CEMENT

BACKGROUND

[0001] In drilling wells for oil and gas exploration, a drilling engineer, usually with input from geologists and others, can choose strategic depths at which the hole will need to be cased in order for drilling to reach a desired depth. This decision is often based on subsurface data such as formation pressures, strengths, and makeup.

[0002] A casing is a relatively large diameter pipe that is assembled and inserted into a recently drilled section of a borehole and typically held into place with cement. It is desirable to detect flaws or contamination in the cement in order to reduce the possibility of the casing not being held securely in place.

BRIEF DESCRIPTION OF THE DRAWINGS

[0003] FIG. 1 shows diagram of an embodiment of a wellbore having a casing with attached sensors.

[0004] FIG. 2 shows diagram of an embodiment of a nuclear magnetic resonance (NMR) in- situ sensor design.

[0005] FIG. 3 shows a diagram of an embodiment of an NMR ex-situ sensor design.

[0006] FIG. 4 shows a diagram of an embodiment of a casing having an attached NMR sensor.

[0007] FIG. 5 shows a diagram of another embodiment of a casing having an attached NMR sensor.

[0008] FIG. 6 shows a diagram of another embodiment of a casing having an attached NMR sensor.

[0009] FIG. 7 shows a diagram of another embodiment of a casing having an attached NMR sensor.

[0010] FIG. 8 shows a diagram of an embodiment of a casing having an expanding NMR sensor.

[0011] FIG. 9 shows various plots of Ti and T ₂ spectra of water-based mud, contaminated cement, and uncontaminated cement. [0012] FIG. 10 illustrates an embodiment of a method for creating a system of cement embedded sensors on a casing in a borehole.

DETAILED DESCRIPTION

[0013] The following detailed description refers to the accompanying drawings that show, by way of illustration and not limitation, various embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice these and other embodiments. Other embodiments may be utilized, and structural, logical, and electrical changes may be made to these embodiments. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments. The following detailed description is, therefore, not to be taken in a limiting sense.

[0014] FIG. 1 is a diagram of an embodiment of a wellbore installation to measure, monitor, and or remove fluids in a subterranean formation. The wellbore 108 comprises an elongated cylindrical borehole 109 drilled through a geological formation and lined with a metallic casing 112 that extends substantially continuously in the borehole 109 for a predetermined depth. The casing 112 forms a hollow cylindrical conduit along which production fluid (e.g., a mixture of liquid and vapor) can be conducted to a wellhead 116 at the Earth's surface.

[0015] An annular space between the production casing 112 and a

substantially cylindrical wall of the borehole 109 may be filled with an impervious material 110 (e.g., cement) for at least some portions of the length of the wellbore 108. The cement 110 reduces the inflow of liquids radially into the fluid conduit provided by the wellbore 108 in such backfilled portions as well as holds the casing 112 in place. Other portions of the wellbore 108, along its length, may be specifically configured to be permeable to radial movement of hydrocarbons from the geological formation into the wellbore 108, thus forming a plurality of production zones spaced along the length of the wellbore 108. In some instances, the wellbore 108 does not necessarily comprise production zones that are segregated by such structural components of the wellbore 108, but naturally occurring production zones may nevertheless exist because of variations in geological formations through which the wellbore 108 extends.

[0016] While injecting the cement into the space between the casing 112 and the geological formation, contaminants such as mud or geological fluids can contaminate the cement. Such contaminates can result in flawed and/or structurally deficient cement. Thus, it is desirable to monitor the cement as it is injected and as it cures. The present embodiments use low magnetic field, magnetic resonance (MR) sensors (including nuclear magnetic resonance (NMR) sensors) attached to the casing 112 and embedded in the cement in order to perform this monitoring. The sensor data can then be transmitted to a sensor data receiver 130 at the surface in order to process the data and determine the quality of the cement as it is setting. The sensor can be powered by a battery or battery pack or by an external power source transmitted to the sensor along the outside of the casing via a wired connection or through a fiber optic line.

[0017] NMR sensors can be used as a tool for probing the microenvironments in heterogeneous media, such as cements. The NMR sensors provide the ability to extract information such as porosity, pore size distribution, permeability, viscosity, and chemical composition. The NMR sensor can monitor any correlates to cement quality and/or contamination level. Since the sensors are both stationary and in direct contact with the sample can significantly reduce the complexity of making relatively high signal-to-noise ratio (SNR) measurements as compared to current wireline and measurement while drilling (MWD) NMR tools.

[0018] NMR can occur when the medium is subjected to a static magnetic field, Bo, and to an oscillating magnetic field, Bi. When subjected to the applied static magnetic field, polarization of nuclear magnetic spins of the medium occurs based on a spin number of the medium and a magnetic field strength. Applying an electromagnetic field to the medium in the static magnetic field can perturb the polarization established by the static magnetic field. In optimal measurements, the static magnetic field and the perturbing field are

perpendicular to each other. Collected responses received from the medium related to the total magnetization of nuclear spins in the medium, in response to these applied fields, can be used to investigate properties of the medium and may provide imaging of the medium. It is noted that magnetization is proportional to polarization.

[0019] NMR measurements can be created by the oscillation of excited nuclear magnetic spins in the transverse plane. The transverse plane is the direction perpendicular to the magnetic field. This oscillation eventually dies out and the equilibrium magnetization returns. The return process is referred to as longitudinal relaxation. The time constant, Ti, for nuclei to return to their equilibrium magnetization, M ₀ , is typically referred to as the longitudinal relaxation time or the spin lattice relaxation time. The magnetization dephasing, that is losing coherence along the transverse plane, is given by the time constant T ₂ and is typically referred to as the spin-spin relaxation time. The loss of phase coherence can be caused by several factors including interactions between spins or magnetic gradients.

[0020] The NMR sensors can include any design that can be implemented in an NMR system. For example, in situ and ex situ are two such sensors.

[0021] FIG. 2 illustrates a diagram of an in situ NMR sensor. In such a sensor, the measurements can provide relatively larger SNR as compared to the ex situ sensor since the measurements are made on cement that has perfused into the bore of the magnet. The in situ sensor measurements are made in a homogeneous field that can increase the transceiver coil' s filling factor as well as enable high- resolution chemical spectroscopy to be performed.

[0022] The in situ sensor of FIG. 2 shows a permanent magnet 200 having a coil 201 wrapped around a hollow core of the magnet. The magnetic field 203 is formed through the core and around the magnet.

[0023] Since the in situ sensor includes a hollow core, measurements can be made of the cement sample inside the homogeneous center 205 of the magnetic field. 203.

[0024] FIG. 3 illustrates a diagram of an ex situ NMR sensor. This sensor includes the coils 301, 302 around the exterior of a permanent magnet 300. The magnetic field 303 is centered at the core of the magnet 300 but, not having a hollow core, the NMR measurements are performed in the fringe field of the permanent magnet. The signal comes from a resonant shell at a radius of investigation determined by the field gradient and measurement frequency. [0025] While the SNR of the ex situ sensor can be lower than the in situ, measurements can be made at a greater distance from the tool as compared to an in situ sensor. Also, since the omnipresent field gradient makes it possible to make measurements of the radial profile of the NMR signal, it is relatively simple to make NMR diffusion measurements.

[0026] In either the in situ or the ex situ sensors, the sensor can protrude from the casing to measure a greater amount of the cement. This can be accomplished using one relatively large sensor or a plurality of relatively smaller sensors (e.g., array of sensors).

[0027] One relatively large sensor can provide a simpler design and can have a greater SNR than an array of smaller sensors. However, the array of sensors can provide a greater spatial resolution and enable a design that rejects common- mode noise. Depending on the cost and the field profile, the array of sensors can be located around a single larger magnet or a plurality of smaller, individual- sized magnets.

[0028] FIGs. 4-7 illustrate various embodiments of an array of sensors. These embodiments are for purposes of illustration only as an array of NMR sensors can include one or more NMR sensors and can take any configuration.

Additionally, the NMR sensors can be in situ, ex situ, or some other sensor design.

[0029] The embodiment of FIG. 4 includes a casing 401 located within a geological formation 402. The sensor array 403 includes a single NMR in situ sensor 420 having a plurality of permanent magnets 410-416. The coil (not shown) is located within the sensor 420. The hollow core of each of the permanent magnets 410-416 enables the cement/fluid mixture to enter the core for analysis. The sensor data transmission circuit 490 for reading the sensor data and transmitting the sensor data to the surface can be located within the sensor housing 420, built in a container adjacent to the casing 401, or attached to the sensor 420 in some other location. The signal containing the cement/fluid mixture sensor data can be transmitted directly to the surface wirelessly, through a wireline logging tool, or with a conductor.

[0030] The embodiment of FIG. 5 includes a casing 501 located within a geological formation 502. The sensor array 503 includes a plurality of ex situ sensors 510-514 on a single magnet 520. The plurality of ex situ sensors 510-514 each include the coils. The sensor data transmission circuit 590 for reading the sensor data and transmitting the sensor data to the surface can be located within the magnet housing 520, built in a container adjacent to the casing 501, or attached to the magnet housing 520 in some other location. The signal containing the cement/fluid mixture sensor data can be transmitted directly to the surface wirelessly, through a wireline logging tool, or through a conductor.

[0031] The embodiment of FIG. 6 includes a casing 601 located within a geological formation 602. The sensor array 603 includes a plurality of ex situ sensors 610-614 each on their respective individual magnet 620-624. The support 629 can include the sensor data transmission circuit 690 for reading the sensor data and transmitting the sensor data to the surface. The transmission circuit 690 can also be built into a container adjacent to the casing 601, as shown, or attached to the support 629 in some other location. The signal containing the cement/fluid mixture sensor data can then be transmitted directly to the surface wirelessly, through a wireline logging tool, or through a conductor.

[0032] The embodiment of FIG. 7 includes a casing 701 located within a geological formation 702. The ex situ sensor array 703 includes a single magnet 720 having a single coil 708. The electronics (not shown) for reading the sensor data can be located within the magnet housing 720, built in a container adjacent to the casing 701 , or attached to the magnet housing 720 in some other location. The signal containing the cement/fluid mixture analysis of the sensor data can then be transmitted directly to the surface wirelessly or through a wireline logging tool.

[0033] FIG. 8 illustrates an expanding sensor array 803. The sensor array 803 of the embodiment of FIG. 8 shows only a single sensor coil 814 on a single magnet 820. However, other embodiments can include any of the embodiments of FIGs. 4-7 (either ex situ or in situ) in an expanding sensor array as illustrated in FIG. 8. Such an expanding array would provide a compact arrangement while the casing 801 is being lowered into the geological formation 802 and then expand after the casing has been built.

[0034] The various stages of expansion 810-813 are shown in FIG. 8. An initial compact stage 810 is illustrated as the magnet 820 being held to the casing 801. Once the casing 801 is in place, a command can be sent to a latch (not shown) that unlatches and enables a hinge 850 and support 830 to push the magnet 820 away from the casing 801. The hinge 850 and support 830 enables the sensor array 803 to expand as far as the geological formation 802 allows. For example, the magnet 820 with the sensor array 803 can stop at any one of the illustrated locations of expansion 810-813. The support 830 would then hold the magnet 820 with the sensor array 803 while the cement/fluid mixture is injected between the casing 801 and the geological formation 802. The hinge 850 can be spring-loaded, mechanically actuated, or chemically actuated.

[0035] For purposes of brevity, the embodiments of FIGs. 4-8 show only one sensor array coupled to the casing. Other embodiments can include multiple sensor arrays located at numerous locations on the casing. These embodiments can have a mixture of different sensor designs (e.g., ex situ, in situ) as well as different types/quantities of sensors in each array.

[0036] In operation, NMR is a versatile technique that can make

measurements of many different physical and chemical quantities, depending on the pulse sequence used. The pulse sequence can be set in software and changed as for different embodiments. One embodiment includes relaxation and diffusion measurements. These types of measurements can be made in low, grossly inhomogeneous fields with relatively inexpensive hardware.

[0037] An NMR measurement depends on the interactions of the Bi and Bo fields through the nuclear spin in a fluid or volume of interest. In identifying the optimum angle (a) for a predetermined configuration of the magnet arrangement and common antenna core of the sensor, one may consider the configuration to exist in an arbitrary R ³ orthonormal basis set, σθ, σΐ, σ2.

[0038] The Bo direction at any point dominates the interaction when Bo» Bi, which is normally the case for down-hole tools. This implies that only portions perpendicular to the Bo direction will matter for Bi. When considering how the Bi field acts this means that only an R ² space will need to be considered. This particular plane will then be called the perpendicular plane, while the direction of Bo is designated as the parallel direction.

[0039] The last part of the NMR measurement is to receive a signal from the excited nuclear spins as detected by the sensor array. This occurs according to the principle of reciprocity where the nuclear spins act as mini transmitting coils.

[0040] The received data, in contemporary down-hole experiments, reveals two characteristics of magnetization: longitudinal recovery and transverse relaxation. Both of these measurements require the magnetization to be perturbed from its equilibrium state and alignment with the static field. These involve measurements of the magnetization rates to equilibrium from the perturbed states in the directions parallel and perpendicular to the Bo field.

[0041] The Ti (spin-lattice) and T ₂ (spin-spin) relaxation rates can be used as probes of the environment of the spin of interest. From the Ti and T ₂ distributions, it is possible to receive information such as chemical species, pore size distribution, and extent of contamination of the cement.

[0042] Many of the short components can be limited by surface relaxation, where the fluid spins interacting with the ferrite on surfaces in the cement pores undergo a more rapid relaxation process than those in the bulk fluid. These spins may also induce relaxation in neighboring spins, effectively "averaging out" the relaxation times within any given pore. As a result of the interactions, the Ti and T ₂ of a given component can be related to the surface-to-volume ratio of the pore within which the component resides, with smaller pores inducing faster relaxation.

[0043] In contrast to the case of oil-based mud, there can be very little long- component signal in uncontaminated or water-based mud contaminated signal, but NMR can still be used to detect contamination by water-based mud from the shorter-component signals as can be seen in FIG. 9.

[0044] FIG. 9 illustrates the Ti spectra 901 and the T ₂ spectra 902 of water- based mud (WBM). This figure further shows 13 parts per gallon (ppg) cement contaminated with 20% WBM 910 and uncontaminated 13 ppg cement 912 as numerous points during the curing process.

[0045] FIG. 9 shows that a signal initially at Ti = 95 ms and T ₂ = 56 ms, present only in the water-based mud contaminated spectra 901, shifts significantly with the nearby peak initially at Ti = 76.8 ms and T ₂ = 36.9 ms, indicating that the mud is being incorporated into the structure of the cement. The fact that the anomalous peaks show up almost exclusively near positions where known peaks from uncontaminated cement also appear likely indicates that the WBM is changing the structure of the cement, either by changing the degree of hydration of the components or changing the pore-size distribution.

[0046] The signal from these telltale components can tend to go down over time as the free water is incorporated into the cement. This can indicate that the embedded NMR sensors can have an advantage in detecting WBM

contamination over the use of wireline or MWD tools. The embedded NMR sensors can further measure water mobility, the pore- size distribution, and the total porosity of the cement. One can then not only identify the degree of the contamination but also its qualitative nature, enabling the determination about the quality of the cement as it cures.

[0047] While the above-described embodiments refer to Ti-T ₂ correlation spectra of setting cement, the embedded NMR sensors are not limited to such measurements. In addition to the spatial measurements of T ₂ , measurements of diffusion, or diffusion- T ₂ correlation spectra, the present embodiments can also measure water mobility and pore size distribution more accurately by running a T ₂ -T ₂ correlation spectrum.

[0048] In a T ₂ -T ₂ correlation spectrum, spins are allowed to relax via their T ₂ spin-spin relaxation mechanism for a period of time. The magnetization is stored using an RF pulse and, after some delay, another RF pulse starts the spin decaying under T ₂ again. During the delay between pulses, spins in an environment decaying under one T ₂ have time to migrate into a region wherein they would decay under a second T ₂ . This can result in cross-peaks between components that are exchanging spins. The rates of exchange can be determined by repeating the measurement with multiple delays and determining when the cross-peaks begin to appear.

[0049] FIG. 10 illustrates an embodiment of a system of NMR sensor arrays being used during a method of embedding the sensor arrays in cement. This embodiment is for purposes of illustration only as other methods can be used to embed the system of NMR sensor arrays in cement as it cures.

[0050] After the borehole has been drilled, the casings 1010, 1011 are lowered into the borehole. One or more NMR sensor arrays 1021-1024 are peripherally coupled around the outside of the casings 1010, 1011 as previously described. If an expandable sensor array is used, it can be unlatched and extended at this time. [0051] The cement 1001, from a cement source 1000, is injected into the borehole around the sides of the downhole casings 1011, 1011, between the casings 1011, 1011 and the geological formation through which the borehole is drilled. The system of NMR sensor arrays 1021-1024 can provide measured data signals to a sensor data receiver 1030 at the surface to enable the cement composition to be adjusted as it is injected. Similarly, as the cement is setting, the system of NMR sensor arrays 1021-1024 can measure the cement composition and transmit this data to the sensor data receiver 1030.

[0052] Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Various embodiments use permutations and/or combinations of embodiments described herein. It is to be understood that the above description is intended to be illustrative, and not restrictive, and that the phraseology or terminology employed herein is for the purpose of description. Combinations of the above embodiments and other embodiments will be apparent to those of skill in the art upon studying the above description.