MXPA99009935A - Apparatus and method for obtaining a measurement of nuclear magnetic resonance during perforation - Google Patents

Abstract

The present invention relates to a tool for drilling with simultaneous sounding by nuclear magnetic resonance. The tool comprises a drill bit, a plurality of FR antennas, and at least one gradient coil. The tool further comprises a plurality of magnets that are polarized in a direction parallel to the longitudinal axis of the tool but opposite each other. The configuration of magnets and antennas provides at least two research NMR regions

Description

APPARATUS AND METHOD FOR OBTAINING A MEASUREMENT OF NUCLEAR MAGNETIC RESONANCE DURING DRILLING Cross-references The present is a continuation in part of Serial US Patent Application No. 09 / 033,965 (Attorney File No. 24,787), filed March 3, 1998.

Background of the Invention The present invention relates in general to an apparatus and method for measuring the nuclear magnetic resonance properties of a ground formation traversed by a borehole and, particularly, with an apparatus for obtaining a nuclear magnetic resonance measurement during borehole drilling. . It is recognized that the atomic particles of a terrestrial formation that possess a magnetic moment of nuclear spin other than zero, for example, protons, have the tendency to align with a static magnetic field imposed on the formation. This magnetic field can be generated naturally, just as it happens with the magnetic field of the Earth, BE. An RF pulse that applies a second transverse magnetic field to BE creates a magnetization component in the transverse plane (perpendicular to BE) that makes a precession around the vector BE with a characteristic resonance known as frequency Lar or,? L, which depends of the intensity of the static magnetic field and the agnatic rotation relation of the particle. The hydrogen nuclei (protons) that are precessing around a magnetic field BE of 0.5 gauss, for example, have a characteristic frequency of approximately 2kHz. If a population of hydrogen nuclei performed a precession in phase, the combined magnetic fields of the protons could generate a detectable oscillating voltage in a receiver coil, conditions known to those skilled in the art as decay of free induction or spin echo. The nuclei of hydrogen present in water and in hydrocarbons located in rocky pores produce nuclear magnetic resonance (NMR) signals that are different from signals from other solids. U.S. Patent Nos. 4,717,878, issued to Taicher and associates, and 5,055,787, issued to Kleinberg and associates, describe NMR tools that use permanent magnets to polarize the hydrogen nuclei and generate a static magnetic field, B0, and antennas RF to excite and detect nuclear magnetic resonance in order to determine the porosity, the free fluid ratio and the permeability of a formation. The atomic nuclei are aligned with the applied field, B0, with a time constant of Ti. After a period of polarization, the angle between the nuclear magnetization and the applied field can be changed by applying an RF field, Bí t perpendicular to the static field B0 in the frequency Larmar fi =? B0 / 2p where? is the gyromagnetic relation of the proton and B0 designates the intensity of the static magnetic field. After the termination of the RF pulse, the protons precess in the plane perpendicular to B < > . A sequence of reconcentration of the RF pulses generates a sequence of spin echoes which produces a detectable NMR signal in the antenna. U.S. Patent No. 5,280,243, issued to Melvin Miller, describes a nuclear magnetic resonance tool for evaluating formations during drilling. This tool comprises a probe section consisting of a permanent magnet that is placed in an annular cavity extending longitudinally out of the drill collar and an antenna located on a non-conductive magnetic sleeve outside the drill collar. The gradient of the magnitude of the static magnetic field is in the radial direction. The antenna produces an RF magnetic field substantially perpendicular to the longitudinal axis of the tool and to the direction of the static field. With the apparatus of the '243 patent the magnet must be long in its axial extent as compared to its diameter for the magnetic fields to approximate its bipolar behavior in two dimensions.

U.S. Patent No. 5,757,186, issued to Taicher et al., Describes a measurement tool during drilling that includes a sensing apparatus that performs nuclear magnetic resonance measurements of the land formation. This NMR sensor apparatus is mounted in an annular cavity on the outer surface of the drill collar. In one of its embodiments, the cavity carries a flow closure. A magnet is placed on the external radial surface of the flow closure. The magnet is formed by a series of radial segments that are magnetized radially outward from the longitudinal axis of the tool. Flow closure is necessary to provide the proper directional orientation of the magnetic field.

The tools described in the '243 and? 186 patents present common problems: both need to use a non-conductive magnet and place said magneto on the outside of the drill collar. For the x243 patent tool, the outer surface of the drill collar must have a recessed area to position the non-conductive magnet. For the tool of the? 186 patent, the outer surface of the drill collar must have a recessed area to place the flow lock, the non-conducting magnet and the antenna. Since the power of the drill collar of a function of its spokes, the reduction of its outer diameter to be able to place the magnet alone or the flow lock, the magnet and the antenna leads to the presence of an unacceptably weak section in the collar that can buckle or break during drilling operations. U.S. Patent No. 5,557,201, issued to Kleinberg et al., Describes a pulsed nuclear magnetism tool for evaluating formations during drilling. This tool includes a drill bit, a drill string and a pulsed nuclear magnetic resonance device housed inside the drill collar and made of a non-magnetic alloy. The tool includes a channel inside the drill string and pulsed NMR device, through which the drilling mud is pumped into the borehole. The pulsed NMR device comprises two tubular magnets that are mounted with equal poles arranged facing each other and surrounding the channel, and an antenna coil mounted on the outside of the drill string between the magnets. This tool is designed to resonate nuclei in a measurement region known as a saddle point by those who know about this matter. U.S. Patent No. 5,705,927, issued to Sezginer et al., Also discloses a pulsed nuclear magnetism tool for evaluating formations during drilling. This tool includes compensating magnets, located inside or outside the tool, which suppress the magnetic resonance signal of the borehole fluids by increasing the magnitude of the static magnetic field in the borehole so that the Larmor frequency in the borehole remains above the frequency of the oscillating field produced by an RF antenna located in a recessed area of the tool. The compensating magnets also reduce the gradient of the static magnetic field in the research region.

Compendium of the Invention The aforementioned advantages of the above inventions are overcome by the invention of an apparatus and a method for obtaining a nuclear magnetic resonance measurement during borehole drilling. The apparatus consists of a perforating element for drilling a hole in the formation, an element for transporting the drilling fluid through the perforating element and a measuring element for carrying out nuclear magnetic resonance measurements while the hole is being drilled. The measuring element produces a series of considerably axisymmetric static magnetic fields through the perforating element and inside the formation in a series of research regions. The measuring element also generates an oscillating magnetic field in the formation. At least one magnetically permeable member is inside the piercing element to mold the static magnetic field so that the contour lines generated byAt least one static magnetic field is substantially straight in the axial direction. The apparatus also comprises a gradient element for applying a magnetic field gradient in order to completely or incompletely offset the spins in a portion of the research regions. The series of substantially axisymmetric static magnetic fields can include the following combinations: low gradient-low gradient, high-gradient-high gradient-low gradient-low gradient-high gradient regions, or a combination of high gradient, low gradient regions gradient and saddle points. The device has a series of antennas and each antenna generates an oscillating magnetic field in a different research region.

Brief description of the illustrations The advantages of the present invention will become more apparent from the description of the appended figures.

It should be understood that the figures should be used for illustrative purposes only and not as a definition of the invention. Figure 1 illustrates a recording apparatus during drilling. Figure 2 shows the low gradient probe. Figures 2a-2d show the contour lines corresponding to four configurations of the low gradient magnet. Figures 3a-3d represent the contour lines of the gradient corresponding to four configurations of the low gradient magnet. Figure 4 shows the high gradient probe. Figure 4a represents the contour lines corresponding to the configuration of the high gradient magnet. Figure 4b represents the contour lines of the gradient corresponding to the configuration of the high gradient magnet. Figure 5 shows the mode of simple data acquisition. Figure 6 shows the modality of interleaved data acquisition. Figure 7 shows the data acquisition mode by broadsides. Figure 8 represents a block diagram of the pulse programmer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 illustrates a nuclear magnetic resonance (NMR) tool 10 for recording during drilling. The tool 10 consists of a drill 12, a drill string 14, a series RF antennas 36, 38, and at least one gradient coil 56. The tool 10 also comprises an electronic circuit 20 housed inside the collar 22. The electronic cirtuitaje 20 is conformed by an RF resonance circuit for the antennas 36, 38, a microprocessor, a digital signal processor and a low voltage bus. The tool 10 further includes a set of tubular magnets 30, 32 and 34 which are biased in a direction parallel to the longitudinal axis of the tool 10 but in the opposite direction to one another, ie with the magnetic poles alike located opposite one another . The magnets 30, 32 and 34 contain a conductive or non-conductive material. The configuration of the magnets 30, 32 and 34 and of the antennas 36, 38 provides, at least, two research NMR regions 60, 62 with a substantially static and RF magnetic field. The element for drilling a hole 24 in the formation consists of a drill 12 and a drill collar 22. The drill collar 22 may include a stabilizer element (not shown) to stabilize the radial displacement of the tool 10 in the hole during the perforation, however, this stabilizing element is not necessary and therefore, the tool 10 can operate being destabilized or stabilized. The sleeve for the flow of slurry 28 provides a channel 90 for transporting the drilling fluid through the drill string 14. A drive 26 rotates the drill 12 and the drill string 14. This drive mechanism is described in a manner described in FIG. suitable in U.S. Patent No. 4,949,045, issued to Clark and associates. However, as a driving mechanism 26, the drilling string can be fitted with a downhole mud drive motor. The invention contemplates combining N + l magnets to obtain, at least, N regions of research in the formation. The combinations contemplated by the present invention include, without being limiting, a region of low gradient-low gradient, high gradient-high gradient, high gradient-low gradient, low gradient-high gradient, or a combination of high gradient regions , low gradient and saddle points. The combination of regions with static magnetic field of high and low gradient offers several advantages. For example, the high gradient region may have a higher signal-to-noise ratio but may experience signal loss when the tool 10 undergoes lateral displacement in the borehole. On the other hand, the low gradient region is less susceptible to problems of signal loss when the tool 10 is moved. Likewise, with a moderate displacement of the tool, echo trains in the lower gradient region can be obtained longer than in the high gradient region, thus obtaining more information about the permeability, the subject and free liquid, and the types of hydrocarbons. In addition, the combination of the acquired data with both gradient regions can provide quantitative information about the amount of lateral displacement experienced by the tool 10 and can be used to correct the NMR data or, at least, control the quality of the data . Measurements of the devices such as strain gauges, accelerometers or magnetometers, or any combination of these devices, can be integrated with the NMR information to control the quality of the data to make corrections to the spin echo train. With the combination of static magnetic fields of something and low gradient, the high gradient region has more diffusion effect and, therefore, is more interesting for hydrocarbon classification techniques than the low gradient region. Finally, the low gradient region has a static magnetic field that has a low amplitude and, consequently, this region with its lower Larmor frequency is less affected by the conductivity of the formation and the borehole fluid.

Low gradient probe Figure 2 shows in a section of the tool, hereinafter referred to as a low gradient probe, a central magnet 30 axially spaced from a lower magnet 32. These magnets 30, 32 generate a substantially axisymmetric static magnetic field which is radial in its polarization and, above a reasonably long cylindrical envelope, the static magnetic field has a fairly constant magnitude. The present invention contemplates exciting a series of cylindrical spin envelopes in the formation where each envelope is resonant at a different RF frequency, and sequentially interrogate each envelope with RF pulse sequences. The area between the magnets 30, 32 is suitable for housing elements such as electronic components, an RF antenna and other similar devices. For example, a series of electronic cells 70 can form an integral part of the mud sleeve 28. These cells 70 can house the RF circuitry (eg, q switch, duplexer and pre-amplifier), preferably very close to the RF antenna. In a preferred embodiment of the invention, the cells 70 form an integral part of the magnetically permeable member 16. In that case, in order to maintain the axial symmetry of the magnetic field, a highly permeable magnet cover 72 is placed on top of each cell 70. The magnetically permeable member 16 is placed inside the drill collar 22 between the magnets 30, 32. The member 16 may consist of a single piece or a series of combined sections between the magnets 30, 32. The member 16 is made of a material magnetically permeable, such as ferrite, permeable steel or other alloy of iron and nickel, corrosion-resistant permeable steel, or a permeable steel that plays a structural role in the design of the member, such as stainless steel 15 -5 Ph. The magnetically permeable member 16 is concentrated in the magnetic field and can also carry drilling fluid through the drill string. No provide structural support to the drill collar. On the other hand, the member 16 improves the shape of the static magnetic field generated by the magnets 30, 32 and minimizes the variations of the static magnetic field due to the vertical and lateral displacement of the tool during the period of acquisition of the NMR signal. The segment of the sleeve 28 located between the magnets 30, 32 may include the magnetically permeable member. Alternatively, a magnetically permeable chassis surrounding the sleeve segment 28 positioned between the magnets 30, 32 defines the member 16. In this case, the segment may consist of a magnetic or non-magnetic material. The present invention contemplates integrating the chassis and the segment to form the member 16. The magnets 30, 32 are biased in a direction parallel to the longitudinal axis of the tool 10 with the equal poles arranged opposite each other. For each magnet 30, 32, the induction magnetic lines travel outward from one end of the magnet 30, 32 into the formation and along the axis of the tool 10, and travel inward to the other end of the magnet 30, 32. In the region located between the central magnet 30 and the lower magnet 32, the induction magnetic lines travel from the center outward into the formation creating a static field in a direction substantially perpendicular to the axis of the tool 10. Then the induction magnetic lines travel in symmetrically above the central magnet 30 and below the lower magnet 32 and converge in the longitudinal direction within the sleeve 28. Due to the separation, the magnitude of the static magnetic field in the region The center between the central magnet 30 and the lower magnet 32 is spatially homogeneous when compared to a field with a saddle point. The amount of separation between the magnets 30, 32 is determined by several factors: (1) selection of the strength of the magnetic field and the necessary homogeneity characteristics, (2) generation of a field that has small radial variations in the region of interest so that the echoes received during a pulse sequence (for example, CPMG, CPI or other sequences) are less sensitive to lateral displacement of the tool, (3) depth of the investigation and (4) minimization of the interference between the resonance circuit and the bus low voltage telemetry to improve the isolation of the receiving antenna that detects the NMR signals coming from the formation. As the separation between the magnets 30, 32 decreases, the magnetic field becomes stronger and less homogeneous. On the contrary, as the separation between the magnets 30, 32 increases, the magnetic field weakens more and becomes more homogeneous.

Figures 2a-2d show the lines of contour corresponding to four configurations modeled in the laboratory of the central magnet 30 and the lower magnet 32. These modeled results were computed using a tool that has a preselected diameter (a constant diameter was used to model all the configurations). The configuration corresponding to Figure 2a comprises a non-magnetically permeable member that separates the central magnet 30 and the lower magnet 32 by 25 inches. The configuration corresponding to Figure 2b comprises a non-magnetically permeable member that separates the central magnet 30 and the lower magnet 32 by 18 inches. The configuration corresponding to Figure 2c comprises a non-magnetically permeable member that separates the central magnet 30 and the lower magnet 32 by eight inches. The low gradient probe, corresponding to Figure 2d, comprises a magnetically permeable member that separates the central magnet 30 and the lower magnet 32 by 25 inches. The aforementioned dimensions were modeled only to illustrate the distance effect and / or a magnetic or non-magnetically permeable member in Figures 3a-3b represent the contour lines of the gradient | VB, or? respectively corresponding to the configurations shown in Figures 2a-2d. In the low gradient probe, the magnetically permeable member 16 deflects a considerable portion of the magnetic flux to the center of the tool 10. By way of example, the magnitude of the B0 field shown in Figure 2d at a distance of approximately seven inches radially from the longitudinal axis of the tool 10 is twice as long as that of the Bc field shown in Figure 2a, which was generated by the same magnet configuration separated by a non-magnetically permeable member. In addition, the low gradient probe produces a longer and more uniform extension of the static magnetic field in the axial direction. The NMR signal measured in this section of the tool is considerably less sensitive to the vertical displacement of the tool. With respect to Figure 3d, with the low gradient probe, a relatively small gradient of about 3 Gauss / cm is measured at a distance of about seven inches radially from the longitudinal axis of the tool. This low gradient results in a measured NMR signal that is considerably less sensitive to the lateral displacement of the tool 10. When the displacement is moderate, longer echo trains can be obtained, thus obtaining more information about the permeability, the subject fluid and free and the types of hydrocarbons. As with other gradient designs, in the case of the low gradient probe, the region of the proton-rich hole surrounding the tool 10 will only resonate at higher frequencies than those applied to the volume of the investigation, ie , there is no proton signal in the hole. Other sensitive NMR cores found in drilling mud, such as sodium-23, resonate in static magnetic field strengths significantly higher than hydrogen when excited at the same RF frequency. For the low gradient probe, these higher field strengths are not produced in the region of the borehole that surrounds the tool or that is close to the antenna where those unwanted signals could be detected.

High gradient probe In Figure 4, in another section of the tool, hereinafter referred to as a high-gradient probe, a central magnet 30 is axially separated from an upper magnet 34. The magnets 30, 34 are biased in a direction parallel to the axis longitudinal of the tool 10 with the equal poles arranged opposite one another. These magnets 30, 34 generate a substantially exisymmetric static magnetic field that is radial in its polarization and, above a reasonably long cylindrical envelope, the static magnetic field has a fairly constant magnitude. The present invention contemplates exciting a series of cylindrical spin envelopes in the formation where each envelope is resonant at a different RF frequency. As illustrated in Figure 2c, if the spacing between the magnets 30 and 34 is about eight inches, the contour lines of the static magnetic field strength are substantially straight and the intensity of is greater than the intensity of the static magnetic field of the magnetic field. the low gradient region. However, the gradient | VB0 [becomes larger, as shown in Figure 3c, at a distance of about seven inches radially from the longitudinal axis of the tool. The contour lines of | VB0 | they are curves indicating variation of the gradient in the axial direction. The high gradient probe is improved by inserting a magnetically permeable member 16 between the magnets 30 and 34. Figure 4 represents the lines of outline of corresponding to a configuration in that the magnetically permeable member 16 separates the upper magnet 34 from the central magnet 30 by eight inches. The contour lines of Figure 4a show a slightly more intense field which indicates a better signal-to-noise ratio and less curvature in the axial direction than the contour lines of Figure 2c. Also, as shown in Figure 4b, the magnetically permeable member 16 produces a more constant gradient | VB0 | in the axial direction that can facilitate the interpretation of NMR measurements influenced by diffusion. As with other gradient designs, in the case of the high-gradient probe, the region of the proton-rich hole surrounding the tool 10 will only resonate at higher frequencies than those applied to the volume of the investigation, ie , there is no proton signal in the hole. The high gradient probe is sensitive to a small part of the sodium from the drilling fluid. For a hole fluid with a NaCl concentration of 30%, possibly the worst case, the estimated porosity error due to the sodium signal is approximately 0.08 pu. In the low gradient probe, the sodium signal is considerably smaller than in the high gradient probe. Consequently, the sodium signal is negligible for both NMR probes.

Antennas and gradient coils In Figures 2 and 4 the antennas 36, 38 which are in the recessed areas 50, 52 create an RF magnetic field in the research regions. The RF field can be produced by one or more segments of RF antennas that transmit and / or receive from different circumferential sectors of the recording device. See U.S. Patent Applications Nos. 08 / 880,343 and 09 / 094,201 (Attorney files Nos. 24,784 and 24,784-CIP) assigned to Schlu berger Technology Corporation. Preferably each antenna 36, 38 has a coil 18 wound circumferentially around the recessed area 50, 52. The RF field created by such a coil arrangement is substantially axisymmetric. The present invention contemplates using the antenna 36, 38 to detect NMR signals. However, a separate antenna or receiver can be used to detect the signals. The recessed area 50, 52 has a non-conductive material 54 below the antenna 36, 38. The material 54 is preferably a ferrite in order to increase the efficiency of the antenna 36, 38. Alternatively, the material 54 can be plastic, rubber or a reinforced epoxy composite material. The antennas 36, 38 are resonated by RF circuitry to create an RF magnetic field in the research regions. The recessed area 52 forms a shallow groove in the drill collar without reducing the S internal diameter thereof, which is commonly done to increase the force in a region of the drill collar when the outer diameter has been recessed to supply an antenna. The recessed area 50 has a greater depth that the recessed area 52. Due to mechanical limitations, it is only possible to have a deeply recessed area when the internal diameter of the drill collar is considerably reduced. The present invention contemplates that the recessed areas 50, 52 have substantially the same depth or that the recessed area 52 has a greater depth than the area 50. The cylindrical sheaths of spins in the research region may be axially segmented or, preferably, azimuthally using, at least, 2o a gradient coil 56 responsive to the direction and located in the recessed area 50 and / or 52. In a preferred embodiment of the invention, there are three gradient coils arranged in a circle around the recessed area and separated by a segment of angular distance of 120 °. 5 Other quantities of gradient coils can be defined, in a number greater than or less than three, and such coils may be separated by angular distances other than 120 ° and / or unequal angular segments. Each coil 56 is made with wire loops which shape the curvature of the outer surface of the material 54. The magnetic field produced by each gradient coil 56 in a region of the formation facing the coil is substantially parallel to the field static magnetic produced by the magnets. As those skilled in the art know, in basic NMR measurement, a pulse sequence is applied to the training under investigation. In U.S. Patent No. 5,596,274, issued to Abdurrahman Sezginer and U.S. Patent No. 5,023,551, issued to Kleinberg and associates, a sequence of pulses, such as the sequence Carr-Purcell-Meiboom-Gill (cpmg), first apply an excitation pulse, a 90 ° impulse, to the formation that spins the spins in the transverse plane. After the spins are rotated 90 ° and begin to phase out, the carrier of the concentration pulses, the 180 ° pulses, is offset with respect to the carrier of the 90 ° pulse sequence according to the following relationship: where the expression in brackets is repeated for n = 1,2, ... N, where N is the number of echoes collected in a single CPMG sequence and the separation between echoes is represents an RF pulse that rotates spins at a 90 ° angle around the + x axis, as commonly defined in the rotating frame of magnetic resonance measurements (alternating phase). The time between the application of the 90 ° impulse and the 180 ° impulse, t0, is less than tcp, half the separation between echoes. The CPMG sequence allows a symmetric measurement to be obtained (ie, a measurement without using the gradient coils). The exact time parameters, t0, ti and t2, depend on several factors (for example, the shape of the applied pulses). In the present invention, a current pulse applied to the gradient coil 56 generates an additional magnetic field, substantially parallel to the static magnetic field. The current pulse is applied between the first 90 ° pulse and the 180 ° phase inversion pulse. This additional field causes an additional lag in the spins. Since the 180 ° phase inversion pulse does not compensate for the additional phase shift, the spins attached to the additional field do not form a spin echo. However, for spins not subject to the additional field, spins echo occurs at 2tcp time with echoes of amplitude spins successively lower at the top time after each phase inversion pulse. The sequence of impulses is where t is the time between the 90 ° pulse and the duration gradient pulse d, tb0 is the time between the gradient pulse and the 180 ° inverting pulse, and tas + d + tbo = t0. Due to successive 180 ° pulses and y to inhomogeneous fields, the x component of the NMR signal will decay after a few echoes. Consequently, we concentrate only on the component and the signal. Thus, leaving relaxation aside, the first NMR echo signal can be represented as: Signal where i is the imaginary complex unit; ? it is the gyromagnetic relation; M ° x and M ° and are respectively the components x and y of the magnetization at the site r at the time of the first echo in the absence of the gradient pulse, G (r) is the component of the gradient field parallel to B0 and at the same site; d is the duration of the gradient pulse and dc (r) indicates the differential sensitivity of the NMR probes. Gradient coils 56 provide a number of advantages for obtaining azimuthal measurements. First, because the axisymmetric antenna detects spin echoes, and the echo trains can be recorded while the tool rotates in the hole. Second, coil 56 simplifies the design of an NMR-L tool D because coil 56 does not have the adjustment requirements of an RF antenna 36, 38. Third, the same antenna 36, 38 can be used to perform symmetric and axisymmetric measurements . Fourth, the coils 56 can be used to obtain NMR measurements with excellent spatial resolution, particularly vertical resolution. The present invention contemplates the different ways of obtaining azimuth NMR measurements. For example, a "simple disturbance" mode uses, at least, a coil 56 to disturb the spins in a given quadrant where a quadrant is defined as a segment of angular distance around the periphery of the tool 10, however, more coils 56 can be used to disturb a series of quadrants. In any case, two measurements are obtained: a sequence of alternating symmetric phase pulses (SIAF) with a fixed waiting time followed by an SIAF gradient with a variable waiting time., with the selected quadrant disturbed by firing coil 56 in the dial. In a preferred embodiment of the invention, the sequence of gradient pulses mentioned above is used. By subtracting the gradient measurement from the symmetric measurement, the azimuth measurement is created. In this mode, a symmetric measurement is obtained for every two SIAF and an azimuth sweep is obtained for every eight SIAF. The measurement noise for azimuthal measurement is greater than the noise in the symmetric or gradient measurement because the two measurements are combined. It is possible to reduce the noise contribution by combining the different perturbation measurements of simple quadrants. For example, four SIAF measurements of gradient perturbed each quadrant can be obtained. The measurements are combined to create a synthetic symmetric and azimuthal measurement. Axially or azimuthally resolved "images" of the array can be generated by combining the measurements made without firing the gradient coils 56 with the measurements made with one or more gradient coils 56 fired. The obtained data, especially in the form of azimuthal images of the porosity of the subject fluid, are very convenient for a better petrophysical interpretation in holes of well deviated and horizontal wells and to make decisions during the drilling for the location of holes of wells with geological base .

Optimization of Impulse Length and Operational Frequency For a given operational RF frequency there is an optimal duration for the 90 ° impulse, t90, as well as for the 180 ° pulses, tiso, which guarantees a desired signal-to-noise ratio. The search for the optimal pulse length can be performed during the master calibration of the tool so that all the pulse lengths will be initialized correctly, or when the static magnetic field changes unpredictably, as in the case of a change due to the accumulation of magnetic waste during the drilling process. See U.S. Patent Application No. 09 / 031,926 (Attorney File No. 24,786) assigned to Schlumberger Technology Corporation. This technique can also be used to choose the appropriate frequency that satisfies other criteria such as keeping research depth constant. The optimal pulse length can be determined by measuring the NMR response of a sample by means of at least two different pulse durations and using a predefined mode independent of the NMR properties of the array. Alternatively, the optimum pulse length can be determined by using at least two different pulse durations, and, additionally, a computed mode from the NMR properties of the array. In the first case, the accumulation of data improves the signal-to-noise ratio; however, the accumulation procedure may require a long period of time to get the data from the training. Preferably, the measured data is accumulated during a stationary time window when the tool 10 pauses in the drilling operation, such as during the time a new section of pipe is added to the drill string. In the second case, if the distribution of T2 of the formation is known, a better obtaining mode can be developed that provides the highest signal-to-noise ratio for a unit of time to obtain and provides an optimal linear combination of the obtained echoes. Lab simulations show that the optimum time for the best mode of obtaining is achieved when the duration of the echo train is approximately equal to T2, max; the T2, predominant of the formation, and when the waiting time Tw, is approximately equal to 2.5 x T2, aax (assuming a constant T? / T2 ratio of 1.5). The best way to obtain determines the optimal pulse length within the range of a minimum percentage over several seconds. A similar technique can be used to optimize the NMR signal with respect to frequency (for example the saddle stitch design). The T2 distribution effectively helps to efficiently adjust the pulse lengths for tool 10.

Data Acquisition Modes As described above, the tool 10 has one. series of antennas 36,38. In a preferred embodiment of the invention, these antennas 36, 38 do not transmit or receive data simultaneously. Preferably, after an antenna 36 acquires data, the other antenna 38 experiences a minimum waiting time while the electrical energy is recharged in order to transmit the next sequence of pulses. The present invention contemplates transmitting or acquiring data simultaneously. In addition, this invention contemplates the acquisition of data without the waiting time requirement. Based on these design preferences, a variety of data acquisition modes can be used. By way of example, three representative times for the acquisition of NMR data are described below: a fast time suitable for water and wet sand areas, a slow time suitable for carbonated zones and a very slow time for areas with hydrocarbons (or invasion of oil-based mud). The times are specified in Table 1.

Table 1 Various different modes can be used with each data acquisition time, including, but not limited to, the following: simple, interleaved, and by broadsides. The simplest way to acquire T2 information with tool 10 is to perform CPMG measurements with both antennas 36,38 using the same time. Figure 5 illustrates the simple mode of data acquisition using the fast drop time, the slow drop time and the very slow drop time of Table I. Each antenna 36.38 alternately acquires a long sequence of pulses that supplies a Effective measurement of porosity from each antenna 36,38. With the interleaved mode, the high-gradient antenna measures at least two cylindrical shells at two different frequencies while the low-gradient antenna obtains a measurement using a single frequency. Figure 6 illustrates an interleaved measurement for fast drop samples, slow drop components and very slow drop components using the time in Table I. The broadside mode increases the signal-to-noise ratio, especially for fast fall components. In addition, the broadside mode provides an i based on the measurement of the subject fluid. See WO 98/29639 assigned to Numar Corporation (describes a method for determining longitudinal relaxation times, Ti). See also U.S. Patent Application No. 0/0/096,320 (Attorney's File No. 24,785) assigned to Schlumberger Technology Corporation (discloses a method for polarizing the fluid subject of a formation). Figure 7 illustrates barrage measurements for fast drop samples, slow drop components, and very slow drop components using the slightly modified times in Table I.

In addition to the simple, interleaved and broadside modes, with the present invention it is possible to optimize the measurements to evaluate the formation by detecting the downhole conditions that create a pause during the drilling operation, determining the drilling mode, and using the way to control the acquisition of data. The common rotary drilling operations contain many natural breaks in which the tool remains stationary: the connection time when a new section of pipe is added to the drill string, the circulation time when the mud is circulated and possibly made Rotate the drill pipe, and time for fishing or shaking while the drill string is stuck and has to be released before resuming drilling. These natural pauses, which occur without interrupting normal drilling operations, or deliberately initiated pauses, are used to perform NMR measurements. The drilling modes include, without limitation, drilling, sliding, firing, circulation, fishing, a short trip (up or down) and drill pipe connections. Determining the drilling mode increases the ability to obtain NMR measurements that take a long time or that benefit from a quiet environment, for example, i, T2, the adjustment of antennas, and the classification of hydrocarbons. See U.S. Patent Application No. 09 / 0312,926 (Attorney File No.24,786), assigned to Schlumberger Technology Corporation. It is also possible to adjust the acquisition modes based on changes in the environment (for example, scour, salinity, etc.) and / or changes in the NMR properties of the formation (for example, a long Ti versus a short Tx). . Spin echo amplitudes are obtained by hardware integration of the receiver's voltages over a time window. The tool 10 uses sensitive phase detection to measure the in-phase components and the quadrature components of the signal-plus-noise amplitudes of the spin echoes. The techniques presented in U.S. Patent No. 5,381,092, issued to Robert Freedman, can be used to compute the sums of downhole windows and transmit the sums of windows to the surface for investment processing and the presentation of T2. Also the techniques set forth in U.S. Patent No. 5,363,041, issued to Abdurrahman Sezginer, can be implemented to use a linear operator to map a relaxation-time distribution to spike echoes, producing a decomposition of the singular values (DVD ) of the linear operator, determine DVS vectors and compress the spin echo data using said vectors. Preferably, the spectrum of T2 is counted downhole and transmitted to the surface. This offers the advantage of eliminating a telemetric bottleneck originated by the transmission of the data necessary to compute the T2 spectrum to the surface. A digital signal processor can be used to invert the T2 data. The amplitudes, Aj, of the spin echoes are characterized with the following relationship: where ? is the noise in the measurement Aj, ai is the amplitude of the distribution T2 taken in T2 / i, the expression represents the elements of the matrix X, where tw is the waiting time and c is a constant (the relation T1 / T2),? is the separation of the echo and j = l, 2, ... N, where N is the number of echoes gathered in a single sequence of pulses. In matrix notation, the equation becomes. Since the noise is unknown,?, A can be approximate looking for a less square solution, that is, a minimum of the functional The solution of this equation is quite affected by the noise present in the data and the solution can have negative components even when the T2 spectrum has no negative components. To overcome this problem a regularizing term is added to functional and functional, it is minimized using an appropriate iterative minimization algorithm (for example, the Projection Method of Conjugated Gradients) under the limitation that a ± > 0 for i = l .... M. See On the Minimization of Quadratic Functions Subject to Box Constraints by Ron S. Dembo and Urrich Tulowitzski, Department of Computing, Yale University. (September 1984) (describes the Method of Projection of Conjugated Gradients). The time required to make the investment of the T2, using a digital signal processor is quite reasonable. For example, assuming that there are 1800 echoes and 30 samples in the T2 domain, the investment in a digital signal processor requires less than two seconds.

Pulse programmer For the measurement of basic NMR with the tool 10, the electronic circuitry applies a sequence of impulses to the formation under investigation. The tool 10 includes a pulse scheduler 80, which selects and controls the pulse sequences applied to the array. The pulse scheduler 80 establishes the pulse sequence using the information found in the Measurement Control Block 82 (see Figure 8) and the operational conditions of the tool 10. Preferably the Measurement Control Block 82 is stored downhole on a memory device. The structure of the block 82 is fixed in order to allow the pulse programmer 80 to adapt and easily change the time of the pulse sequences autonomously downhole. It is advantageous to divide a portion of the block 82 into a series of tables 84, 86 and 88. Instead of controlling all the operations of the tool which depend on the sequence of pulses from the pulse programmer 80, tables 84, 86 and 88 they are used to control such operations. This allows the pulse programmer 80 to vary the pulse sequences without introducing contradictions in the tool configuration. The series of tables 84, 86 and 88 may include, but are not limited to, the following: an intermediate table that describes the arrangement of the accumulator compensators, an acquisition table that defines the acquired signals accumulated in the compensators, a table of the filter coefficients prescribed by the detection filter used with a signal acquisition, a table of correction of the dynamics of the spins, which determines the correction of the spin dynamics to be used for each compensator, and a table data processing designating the nuclear magnetic resonance characteristic calculated from the acquired compensators. The pulse programmer 80 includes a template of pulse sequences 94 useful for generating pulse sequences and which comprises a sequence of states that depend on the repetition and time variables. These variables are calculated from the sequence configuration parameters using the calculation block 92. The calculation block 92 can be applied as an executable or interpretive structure. Based on the physical quantity that will be measured, for example, T2, the time variables, such co or the waiting time, tW the separation between techno-echoes, and the number of acquired echoes, can be defined. The configuration parameters include, but are not limited to, t90, the amplitude of the pulse and the shape of the pulse. These parameters can be measured periodically during the calibration of the tool 10 or during the operation of the tool 10 since they can vary when varying the operational conditiof the tool 10. For example, the amplitude and shape of the pulse depends on the quality factor of the antenna and, therefore, of the conductivity of the formation surrounding the tool 10. In general, after the pulse programmer 80 initiates a sequence of pulses, the sequence runs in a deterministic manner until its completion. In order to implement certain parallel measuring modes with the tool 10, the pulse programmer 80 can vary the sequence of pulses during the execution of the sequence. The pulse programmer 80 can stop the execution of the pulse sequence and enter a STOP state until an external signal ends the state at time tc or until a maximum period of time has expired. As already indicated previously in the section of Data Acquisition Modes of the present specification, since at least one of the different modes (interleaving) that can be used with the data acquisition time contemplates the interleaving of several measurements, programmer 80 compensates for the time elapsed during the STOP state. Preferably compensation is achieved by grouping HIGH events. For example, a grouping can comprise a pair of events HIGH where a HIGH event operates in the manner described above and the other event HIGH is a normal event of duration of tmax-tc. The grouping of events allows the programmer 80 to combine the sequences that have variable time and deterministic ones. In addition, the sequence of states, defined in the pulse sequence template 94, may comprise several alternatives for the parts of the sequence. In real time, one of the alternatives is selected (branch) that depends on the external conditions of the tool (for example, the azimuth of the tool). The foregoing description of the preferred and alternative embodiments of the present invention has been made for illustrative and descriptive purposes and is not intended to be exhaustive or to limit the invention to the precise form presented. Obviously, for those who know about the matter, many modifications and variations will be evident. The embodiments were chosen and described in order to explain the principles of the invention and their practical application in the best way so as to enable other experts in the field to understand the invention in different embodiments and with different modifications that are suitable for the particular use. contemplated. It is intended that the scope of the invention be defined by the appended claims and their equivalents.

Claims (16)

RE I V I ND I CA C I ONE S

1. An apparatus for the determination of a nuclear magnetic resonance property in a research region of land formations surrounding a borehole, comprising: a) a drilling means for drilling a hole within the formation; b) means for transporting a drilling fluid through the drilling means; c) a measuring means, connected to the drilling medium, for performing nuclear magnetic resonance measurements while the drill hole is being drilled, the measuring means comprises: i) a means for the production of a plurality of substantially axial symmetrical static magnetic fields through the perforation means and within the formation in a plurality of research regions where the measurement of nuclear magnetic resonance is obtained in such a way that the lines contours generated by at least one static magnetic field are substantially straight in the axial direction; and ii) a means for the production of an oscillating magnetic field in the formation; d) at least one magnetically permeable member located within the piercing means for the conformation of the static magnetic field; e) half gradient in the probing device for the application of a magnetic field gradient to offset the spins in a portion of the investigation regions; and, f) a means for the detection of nuclear magnetic resonance signals from the research regions.

2. The apparatus of Claim 1, wherein the means for production is a plurality of symmetrical axial static magnetic fields further comprises a means for the production of at least one static magnetic field having a low gradient point, high gradient or singular point. in a first region of the investigation.

The apparatus of any of the preceding Claims wherein the means for the production of a plurality of symmetric axial static magnetic fields further comprises means for the production of at least one static magnetic field having a high gradient, low gradient, or point singular in a second research region.

4. The apparatus of any of the preceding claims wherein the piercing means further comprises a tubular piercing collar having a generally cylindrical internal surface having an internal diameter and a generally cylindrical external surface having an external diameter in which an antenna is disposed in a socket comprising an axial extension on the outer surface, the outer surface having a diameter that is reduced from the external diameter on the axial extension of the socket, and the inner surface of the drill collar having a diameter that is not substantially reduced from the internal diameter on the axial extension of the socket.

The apparatus of Claims 1-3 wherein an antenna is disposed in a socket that encompasses an axial extension on an outer surface of the piercing means, the outer surface having a diameter that is reduced from the outer diameter of the medium of perforation on the axial extension of the socket.

6. The device of any of the Precedent claims wherein the gradient means further comprises a plurality of gradient means 'positioned around the circumference of the drilling medium.

7. - A method for the determination of a nuclear magnetic resonance property in a research region of terrestrial formations surrounding a drill hole, comprising the steps of: a) the provision of a device that moves through the drill hole; b) generation, from the device, of a plurality of substantially axial symmetric static magnetic fields within the array in a plurality of investigation regions; c) generation from the device of an oscillating magnetic field in the formation; d) the conformation of at least one static magnetic field in such a way that the contour lines generated by the field are substantially straight in the axial direction; and, e) the detection of nuclear magnetic resonance signals from the research regions.

8. The method of Claim 7 wherein the step (b) further comprises the step of producing at least one static magnetic field having a low gradient, high gradient, or singular point in a first research region.

9. The method of Claims 7-8 wherein step (h) comprises the step of producing at least one static magnetic field having a high gradient, low gradient, or singular point in a research region.

The method of Claim 9 further comprising the step of applying a magnetic field gradient to offset the spins in a portion of at least one region of investigation.

The method of Claim 10 further comprising the step of separating a cross section from the array into a plurality of angular distance segments around the drill hole and spatially varying the intensity of at least one static magnetic field in at least one a segment.

The method of Claim 10 further comprising the step of separating a cross section from the array into a plurality of axial segments and spatially varying the intensity of at least one static magnetic field in at least one axial segment.

The method of Claims 11-12 further comprising the steps of applying a plurality of FR pulses having a frequency, fi, in the first investigation region; applying a plurality of FR pulses having a different frequency, f2, in the first investigation region; and applying a plurality of FR pulses in the second investigation region.

14. The method of Claims 11-12 further comprising the steps of applying a plurality of RF pulses in the first investigation region; and, during a waiting time, the application of a second plurality of RF pulses in the second research region.

The method of Claims 7-14 further comprising the step of integrating the information obtained from at least one device with the nuclear magnetic resonance signals detected for the quality control of the signals.

16. The method of Claims 7-15 further comprising the step of correcting the detected signals with respect to the effect of the movement of the device on the detected signals.