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BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates generally to devices and methods used in a wellbore operation. 
         [0003]    2. Background Art 
         [0004]    During production of hydrocarbons from earth formations, it is important to monitor and control the production on a regular or permanent basis to prevent any potential problems and to determine the causes of any problem (e.g., stoppage or reduction in production) in order to attempt to provide a remedy. For example, the positions of the contact surface between oil and water, and between oil and gas, within the reservoir should be monitored to prevent water or gas from mixing in the oil production. 
         [0005]    Traditionally, measurement devices/sensors may be placed in the wellbore to perform continuous monitoring or measurements of reservoir conditions. For example, resistivity may be measured by measuring the potential difference between a measurement electrode and a reference electrode. These devices or sensors would be disposed outside a casing in a production well. These devices or sensors may be connected via contact pieces and wirings to electronic means or equipment uphole. The wirings are also disposed on the outside of the casing. These devices are typically fixed in the wellbore when cement is injected to fill the annular space between the casing and the wellbore. 
         [0006]    However, discontinuous cement formation may occur outside the casing due to the measurement devices located on the outside wall of the casing. As a result of the discontinuous cementing in the annulus, fluids from the rock formation may infiltrate between the cemented annulus and the casing, causing damages to the casing and the measurement devices. Furthermore, such discontinuous cementing may also create a path that can cause such fluids to rise towards the surface, thereby not only causing damages to the equipment, but also endangering personnel in the vicinity of the well. 
         [0007]    One approach to address this issue is described in U.S. Pat. No. 7,071,696, issued to Gambier et al. For example,  FIG. 1  shows the outside wall (jacket)  17  of a casing contains a succession of segments having measurement devices  10 . The jacket  17  of the casing provides annular recesses suitable for receiving measurement electrodes  6 . The recesses are of dimensions that the electrodes  6  are no more than flush relative to the outside diameter of the jacket  17 . The electrodes  6  are installed in the recesses prior to the device being lowered down the well. At least one of the devices  10  has measurement electrodes  6 , and each of the devices  10  also has an axial recess for receiving a wire connection  7 , which transmits the data received by the electrodes  6  to means  16  located on the surface for processing the measurements and for supplying power. 
         [0008]    By placing the measurement devices in the recesses, these devices may not create physical obstruction on the surface of the casing, thus, avoiding discontinuous cementing. As a result, the integrity of the cemented annulus  15  may be preserved, preventing the fluids from the reservoir  13  or from the formations  12  from infiltrating between the annulus  15  and the casing. The permanently fixed measurement means, which are located on the recesses of the casing, would be protected from the fluid-induced damages. 
         [0009]    While the methods known in the art are useful, sometimes it is desirable to have the sensors contacting a wellbore wall. Therefore, there is still a need for improved sensor deployment in the wellbores. 
       SUMMARY OF INVENTION 
       [0010]    One aspect of the invention relates to downhole equipment. A downhole equipment in accordance with one embodiment of the invention includes a tubing configured for deployment in a wellbore; and a measurement unit disposed on outside of the tubing, wherein the measurement unit comprises a detector embedded in a swellable material. 
         [0011]    Another aspect of the invention relates to methods for formation property measurements. A method in accordance with one embodiment of the invention includes deploying a downhole equipment to a predetermined location inside a wellbore, wherein the downhole equipment comprising a tubing and a measurement unit dispose on outside of the tubing, wherein the measurement unit comprises a detector embedded in a swellable material; allowing the swellable material to swell; and performing measurements. 
         [0012]    Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  shows an example of a prior art downhole measurement system. 
           [0014]      FIG. 2  shows a wellbore equipment in accordance with one embodiment of the invention prior to swelling of a swellable material. 
           [0015]      FIG. 3  shows a wellbore equipment in accordance with one embodiment of the invention after swelling of a swellable material. 
           [0016]      FIG. 4  shows a wellbore equipment containing measurement devices in accordance with one embodiment of the invention after swelling of the swellable materials. 
           [0017]      FIG. 5  shows a wellbore equipment containing measurement devices in accordance with one embodiment of the invention after swelling of the swellable materials. 
           [0018]      FIG. 6  shows a wellbore equipment containing measurement devices in accordance with one embodiment of the invention for use in intelligent completion operations. 
           [0019]      FIG. 7  shows a wellbore equipment containing a resistivity array on a section of a tubing coated with a swellable material in accordance with one embodiment of the invention. 
           [0020]      FIG. 8  shows an example of quadrupole electrodes that may be used in resistivity devices in accordance with one embodiment of the invention. 
           [0021]      FIG. 9  shows an example of single electrode arrays that may be used with a resistivity device in accordance with one embodiment of the invention. 
           [0022]      FIG. 10  shows a method for performing formation property measurements in accordance with one embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Embodiments of the invention relate to methods and systems for deploying sensors, measurement devices, or equipment in wellbores. These wellbore equipment, sensors, or measurement devices may be used to monitor the formation properties, the well conditions, or other production parameters. Embodiments of the invention may be used on land or off shore. For clarity of illustration, the following description uses the devices for measuring pressure and/or resistivity in the well or formation to illustrate embodiments of the invention. However, one of ordinary skill in the art would appreciate that the same approaches may be used with other kinds of sensors or devices for performing measurements on other wellbore or formation parameters, such as porosity, permeability, temperature, etc. 
         [0024]    In accordance with embodiments of the invention, a tubing or casing for deployment downhole may include one or more measurement units, each of which may include one or more detectors or sensors embedded in a swellable material. The swellable material may be oil swellable (solvent swelling), water swellable (osmotic swelling), or oil/water swellable. The swellable material compositions may be those known in the art, including rubbers and elastomers. Rubbers may be natural or synthetic rubbers. Elastomers may include those listed in ASTM D1418. In addition, a swellable material may be a mixture that includes a filler, plasticizer, accelerant, fiber, nanoflake and/or nanoplatelet, as long as the material retains the swellability. 
         [0025]    Preferably, the swellable materials would swell, but do not substantially degrade or disintegrate upon long term exposure to the downhole conditions or downhole fluids, e.g., water and water-based fluids, such as brines, or hydrocarbons. Preferably, swelling maintains the basic material properties and the cross-links in the molecular structures of the swellable materials, thereby maintaining the positive swelling pressure for a period of time. 
         [0026]    By using swellable materials, embodiments of the invention can be deployed, before swelling, in a wellbore with relative ease and with less danger of damages to the sensors or devices because the tubings (or casings) with the attached measurement units have diameters smaller than the diameters of wellbores. Once the tubings with the measurement units are in position, the swellable materials may be allowed to swell. Once the swellable materials expand, they help to place the detectors or sensors in positions. This is particularly advantageous when the sensors or detectors need to be in contact with the wall of a wellbore. For example, for measurements of formation pressures and permeability, it would be necessary to press the pressure sensors against the wall of the wellbore. Even with other measurements where the sensors do not need to contact the formation, it might still be advantageous to put the sensors closer to the formations. For sensors that need to contact the formations, the sensors are embedded in a manner that the contact sides of the sensors are exposed, i.e., not embedded in the swellable materials. 
         [0027]    In accordance with some embodiments of the invention, the sensors or measurement devices embedded in a swellable material may be used by themselves without additional cementing. Yet, in accordance with other embodiments of the invention, cementing may be used in conjunction with the swellable materials. The following describes several examples to illustrate embodiments of the invention. One skilled in the art would appreciate that these examples are not meant to limit the scope of the invention and that various modifications or variations of these examples are possible without departing from the scope of the invention. 
         [0028]    Sensors Embedded in a Swellable Material for Measuring Pressure 
         [0029]      FIG. 2  illustrates one embodiment of the invention. As shown, a measurement unit  20  (e.g., a pressure sensor) in accordance with one embodiment of the invention is disposed on the outside of a tubing or casing  26 , and the tubing is deployed to a desired location in a wellbore  22  penetrating a formation  24 . As shown in this example, the tubing or casing  26  includes retention elements  28  on the outside of the casing for retaining the measurement unit  20 , which includes a pressure probe  21  embedded in a swellable material  23 . Thus, the measurement unit  20  may be deployed in the wellbore  22  when the tubing or casing  26  is run in the wellbore  22 . 
         [0030]    As shown in this example, the measurement unit  20  includes a pressure probe  21  connected to a pressure gauge  29 , which is connected to a downhole electrical cable  25 . The electrical cable  25  may supply the power and/or transmit the pressure measurements from the measurement device  20  to the surface for processing. 
         [0031]    The system shown in  FIG. 2  can be run in the wellbore before the swellable material is allowed to expand. Once the measurement device is deployed at the desired depth, the swellable material may be allowed to swell and expand.  FIG. 3  shows the measurement unit  20  of  FIG. 2  after the swellable material  23  has expanded. The swellable materials  23  expands after being exposed to a swelling fluid, which may be water, water-based fluids, hydrocarbons, or water-hydrocarbon mix fluids. When the swelling material  23  absorbs the swelling fluids, the swelling material  23  may expand to press against the wall of wellbore  22  and create an annular seal  30  inside the wellbore  22 . As a result, the pressure probe  21  is urged against the formation  24  to facilitate formation pressure measurements. 
         [0032]    Note that the expanded swelling material  23  may form a tight seal around the annulus, this tight seal may prevent fluids from flowing into different zones in the well. In addition, these tight seals may also help to fix the tubing or casing  26  in the wellbore. Therefore, cementing may become optional. However, in some cases, cementing may still be performed to secure the structure of the well. 
         [0033]    Although the examples shown in  FIG. 2  and  FIG. 3  include only one sensor device, one skilled in the art would appreciate that embodiments of the invention may involve more than one sensor devices. For example,  FIG. 4  shows another example in accordance with embodiments of the present invention. As shown, a tubing or casing  40  may contain a series of pressure measurement units (or pressure substations)  42 ,  42 ′ (two are shown here for illustration), which may be located in tandem or in any spacing configurations on the outside of the tubing or casing  40 . Each measurement unit  42 ,  42 ′ may contain a pressure probes  44 ,  44 ′. These probes  44 ,  44 ′ may be embedded in a swellable material  48 ,  48 ′ and held by retaining elements  46 ,  46 ′. The probes  44 ,  44 ′ may be connected to pressure gauges  45 ,  45 ′, which may be connected to a downhole electrical cable  47 . 
         [0034]    In some embodiments, the pressure gauges  45 ,  45 ′ and/or part of the cable  47  may be also embedded in the swellable material  48 ,  48 ′. The cable  47  may supply the power and/or transmit the pressure data from the devices  42 ,  42 ′ to the surface for processing. 
         [0035]    After the devices  42 ,  42 ′ are exposed to swelling fluids, such as water, water-based, and/or oil fluids, the swellable material  48 ,  48 ′ may expand to press against the formation  49  and create pressure-holding annular seals inside the wellbore  43 . The swelling of the swellable materials may urge the pressure probes  44 ,  44 ′ against the formation  49  to facilitate the pressure measurements. 
         [0036]    The systems illustrated in  FIG. 3  and  FIG. 4  may be instrument liners or tubings that are not intended to be cemented in the wellbore. On the other hand, some instrument strings (liners) may be intended to be used with cementing. Such liners may be made up of multiple sections of tubings. The joints of these sections of tubings may be protected with cross coupling protectors. Once such liners are deployed in a well, the liners may be cemented in the well. Embodiments of the invention can also be used with these liners. In these cases, the swellable materials and sensors may be conveniently included on the outside of the liner in between the cross coupling protectors. In this manner, the cross coupling protectors may also help to protect the measurement units during deployment. 
         [0037]    For example,  FIG. 5  shows an example in accordance with embodiments of the present invention. As shown, a typical tubing or liner  50  is made up of multiple sections of tubings. The joints of the sections are protected by cross coupling protectors  52 ,  52 ′, and  52 ″ (three are shown here for illustration). Three pressure measurement units (or subs)  54 ,  54 ′,  54 ″ are shown positioned on the outside of the tubing or liner  50 . Each pressure measurement subs  54 ,  54 ′,  54 ″ may contain a reservoir pressure probe  56 ,  56 ′,  56 ″ connected to a pressure gauge  55 ,  55 ′,  55 ″. In some embodiments, both the probe  56 ,  56 ′,  56 ″ and the gauges  55 ,  55 ′,  55 ″ may be embedded in a swellable material  58 ,  58 ′,  58 ″. In other embodiments, the gauges  55 ,  55 ′,  55 ″ may be outside the swellable material  58 ,  58 ′,  58 ″. The probes  56 ,  56 ′,  56 ″ and the gauges  55 ,  55 ′,  55 ″ may be connected to a downhole electrical cable  57 , which may supply the power and/or transmit the pressure data obtained from the devices  54 ,  54 ′,  54 ″ to the surface for processing. 
         [0038]    After the swellable materials  58 ,  58 ′,  58 ″ are exposed to a swelling fluid, such as water, water-based, and/or oil fluids, the swelling material  58 ,  58 ′,  58 ″ would expand to push against the formation  59  to create pressure-holding seals within the wellbore  53 . As a result, the flexible probes  56 ,  56 ′,  56 ″ may be pressed against the formation  59  for performing pressure measurements. 
         [0039]    Embodiments of the invention can be advantageously used in intelligent completion (IC) applications. In intelligent completion (IC) applications, pressure sensors and resistivity sensors are deployed in the well to monitor the conditions of the well in various zones and to maximize the production. For example,  FIG. 6  shows an exemplary intelligent completion string  60  that may include a series (only two shown for illustration) of flow control valves  64 ,  64 ′, swellable packers  69 ,  69 ′, and pressure measurement subs  66 ,  66 ′ for monitoring formation pressures. The pressure measurement subs may comprise pressure sensors embedded in swellable materials, as described above. Thus, once the IC string  60  is in place and the swellable materials expand, the sensors will be urged against the formation for monitoring the formation pressures. Similarly, the sensors may include resistivity or other type of sensors for monitoring or measuring other formation properties. 
         [0040]    Sensors Embedded in Swellable Material for Measuring Resistivity 
         [0041]    Although the above description uses pressure sensors to illustrate embodiments of the invention, one skilled in the art would appreciate that embodiments of the invention may use other types of sensors, such as resistivity sensors. For example,  FIG. 7  shows a resistivity array  70  in accordance with one embodiment of the invention. The resistivity array  70  may contain any numbers of resistivity subs  71  disposed on a string, interspersed with a swellable material  72 . The resistivity subs  71  may be connected by an electric cable  73 , which may be embedded in the swellable material  72 . 
         [0042]    The swellable material  72  in the embodiment shown in  FIG. 7  may be coated on a casing or liner  74  and may function as insulators for the resistivity sensors (e.g., electrodes). In addition, the swellable material  72  may replace cements for zonal isolation in a well. When the cable  73  is embedded in the swellable material  72 , the swellable material  72  also helps to protect the cable  73 . 
         [0043]    The resistivity subs  71  may contain any suitable number or types of resistivity sensors/detectors, e.g., electrodes or coils (antennas). For example,  FIG. 8  shows an example of a resistivity array  80 , in which each resistivity sub comprises four electrodes  81  in a quadrupole electrode configuration. In contrast,  FIG. 9  shows an example of a resistivity array  90 , in which each resistivity sub includes a single electrode  91 . These are examples for illustration only. One skilled in the art would appreciate that other configurations may also be used without departing from the scope of the invention. Furthermore, while the examples use electrodes as resistivity sensors/detectors, embodiments of the invention include those using electromagnetic (EM) coils (antennas) or combinations of electrodes and antennas. When EM coils are used, they may be longitudinal magnetic dipole (LMD) or transverse (or tilt) magnetic dipole (TMD) coils. 
         [0044]    Although the above examples describe the use of pressure sensors and resistivity sensors with swellable materials, modern intelligent well completion may employ various downhole sensors, including seismic or acoustic sensors, pressure sensors, resistivity sensors, temperature sensors, flow-rate sensors/gauges, gamma ray detectors, optical sensors, pH sensors, vibration sensors, chemical composition sensors, water or gas cut rate meters, mechanical stress sensors, radioactive tracer sensors, etc. Embodiments of the invention may be used with any of these devices or a combination thereof. 
         [0045]    Embodiments of the present invention may be used with or without cementing after the wellbore equipment is deployed to a predetermined location inside the wellbores and swelling materials have expanded. Thus, if desired, the casing or the liner may still be cemented in place using conventional methods, such as stage cementing techniques. 
         [0046]    Some embodiments of the invention relate to methods for using an equipment or system of the invention in a wellbore. For example,  FIG. 10  shows a method  100  for deploying a wellbore equipment inside a wellbore in accordance with one embodiment of the present invention. The wellbore equipment carrying measurement units (subs), some or all of which may be embedded in a swellable material on the outside wall of a casing or a liner, may be deployed to a predetermined location inside a wellbore (step  102 ). Once the measurement device reaches the predetermined location, a swelling fluid, such as water, water-based, oil, and/or water/oil fluids, may be used to swell the swellable material or the swellable material may be allowed to contact a wellbore fluid to expand (step  104 ). Thus, the swellable material may expand and press against the formation to create pressure-holding annular seals inside the wellbore. As a result, the measurement devices may be urged against the formation to contact with the formation (step  106 ). The measurement devices may then be used to perform measurements related to the formation properties, such as pressure, resistivity, porosity, permeability, pH, temperature, etc. (step  108 ). 
         [0047]    Advantages of embodiments of the invention may include one or more of the following. By embedding the measurement devices in a swellable material, the probes, the cables, the gauges, the casing and/or the liner may be protected from damages during run-in of the casing or liner. In addition, the measurement devices may be brought into contact with the formation by the swelling material, thus, providing more accurate measurements. While the sensors that need to be in contact with the formation would benefit most from embodiments of the invention, even sensors that do not need to be in contact with the formation would benefit from the approach disclosed herein. For example, when these sensors are deployed in a state embedded in a swellable material, they are less likely to be damaged during deployment. In addition, when the swellable material expands, these sensors would be placed closer to the formation. Even if direct contact is not required, many sensors benefit from being in closer proximity to the formation to be measured. 
         [0048]    Furthermore, compared with methods using cement to permanently fix the measurement devices in place, the devices and methods in accordance with the embodiments of the invention can provide a less labor intensive (thus, cheaper), faster, and safer way to deploy the sensors and to perform measurements or monitoring. For example, the expanded swellable materials can seal the wellbore into different zones, thereby functioning as inflatable packers. In some situations, the swellable materials when expanded can provide seals and support such that cementing may not be needed. 
         [0049]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A downhole equipment includes a tubing configured for deployment in a wellbore; and a measurement unit disposed on outside of the tubing, wherein the measurement unit comprises a detector embedded in a swellable material. A method for formation property measurement includes deploying a downhole equipment to a predetermined location inside a wellbore, wherein the downhole equipment comprising a tubing and a measurement unit dispose on outside of the tubing, wherein the measurement unit comprises a detector embedded in a swellable material; allowing the swellable material to swell; and performing measurements.