System and method for measuring borehole geometry while drilling

A method and system for determining a geometry of a borehole includes forming an nuclear magnetic resonance (NMR) caliper with a plurality of coils and coupling the NMR caliper to a borehole assembly. The NMR caliper may be calibrated for porosity and the T2 of the drilling mud, prior to drilling, at the surface. After drilling commences, scans of the borehole may be conducted with each coil of the NMR caliper. Each scan may include propagating RF energy across a range of frequencies with each coil in order to excite a NMR signal at varying depths. Borehole wall distances from the NMR caliper may be determined by reviewing a plurality of T2 distributions from CPMG measurements derived from the scans. In some embodiments, borehole wall distances from the NMR caliper may be determined by reviewing porosity values derived from the scans.

BACKGROUND

Several conventional logging while drilling (“LWD”) calipers currently exist. Of these, the ultrasonic calipers generally offer a good and direct measurement. This caliper may offer precision azimuthal hole shape definition in suitable environments and is generally not restricted to specific mud types.

For robust ultrasonic caliper measurements, it is desirable that acoustic properties of the mud are known or can be derived, and that there is sufficient contrast in acoustic impedance between the mud and formation wall. Unfortunately, these boundary conditions associated with mud sometimes cannot be calculated.

Meanwhile, other conventional azimuthal LWD calipers have a limited depth of investigation range and are susceptible to mud types, and in particular, high barite muds. In other caliper options, density and neutron based measurements can be used to derive non-directional caliper information. These LWD calipers may have the advantage that they are feasible in sliding mode, but as with most neutron log measurements, the neutron caliper is sensitive to mud properties and eccentering. The azimuthal information from the density caliper cannot be obtained when the tool is sliding as there is generally one sensor. The term sliding refers to non-rotation of the bottomhole assembly, such as occurs when drilling with a mud motor, tripping into a well, or tripping out of a well.

Other calipers include propagation resistivity tools. Such resistivity tools used as calipers may offer good quality caliper information in water based muds and can be derived from conventionally acquired data as a byproduct of an inversion. However, these resistivity tools as well as the other conventional calipers mentioned above are limited in that they cannot provide consistent and dependable high quality caliper measurements across various types of conditions, including different mud types and during washout conditions while providing measurements at different depth ranges.

SUMMARY

A method and system for determining a geometry of a borehole includes forming an nuclear magnetic resonance (NMR) caliper with a plurality of coils and coupling the NMR caliper to a borehole assembly. After drilling commences, NMR scans of the borehole may be conducted with each coil.

DETAILED DESCRIPTION

Referring initially toFIG. 1A, this figure is a diagram of a system102for measuring borehole geometry while drilling. The system102includes a controller106, a nuclear magnetic resonance (“NMR”) caliper processing module101, and an NMR caliper111. Further details of the NMR caliper111will be described below in connection withFIGS. 1C-1E.

The system102also includes a drilling system104which has a logging and control module95. The controller106further comprises a display147for conveying alerts110A and status information115A that are produced by an alerts module110B and a status module115B. The controller106may communicate with the drilling system104via a communications network142.

The controller106and the drilling system104may be coupled to the communications network142via communication links103. Many of the system elements illustrated inFIG. 1Aare coupled via communications links103to the communications network142.

The links103illustrated inFIG. 1Amay comprise wired or wireless couplings or links. Wireless links include, but are not limited to, radio-frequency (“RF”) links, infrared links, acoustic links, and other wireless mediums. The communications network142may comprise a wide area network (“WAN”), a local area network (“LAN”), the Internet, a Public Switched Telephony Network (“PSTN”), a paging network, or a combination thereof. The communications network142may be established by broadcast RF transceiver towers (not illustrated). However, one of ordinary skill in the art recognizes that other types of communication devices besides broadcast RF transceiver towers are included within the scope of this disclosure for establishing the communications network142.

The drilling system104and controller106of the system102may have RF antennas so that each element may establish wireless communication links103with the communications network142via RF transceiver towers (not illustrated). In some embodiments, the controller106and drilling system104of the system102may be directly coupled to the communications network142with a wired connection. The controller106in some instances may communicate directly with the drilling system104as indicated by dashed line99or the controller106may communicate indirectly with the drilling system104using the communications network142.

NMR caliper processing module101may comprise software or hardware (or both). The NMR caliper processing module101may generate the alerts110A relating to borehole shape that may be rendered on the display147. The alerts110A may be visual in nature but they may also comprise audible alerts as understood by one of ordinary skill in the art.

The display147may comprise a computer screen or other visual device. The display147may be part of a separate stand-alone portable computing device that is coupled to the logging and control module95of the drilling system104. The logging and control module95may comprise hardware or software (or both) for direct control of a borehole assembly100as understood by one of ordinary skill in the art.

FIG. 1Billustrates a wellsite drilling system104that forms part of the system102illustrated inFIG. 1A. The wellsite can be onshore or offshore. In this system104, a borehole11is formed in subsurface formations by rotary drilling in a manner that is known to one of ordinary skill in the art. Embodiments of the system104can also use directional drilling, as will be described hereinafter. The drilling system104comprises the logging and control module95as discussed above in connection withFIG. 1A.

A drill string12is suspended within the borehole11and has a bottom hole assembly (“BHA”)100which includes a drill bit105at its lower end. The surface system includes platform and derrick assembly10positioned over the borehole11, the assembly10including a rotary table16, kelly17, hook18and rotary swivel19. The drill string12is rotated by the rotary table16, energized by mechanisms not shown, which engages the kelly17at the upper end of the drill string. The drill string12is suspended from a hook18, attached to a traveling block (also not shown), through the kelly17and a rotary swivel19which permits rotation of the drill string12relative to the hook18. As is known to one of ordinary skill in the art, a top drive system could be used instead of the kelly17and rotary table16to rotate the drill string12from the surface. The drill string12may be assembled from a plurality of segments125of pipe and/or collars threadedly joined end to end.

In the embodiment ofFIG. 1B, the surface system further includes drilling fluid or mud26stored in a pit27formed at the well site. A pump29delivers the drilling fluid26to the interior of the drill string12via a port in the swivel19, causing the drilling fluid to flow downwardly through the drill string12as indicated by the directional arrow8. The drilling fluid exits the drill string12via ports in the drill bit105, and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows9. In this system as understood by one of ordinary skill in the art, the drilling fluid26lubricates the drill bit105and carries formation cuttings up to the surface as it is returned to the pit27for cleaning and recirculation.

The bottom hole assembly100of the illustrated embodiment may include a logging-while-drilling (LWD) module120, a measuring-while-drilling (MWD) module130, a roto-steerable system and motor150, and drill bit105.

The LWD module120is housed in a special type of drill collar, as is known to one of ordinary skill in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD120and/or MWD module130can be employed, e.g. as represented at120A and120B. (References, throughout, to a module at the position of120A may include a module at the position of120B as well.) The LWD module120includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the embodiment ofFIG. 1B, the first LWD module120A includes a directional resistivity measuring device. The second LWD module120B may include an NMR caliper111as will be described below. While the position of the NMR111caliper has been illustrated in the second LWD module120B, the NMR caliper may easily be positioned in the first LWD module120A as desired by one of ordinary skill in the art. The spatial arrangement and sequence of the LWD modules120relative to other parts of the borehole assembly (“BHA”)100may be interchanged as recognized by one of ordinary skill in the art.

The MWD module130is also housed in a special type of drill collar, as is known to one of ordinary skill in the art, and can contain one or more devices for measuring characteristics of the drill string12and drill bit105. The MWD module130may further includes an apparatus (not shown) for generating electrical power to the downhole system100.

This apparatus may include a mud turbine generator powered by the flow of the drilling fluid26, it being understood by one of ordinary skill in the art that other power and/or battery systems may be employed. In the embodiment, the MWD module130includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device.

The foregoing examples of wireline and drill string conveyance of a well logging instrument are not to be construed as a limitation on the types of conveyance that may be used for the well logging instrument. Any other conveyance known to one of ordinary skill in the art may be used, including without limitation, slickline (solid wire cable), coiled tubing, well tractor and production tubing.

FIG. 1Cillustrates a nuclear magnetic resonance (“NMR”) caliper111and its associated RF pulses188generated within a borehole. As understood by one of ordinary skill in the art, the NMR caliper can be an NMR tool or device, and may comprise a plurality of coils107(not illustrated in this Figure but seeFIGS. 1D & 1E) which produce the RF pulses188. The range of frequencies that may be generated by the NMR caliper111may range between about 100 kHz to about 2 MHz. The NMR signal created by the appropriate application of the RF pulses188may be used by the NMR caliper111to determine the distance X and distance Y between the NMR caliper111and the borehole wall11.

As understood by one of ordinary skill in the art, the frequency is dependent on the static magnetic field provided by the tools magnets. For a gradient field, the frequency decreases with increasing distance from the tool face. Thus, the lower frequency ranges expand an investigation range to longer distances while the higher frequency ranges contract or shorten the investigation range relative to a surface or face of the NMR caliper111. For example, the frequency of about2MHz may provide an investigation of approximately ½ inch (approx. 1.27 cm) relative to a face of the caliper111while the frequency of about 100 kHz may extend the investigation range to between about 4 to 5 inches (approx. 10.16 cm to approx. 12.70 cm) relative to the face of the caliper111. Other frequencies expanding or contracting the investigation range of the NMR caliper111are within the scope of this disclosure. In some embodiments, the depth of investigation can be changed (whether from shallow to deep or vice-versa) by dynamically changing the static magnetic field while keeping the frequency constant.

One advantage of the NMR caliper111is that it can calibrate its measurements against mud189as illustrated inFIG. 1C. That is, mud189at the surface or near the surface of the well such as near the platform and derrick assembly10as illustrated inFIG. 1Bmay be measured and then accounted for as the bottom hole assembly100penetrates through the earth. It has been found that drilling mud as measured on surface189has similar characteristics of the drilling mud used in the borehole.

FIG. 1Dillustrates a side view of coils107for a nuclear magnetic resonance (“NMR”) caliper111. According to this embodiment and the embodiment illustrated inFIG. 1E, the number of separate coils107is four such that each coil covers a sector shaped region (relative to theoretical geometric rays that would originate from a geometric center of the BHA100) around the cylindrically shaped BHA100. The size of each sector is approximately ninety degrees. However, other NMR calipers111having fewer or a greater number of coils107are included within the scope of this disclosure as understood by one of ordinary skill in the art. Further the NMR caliper111is not limited to partial cylindrical shapes. Other geometric shapes, especially when more than four coils107are deployed, are within the scope of this disclosure.

FIG. 1Eis an elevational view of the four coils107illustrated inFIG. 1D. The four coils107A-107B each cover a sector shaped region relative to the cylindrical shaped BHA100to which they are attached. The coils107may be produced from any suitable metal, such as copper. An appropriate number of windings for each coil107may be made as understood by one of ordinary skill in the art. The coils may be controlled by the NMR control module101. The number of windings for each coil depends on the size of the collar and the requisite inductance. As may be recognized by one of ordinary skill in the art having benefit of the present disclosure, an appropriate number of windings can be calculated and/or determined based on the size of the collar and the number of coils used in the array. In some embodiments, the number ranges between one and ten windings.

The coils107may be designed and operated such NMR processing/control module101produces sweeps across the frequency range of about 2 MHz to about 100 kHz for each coil and at the same time. That is, each coil will operate at the same frequency for a given instant of time while the sweep across the disclosed frequency range is made by NMR processing module101. In this way, each coil107is measuring about the same distance from its surface at a given instant in time. In the embodiment ofFIG. 1E, the coils107overlap one another along their edges. The coils are designed to overlap each other so as to cancel the field created by the wire in the long axis of the coils. By having opposing directions of current flow in adjacent coils, we can reduce this unwanted EM field contribution.

FIG. 1Fis a diagram of some computer based elements in the controller106for controlling a nuclear magnetic resonance (“NMR”) caliper111of the wellsite drilling system104ofFIG. 1A. The operating environment for the controller106may include a general-purpose computing device in the form of a conventional computer as understood by one of ordinary skill in the art.

Generally, the computer forming the controller106includes a central processing unit121, a system memory122, and a system bus123that couples various system components including the system memory122to the processing unit121.

The system bus123may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The system memory includes a read-only memory (“ROM”)124and a random access memory (“RAM”)127. A basic input/output system (“BIOS”)126, containing the basic routines that help to transfer information between elements within computer, such as during start-up, is stored in ROM124.

The computer106can include a hard disk drive127A for reading from and writing to a hard disk, not shown, a USB port128for reading from or writing to a removable USB drive129, and an optical disk drive130for reading from or writing to a removable optical disk131such as a CD-ROM, a DVD, or other optical media. Hard disk drive127A, USB drive129, and optical disk drive130are connected to system bus123by a hard disk drive interface132, a USB drive interface133, and an optical disk drive interface134, respectively.

Although the environment described herein employs hard disk127A, removable USB drive129, and removable optical disk131, it should be appreciated by one of ordinary skill in the art that other types of computer readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, Bernoulli cartridges, RAMs, ROMs, and the like, may also be used in the operating environment without departing from the scope of the system102. Such uses of other forms of computer readable media besides the hardware illustrated will be used in internet connected devices such as in a portable computing device, like a laptop computer or a handheld computer.

The drives and their associated computer readable media illustrated inFIG. 1Fprovide nonvolatile storage of computer-executable instructions, data structures, program modules, and other data for computer or client device102A. A number of program modules may be stored on hard disk127, USB drive129, optical disk131, ROM124, or RAM137, including, but not limited to, an NMR processing module101and an alert module110, and other drilling control modules177. Program modules may include, but are not limited to, routines, sub-routines, programs, objects, components, data structures, etc., which perform particular tasks or implement particular abstract data types.

A user may enter commands and information into the computer106A through input devices, such as a keyboard140and a pointing device142. Pointing devices may include a mouse, a trackball, finger input, and/or an electronic pen that can be used in conjunction with an electronic tablet. Other input devices (not shown) may include a joystick, game pad, satellite dish, scanner, or the like. These and other input devices are often connected to processing unit121through a serial port interface146that is coupled to the system bus123, but may be connected by other interfaces, such as a parallel port, game port, a universal serial bus (USB), or the like.

The display147may also be connected to system bus123via an interface, such as a video adapter148. As noted above, the display147can comprise any type of display devices such as a liquid crystal display (LCD), a plasma display, an organic light-emitting diode (OLED) display, and a cathode ray tube (CRT) display.

The camera175may also be connected to system bus123via an interface, such as an adapter170. The camera175may comprise a video camera. The camera175can be a CCD (charge-coupled device) camera or a CMOS (complementary metal-oxide-semiconductor) camera. In addition to the monitor147and camera175, the client device100A, comprising a computer, may include other peripheral output devices (not shown), such as a printer.

The computer may also include a microphone111that is coupled to the system bus123via an audio processor113is understood by one of ordinary skill in the art. A microphone111may be used in combination with the voice recognition module206in order to process audible commands received from an operator.

The computer forming the central controller106A may operate in a networked environment using logical connections to one or more remote computers, such as a web server. A remote computer106B may be another personal computer, a server, a mobile phone, a router, a networked PC, a peer device, or other common network node. While the web server or a remote computer106B may include many of the elements described above relative to the controller106A, a memory storage device127B has been illustrated in thisFIG. 1C. The logical connections depicted inFIG. 1Cinclude a local area network (LAN)142and a wide area network (WAN)142B. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the Internet.

When used in a LAN networking environment, the computer forming the controller106A is often connected to the local area network142A through a network interface or adapter153. When used in a WAN networking environment, the computer106A may include a modem154or other means for establishing communications over WAN142B, such as the Internet. Modem154, which may be internal or external, is connected to system bus123via serial port interface146. In a networked environment, program modules depicted relative to the server102B, or portions thereof, may be stored in the remote memory storage device127A. It will be appreciated that the network connections shown are just examples and other means of establishing a communications link between the computers may be used.

Moreover, those skilled in the art will appreciate that the system102may be implemented in other computer system configurations, including hand-held devices, multiprocessor systems, microprocessor based or programmable consumer electronics, network personal computers, minicomputers, mainframe computers, and the like. The system102may also be practiced in distributed computing environments, where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules may be located in both local and remote memory storage devices.

FIG. 2Ais a diagram illustrating surface relaxation as it relates to nuclear magnetic resonance (“NMR”) measurements. As understood by one of ordinary skill may art, surface relaxation relates to how protons (a single proton is identified by its movement trace205A is illustrated inFIG. 2A) interact with the wall or walls215of a pore210A. It is assumed that the pore210A is filled with a fluid. The pore210may be formed by within a rock. As the proton hits the walls215of the pore210A, it loses energy as indicated by the movement trace205A caused by an RF pulse188.

The time measured while the proton is losing energy is often referred to as relaxation time as understood by one of ordinary skill in the art. Relaxation time may include T1time and T2time.

T1relaxation time, as understood by one of ordinary skill in the art, refers to the spin-lattice relaxation and the decay constant for the recovery of the z component (longitudinal) of the nuclear spin magnetization vector M parallel to the external magnetic field, Bo. Once the nuclear spins in a population of atoms for a pore210A is relaxed, the population can be probed again with an RF signal, since the population has returned to an initial, equilibrium (mixed) state.

T2relaxation time refers to the spin-spin relaxation time and is the decay constant for the component of the magnetization perpendicular to the static magnetic field, Bo. Because of the difference in the actual relaxation mechanisms involved (for example, inter-molecular versus intra-molecular magnetic dipole-dipole interactions) T1time is, in many cases, longer than T2time. The T2relaxation time in a pore system with one fluid type, is a sum of relaxation times that can be correlated with the pore size distribution from the surface relaxation.

T1and T2times are calculated from the surface relaxation illustrated inFIG. 2Aas well as from bulk relaxation as illustrated inFIG. 2B. Specifically,FIG. 2Bis a diagram illustrating bulk relaxation as it relates to nuclear magnetic resonance (“NMR”) measurements. InFIG. 2B, fluid is contained within walls215of a much larger pore210B. According to bulk relaxation, protons within the fluid are being measured and do not interact with the walls215of the pore210B.

In addition to bulk relaxation in surface relaxation, T2relaxation also involves diffusion-based relaxation. If the nuclear spins are moving within a volume in which the magnetic field is changing, i.e a gradient, then the precessional frequency is changing during the NMR sequence. When this happens, the NMR signal can be lost or reduced.FIG. 2Cis a diagram illustrating diffusion relaxation as it relates to nuclear magnetic resonance (“NMR”) measurements.

However, with diffusion relaxation, many gradients may exist and therefore, this variable is, in many cases, not calculated in NMR measurements. Equation250ofFIG. 2Cillustrates that T1time is a function of both bulk relaxation (“B”) and surface relaxation (“S”). Specifically, T1may be calculated from the sum of the bulk and surface relaxations. Meanwhile, as indicated by equation255, T2time is a function of bulk relaxation (“B”), surface relaxation (“S”), and diffusion relaxation (“D”), although the diffusion relaxation parameter may be ignored in fields in which there is a low magnetic field gradient.

FIG. 3is a diagram illustrating nuclear magnetic resonance (“NMR”) Carr-Purcell-Meiboom-Gill (CPMG) measurements. NMR CPMG measurements refer to pulse sequences188that are applied after maximum T1time buildup which occurs at point301of graph300. Graph300plots maximum amplitude/intensity (y-axis) of an echo intensity (Mo) against time on the x-axis. The time segment taken to reach the maximum T1time buildup may be referred to as polarization time. After polarization time and as CPMG pulse sequences188are applied, T2decay is measured. Specifically, CPMG pulse sequences188are widely used to measure spin-spin relaxation time T2.

A 180° pulse is applied around the rotating imaginary axis that time τ (tau) to refocus the spins which leads to the formation of the “Hahn” echo time 2τ (tau). Then further applications of 180° pulses188B,188C, etc. times 3τ (tau), 5τ (tau), etc. are generated. The NMR echoes occur at the odd tau times (e.g., 3×, 5× etc). When spins are not diffusing, CPMG measurements completely compensate the dephasing of spins due to the local magnetic field inhomogeneities.

Equation350ofFIG. 3defines how each echo amplitude305is related to its corresponding T2relaxation time. Equation350corresponds to the decay as demonstrated by the decreasing echo amplitude305ofFIG. 3. The decreasing echo amplitudes305ofFIG. 3correspond to the loss of energy described above in connection withFIGS. 2A and 2Bwhich illustrate bulk and surface relaxation.

From the echo amplitude data illustrated inFIG. 3, the T2values may be calculated by taking the integral of the areas of each echo305illustrated inFIG. 3. This integral of each area defined by an echo305is plotted inFIG. 4A.FIG. 3corresponds to blocks1005-1020of submethod or routine930as will be discussed below in connection withFIGS. 9-10.

FIG. 4Ais graph400A of time (x-axis) plotted against amplitude (y-axis) for the measurements calculated inFIG. 3at one frequency of RF pulses188. Specifically, each point405represents the value for the integral found for each echo305plotted inFIG. 3. An exponential (best fit) line410is calculated and runs through each of the points405. This can be calculated in a variety of ways and is well known to those skilled in the arts of signal processing. As equation415reflects, the best fit line410is equal to the sum of the exponential values represented by each point405. Meanwhile, the amplitude forming the y-axis may represent porosity and can be calibrated back to the surface measurement of 100% water. The data plotted inFIG. 4Amay be transformed using a Laplace transform, resulting in a T2distribution.

Specifically,FIG. 4Bis a T2distribution (T2time plotted against amplitude) derived from the data inFIG. 4Ain which the data ofFIG. 4Ais transformed using a Laplace transform. A Laplace transform is often interpreted, by one of ordinary skill in art, as a transformation from the time-domain, in which inputs and outputs are functions of time, to the frequency-domain, where the same inputs and outputs are functions of complex angular frequency, in radians per unit time.FIG. 4Acorresponds with blocks1025and1030of submethod930as discussed below in connection withFIG. 10.

FIG. 4Breflects one embodiment of a T2distribution at one frequency. The x-axis comprises a unit of time such as seconds. Shorter T2amplitudes435A (smaller amplitudes or intensities) are on the left side of the graph400B correspond with lower T2times while taller T2amplitudes435B on the right side of the graph400B correspond with longer T2times in this embodiment. The size of the peaks represents the amplitude of the signal of each T2component. T2times may range between about 0.3 milliseconds (shorter amplitudes) to about 5000 milliseconds (taller amplitudes). Short T2times correspond to formations and measure between about 3.0 ms and 33.0 ms while fluids are measured at above about 33 ms. The T2distribution is plotted over a range of time on the x-axis. When there are largely or completely fluids, the range is generally from 0.3-3000 ms. For a gas, the range can go up to about 5000 ms. Free fluid is anything with a T2above 33 ms (in sandstones) and 100 ms in carbonates. Water can be ˜1000 ms and light oil/gas up to 5000 ms. As indicated in graph400B, the sum of the amplitudes is equal to porosity multiplied against the hydrogen index (HI).

As noted above,FIGS. 4A-4Bare calculated from the NMR caliper111operating at one particular frequency. Multiple calculations meaning that multiple graphs400are calculated for each frequency over a range or sweep of frequencies. A range of frequencies is used to measure various distances from the NMR caliper111as illustrated inFIG. 1C. A particular frequency may represent a single distance measurement relative to a face of the NMR caliper111.

FIG. 5is a three-dimensional graph500of several T2distributions plotted against various depths from an NMR caliper tool face represented by the z-axis. Each curve435of graph of500comprises a T2distribution, similar to graph400B ofFIG. 4B. That is, if one were to stack a plurality of graphs400B ofFIG. 4Bin sequence, then such stacking of these graphs400B would generate a three-dimensional plot similar to graph500ofFIG. 5. However, each curve435of graph500has been truncated to illustrate the highest elements of each T2distribution.

As noted above, each T2distribution represented by each curve435inFIG. 5corresponds to one frequency of operation for the NMR caliper111. For the T2distributions435closest to the X-and Y-axes, these were generated with higher frequencies while T2distributions farthest from the X- and Y-axes were generated with lower frequencies of the RF pulses188.

As the T2distributions are plotted along the z-axis, transitions between relatively short T2distributions to relatively long T2distributions will indicate the presence of the borehole wall11as indicated by line505. Specifically, the transition between the T2distributions where relatively short distributions give way to relatively tall distributions occurs between point “B” and point “W” forming the segment BW as illustrated inFIG. 5. Therefore,FIG. 5illustrates the depth or distance of the borehole wall11from a face from one of the coils107such as illustrated inFIG. 1E. The calculations forFIG. 5are completed by the NMR processing module101for each of the four coils107as illustrated inFIG. 1E.FIG. 5corresponds to submethod1030A ofFIG. 11which will be described in further detail below.

FIG. 6is a graph600of caliper data for an NMR caliper111having four coils107represented by the four segments of the graph600. Specifically, the center of the Cartesian coordinate system for graph600may correspond to the center of an NMR caliper11. A distance of the borehole wall11relative to each of the four coils107calculated by the NMR processing module101may be plotted in this graph600. So for each three-dimensional plot500of T2distributions as illustrated inFIG. 5, the borehole wall distance or depth determined from the plot500may be represented as a point in the graph600ofFIG. 6.

If graph600represented data from a horizontal well, then the point BW4would represent a top portion of the well while the point BW2would represent a bottom portion of the horizontal well. Points BW1and BW3would represent sidewalls of the horizontal well. For a vertical well, point BW4would represent the true north coordinate and point BW2would represent the true south coordinate as understood by one of ordinary skill in the art.

One of ordinary skill in the art recognizes that additional coils107increase the number of points BW that are used to determine the profile or geometry of the borehole wall11. As more coils107are used, then the geometry of the borehole wall11may become more accurate.FIG. 6corresponds with block1120ofFIG. 11and with block1210ofFIG. 12as will be described in more detail below.

FIG. 7is a graph of a depth or distance from the face of the NMR caliper111plotted against porosity for one coil107.FIG. 7illustrates an approach to using the T2distribution data determined inFIG. 4B. As noted inFIG. 4B, the sum of the amplitudes illustrated inFIG. 4Bequals the measured porosity times HI. Porosity has units of measurement equal to the percentage of pore space in a unit volume of rock. It is abbreviated to p.u. and lies between about 0 and about 100. A fluid has a p.u. value of about 100% while rock formations will have percentages of about 40% p.u. and lower. Each T2distribution illustrated inFIG. 4Brepresents pulses for one particular frequency at a particular distance from the face of a coil107of the NMR caliper111.

So each point702of graph700may represent the porosity calculated from a given T2distribution, such as the T2distribution illustrated inFIG. 4B. As illustrated inFIG. 7, as the porosity values begin to drop, this indicates the possibility of the detection of a rock formation. Therefore, between point702C and point702D, a transition occurs which is an indicator of the borehole wall represented by line BW inFIG. 7. Once line BW is determined, its x-axis value directly corresponds with the depth or distance from the face of the particular coil107which produced the various T2distributions plotted in this graph700.

Like the data point determined from graph500, the x-axis value of graph700may be plotted on a Cartesian coordinate system similar to graph600ofFIG. 6which represents the four different coils of the NMR caliper111ofFIG. 1E.FIG. 7corresponds with block1205ofFIG. 12which will be described in further detail below.

FIG. 8Ais an example of a display800A showing a calculated shape of a borehole11A from an NMR caliper111. This display800A may be generated by the NMR processing module(s)101described above which operate the NMR caliper111. This display800A may be projected on the display device147described above in connection withFIG. 1A.

FIG. 8Bis an example of a display800B showing a calculated shape of a borehole11B from an NMR caliper111. This display800B may be generated by the NMR processing module(s)101described above which operate the NMR caliper111. This display800B may be projected on the display device147described above in connection withFIG. 1A. The shape of the second borehole11B ofFIG. 8Bis different relative to the borehole11B illustrated inFIG. 8Abecause the second borehole11B is in closer proximity to the drill bit compared to the borehole11A ofFIG. 8A.

The second borehole11B has more of a “round” shape while the first borehole11A has more of an “oval” shape. Changes in borehole shape are often used to explain geomechanical aspects of the formations and is well understood by those skilled in the art.

FIG. 9is a flowchart illustrating a method900for calculating borehole geometry with an NMR caliper111. Block905is the first block of the method900for calculating borehole geometry with an NMR caliper111. In block905, a plurality of coils107is formed in order to segment or divide the 360° field of view around the NMR caliper111. Block905generally corresponds withFIGS. 1D-1Ewhich illustrate examples of the coils107used in the NMR caliper111. As discussed above, one of ordinary skill in the art recognizes that any number of coils107may be employed and is within the scope of this disclosure.

Next, in block910, the NMR caliper111is coupled to the borehole assembly100as illustrated inFIG. 1B. As noted previously, the relative position of the NMR caliper111along the borehole assembly100may be determined by one of ordinary skill in the art and may be adjusted depending upon the other components and systems that work together to form the borehole assembly100as illustrated inFIG. 1B.

In block915, preliminary calibrations of one or more materials found at the borehole site (near reference numeral12as illustrated inFIG. 1B) may be conducted with the NMR processing module(s)101. Specifically, dirt and/or mud found at the borehole site prior to drilling may be analyzed with the NMR processing modules101. The NMR caliper111may then be calibrated based on the scanning of dirt and/or mud found that the borehole site prior to drilling.

By calibrating the NMR caliper111with drilling mud found at the surface of the borehole site prior to drilling, the NMR caliper111will be able to use this information to help assign T2cutoffs for mud and formation T2times

Next, and block920, the borehole assembly100may be activated and drilling may begin at the borehole site. Next, in block125, the NMR processing module(s)101may conduct scans of the borehole over a range of RF frequencies for each coil107. In this block920, the NMR caliper111generates the series of RF pulses189that are used to generate a NMR signal as illustrated inFIG. 1C. As the frequencies decrease, the RF pulses189extend further out relative to the face of the NMR caliper111, based on the Lamour frequency as determined by the static magnetic field. Meanwhile, for very small radiofrequencies, the RF pulses189extend very close to the face of the caliper111.

Next, in routine or submethod930, the NMR processing module(s)101calculate the NMR CPMG measurements at each frequency for each coil107. Further details of submethod930will be described below in connection withFIGS. 10-12. During this routine or submethod930, each of the coils107generates RF pulses188as illustrated inFIG. 1Cand are at the same frequency for each distance pulsed/scanned. The NMR caliper111is operated such that it scans or creates RF pulses over a range of frequencies to cover a range of distances relative to the face of the NMR caliper111.

In block935, the NMR processing module(s)101identify the profile of the borehole from the NMR CPMG measurements calculated in block930. This block935may correspond with the caliper data represented by the four segments of the graph600illustrated inFIG. 6.

Next, and block940, the NMR processing module(s)101may store the borehole profile data in memory, such as in RAM124, ROM137, or other storage devices. In block945, the NMR processing module(s)101may generate a display800A,800B for displaying on a display device147such as illustrated inFIGS. 8A-8B. The method or process900then returns to block925for conducting additional scans at the next frequency while the borehole assembly100is drilling. The data may also be sent uphole to a surface computer in real-time via a range of communication means known to those skilled in the art. This may include mud telemetry or wired drill pipe.

FIG. 10is a flowchart illustrating a submethod or routine930ofFIG. 9for calculating NMR CPMG measurements as illustrated inFIGS. 3-4. Block1005is the first block of submethod930. In block1005, radiofrequency energy is applied by each coil107at a partial frequency to generate RF pulses188as illustrated inFIG. 1C. The NMR processing module(s)101issue the commands to generate these RF pulses188. Block1005corresponds to time T1of the polarization time as illustrated inFIG. 3. Next, in block1010, the NMR processing module(s)101wait for a predetermined amount of time for the T1time buildup as illustrated inFIG. 3.

Subsequently, in block1015, the NMR processing module(s)101may initialize CPMG pulse trains188as illustrated inFIG. 3. Next, and block1020, the NMR processing module(s)101may measure the echoes305that occur after each180degree RF pulse188as illustrated inFIG. 3. In block1025, the NMR processing module(s)101may integrate each of the echo measurements305and plot them on a graph such as illustrated inFIG. 4A.

In block1030, the NMR processing module(s)101may extrapolate from the data presented inFIG. 4Aand form a best fit curve410among these echo measurements as understood by one of ordinary skill in the art. Next, in submethod or routine1030, the NMR processing module(s)101may generate a two-dimensional caliper graph of a borehole11at a given instant of time such as graph600as illustrated inFIG. 6or a full view of a borehole11may be generated similar to displays800A,800B ofFIG. 8. Further details of submethod1030will be described below in connection withFIGS. 11-12.FIG. 11will illustrate a first version of the submethod1030whileFIG. 12illustrate a second version of submethod1030.

FIG. 11is a flowchart illustrating a first submethod or routine1030A ofFIG. 9for generating a 2-D caliper graph600(seeFIG. 6) outlining dimensions of a borehole11at a given instant of time. Block1105is the first block of submethod1030A. In block1105, the NMR processing module(s)101may conduct a Laplace transform inversion of the echo data illustrated inFIG. 4Ato form a T2distribution as illustrated inFIG. 4Bas understood by one of ordinary skill in the art. The T2distribution ofFIG. 4Bis for a single frequency and for a single coil107.

In block1110, the NMR processing module(s)101may plot T2distributions for each frequency scanned and for each coil107of the NMR caliper111and provide them on a three-dimensional graph as illustrated inFIG. 5. This means that the NMR processing module(s)101produce multiple versions ofFIG. 4Bfor each coil as a scan across RF frequencies progresses and the multiple versions ofFIG. 4Bare projected as geometrical planes along the z-axis as illustrated inFIG. 5.

Next, in block1115, the NMR processing module(s)101may determine the borehole wall values from the T2distributions projected along the z-axis ofFIG. 5. As noted previously, the NMR processing module(s)101may determine the points or segments, such as segment BW ofFIG. 5, which show a transition between T2distributions. The transition between T2distributions defined by shorter T2times to longer T2times, like segment BW ofFIG. 5, indicates the presence of a borehole wall11for a particular coil107. Each coil107of an NMR caliper111generates the T2distributions for a given graph500as illustrated inFIG. 5. This means that four separate graphs500would be generated for an NMR caliper having four coils107.

Next, in block1120, the NMR processing module(s)101may plot the borehole wall values on a two-dimensional caliper graph for each coil107such as illustrated by graph600ofFIG. 6. The first submethod1030A then returns back to block935ofFIG. 9.

FIG. 12is a flowchart illustrating a second sub-method or routine1030B ofFIG. 9for generating a 2-D caliper graph600(seeFIG. 6) outlining dimensions of a borehole11at a given instant of time. Block1205is the first block of this second submethod1030B. In block1205, the NMR processing module(s)101may calculate porosity values against depth for each coil107at each frequency scanned by the NMR caliper111.

Specifically, the NMR processing module(s)101may review the amplitude values calculated for each graph400B ofFIG. 4B. As noted previously, each graph400B is produced by a coil for each frequency. The NMR processing module(s)101may convert the amplitudes ofFIG. 4Binto a single porosity value702that indicates porosity at a specific depth or distance (x or y—seeFIG. 1C) relative to a face of the NMR caliper111. Each porosity value702may be plotted on a graph such as graph700as illustrated inFIG. 7.

Next, in block1210, after the porosity values702across a range of frequencies are determined by the NMR processing module(s)101for a single coil107are plotted, then borehole wall values may be determined from the graph700. A transition between the porosity values, such as defined by the segment BW as illustrated inFIG. 7, indicates the presence of a borehole wall111.

The X-axis of graph700illustrates the distance of the borehole wall relative to the face of a single coil107of the NMR caliper111. This distance may then be projected onto a two-dimensional caliper graph, and specifically, onto a single segment of the four segments illustrated in the graph600ofFIG. 6. This projection of distance from a face of a coil107on to graph600may be repeated for the remaining coils107of the NMR caliper111. The second submethod then returns to block935ofFIG. 9.

In this description, the term “application” may also include files having executable content, such as: object code, scripts, byte code, markup language files, and patches. In addition, an “application” referred to herein, may also include files that are not executable in nature, such as documents that may ought to be opened or other data files that ought to be accessed.

The term “content” may also include files having executable content, such as: object code, scripts, byte code, markup language files, and patches. In addition, “content” referred to herein, may also include files that are not executable in nature, such as documents that may ought to be opened or other data files that ought to be accessed.

In this description, the term “application” may also include files having executable content, such as: object code, scripts, byte code, markup language files, and patches. In addition, an “application” referred to herein, may also include files that are not executable in nature, such as documents that may ought to be opened or other data files that ought to be accessed.

The term “content” may also include files having executable content, such as: object code, scripts, byte code, markup language files, and patches. In addition, “content” referred to herein, may also include files that are not executable in nature, such as documents that may ought to be opened or other data files that ought to be accessed.

Certain steps in the processes or process flows described in this specification naturally precede others for the invention to function as described. However, the invention is not limited to the order of the steps described if such order or sequence does not alter the functionality of the invention. That is, it is recognized that some steps may performed before, after, or parallel (substantially simultaneously with) other steps without departing from the scope and spirit of the disclosure. In some instances, certain steps may be omitted or not performed without departing from the invention. Further, words such as “thereafter”, “then”, “next”, etc. are not intended to limit the order of the steps. These words are simply used to guide the reader through the description of the sample methods described herein.

Additionally, one of ordinary skill in programming is able to write computer code or identify appropriate hardware and/or circuits to implement the disclosed invention without difficulty based on the flow charts and associated description in this specification, for example.

Therefore, disclosure of a particular set of program code instructions or detailed hardware devices is not considered requisite for an adequate understanding of how to make and use the invention. The inventive functionality of the claimed computer implemented processes is explained in more detail in the above description and in conjunction with the Figures which may illustrate various process flows.

Also, any connection is properly termed a computer-readable medium. For example, if the software is transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (“DSL”), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium.

Disk and disc, as used herein, includes compact disc (“CD”), laser disc, optical disc, digital versatile disc (“DVD”), floppy disk and blu-ray disc where disks may reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

Although just a few embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from this invention. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the following claims.

For example, while multiple coils have been described above, the system may be implemented with a single coil as understood by one of ordinary skill in the art. As the NMR caliper tool111rotates, the tool111may generate scans that correspond to the four sectors generated by the coils107A-D as illustrated inFIG. 1Edescribed above. However, using one coil instead of four coils for the NMR caliper tool111may increase scanning time four-fold in view of the reduction of the number of coils as understood by one of ordinary skill in the art.