Abstract:
An NMR sensor and method is disclosed for analyzing a core sample from a subsurface formation. Embodiments of the method utilize two or more magnets disposed proximate to each other. The configuration of the magnets allows for increased detection frequency, and creates a strong field with much finer resolution than existing designs. In addition, embodiments of the sensor may be used at the well site due to its small size and simple hardware. Further details and advantages of various embodiments of the method are described in more detail herein.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable 
       BACKGROUND 
       [0003]    Field of the Invention 
         [0004]    This invention relates generally to the field of geological exploration for hydrocarbons. More specifically, the invention relates to a method of determining petrophysical properties of rock samples. 
         [0005]    Background of the Invention 
         [0006]    Traditionally, nuclear magnetic resonance (NMR) devices are designed to enclose a sample, producing a strong, homogeneous magnetic field that is used to analyze that sample for purposes including chemical analysis and anatomical imaging. The reverse (“inside-out” or “single-sided”) geometry, where the sample is located outside the NMR device, is technologically more challenging because of the difficulty in generating a homogeneous field over a large spatial region outside the magnet; as a result, NMR spectral linewidths tend to be too broad for chemical analysis, and images tend to be very blurry and have distortions. One area in which single-sided NMR has found application is oilfield borehole logging, where fluid typing is conducted within a small, shallow region in an underground rock formation in order to locate and characterize deposits of extractable hydrocarbon materials. Rather than the high magnetic fields (1 Tesla (T) or higher) used by most chemical and medical devices, logging tools employ “low-field” detection (generally below 1 T), with a field generated by one or more permanent magnets and detection occurring over a range of several inches into the formation wall. Fluids are characterized based on their diffusion and spin relaxation properties, a modality that is compatible with the field inhomogeneity endemic to the tool configuration. Recently, there has been increasing development on single-sided NMR devices for portable applications such as materials analysis. These tend to be designed for more general applicability than the specialized logging tools, although they operate on the same basic principles. Currently, there is no sensor available that provides a high-resolution porosity map of a subsurface rock sample, which will be critical for integrating the measured petrophysical properties and performing multi-scale analysis on the range of micro to field scale. A single-sided NMR sensor is the only solution for efficiently measuring a porosity map on the millimeter-to-centimeter scale. 
         [0007]    Consequently, there is a need for improved sensors and methods to analyze rock and/or core samples from subsurface formations. 
       BRIEF SUMMARY 
       [0008]    An NMR sensor and method is disclosed for analyzing a core sample from a subsurface formation. Embodiments of the method can utilize two or more magnets disposed adjacent to each other. The configuration of the magnets allows for increased detection frequency, and creates a strong field with much finer resolution than existing designs. In addition, embodiments of the sensor may be used at the well site due to its small size and simple hardware. Further details and advantages of various embodiments of the method are described in more detail herein. 
         [0009]    In an embodiment, a nuclear magnetic resonance (NMR) sensor comprises two or more permanent magnets disposed proximate to each other. The magnets are configured to create a magnetic field. The sensor further comprises a sample holding member coupled to the magnets for holding a core sample. The magnetic field is located in a position between the magnets and proximate the surface of the sample holding member. The sensor also comprises an antenna disposed in between the magnets. 
         [0010]    In another embodiment, a system for analyzing a sample from a subsurface formation comprises an NMR sensor comprising two or more permanent magnets disposed proximate to each other. The magnets are configured to create a magnetic field. The sensor further comprises a sample holding member coupled to the magnets for holding a core sample. The magnetic field is located in a position between the magnets and proximate the surface of the sample holding member. The sensor also comprises an antenna disposed in between the magnets. The system additionally comprises an interface for receiving one or more user inputs. The system also comprises a memory resource. The system further comprises input and output functions for presenting and receiving communication signals to and from a human user. In addition, the system comprises one or more central processing units for executing program instructions and program memory, coupled to the central processing unit, for storing a computer program including program instructions that, when executed by the one or more central processing units, cause the computer system to perform a plurality of operations for analyzing a fluid or core sample from a subsurface formation. 
         [0011]    In another embodiment, a method of analyzing a sample from a subsurface formation, the method comprises a) extracting a sample from a subsurface formation. The method also comprises b) using a NMR sensor to scan the sample to determine one or more properties of the sample. The NMR sensor comprises two or more permanent magnets disposed proximate to each other. The magnets are configured to create a magnetic field. The sensor further comprises a sample holding member coupled to the magnets for holding a core sample. The magnetic field is located in a position between the magnets and proximate the surface of the sample holding member. The sensor also comprises an antenna disposed in between the magnets. 
         [0012]    The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which: 
           [0014]      FIG. 1A  illustrates an embodiment of an NMR sensor; 
           [0015]      FIG. 1B  illustrates another view of an embodiment of an NMR sensor; 
           [0016]      FIG. 1C  illustrates an exploded perspective view of an embodiment of an NMR sensor; 
           [0017]      FIG. 1D  illustrates a cross-sectional view of another embodiment of an NMR sensor; 
           [0018]      FIG. 1E  illustrates an embodiment of the NMR sensor with a cylindrical antenna; 
           [0019]      FIG. 1F  illustrates an embodiment of the NMR sensor with a semi-toroidal antenna 
           [0020]      FIG. 1G  illustrates an embodiment of the NMR sensor with tilted or angled magnets, including a computer simulation of the sweet spot of the sensor; 
           [0021]      FIG. 1H  illustrates an embodiment of the NMR sensor; including a computer simulation of the sweet spot of the sensor; 
           [0022]      FIG. 2  illustrates another embodiment of the NMR sensor; 
           [0023]      FIG. 3  illustrates the magnetic field component B x  measured at the surface of an embodiment of the sensor, where x is the horizontal axis, showing a saddle point with field of 0.27 T 
           [0024]      FIG. 4A  shows an NMR echo train signal of relaxed water taken with an embodiment of the NMR sensor, with TE=60 μs; 
           [0025]      FIG. 4B  shows and inversion of the echo train data, showing a spin-spin relaxation time (T 2 ) peak at 7.5 millisecond (ms); 
           [0026]      FIG. 5A  shows the T 2  relaxation spectrum of a bulk relaxed water calibration; 
           [0027]      FIG. 5B  show the T 2  relaxation spectrum of a carbonate core plug which yields a porosity for the plug of 60 porosity units (pu); 
           [0028]      FIG. 6  shows a demonstration of T 1 T 2  two-dimensional (2D) NMR with an embodiment of the NMR sensor for three bulk fluid samples: relaxed water (the same sample as in  FIG. 5A ), mineral oil, and deionized water. T 1  refers to the spin-lattice relaxation time as is known in the art; 
           [0029]      FIG. 7  illustrates a demonstration of diffusion-T 2  2D NMR with an embodiment of the NMR sensor for three bulk fluid samples: relaxed water (the same as in  FIG. 5A ), mineral oil, and deionized water; 
           [0030]      FIG. 8  illustrates a schematic of a tuning circuit that may be used with embodiments of the NMR sensor; 
           [0031]      FIG. 9  illustrates a schematic of a system which may be used in conjunction with embodiments of the NMR sensor and associated methods; and 
           [0032]      FIG. 10  illustrates a schematic of an NMR console which may be used with embodiments of the NMR sensor. 
       
    
    
     NOTATION AND NOMENCLATURE 
       [0033]    Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. 
         [0034]    In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    Referring now to the Figures, embodiments of the disclosed methods will be described. As a threshold matter, embodiments of the methods may be implemented in numerous ways, as will be described in more detail below, including for example as a system (including a computer processing system), a method (including a computer implemented method), an apparatus, a computer readable medium, a computer program product, a graphical user interface, a web portal, or a data structure tangibly fixed in a computer readable memory. Several embodiments of the disclosed methods are discussed below. The appended drawings illustrate only typical embodiments of the disclosed methods and therefore are not to be considered limiting of its scope and breadth. 
         [0036]    In an embodiment, referring to  FIGS. 1A-1F , sensor  100  generally includes permanent magnets  105 A- 105 B, a sample holding member  120 , antenna  125 , wall members  115 ,  117 , and base  103 . In this embodiment or arrangement, as better shown in  FIG. 1C , which depicts an exploded view of an embodiment of sensor  100 , wall members  113 ,  114 ,  115 ,  117  enclose two or more magnets  105 A- 105 B. Base  103  supports magnets  105 A- 105 B and may be coupled to magnets  105 A- 105 B. Sample holding member  120  is disposed on magnets  105 A- 105 B and the geological samples may be placed on sample holding member  120  so as to coincide with the “sweet spot”  123 , shown in  FIG. 1A . Antenna  125  may be coupled to sample holding member  120 . In an embodiment, antenna  125  is coupled beneath member  120  and may be disposed or positioned in between magnets  105 A- 105 B. 
         [0037]    As shown in  FIGS. 1A-F , embodiments of the apparatus  100  may contain two or more permanent magnets  105 A- 105 B. In an embodiment, the magnets may be neodymium magnets. Neodymium magnets may be used because of their very large remnant magnetization to produce a strong field and thus large NMR signal. However, any magnets known to those of skill in the art may be used. In an embodiment, the magnets can be permanent magnets. More specifically, examples of permanent magnets include without limitation, neodymium magnets, rare earth magnets, ceramic magnets, iron alloy magnets, or combinations thereof. In an embodiment, magnets  105 A- 105 B may be cuboidal in geometry (i.e. have a rectangular cross-section). However, other suitable geometries known to those of skill in the art may be used. In one embodiment, the magnets  105 A- 105 B may have dimensions of 1″×1″×2″ (with magnetization along one of the short axes). However, magnets  105 A- 105 B may be constructed or configured with any suitable dimensions. The remnant magnetization or magnetic strength of the magnets, the physical size, and spacing of the magnets help determine the magnetic field profile, and thus the sweet spot size, shape, and field strength. Accordingly, these parameters may all be configured in order to achieve the desired sweet spot size, shape, and field strength for the sensor  100 . As used herein, the “sweet spot”  123 , as shown in  FIG. 1B , refers to the region of the sensor  100 , distributed around the local extremum in the magnetic field strength, where the magnetic field is most uniform and so the largest volume of spins can be excited and detected simultaneously. Although shown in the center of the sensor  100  in the Figures, the sweet spot  123  may be located in any position of the sensor  100  according to the configuration of magnets  105 A- 105 B. The core or fluid samples to be analyzed are generally disposed or placed in the sweet spots  123  for analysis. In an embodiment, sweet spot  123  may be disposed proximate to the member  120  so that the sweet spot  123  coincides with location of member  120 . In this way, a sample may be placed on the member  120  for analysis. 
         [0038]    Generally, the magnets are disposed proximate to each other so as to create a sweet spot. In other words, “proximate” in this context means the magnets  105 A- 105 B are within sufficiently close vicinity to each other to create a magnetic field and a sweet spot. In an embodiment, the magnets  105 A- 105 B may be arranged in parallel as shown in  FIG. 1A , with magnetic fields aligned vertically but with opposite polarity or in the opposite direction as indicated by the arrows. This alignment generates a strong field above the device, pointing parallel to the direction between the magnets. The field in such an embodiment has maximum homogeneity at a saddle point  123 , depicted by the curved surfaces in this region, creating a sweet spot that serves as the detection volume for the NMR measurement. 
         [0039]    Magnets  105 A,  105 B may have any suitable magnetic strength. In some embodiments, the magnets may have a magnetic strength ranging from about 0.1 Tesla (T) to about 2.0 T, alternatively, from about 0.2 T to about 1.7 T, alternatively from about 0.5 T to about 1.5 T. 
         [0040]    In an embodiment, a magnetic metallic base member  103  may act as the base of the sensor  100 , returning the magnetic flux lines between the magnets  105 A- 105 B to enhance the strength of the field at the sweet spot  123 . Base member  103  may be made of any metallic magnetic material known to those of skill in the art. Examples may include without limitation, iron, steel, nickel, cobalt, gadolinium, any ferromagnetic metal alloys, or combinations thereof. The detection region profile is well-suited for both wellsite (onsite) and laboratory measurements, combining both a large detection volume for high signal-to-noise with high spatial resolution. In contrast, many existing devices exhibit a broad, flat measurement profile which may be ideal for high vertical resolution, but provide poor lateral resolution. 
         [0041]    A spacer  112  may be disposed between magnets  105 A- 105 B. Spacer  112  may serve to provide physical support between magnets  105 A- 105 B to prevent magnets  105 A- 105 B from attaching to each other. In embodiments, spacer  112  may be made of a non-magnetic material. Examples may include without limitation, nylon, polytetrafluoroethylene (PTFE), polyaryletherketone (PAEK), and/or other polymers. Spacer  112  may be configured with any shape suitable to fit in between magnets  105 A- 105 B. 
         [0042]    In an embodiment, antenna  125  may be coupled to sample holding member  120 . As shown in  FIGS. 1B-D , antenna  125  is depicted as having a cuboidal geometry. However, it is contemplated antenna  125  may be any suitable geometry such as without limitation, rectangular, toroidal, circular, semi-circular, hemispherical, etc. In an embodiment, antenna  125  is a radiofrequency (RF) antenna. Generally, antenna  125  may be positioned in between magnets  105 A- 105 B and beneath member  120 . However, it is contemplated that antenna  125  may be disposed in any position on the member  120 . In embodiments, antenna  125  can be a coil or wire that is wound around a holder, which may be fabricated from ferrite or any other suitable material. In other embodiments, antenna  125  may be fabricated using printed circuit boards (PCBs) mounted on the top of the device. In some embodiments, the sensitive region of the coil may not overlap with the sweet spot of the magnet. In other embodiments, the sensitive region of the coil may overlap with the sweet spot of the magnet. In another embodiment, antenna  125  may have a semi-cylindrical geometry such as shown in  FIG. 1E . In an embodiment, sensor  100  may include an internal antenna, to prevent the exposure of the antenna to fluids during measurement. In one embodiment, antenna  125  may have a semi-toroidal geometry as shown in  FIG. 1F . The semi-toroid antenna  125 A may be made of a high-permeability material, such as ferrite or iron powder, shaped to capture magnetic flux from a particular sensing zone that coincides with the sweet spot of the magnets. As used herein, high-permeability refers to a material with a relative magnetic permeability of at least 10. This may serve to guide the magnetic flux from the sweet spot to the inside of the device, concentrating the NMR signal so that it can be detected by a coil wrapped around the torus; similarly, it will guide fields produced by the antenna to the sweet spot for optimal spin excitation. 
         [0043]      FIG. 1G  illustrates another embodiment of sensor  100  in which magnets  105 A- 105 B are angled or tilted. In this embodiment, magnets  105 A- 105 B may be angled at any suitable angle. The magnets may be angled toward one another or away from one another. By considering the field strength, spot size, and relative NMR signal intensity, this embodiment shows that the sensor can be tailored for the design for a particular application. For example, a larger sweet spot size may correlate with the spot being further away from the magnets. By tilting the magnets inward, as in  FIG. 1G , the sweet spot can be brought closer to the device without significant loss in size; compare with the sweet spot shown in  FIG. 1H , which presents the case with the same conditions as  FIG. 1G  except the magnets are not tilted. 
         [0044]      FIG. 2  illustrates another schematic of an embodiment of sensor  100  with additional magnets  105 A- 105 D. In this embodiment, sensor  100  includes four magnets  105 - 15 D arranged in a rectangular array. However, any number of additional magnets may be used and/or configured as desired. 
         [0045]    Referring now to  FIGS. 1D and 8 , a tuning circuit may be used with embodiments of the NMR sensor to achieve the radiofrequency (RF) amplitudes and pulses. Specifically, sensor  100  may include a tuning capacitor  135  disposed within sensor  100 . Alternatively, the tuning circuit can be disposed external to sensor  100 . In addition, wires  131  may be disposed within sensor  100 . In an embodiment, wires  131  may serve as an antenna. In an embodiment, as shown in  FIG. 1D , capacitor  100  and wires  131  may be located in between magnets  105 A- 105 B. In an embodiment, the capacitances can be set by variable capacitors in parallel with fixed capacitors, to give adjustability around a central value. The values for the tuning and matching capacitances can be determined empirically, as they are determined by a combination of factors including the properties of the RF coil, the sample, and any cables used. 
         [0046]    In an embodiment of a method of using sensor, the sensor  100  can be scanned either manually or automatically over the surface of a rock sample to create a high-resolution map of porosity, with voxel size determined by the spatial resolution of the sensor and the spacing of the measurements. These porosity values can be combined with other petrophysical measurements, such as resistivity, permeability, and hardness, taken at the same locations to create an overall petrophysical model of the sample. For example, as a compliment to the current, time-consuming practice of shipping core samples from the wellsite to the laboratory for high-quality NMR testing, a mobile NMR device incorporating sensor  100  can be quickly scanned (manually or automatically) over the entire length of core extracted from a well, in order to get an approximate porosity evaluation. This quick scan may allow for immediate core evaluation, and may be used for prompt verification of petrophysics logging measurements. If numerous cores are taken from various locations within an oil field, then a field-wide “sweet spot” can potentially be determined. 
         [0047]      FIG. 9  illustrates, according to an example of an embodiment of a system  20 , which may perform the operations described in this specification to perform the operations disclosed in this specification. In this example, system  20  is as realized by way of a computer system including NMR sensor  100  connected to a workstation  21  which may be connected to server  30  by way of a network. As mentioned, NMR sensor  100 , although depicted as a block, may be any embodiment of the sensor  100  disclosed herein. Of course, the particular architecture and construction of a computer system useful in connection with this invention can vary widely. For example, system  20  may be realized by a single physical computer, such as a conventional workstation or personal computer, or alternatively by a computer system implemented in a distributed manner over multiple physical computers. Accordingly, the generalized architecture illustrated in  FIG. 9  is provided merely by way of example. 
         [0048]    As shown in  FIG. 9  and as mentioned above, system  20  may include workstation  21 , NMR sensor  100 , and server  30 . In an embodiment, NMR sensor  100  may be coupled to an NMR console  50 , which also may be coupled to a system  20 .  FIG. 10  illustrates an NMR console  50  which may be used in conjunction with embodiments of the sensor. NMR console  50  may include without limitation, a digital controller  51 , a pulse sequence generator  53 , a digital receiver  55 , RF power amplifier  59  and an RF preamplifier  57 . Other components as are known to those of skill in the art may be included in the NMR console. Although one configuration of an NMR console is shown in  FIG. 10 , any NMR consoles known to those of skill in the art may be used. 
         [0049]    Of course, the particular architecture and construction of a computer system or NMR console  50  useful in connection with this invention can vary widely. Workstation  21  includes central processing unit  25 , coupled to system bus. Also coupled to system bus is input/output interface  22 , which refers to those interface resources by way of which peripheral functions P (e.g., keyboard, mouse, display, etc.) interface with the other constituents of workstation  21 . Central processing unit  25  refers to the data processing capability of workstation  21 , and as such may be implemented by one or more CPU cores, co-processing circuitry, and the like. The particular construction and capability of central processing unit  25  is selected according to the application needs of workstation  21 , such needs including, at a minimum, the carrying out of the functions described in this specification, and also including such other functions as may be executed by computer system. In the architecture of allocation system  20  according to this example, system memory  24  is coupled to system bus, and provides memory resources of the desired type useful as data memory for storing input data and the results of processing executed by central processing unit  25 , as well as program memory for storing the computer instructions to be executed by central processing unit  25  in carrying out those functions. Of course, this memory arrangement is only an example, it being understood that system memory  24  may implement such data memory and program memory in separate physical memory resources, or distributed in whole or in part outside of workstation  21 . In addition, as shown in  FIG. 9  parameter inputs  28  may be input via input/output function  22 , and stored in a memory resource accessible to workstation  21 , either locally or via network interface  26 . 
         [0050]    Network interface  26  of workstation  21  is a conventional interface or adapter by way of which workstation  21  accesses network resources on a network. As shown in  FIG. 9  the network resources to which workstation  21  has access via network interface  26  includes server  30 , which resides on a local area network, or a wide-area network such as an intranet, a virtual private network, or over the Internet, and which is accessible to workstation  21  by way of one of those network arrangements and by corresponding wired or wireless (or both) communication facilities. In this embodiment of the invention, server  30  is a computer system, of a conventional architecture similar, in a general sense, to that of workstation  21 , and as such includes one or more central processing units, system buses, and memory resources, network interface functions, and the like. According to this embodiment of the invention, server  30  is coupled to program memory  34 , which is a computer-readable medium that stores executable computer program instructions, according to which the operations described in this specification are carried out by allocation system  30 . In this embodiment of the invention, these computer program instructions are executed by server  30 , for example in the form of a “web-based” application, upon input data communicated from workstation  21 , to create output data and results that are communicated to workstation  21  for display or output by peripherals P in a form useful to the human user of workstation  21 . In addition, library  32  is also available to server  30  (and perhaps workstation  21  over the local area or wide area network), and stores such archival or reference information as may be useful in allocation system  20 . Library  32  may reside on another local area network, or alternatively be accessible via the Internet or some other wide area network. It is contemplated that library  32  may also be accessible to other associated computers in the overall network. 
         [0051]    The particular memory resource or location at which the measurements, library  32 , and program memory  34  physically reside can be implemented in various locations accessible to allocation system  20 . For example, these data and program instructions may be stored in local memory resources within workstation  21 , within server  30 , or in network-accessible memory resources to these functions. In addition, each of these data and program memory resources can itself be distributed among multiple locations. It is contemplated that those skilled in the art will be readily able to implement the storage and retrieval of the applicable measurements, models, and other information useful in connection with this embodiment of the invention, in a suitable manner for each particular application. 
         [0052]    According to this embodiment, by way of example, system memory  24  and program memory  34  store computer instructions executable by central processing unit  25  and server  30 , respectively, to carry out the disclosed operations described in this specification. These computer instructions may be in the form of one or more executable programs, or in the form of source code or higher-level code from which one or more executable programs are derived, assembled, interpreted or compiled. Any one of a number of computer languages or protocols may be used, depending on the manner in which the desired operations are to be carried out. For example, these computer instructions may be written in a conventional high level language, either as a conventional linear computer program or arranged for execution in an object-oriented manner. These instructions may also be embedded within a higher-level application. Such computer-executable instructions may include programs, routines, objects, components, data structures, and computer software technologies that can be used to perform particular tasks and process abstract data types. It will be appreciated that the scope and underlying principles of the disclosed methods are not limited to any particular computer software technology. For example, an executable web-based application can reside at program memory  34 , accessible to server  30  and client computer systems such as workstation  21 , receive inputs from the client system in the form of a spreadsheet, execute algorithms modules at a web server, and provide output to the client system in some convenient display or printed form. It is contemplated that those skilled in the art having reference to this description will be readily able to realize, without undue experimentation, this embodiment of the invention in a suitable manner for the desired installations. Alternatively, these computer-executable software instructions may be resident elsewhere on the local area network or wide area network, or downloadable from higher-level servers or locations, by way of encoded information on an electromagnetic carrier signal via some network interface or input/output device. The computer-executable software instructions may have originally been stored on a removable or other non-volatile computer-readable storage medium (e.g., a DVD disk, flash memory, or the like), or downloadable as encoded information on an electromagnetic carrier signal, in the form of a software package from which the computer-executable software instructions were installed by allocation system  20  in the conventional manner for software installation. 
       Example 
     Experimental 
       [0053]    Experiments were performed using a Tecmag NMR console (either Apollo or LapNMR) and a Tomco RF amplifier. The consoles were controlled using Tecmag&#39;s TNMR software. For the simple CPMG sequence, a wait time of 200 ms was used for samples with relaxed water (water doped with MnCl2), and 1 s was used for samples with standard brine. For the 2D NMR demonstrations, we used an in-house pulse sequence for T 1  T 2  measurements, and an in-house pulse sequence for DT 2  measurements. Pulse lengths of 3.0 μs and 4.5 μs for the 90° and 180° pulses were used, respectively (note that in the presence of a strong magnetic field gradient, the optimal tip angle for the refocusing pulse is in fact less than 180°). The circuit was tuned with a custom built tuning box based on the circuit drawn in  FIG. 8 , with two variable capacitors for tuning (grounded to the chassis) and matching (kept floating). 
         [0054]    The prototype design was optimized using a numerical computer model of the field produced by the magnets in a given configuration, with examples shown in  FIGS. 1G-1H . In the case of the prototype, the magnets  105 A and  105 B are separated by 1.2 inches. The prototype model did not include the effects of the steel base, so the calculated field of 0.21 T (2100 G) at the saddle point is slightly lower than the measured field of 0.23 T given by the assembled device. The saddle point had a lateral size of several mm, setting the resolution of the sensor (which may also be referred to as the Scanning Porosity Tool (SPOT) for purpose of this specification), but again this does not consider the effects of the steel base. Modeling with a multiphysics package may be performed for a more accurate calculation of the saddle point size and shape. By considering the field strength, spot size, and relative NMR signal intensity, the sensor may be tailored for a particular application. For example, a larger spot size generally correlates with the spot being further away from the magnets, and thus, the weaker field strength; by tilting the magnets inward, as in  FIG. 1G , the sweet spot may be brought closer to the device without significant loss in size. 
         [0055]    Results 
         [0056]    A prototype of the sensor was assembled with magnet separation of 1.0 inches, the “SPOT-B”. The measured magnetic field profile of the device at and near the detection region (right above the surface of the device) is shown in  FIG. 3 . As expected, the magnetic field exhibits a saddle point in between the magnets that forms the sweet spot; the field has a local minimum at this point, but it is also more homogeneous than in the regions with maximal field. The strength of the field at the saddle point was measured to be 0.27 T, corresponding to a proton resonance frequency of 11.5 MHz. 
         [0057]    Another prototype was assembled with a magnet separation of 1.2 inches (the “SPOT-C”), and was used to successfully demonstrate proton NMR detection. The signal was optimized at approximately 9.7 MHz, higher than expected from the computer simulation and slightly lower than expected from the field measurement (0.23 T, corresponding to 9.8 MHz). The prototype left space between the magnets and the detection region; other embodiments of the sensor may minimize this space to further increase the detection frequency, and thus potentially enhance the NMR signal. Data from a simple Carr-Purcell-Meiboom-Gill (CPMG) echo sequence taken with water (“relaxed” water, which has been doped with MnCl 2  salt to have a short spin relaxation time) are shown in  FIGS. 4A-4B . Here the spacing between echoes (TE, or “echo spacing”) was 60 μs, the wait time between CPMG sequences was 200 ms, and there were 256 scans. For comparison, for any logging tool TE is at least several hundred μs, and for laboratory equipment it tends to also be at least 200 μs (for example, on many 2 MHz NMR core analyzers). Shorter TE enables measurement of shorter relaxation time components, such as very heavy oils and fluids in very small pores (e.g., in shales). Based on inversion of the data, sensitivity is estimated to be approximately 1.5 pu after measuring for 1 minute.  FIGS. 5A-5B  show a comparison of data taken of a bulk water calibration (defined as 100 pu; a different amount of doping results in a longer relaxation time than in  FIGS. 4A-4B ) and of a carbonate core plug, yielding a porosity measurement of 60 pu for the plug, in good agreement with a standard laboratory 2 MHz NMR measurement of 57.8 pu. Here the echo spacing was 60 μs, the wait time was 1 s, and there were 64 scans. 
         [0058]    The “SPOT-C” sensor prototype was also applied to two-dimensional NMR. In  FIG. 6 , we show T 1 T 2  spectra recorded for three bulk fluid samples: relaxed water (the same sample as used for the data in  FIG. 5A ) with T 1 =19.8 ms and T 2 =10.5 ms, the latter in good agreement with the value T 2 =7.5 ms as measured by CPMG; mineral oil with T 1 =84.5 ms and T 2 =48.7 ms; and deionized water with T 1 =1885 ms and T 2 =165.2 ms.  FIG. 7  also shows diffusion-T 2  spectra measured with the SPOT-C using the same three fluid samples, demonstrating the usefulness of this method for discriminating between water and oil; both water samples lie along the horizontal line, which represents the expected diffusion coefficient of water, while the mineral oil sample lies along the diagonal solid line that represents the typical trend for oil. Note that the diffusion measurements better account for the effect of the strong magnetic field gradient on T 2  relaxation time, so the measured value of T 2  for the deionized water sample (1.3 s) is significantly greater than in the T 1 T 2  measurement. 
         [0059]    While the embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. 
         [0060]    The discussion of a reference is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.