosity, T₁and T₂relaxation measurements, bound fluid volume, etc. These measurements characterize the pore space, the type and volume of bound fluids and volume available for producible (movable) fluids. Deep volumes enable quantification of hydrocarbon saturation substantially independent of the borehole effects, such as formation invasion by borehole fluids. To this end, the deep measurements may be used to supplement the shallow free-fluid measurements by quantifying the amount of oil and gas present at a distance from the borehole. Taken together, shallow and deep volume measurements may be used as basis for stand-alone formation evaluation, enabling more accurate estimates of porosity quantification of producible hydrocarbons, for borehole invasion profiling, and others.

In a preferred embodiment, the NMR tool comprises two permanent magnets polarized along a longitudinal axis and aligned with like poles facing each other. One or both poles of each magnet are extended longitudinally with one or more pole pieces. Pole pieces used in this application have high permeability to intensify the static magnetic field of the magnets and make it more homogeneous in the formation region surrounding the space between the magnets. Interposed between the inner set of poles is an RF antenna assembly.

There are several possible implementations for the antenna assembly in accordance with different embodiments. By way of illustration, such an assembly comprises a plurality of independently addressable antenna segments. The antenna segments can be implemented as coils that may be wound around a magnetic core or saddle antenna pieces. In various embodiments the antenna segments may be disposed about a suitable reference axis, such as the tool axis, which usually is substantially parallel to the borehole axis. Preferably, when so disposed, the antenna segments are distributed symmetrically. This results in dividing the addressable sensitive volume sections in the formation in a regular manner. However, this preference is not intended as a limitation. In preferred embodiments, either three or four antenna segments are disposed about such an axis. Alternatively, the antenna assembly comprises a tubular ferromagnetic core and three coils wound around the core. The coils include at least one solenoid and at least two orthogonally-placed saddle coils. The antenna assembly is enclosed into a non-conductive, non-magnetic shield, which protects it from the borehole environment. Finally, the entire magnetic assembly may be mounted on a drill collar having mud flow conduit therein to facilitate mud circulation in the borehole.

In another embodiment, the NMR tool comprises a single, axially polarized magnet assembly having a permanent magnet longitudinally extended, at both poles, with one or more soft-magnetic pole pieces, thus simulating a magnetic monopole in the near regions of the formation. One or both pole pieces may in turn be adjacent to one or more RF antenna assemblies. Each antenna assembly further comprises a (tubular) ferromagnetic core and one or more antenna coils wound around the core. The coils include preferably at least one solenoid coil and at least two orthogonally-placed saddle coils. Each antenna assembly is enclosed into a non-conductive, non-magnetic protective shield. Finally, the entire magnetic assembly may be mounted on a drill collar having mud flow conduit therein to facilitate mud circulation in the borehole.

In a preferred embodiment, the NMR tool is capable of directional NMR detection. In particular, the directional sensitivity of the tool is provided by a unique design of RF transceiver antenna and a novel data processing method. In particular, a solenoid coil is capable of imparting an azimuthally symmetric magnetic field B₁. Directionally-sensitive antenna segments, in turn, are capable of detecting NMR signals from different sections of the sensitive volume. The data processing method in a preferred embodiment combines NMR signals detected by the antenna coils to enable analysis of the received NMR signals to determine which regions of formation are characterized by the given NMR response. Such directional NMR detection, combined with conventional accelerometer and magnetometer readings, enables real-time payzone steering during borehole drilling.

In another aspect, this application is directed to a method for optimizing the reception of signals from a deep sensitive volume of the tool with reduced power requirements. The method is based on a novel pulse sequence for NMR relaxation measurements. In particular, by lengthening each refocusing pulse (Ï€) of the standard CPMG pulse sequence and frequency-modulating each pulse, a substantial improvement in signal-to-noise ratio (SNR) results, thus improving the quality of deep NMR measurements. In another aspect, various signal-processing techniques are disclosed suitable to further improve SNR from deep measurements. Additional aspects of the invention(s) in this disclosure will become apparent to those skilled in the art upon reviewing the description in light of the illustrative figures described below.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the technical approaches disclosed in this patent application will become apparent from the following description of the accompanying drawings. It is to be understood that the drawings are to be used for the purpose of illustration only, and not as limitations on the scope of the attached claims.

FIG. 1

is block/schematic diagram of electrical components of a NMR tool in accordance with a one embodiment.

FIG. 2

is schematic diagram of the magnetic assembly of the NMR tool in accordance with a preferred embodiment.

FIG. 3A-3D

provide various views of the static magnetic field flux lines, oscillating magnetic field flux lines, and sensitive volumes generated by the NMR tool of the embodiment illustrated in

FIG. 2

FIG. 3E

illustrates the distribution of the static magnetic field strength and static magnetic field gradient as a function of distance from the tool.

FIG. 4A

is schematic diagram of the magnetic assembly of the NMR tool in accordance with a second embodiment.

FIG. 4B

illustrates static magnetic field flux lines and sensitive volumes generated by the NMR tool of the second embodiment.

FIG. 5

is an elevated view of a transceiver antenna assembly in a preferred embodiment.

FIG. 6

is top view of a transceiver antenna in a preferred embodiment, with direction of magnetic fields indicated.

FIG. 7

shows sectional and elevated views of the transceiver antenna assembly in accordance with a second embodiment;

FIG. 7A

shows a cross-sectional view.

FIG. 8

illustrates a frequency-modulated CPMG pulse sequence used in one embodiment.

FIG. 9

illustrates a schematic cross-sectional view of an antenna according to another embodiment.

FIG. 10

illustrates the relative placement of sensitive volumes and a simulated monopole magnetic field.

FIG. 11

schematically illustrates an example MRIL wireline tool (not to scale) placed against a borehole wall with standoffs and a bowspring.

FIG. 12

illustrates six possible modes in one embodiment directed to obtaining directional resonance data.

FIG. 13

provides comparison between data obtained with MRIL-Prime and directional MRIL tool embodiments.

FIG. 14

illustrates results of the repeatability test comprising repeat logs of a well taken at different points in the borehole.

FIG. 15

shows data from different sensitive volumes of the directional tool.

FIG. 16

shows a cross section of a logging tool close to a side of a borehole and corresponds to the illustration in

FIG. 11

FIG. 17

illustrates directional drilling using a directional antenna configuration combined with the deep-looking capabilities in an exemplary embodiment.

FIG. 18

provides an illustration of a general logging tool, including in a logging while drilling context.

FIG. 19

shows an illustrative magnetic system, suitable for deployment in a borehole, with a single magnet and two attached soft magnetic poles, which system is suitable for generating substantially radial magnetic flux lines in a region of a formation about the magnetic system.

DETAILED DESCRIPTION

The Magnetic Assemblies

FIG. 2

illustrates a preferred embodiment of an NMR tool, which is suitable for logging-while-drilling of a geologic formation. Tool 200 comprises the following components: two magnetic configurations each comprising a permanent magnet (magnets 210 and 220) positioned with like poles facing each other and a plurality of pole pieces 230A, 230B, 230C, and 230D (subsequently referred to as pole pieces 230), longitudinally extending one or both poles of a magnet. Interposed between the magnetic configurations is a RF transceiver antenna assembly 240, which comprises a soft-magnetic core 250 and a plurality of coil windings 260. Antenna assembly 240 is preferably surrounded by a protective shield 270. All aforementioned components are centered along longitudinal axis 205 of tool 200, which in several coincides with longitudinal axis of the borehole. In LWD embodiments, tool 200 is mounted within a tubular drill collar (not shown) having mud flow conduit 285 therein to facilitate mud circulation in the borehole.

(1) Magnets

With reference to

FIG. 2

, in accordance with a preferred embodiment, magnets 210 and 220 are cylindrical magnets having a substantially circular cross section, each magnet being generally elongated and centered along the longitudinal axis 205 of the magnetic assembly 200. In operation, this axis is usually aligned with the axis of the wellbore. Magnets 210 and 220 are preferably polarized in a direction parallel to the longitudinal axis of the magnetic assembly 200 and mounted with like magnetic poles facing each other. For each magnet 210 and 220, the magnetic field lines travel as shown in

FIG. 3B

. As shown, in the region between the magnets, the magnetic field lines extend radially in all azimuthal directions to create a static field B₀in a substantially cylindrical region of the formation surrounding the borehole, as also shown in

FIG. 3A

. Because of the separation, the magnitude of the static magnetic field B₀in the region between magnets 210 and 220 (in the vicinity of any given radius) is substantially homogeneous.

Magnets 210 and 220 are preferably strong Alnico magnets with remanence induction of approximately 1.25 Tesla. Alnico magnets are composed primarily of alloys of Aluminum, Nickel, and Cobalt. Alnico materials are generally characterized by excellent stability over a wide temperature range, high residual induction, and relatively high energies. Such magnets are manufactured through either a casting or sintering process. Cast magnets may be manufactured in complex shapes, in a preferred embodiment magnets 210 and 220 are tubular (i.e., rod-shaped). It should be noted that sintered Alnicos offer slightly lower magnetic properties but better mechanical characteristics than cast Alnicos. For most practical purposes, either manufacturing method is suitable.

For optimized performance, the length of the magnets is preferably about 4-5 times of the diameter. To this end, in a preferred embodiment, magnets 210 and 220 are preferably about 13 cm in diameter and approximately 60 cm long. It should be understood by those of skill in the art that the length of the magnets may vary depending on the specific design needs, provided of course that the magnet's length-width ratio is selected to optimize its performance. The above-described Alnico magnets are manufactured, for example, by the Alnico Products Division of Group Arnold, magnetic products group of SPS Technologies, Inc. based in Marengo, Ill.

In alternative embodiments, magnets 210 and 220 may comprise permanent magnets, such as hard ferrite (SrO-6(Fe₂O₃)) magnets having strontium carbonate and iron oxide composition, neodymium-iron-boron (NdFeB) magnets, or samarium cobalt (SmCo) magnets. In other embodiments, magnets 210 and 220 may comprise electromagnets that are made of thin-gauge copper or aluminum foils. Furthermore, the shape of magnets 210 and 220 may vary depending on the specific design need, for example, it may be a solid cylinder or a cylindrical annulus (i.e., tubular). In different embodiments, magnets may be integral, or may comprise a plurality of smaller magnets bonded together. In the latter design case, magnetization direction of each smaller magnet may vary as long as total magnetic moment is aligned longitudinally with the axis of the magnetic configuration 200 and like poles of magnets 210 and 220 face each other. The above-described magnets and variations thereof are can be made, for example, by Group Arnold, magnetic products group of SPS Technologies, Inc. based in Marengo, Ill.

(2) Pole Pieces

In accordance with a preferred embodiment, magnets 210 and 220 are extended at one or both poles by a plurality of pole pieces 230 (see pole pieces 230A, 230B, 230C, and 230D in

FIG. 2

). A pole piece in this application is a piece of (ferromagnetic) material attached to one end of a magnet and so shaped that the distribution of the magnetic flux in the adjacent medium is appreciably controlled. Pole pieces 230 are composed of high permeability material, preferably greater than about 100. Suitable materials for the pole pieces may comprise iron, permeable steel or other soft-magnetic alloy of iron and nickel. In the specific embodiment discussed above, pole pieces are cylindrically shaped rods about 30 cm long (which may vary depending on the specific design requirements). Pole pieces 230 may be attached to a magnet pole face by adhesive bonding or with studs or bolts. For greater efficiency, a non-magnetic gap between the magnet poles and the pole pieces 230 should be as minimized, and preferably made as small as practically possible. While the pole pieces can be allowed to carry some load, the connection between pole pieces 230 and magnets 210 and 220 should be done in a manner that minimizes the structural load on the magnets.

In one aspect of the invention, pole pieces 230 shape the static magnetic field B₀generated by magnets 210 and 220. With reference to

FIG. 3B

, for each magnet 210 (220), the magnetic field lines travel outward from an end of the magnet through the respective inner set of pole pieces 230 and into the formation, and travel inward to the other end of the magnet. In general, pole pieces 230 make the magnetic field intensity more uniform and homogeneous in the annular regions of the formation surrounding the borehole.

To this end, in accordance with a preferred embodiment, the longitudinal separation between the inner pole pieces of magnets 210 and 220 is about 80 cm. The amount of separation between magnets 210 and 220 is determined based on several factors, including: (1) selecting the requisite magnetic field strength; (2) received depth of investigation; and (3) generating a field having small radial variations in the region of interest so that the echoes received during a pulse sequence are less sensitive to lateral tool motion. In general, as the separation between magnets 210 and 220 decreases, the magnetic field becomes stronger and less homogeneous. Conversely, as the separation between magnets 210 and 220 increases, the magnetic field becomes weaker and more homogenous.

FIG. 3E

illustrates how the field strength (B₀) and field gradient (G₀) of the static magnetic field change as functions of radius (R) from the tool in accordance with a preferred embodiment. It can be seen from

FIG. 3E

that magnetic assembly 200 provides higher field strengths and lower field gradients further away from the tool than prior art NMR tools. In particular, magnetic assembly 200 imparts an azimuthally symmetric static magnetic field B₀suitable for measuring NMR signals in a region extending 20 cm away from the tool to about 80 cm and beyond into the formation, in some instances about 1 meter deep. For instance, prior art discloses field strengths of about 10 G or lower at a distance of 30 cm from the tool. In contrast, an embodiment of the tool in accordance with the principles disclosed in this application shows field strength of over 10 G at 80 cm or more away from the tool.

A general structure of the logging tool in a specific embodiment is shown in illustrative

FIG. 18

. Borehole 1800 in formation 1805 has logging tool 1810 comprising magnet 1815 with pole piece 1820. Antenna core 1825 has antenna coils 1830 with the antenna assembly protected by antenna sleeve 1835. The antenna is mounted on yoke 1845. In operation, the deep looking logging tool is able to collect data from shallow sensitive volume 1850 and deep sensitive volume 1855. The entire magnet and antenna assembly may be mounted on a drill collar 1860 in a LWD application. Also shown is flow channel 1840, which is important for use of the logging tool 1810 while drilling to allow mud to flow to the drill bit.

Alternative Magnet Design Construction

It should be noted that the embodiment described in

FIG. 2

with two opposed magnets, although preferred, may be replaced to a good approximation by a single magnet with pole extended by pole pieces. The shaping of the magnetic flux lines by pole pieces even in the absence of opposing poles provides a substantially radial magnetic field in a portion of the formation surrounding a borehole. Thus, useful NMR data may be obtained for sensitive volumes in the vicinity of a logging tool with a single permanent magnet having pole pieces. This simplifies the construction of a suitable magnet that provides a good approximation to magnetic flux lines directed outwards in a substantially radial direction.

One such embodiment of a NMR tool is shown in

FIG. 4A

. Tool 400 comprises a single permanent magnet system having a tubular, axially polarized magnet 405 and one or more soft-magnetic pole pieces 410A and 410B extending the poles of magnet 405. Tool 400 further comprises a RF transceiver antenna 415 surrounding one or more pole pieces 410A. Antenna 415 comprises antenna core 420 and one or more antenna coils 425 wound around core 420. Tool 400 may further comprise another antenna 430 surrounding one or more pole pieces 410B. Antenna 430 may also comprise antenna core 435 and one or more antenna coils 440. Finally, tool 400 is mounted on a tubular drill collar 445 having a mud flow channel therein.

FIG. 19

shows another view of the magnet configuration in this embodiment. In particular, logging tool 1900 has central channel 1905, optional antenna core 1910 adjacent to pole piece 1915 extending a pole of magnet 1920. The other pole of magnet 1920 is extended by pole piece 1925, which is adjacent to core 1930 with wound coil 1935 forming an antenna. As indicated, a pole piece, preferably iron, may be used doubling as an antenna core and pole extender.

The magnetic components of the tool 400 are preferably made of substantially similar materials and have substantially similar magnetic and electrical characteristics as the corresponding components in the tool 200 of the embodiment, which was described above.

FIG. 4B

shows the flux lines of the NMR tool 400. The tool 400 creates two sets of the sensitive regions suitable for NMR measurements. The two sensitive regions are located in front of upper and lower soft magnetic poles 410A and 410B. In these regions, the direction of the static field is substantially radial. In

FIG. 4B

, only one region of the nested sensitive volumes (SV) is denoted in front of the pole pieces 410A. The shapes of the sensitive volumes are different from ideal cylinders and their boundaries are not exactly parallel to the tool's axis.

Antenna Assembly in the First Embodiment

(1) Yoke

In accordance with a one embodiment, the proximal ends of the inner pole pieces 230 are attached to a yoke (not shown). The yoke may have cylindrical shape with its longitudinal axis aligned in coincidence with the axis of the tool 100. Preferably, pole pieces 230 are not joined rigidly with the yoke, but can move some against each other to provide overall bending flexibility. Because the yoke is interposed between the inner pole pieces of magnets 210 and 220, the entire magnetic assembly is symmetric around the yoke. The length of the yoke is preferably about 80 cm. Thus, the length of the entire magnetic assembly in this embodiment is about 315 cm. In general, the yoke may provide support for RF transceiver antenna 240, as described next. The yoke is preferably made of a mechanically durable soft-magnetic material.

(2) Antenna Core

With reference to

FIG. 2

, in accordance with a preferred embodiment, magnetic assembly 200 further includes transceiver antenna 240 mounted on the yoke or directly on drill collar 280. Transceiver antenna 240 comprises antenna core 250 and coil windings 260. Core 250 is preferably shaped as an annular cylinder with drill collar 280 (or yoke) passing through the longitudinal cavity extending within core 250. The total length of core 250 is about 72 cm. In a preferred embodiment, core 250 preferably comprises several rings stacked on a drill collar (or yoke) 280. The rings are preferably made of soft-magnetic material having high magnetic permeability. The soft-magnetic rings may be made from soft ferrite, ferrite polymer composites, powdered iron or nickel cores, etc.

In a preferred embodiment, core 250 has magnetic permeability of about 100. It is important that the core material maintains its permeability to limit RF power losses and does not saturate due to the static magnetic field B₀generated by magnets 210 and 220. It will be appreciated that the permeability values of core 250 may vary depending on the specific design requirements.

In conjunction with pole pieces 230, core 250 shapes the static magnetic field generated by magnets 210 and 220 in the sensitive region. With reference to

FIGS. 3B and 3C

, core 250 enables even and azimuthally symmetric distribution of magnetic field lines along its entire length. As a result, magnetic field intensity is distributed evenly along core 250, which makes the static magnetic field B₀more uniform in a plurality of sensitive volumes extending as far as one meter into the formation surrounding the borehole, as shown in

FIG. 3A

(3) Antenna Coils

With reference to

FIG. 5

, in a preferred embodiment, RF transceiver antenna assembly 500 comprises at least three coil windings 510, 520, and 530 wound around antenna core 550 (corresponding to core 250 in

FIG. 2

). Antenna coils 510 and 520 are preferably saddle coils, each having one or more windings. Saddle coils 510 and 520 are preferably rotated 90Â° with respect to each other along the longitudinal axis of the tool (z-axis in

FIG. 5

), so that coil 510 is substantially co-planar with the x-axis, and coil 520 is substantially co-planar with the y-axis. Antenna coil 530 is a solenoid, preferably having a plurality of windings lying in planes substantially orthogonal to the longitudinal axis of the tool. In a preferred embodiment, solenoid coil 530 comprises eight-windings having total inductance of about 27 Î¼H. In alternative embodiments, two or more solenoid coils having different inductances may be used.

In an important aspect of the novel approaches in this application, antenna 500 is suitable for azimuthally symmetric and directionally sensitive NMR signal detection. In particular, during transmission, the transmitter output signal may be routed to either one of the coils. In a preferred embodiment, solenoid coil 530 is used to impart RF field B₁orthogonal to the static field B₀in the entire sensitive volume, whereas saddle coils 510 and 520 are used to impart RF field B₁orthogonal to the static field B₀in restricted sections of the sensitive volume. During reception, the NMR signals from all three coils are preferably detected, amplified, and processed, as discussed below. It should be noted that coils 510, 520, and 530 may be combined in a plurality of configurations for transmitting RF pulses and for receiving NMR signals from shallow and the deep volumes. Moreover, two or more solenoid coils having different lengths (i.e., number of windings) may be used to excite NMR signals in shallow and deep volumes.

To achieve azimuthally symmetric nuclear magnetic resonance in the surrounding formation, a solenoid coil 530 is preferably used. When RF power pulses are conducted through solenoid coil 530, the coil generates an RF equivalent magnetic dipole centered at the origin and directed along the z-axis, as shown in

FIG. 3C

. The equivalent magnetic dipole generates an RF magnetic field directed opposite to the dipole direction and of substantially equal amplitude within a sensitive volume in the formation (the sensitive volume is a cylindrical shell with thickness determined by the bandwidth of the RF pulse). Since axially-oriented B₁field lines are substantially orthogonal to the radially-oriented static magnetic field B₀in all azimuthal directions, the nuclear magnetic resonance is induced in the entire sensitive volume.

Saddle coils 510 and 520 can be used in a specific embodiment to achieve azimuthally focused nuclear magnetic response in the surrounding formation. When RF power pulses are conducted through saddle coils 510 and 520, they produce RF dipole in a direction transverse to the longitudinal axis of the tool. Unlike solenoid coil 530, saddle coils 510 and 520 generate radially-oriented B₁field lines orthogonal to the static field B₀along their respective planes. (This characteristic is exploited to provide directional sensitivity to the antenna 500, as discussed below.) Thus, in a vertical borehole, for example, saddle coil 510 may be sensitive in the east-west direction, while saddle coil 520 may be sensitive in the north-south direction as indicated in

FIG. 3D

It should be noted that core 550 significantly improves the efficiency of RF antenna 500 by offsetting the reduction in the antenna aperture due to the presence of conductive drill collar 580 (or yoke). In particular, coil windings of antenna 500 are magnetically coupled to ferromagnetic core 550 such that, when the electrically conductive coils 510, 520, and 530 are energized with AC current, a magnetic dipole of increased magnetic moment is formed in core 550, thereby increasing the strength of RF magnetic field B₁. As a result, antenna 500 is capable of generating a much larger magnetic moment than typical nonmagnetic-cored antenna, for the same power input. In the absence of antenna core 550, large eddy currents would be induced in drill collar 580 (or yoke), which would tend to oppose the antenna currents and would result in significantly reduced sensitivity during reception, and in much larger current and power requirements during transmission.

The illustrative antenna of the first embodiment is but one of many possible antenna configurations. Some illustrative examples of directional antennas including those using combinations of multiple solenoids are described next.

Alternative Antenna Assembly Embodiments

(1) Yoke

With reference to

FIG. 7

, the antenna assembly 700 in accordance with a second embodiment is set on a ribbed yoke 710. The yoke 710 has a plurality of guide projections 715, which are formed along its outer surface. The guide projections 715 extend outwardly away from opposite sides of yoke 710, as illustrated, and are spaced apart from one another at a predetermined interval along the longitudinal extent of yoke 710. Guide projections 715 are positioned to form two or more spaced-apart rows of guide projections along each side of yoke 710, with each row extending along the length of yoke 710. In a specific embodiment, three guide projections are formed on the surface of yoke 710, thereby creating three spaced-apart rows. As a result, three generally centrally positioned grooves or recessed region 720 are provided along the outer periphery of yoke 710, extending along of the sides of yoke 710, as well as along the opposite ends of yoke 710.

The surface of yoke 710 is preferably coated with copper, which provides an RF screen. Within the gaps between adjacent coils, the copper screen can be extended up to the inner diameter of the antenna sleeve 750, which preferably encloses the entire antenna assembly 700, to reduce the magnetic coupling between antenna coils 730. The radial extension of the RF screen in the space between the adjacent coils 730 allows for correction of the RF field pattern close to the tool. For the deep-looking tool, the air gap between core segments 725 has insignificant influence on the field variations within the deep sensitive volume.

(2) Antenna Core

With reference to

FIG. 7

, antenna assembly 700 in accordance with a second embodiment comprises two or more core segments 725 adjacent to yoke 710 and extending longitudinally within recessed regions 720 along the surface of yoke 710. The shape of each core segment 725 resembles a sector of a tubular cylinder cut along its longitudinal axis. Core segments 725 are separated from each other by guide projections 715 of yoke 710.

Core segments 725 are preferably made of soft-magnetic material(s) having high magnetic permeability. The soft-magnetic core sectors 725 may be made from soft ferrite, ferrite polymer composites, powdered iron or nickel cores, or others. In a preferred embodiment, core 725 has magnetic permeability of about 100. It is important that the core material maintains its permeability to limit RF power losses and do not saturate due to the static magnetic field B₀generated by the magnetic assembly (not shown). It should be understood that the permeability of core 725 may be different depending on the specific design requirements. It is desirable, however, that all core sectors 725 be made of the same material and have the same permeability.

(3) Antenna Coils

Antenna assembly 700 further comprises a plurality of antenna coils 730 wound around core segments 725. In particular, each core segment 725 carries at least one antenna coil 730. Each antenna coils 730 resembles a deformed solenoid. Each coil 730 preferably has a plurality of windings lying in planes substantially orthogonal to the longitudinal axis of the tool. In a preferred embodiment, each coil 730 comprises of the same winding having with a small magnetic coupling between the coils. In an alternative embodiment, two or more solenoid coils having different inductances may be used. Since coils 730 are wound in transverse planes, the produced RF field is similar to the RF field produced by a solenoid. To this end, coils 730 are preferably substantially similar and carry similar currents.

Antenna 500 is preferably suitable for azimuthally symmetric and azimuthally focused NMR detection. In transmit mode, azimuthally uniform excitation of the formation is desired. Accordingly, all coils 730 may transmit in parallel. As a result, coils 730 generate an RF equivalent magnetic dipole centered at the origin and directed along the longitudinal axis of the tool. Since the axially-oriented B₁field lines are substantially orthogonal to the radially-oriented static magnetic field B₀in all azimuthal directions, NMR is induced in the entire sensitive volume. During reception of NMR echo signals, the signals from all coils are detected, amplified, and processed separately. A typical pulse sequence would be based on the CPMG sequence; a specific pulse sequence is described below.

Due to the sectional design of the antenna, the echo signals received by each coil correspond to subtending sectors of the sensitive volume. Certain azimuthal overlapping of the sensitive volume sectors may exist and should be considered during data processing and log interpretation. Radial extensions of the RF screens within recess regions 720 between the adjacent coils 730 allow some control over how much adjacent sectors overlap. By increasing the number core sectors 725, it is possible to achieve a fine azimuthal resolution on the NMR sensor.

Antenna Sleeve

In a preferred embodiment, antenna assembly 240 (or 700) is enclosed in a sleeve 270. An example construction of antenna sleeve 270 is described, for example, in U.S. Pat. No. 6,008,646, which is incorporated herein by reference. In particular, sleeve 270 is suitable to protect antenna assembly 240 against abrasions from particles in the drilling mud and impact against the earth formation. In a preferred embodiment, sleeve 270 is composed of a non-conductive, impact and wear resistant material, such as fiberglass or transition toughened zirconia. In an alternative embodiment, sleeve 270 may be composed of steel. In this embodiment, sleeve 270 should preferably have vertical slots cut therein at the top and the bottom of the antenna 260 to allow flux of magnetic field B₁to enter and exit the magnetic assembly.

It should be noted that shield 270 may be extended to cover the entire magnetic assembly 200.

Alternative Antenna Assembly Embodiment(s)

FIG. 9

illustrates another embodiment of an antenna assembly in accordance with the principles disclosed herein, with four longitudinal segments (strips) interconnected at one of their end. The use of this strips' arrangement allows NMR investigation of four quadrants about an axis of the logging tool. In particular, it is useful for determining directional properties of the surrounding formation with the aid of signals received at two or more, preferably adjacent antenna strips.

As shown in

FIG. 9

, antenna assembly 900 comprises four longitudinal strips 930A, 930B, 930C, and 930D spaced apart by 90 degrees and interconnected by means of an end ring 910. On the opposite end of the strips, four antenna terminals are located (T1, T2, T3, and T4). In accordance with a specific embodiment, antenna assembly 900 can include an antenna core placed inside the strips.

The four terminals (T1, T2, T3, and T4) of the antenna 900 can be connected to transmitter in eight different fashions: T1+T2, T2+T3, T3+T4, T4+T1, T1+T3, T2+T4, T1&T2+T3&T4, and T1&T4+T2&T3. In addition, the terminals of the antenna 900 that are not currently connected to transmitter can be shunted. The RF field produced by antenna 900 is similar to that of a saddle shaped antenna. For example, in order to get an RF field pattern shown in

FIG. 3D

, the T1 &T2+T3&T4 strips combination should be connected to transmitter (T1&T2 means that two antenna terminals, T1 and T2, are shunted and connected to first transmitter terminal, while T3&T4 means that another two antenna terminals, T3 and T4, are also shunted and connected to second transmitter terminal).

This antenna design further breaks down the typically concentric sensitive volumes into four azimuthally-distinguishable quadrants. This antenna enables operation in large (12Â¼ in. and up) and potentially badly washed-out boreholes. This design also exploits the simulated-monopole static magnetic field discussed above. As noted, the magnetic monopole is simulated by the radially directed static magnetic field capable of reaching deep into the formation. In a preferred embodiment, a magnetic monopole is simulated by a logging tool having a substantially radial static magnetic field extending beyond about 60 cm into a formation surrounding the logging tool.

FIG. 10

illustrates the relative location of sensitive volumes 1000 in a simulated monopole magnetic field represented by static magnetic field lines 1005.

In a preferred embodiment, radial separation between sensitive volumes is achieved by switching operating frequencies to selectively excite resonance at different radii. In a preferred embodiment, deploying the example antenna configuration disclosed herein, there are five (5) radial depths possible and four (4) azimuthal positions for a total of twenty (20) distinct sensitive volumes that may be addressed by the antenna.

In particular, there are two omnidirectional modes (N-S and E-W) and four directional modes (N, S, E and W). Notably, in a preferred embodiment individual quadrants are not being addressed; instead, two adjacent or all four quadrants resonate simultaneously. The signals from each measurement are, typically, not independent of each other. Decomposition into independent quadrant readings requires some additional steps, which are outlined below.

The change in magnet configuration requires some adjustments in field strength, operating frequencies and magnetic field gradient. For comparison, Table 1 below lists some of the operating parameters for the familiar omnidirectional frequency hopping designs.

TABLE 1Sensitive VolumeRadialFrequencyGradientPositionPosition (in)(kHz)(gauss/cm)Innermost7.875017.0Middle8.268014.7Outer8.760012.3

TABLE 2Sensitive VolumeRadialFrequencyGradientPositionPosition (in)(kHz)(gauss/cm)Innermost6.05008.0Middle6.54506.6Outer7.54005.3

Table 2 above lists comparable values for the directional design. As is readily seen, the frequencies have dropped by 200-250 kHz, and the gradient values are roughly cut in half. Therefore, all calculations that either explicitly calculate diffusivity values or otherwise rely on diffusion contrast to differentiate between fluid types are modified to take into account this change. Alternatively, the reduced magnetic field gradient can be compensated for by an increase in echo-to-echo spacing (T_e). The ratio of new echo spacing for the directional tool to that for omnidirectional familiar near borehole investigating tools should equal the ratio of new (and lower) field gradient to the old field gradients.

The azimuthal sensitivity for the example directional antenna, however, has been reduced from 360Â° for the omnidirectional tool to less than 180Â°. In an example embodiment, the directional tool typically acquires data in 10 different volumes, staggered in 5 different depths of investigation (DOI). DOI ranges from 2.5 in. to 4 in.

FIG. 12

illustrates six modes A-F of operation for the directional antenna having four longitudinal strips (

FIG. 9

). Also shown in

FIG. 12

are sensitive volumes 1200 for each of the modes A-F. Each of the sensitive volumes corresponds to a different frequency NMR signal. However, it should be noted that although a plurality of sensitive volumes are depicted in the illustrative figures, this is not intended to be a limitation on the scope of the attached claims unless so indicated expressly. Also shown in

FIG. 12

, although not marked as such is a schematic antenna core in cross-section. These six modes are further described in Table 3 below:

TABLE 3Terminalsused forAntennatransmitterModeconnectionSignal Comes from Quadrants . . .OmniT1&T2 + T3&T4All (represented by A in FIG. 12)North (N)T1 + T2NW and NE (represented by D in FIG. 12)