Abstract:
A system for determining a curing state for cement disposed in a borehole penetrating the earth that includes a transceiver that includes a transmitting coil and that is configured to provide an input signal to the coil that sweeps through a frequency range and to measure the magnitude of the voltage across the coil. The system also includes a plurality of sensor nodes disposed in the concrete. The sensor nodes include a receiving coil and a capacitor coupled to the receiving coil to form a receiving circuit and that has a capacitance that changes as one of pressure or strain in the cement changes.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to sensors and, in particular, to systems that utilize sensors to measure the state of cement. 
         [0003]    2. Description of the Related Art 
         [0004]    Boreholes are drilled deep into the earth for many applications such as carbon dioxide sequestration, geothermal production, and hydrocarbon exploration and production. In all of the applications, the boreholes are drilled such that they pass through or allow access to a material (e.g., a gas or fluid) contained in a formation located below the earth&#39;s surface. Many different types of tools and instruments may be disposed in the boreholes to perform various tasks and measurements. 
         [0005]    Boreholes are typically completed such that they exhibit a composite pipe in pipe construction that can resist high pressures and temperatures. The construction consists of an inner and an outer pipe whose annular gap is filled with cement. When a section of the well is cased and cemented, resumption of drilling must be delayed until the cement cures. If the drilling is resumed prematurely, the cement, which has not had sufficient time to set, can be destroyed. Unnecessary delay however, is associated with excess cost that is preferably avoided. Therefore, it is important to determine the state of the cement as accurately as possible. 
         [0006]    The curing time of cement is affected by the pressure, shear stresses and the temperature of the cement. The pressure and shear stresses depend on process parameters i.e. altitude of the cement and volume flow. The temperature is affected by the temperature of the cement just prior to application, the heat released by the exothermic reaction and the heat transfer of the formation to the cement. As long as the pressure, shear stresses and temperature are known, curing time can be determined. 
         [0007]    In addition to knowing when the cement has cured, it is also desirable to know the properties of the cured cement right after curing and over its entire life. The properties can include, for instance, the presence of bubbles or gaps in the cement. 
       BRIEF SUMMARY 
       [0008]    Disclosed is a system for determining a curing state for cement disposed in a borehole penetrating the earth that includes a transceiver that includes a transmitting coil and that is configured to provide an input signal to the coil that sweeps through a frequency range and to measure the magnitude of the voltage across the coil. The system also includes a plurality of sensor nodes disposed in the concrete. The sensor nodes include a receiving coil and a capacitor coupled to the receiving coil to form a receiving circuit and that has a capacitance that changes as one of pressure or strain in the cement changes. 
         [0009]    Also disclosed is a method of determining a curing state of cement disposed in a borehole penetrating the earth. The method includes disposing a plurality of sensor nodes that includes a receiving coil and a capacitor that form a receiving circuit in a cement slurry, the capacitor having a capacitance that changes based on one of a pressure or a strain; disposing the cement slurry into the borehole; lowering a transceiver into the borehole internal to the cement slurry, the transceiver including a transmitting coil; sweeping a frequency of a voltage applied to the transmitting coil over a frequency range as the transceiver is being lowered in the borehole; measuring a magnitude of the voltage; and determining the curing state based on dips in the measured magnitude. 
         [0010]    Further disclosed is a system for determining a property of cement disposed in a borehole penetrating the earth that includes a transceiver that includes a transmitting coil and that is configured to provide an input signal to the coil that sweeps through a frequency range and to measure the magnitude of the voltage across the coil. The system also includes a plurality of sensor nodes disposed in the concrete. The sensors include a receiving coil and a capacitor coupled to the receiving coil to form a receiving circuit and that has a capacitance that changes as one of pressure or strain in the cement changes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0012]      FIG. 1  is a cut-away side view of a borehole that includes casing and production casing cemented to the casing; 
           [0013]      FIG. 2  is a cut-away side view of another borehole that includes casing and production casing cemented to the casing; and 
           [0014]      FIG. 3  is a circuit diagram showing a transceiver and a sensor node that can be utilized to carry out embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    A detailed description of one or more embodiments of the disclosed apparatus and method presented herein is by way of exemplification and not limitation with reference to the Figures. 
         [0016]      FIG. 1  shows a borehole  100  that includes a completed section  102  and a non-completed section  104 . In the completed section  102  there exists at least two casing layers  106 ,  108 . As illustrated, an outer casing  106  contacts a wall  112  of the borehole  100 . Of course, other layers or elements could be disposed between the outer casing  106  and the wall  112 . The exact configuration of the casing layers  106 ,  108  can vary from that illustrated in  FIG. 1  as long as at some location along the depth of the borehole  100 , the two casing layers  106 ,  108  overlap and are separated from one another by a layer of cement  110 . 
         [0017]    According to one embodiment, at least a portion of the inner casing  108  is disposed within the outer casing  106 . For example, and with reference now to  FIG. 2 , the inner casing  108  includes a top portion  202  that fits within a bottom portion  204  of the outer casing  106 . As illustrated in  FIG. 2 , both the inner and outer casing  108 ,  106 , surround a production tube  206 . A parent drilling liner  208  contacts a formation  200  through which the borehole  100  passes. In  FIG. 2 , several voids  210 ,  212 ,  214 , respectively, exist between the outer casing  106  and the parent drilling liner  208 , the inner and outer casings  108 ,  106  and the outer casing  106  and the production tube  206 . It shall be understood that one or more of the voids  210 ,  212 ,  214  are filled with cement  110  at some point during the completion of the borehole. 
         [0018]    In both of the cases illustrated in  FIGS. 1 and 2 , according to an embodiment of the present invention, the cement  110  includes one or more sensing nodes  120  disposed therein. Referring again to  FIG. 1 , the sensing nodes  120  can be activated and read by transceiver  130  that is lowered into the borehole  100 . The transceiver  130 , described in greater detail below, interacts with the sensing nodes  120  to determine temperature, strain or both of the cement  110  in an area directly surrounding the particular sensor node  120 . 
         [0019]    In one embodiment, the sensing nodes include a coil (inductor) coupled to a capacitor that varies in capacitance based on the surrounding temperature or strain. As the transceiver  130  is lowered into or raised out of the borehole  100 , the temperature/strain values measured by the sensor nodes  120  is transmitted to a computing device  140  via a wireline  135 . At the computing device  140 , a determination as to whether the cement  110  has cured sufficiently such that drilling can resume can be made based on the one or both the temperature and strain values. It shall be understood that, in one embodiment, the location of the computing device  140  can be moved to another surface  150  location (i.e., it can be remote from the transceiver  130 ). In another embodiment, the computing device  140  can be part of the transceiver  130 . 
         [0020]      FIG. 3  shows transceiver  130  arranged to communicate with a sensor node  120 . It shall be understood, that the sensor node  120  can be encapsulated in a material that allows it to be mixed into a cement slurry. The cement slurry is flowed into the annulus of a borehole  100  where it becomes the cement  110  illustrated in  FIGS. 1 and 2  when cured. The sensor node  120  assists in making the determination that the cement  110  has cured. 
         [0021]    The sensor node  120  includes a coil  308  coupled to a capacitor  310  that form a receiving circuit  306 . The coil  308  an the capacitor  310  form an LC circuit that has a resonant frequency ω that generally is generally defined as shown in equation 1: 
         [0000]    
       
         
           
             
               
                 
                   ω 
                   = 
                   
                     1 
                     
                       LC 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where L is the inductance of the coil  308  and C is the capacitance of the capacitor  310 . 
         [0022]    According to one embodiment, the transceiver  130  includes a control circuit  302  that drives a transmitting coil  304 . According to one embodiment, the control circuit  302  varies the frequency at which the transmitting coil  304  transmits. In particular, the control circuit  302  can provide an input signal to the transmitting coil  304  that has a frequency that sweeps through a frequency range. 
         [0023]    The control circuit  302  can measure the voltage v across the coil  308 . As the input to the transmitting coil  308  is swept through the resonant frequency of the receiving circuit  306 , the magnitude of the voltage v across the transmitting coil  308  will drop due to the increased coupling between the transmitting coil  304  and receiving circuit  306 . The voltage can be continuously measured or measured at times when the transmitting coil  308  is being driven at specific frequencies. 
         [0024]    According to one embodiment, the capacitor  310  is formed such that its capacitance varies with strain or pressure. For instance, and as will be understood of one of skill in the art, the capacitor  310  can include a flexible plate the compression or shearing of which causes the distance between it an the other plate to vary. The variation in distance between the capacitor plates will, of course, cause the capacitance (C) of the capacitor  310  to change. As the capacitor  310  changes is capacitance, the resonant frequency ω of the receiving circuit  306  changes. Thus, by monitoring voltage levels at the transmitting coil  304 , the capacitance of capacitor  310  can be determined. In more detail, the frequency at which the voltage, v. drops is the resonant frequency ω of the receiving circuit  306 . From equation 1, the capacitance of the capacitor  310  can be determined at the resonant frequency assuming L is known. Given the capacitance of the capacitor  310 , the pressure or strain can be determined based on known responses of the capacitor  310  to either temperature or pressure. In one embodiment, in contrast to some prior art devices, the sensor nodes  120  do not include a power supply such as a battery or storage capacitor coupled to the receiving circuit  106 . 
         [0025]    It shall be understood that in one embodiment, the capacitance may be fixed such that some or all of the sensors nodes  120  has a fixed resonant frequency. This fixed resonant frequency can be used to identify each node  120  individually. In such a case, the general location of each of the sensors nodes can be determined. Based on the position, the speed of movement of the cement can be determined. From speed, viscosity can be determined. 
         [0026]    It shall be understood that while the term “cement” is used through out the above description, that term can be interpreted to include any filling material between downhole tubing and a formation or other tubing or that serves to strengthen or seal a borehole. It shall further be understood that while pressure and strain are measured, the location of the detected sensor nodes  120  can also be used to determine the volumetric distribution of the cement in the borehole. 
         [0027]    Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list of at least two terms is intended to mean any term or combination of terms. The terms “first,” “second,” and “third” are used to distinguish elements and are not used to denote a particular order. 
         [0028]    It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed. 
         [0029]    While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.