Abstract:
A system, method and device for interrogating a downhole environment in a borehole beneath a surface includes a source of electromagnetic energy operable to transmit an electromagnetic signal in the borehole, a sensor module, including a passive resonating circuit including a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment in response to a condition in the downhole environment in the borehole and a detector positionable to receive the reflected modulated electromagnetic signal. In an embodiment, a solids-free dielectric medium is provided within an annular volume in the borehole defined by the casing through which the electromagnetic signal is transmitted.

Description:
[0001]    This application is a Continuation-In-Part of U.S. Pat. App. Ser. No. 12/627,639, filed Nov. 30, 2009, herein incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    The present invention relates generally to an insulating packer fluid for use in a borehole and more particularly to a system and method for remote sensing in an environment containing the insulating packer fluid. 
         [0004]    2. Background 
         [0005]    In resource recovery, it may be useful to monitor various conditions at locations remote from an observer. In particular, it may be useful to provide for monitoring conditions at or near to the bottom of a borehole that has been drilled either for exploratory or production purposes. Because such boreholes may extend several miles, it is not always practical to provide wired communications systems for such monitoring. 
         [0006]    U.S. Pat. No. 6,766,141 (Briles et al) discloses a system for remote down-hole well telemetry. The telemetry communication is used for oil well monitoring and recording instruments located in a vicinity of a bottom of a gas or oil recovery pipe. Modulated reflectance is described for monitoring down-hole conditions. 
         [0007]    As described in U.S. Pat. No. 6,766,141, a radio frequency (RF) generator/receiver base station communicates electrically with the pipe. The RF frequency is described as an electromagnetic radiation between 3 Hz and 30 GHz. A down-hole electronics module having a reflecting antenna receives a radiated carrier signal from the RF generator/receiver. An antenna on the electronics module can have a parabolic or other focusing shape. The radiated carrier signal is then reflected in a modulated manner, the modulation being responsive to measurements performed by the electronics module. The reflected, modulated signal is transmitted by the pipe to the surface of the well where it can be detected by the RF generator/receiver. 
         [0008]    In a borehole, production tubing is generally placed inside the casing string defining an annulus therebetween (the “A annulus”). At the bottom of the casing, the A annulus is usually sealed using a packer. The annulus is then often filled with a fluid such as crude oil, diesel, drilling mud or the like. 
       SUMMARY 
       [0009]    In an aspect of an embodiment of the present invention, a system for interrogating a downhole environment in a borehole beneath a surface, includes a source of electromagnetic energy, operable to transmit an electromagnetic signal in the borehole, a pair of conducting tubes, positioned within the borehole and together defining an annular volume therebetween, a packer, arranged at a downhole end of the conducting tubes and constructed and arranged to seal a distal end of the annular volume, a substantially solids-free dielectric packer fluid, disposed within the sealed annular volume, the packer fluid comprising a halogenated hydrocarbon and having a density between about 12 ppg and about 16 ppg, a sensor module, comprising a passive resonating circuit, the passive resonating circuit comprising a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment to, in use, reflect the electromagnetic signal and to modulate the electromagnetic signal in response to a condition in the downhole environment in the borehole, and a detector positionable to receive the reflected modulated electromagnetic signal. 
         [0010]    In another aspect of an embodiment of the present invention, a method of interrogating a downhole environment in a borehole beneath a surface, includes providing a substantially solids-free dielectric fluid medium within an annular volume in the borehole, the packer fluid comprising a halogenated hydrocarbon and having a density between about 12 ppg and about 16 ppg, transmitting an electromagnetic signal in the borehole and through the fluid medium, reflecting the electromagnetic signal with a sensor module, comprising a passive resonating circuit, the passive resonating circuit comprising a crystal oscillator having a resonant frequency that varies with changes in the condition in the downhole environment, modulating the electromagnetic signal in accordance with the varying resonant frequency in response to the condition in the downhole environment in the borehole, and receiving the reflected modulated electromagnetic signal. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0011]    Other features described herein will be more readily apparent to those skilled in the art when reading the following detailed description in connection with the accompanying drawings, wherein: 
           [0012]      FIG. 1  is a schematic illustration of a system for interrogating a downhole environment in a borehole beneath a surface in accordance with an embodiment of the present invention; 
           [0013]      FIG. 2  is a schematic illustration of a sensor package incorporating a pressure or temperature sensor in accordance with an embodiment of the present invention; 
           [0014]      FIG. 3  is a schematic illustration of a circuit incorporating a crystal oscillator based sensor in accordance with an embodiment of the present invention; 
           [0015]      FIG. 3A  is a schematic illustration of a circuit incorporating a crystal oscillator based sensor and a capacitive sensor in accordance with an embodiment of the present invention; and 
           [0016]      FIG. 4  is a schematic illustration of a package incorporating a plurality of sensors in accordance with one or more embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  illustrates an example of an apparatus  100  for monitoring a condition in a subsurface borehole. The apparatus  100  includes an electromagnetically transmissive medium, such as a conductive line  102 , for conducting electromagnetic energy through the borehole. It will be appreciated by those having ordinary skill in that art that the conductive line  102  may take different forms or embodiments, depending on the state of the borehole. Thus, for example, the conductive line  102  may comprise a production tubing string in a completed borehole or a drillstring in a borehole under construction. Near the top of the conductive line  102 , a transformer  104  is provided to couple the conductive pipe to a source of electromagnetic energy. Alternate coupling methods to the transformer  104  may be employed. For example, the transmission line may directly couple to a coaxial cable or any other suitable cable. 
         [0018]    In the example embodiment as shown, the transformer  104  includes a stack of ferrite rings  106 , and a wire  108  wound around the rings. The wire  108  includes leads  110  that may be coupled to a signal generator  112  which may be configured to produce a pulsed or a continuous wave signal, as necessary or desirable. The wire  108  may further be coupled to a receiver  114 . The receiver  114  may be embodied as a computer that includes a bus for receiving signals from the apparatus  100  for storage, processing and/or display. In this regard, the computer  114  may be provided with a display  118  which may include, for example, a graphical user interface. 
         [0019]    The computer  114  may be programmed to process the modulated frequency to provide a measure of the sensed characteristic. The computer  114  may perform any desired processing of the detected signal including, but not limited to, a statistical (e.g., Fourier) analysis of the modulated vibration frequency, a deconvolution of the signal, a correlation with another signal or the like. Commercial products are readily available and known to those skilled in the art that can be used to perform any suitable frequency detection. Alternately, the computer may be provided with a look-up table in memory or in accessible storage, that correlates received modulated frequencies to sensed acoustic energy. 
         [0020]    In a typical drilling application, the borehole will be lined with a borehole casing  120  which is used to provide structural support to the borehole. This casing  120  is frequently made from a conductive material such as steel, in which case it will cooperate with the line  102  in order to form a coaxial transmission line, and it is not necessary to provide any additional conductive medium. Where the casing is not conductive, a conductive sleeve (not shown) may be provided within the casing in order to form the coaxial structure. In order to maintain a spacing between the line  102  and the casing  120 , the apparatus  100  may include dielectric rings  122  disposed periodically along the conductive line  102 . 
         [0021]    The spacers can, for example, be configured as insulated centralizers which can be disks formed from any suitable material including, but not limited to, nylon or polytetrafluoroethylene (PTFE). Though the illustrated embodiment makes use of a coaxial transmission line, it is contemplated that alternate embodiments of a transmission line may be employed, such as a single conductive line, paired conductive lines, or a waveguide. For example, the casing alone may act as a waveguide for certain frequencies of electromagnetic waves. Furthermore, lengths of coaxial cable may be used in all or part of the line. Such coaxial cable may be particularly useful when dielectric fluid cannot be used within the casing  120  (e.g., when saline water or other conductive fluid is present in the casing  120 ). 
         [0022]    A probe portion  124  is located near the distal end of the apparatus  100 . In principle, the probe portion may be located at any point along the length of the transmission line. Indeed, multiple such probe portions may be placed at intervals along the length, though this would tend to create additional signal processing burdens in order to differentiate signals from the several probes. Setting a natural resonance frequency of each probe at a different frequency would, in principle, allow for a type of wavelength multiplexing on the coaxial line that could simplify the processing. 
         [0023]    The probe portion includes a port  126  that is configured to communicate ambient pressures from fluid present in the borehole into the probe where it may be sensed by the sensor (not shown in  FIG. 1 ). Below the probe is illustrated a packer  128  and packer teeth  130 . 
         [0024]    In use, the signal generator  112  generates an electromagnetic pulse that is transmitted through the transmission line to the probe  124 . In an alternate arrangement, the pulse may be generated locally as described in U.S. patent application Ser. No. 11/898,066, herein incorporated by reference. 
         [0025]    The probe includes a sensor that includes a resonant circuit portion that, upon receiving the pulse, modulates and re-emits or reflects the pulse back up the transmission line. The resonant circuit may be, for example, a tank circuit that includes inductive and capacitive components. 
         [0026]    In an embodiment, illustrated in  FIG. 2 , a crystal-based oscillator  200  acts as the L-C tank circuit. The structure of the housing  202  has at one end a pressure feed-in tube  204  that allows pressure from the borehole environment that has entered via the port  126  to pass into an interior space  206  of the sensor  200 . In the interior space, the pressure is transmitted to a flexible diaphragm  208  or otherwise pressure-reactive structure. 
         [0027]    Motion of the diaphragm  208  is transmitted to a quartz crystal  210 , or other piezoelectric crystal such as gallium phosphate. As pressure is transmitted to an edge of the quartz crystal, its resonant frequency changes. By correct selection of a direction of the face of the crystal, the sensor may be made to be more sensitive to pressure or to temperature (e.g., AC-cut). For pressure monitoring, the crystal should be preferentially sensitive to pressure and relatively less sensitive to temperature (e.g., AT-cut). Furthermore, for monitoring of pressure changes with a relatively high frequency response (e.g., monitoring of acoustic frequencies), it is useful for the crystal to be generally relatively thin (e.g., 0.2-2.0 mm) and a typical size is on the order of 1 cm in diameter. 
         [0028]    A return spring mechanism  214  may be provided to bias the crystal  210  and its holders towards the feed-in tube  204  and thereby to tend to cause the diaphragm to return to a neutral position. An electrical feed through  216  is provided to couple the sensor  200  to the sensor circuit (not shown). 
         [0029]    The sensor  200  may be coupled to the transmission line via an inductive ferrite ring  400  as illustrated in  FIG. 3 . Electrical leads  402  are provided through the electrical feed through  216  of the sensor module. The leads  402  couple wire loops around the ferrite ring  400 . In this embodiment, the oscillator has the characteristics of an L-C circuit and the ferrite ring essentially acts as a transformer to couple the oscillator to the transmission line. 
         [0030]      FIG. 3A  illustrates an alternate embodiment directed to a pressure sensor configuration. In this embodiment, the relatively temperature-insensitive crystal (e.g., AT cut crystal) is isolated from the ambient pressure, and a capacitive pressure-responsive element  404  is provided in series with the sensor  200 ′ and exposed to the ambient pressure. In this configuration, the ferrite ring  400  again acts as a transformer, while the capacitive sensor  404  in combination with the crystal sensor  200 ′ acts as the L-C tank circuit. The crystal sensor  200 ′ will resonate with a frequency that depends in large part on the capacitance of the capacitive sensor  404 . In this case, the capacitive sensor acts to pull the base frequency of the crystal oscillator as a function of the pressure sensed at the capacitor. 
         [0031]      FIG. 4  illustrates a package for sensors in accordance with embodiments of the present invention. A number of sensors  500  are disposed within a common housing  502 . For each sensor  500 , there is a corresponding ferrite ring  400 , which is disposed in a portion  504  of the housing  502  that is made from a dielectric material, for example PTFE. While ordinarily there will be a one-to-one ratio of sensors to rings, it is also possible to have one ring correspond to two or even more sensors. As described above, the rings  400  couple the sensors to the transmission line  102 . The sensors, in turn, are held in a metal block portion  506  of the sensor module. Tubing  508  is threaded into the metal block in order to positively locate the sensor package. In a typical application, this tubing may constitute either the production tubing itself, or an extension of the production string. 
         [0032]    As will be appreciated, it is possible to combine pressure and temperature sensors in a single package, such that the temperature measurements may be used to help account for temperature related drift of the pressure sensor. 
         [0033]    To account for variations in response that are well-dependent rather than temperature or pressure dependent, a calibration crystal sensor may be included along with the primary sensor. In this approach, the calibration crystal sensor is provided with its own power source, for example a battery. The resulting sensor is isolated from the well impedance, eliminating well-dependent effects. As an example, the sensor circuitry may include transistors that, in part, act to isolate the calibration crystal sensor when under power. Though the battery may be of limited life, it is possible to use measurements from the calibration crystal sensor during the battery lifetime, and then apply the generated calibration data to ongoing measurements after the calibration sensor has expired. In this regard, a calibration curve or calibration lookup table may be generated over the battery lifetime and stored for use in later measurements. 
         [0034]    Another approach is to make use of a temperature insensitive crystal that is isolated from the ambient pressure, similar to that used in the pressure sensor of  FIG. 3A . In this variation, the crystal signal, isolated from pressure and relatively insensitive to temperature, will only react to the particular electromagnetic transmission characteristics of the well in which it is positioned. Therefore, its output can be regarded as being representative of the well shift only, unaffected by the other environmental factors. As will be appreciated, this approach may be used in conjunction with the powered calibration sensor previously described to provide additional information regarding the nature of the well-shift phenomenon. In this regard, the powered sensor may be used for calibrating the well-shift monitoring crystal sensor during the period in which the power supply is active. Once the power supply is exhausted, then the unpowered well-shift monitoring crystal sensor may continue to be used in accordance with the previously measured and stored calibration information. 
         [0035]    As noted above, the annulus may be filled with a dielectric fluid to allow for transmission without the use of a separate coaxial cable. In particular, the A annulus may include a dielectric fluid retained by a packer at the distal (formation side) end of the string. In general, for shallow wells relatively low-density fluids such as oils (including crude and/or diesel) or the like may be used. For deeper wells (i.e., higher pressure environments) denser materials should be used. For example, oil-based drilling muds incorporating density-increasing solids such as barite, calcium carbonate, hematite or other minerals may be used. In particular, such fluids may be selected such that they are in the neighborhood of 12-16 ppg (pounds per gallon), depending on expected or measured pressures present in the formation or reservoir. 
         [0036]    In a particular embodiment, the dielectric fluid is selected to be solid-free, thereby reducing the possibility of changes in properties due to settling of suspended solids over time. Furthermore, such fluids may be selected to be weighted to match drilling fluids based, for example, on depths of deployment and pressures present at those depths. By way of example, weights between about 10 ppg and about 18 ppg and more particularly between about 12 ppg and about 16 ppg may be useful. 
         [0037]    In an embodiment, the fluid is selected to be a halogenated hydrocarbon. Halogenated hydrocarbons should be understood to include chlorinated, brominated, fluorinated and/or iodinated hydrocarbons and blends thereof. Such halogenated hydrocarbons may be produced by adding a halogen to crude oil, diesel and/or more generally, fuel oil. In particular, blends of various halogenated materials may be useful in allowing the user to reach a particular target density, depending on the particular down-hole conditions. In particular embodiments, an emulsifier may be used in order to improve the miscibility of the halogenated compounds, though in general, this is not a requirement. 
         [0038]    In an example, perchlorethylene (about 13 ppg) may be added to a hydrocarbon base selected to create a packer fluid having a density of between about 7 and about 20 ppg. Hydrocarbon bases used for this purpose can be selected for their density, in order to render a final product of higher or lower density. For example, diesel, mineral oil, paraffins, olefins, esters, or combinations thereof tend to produce lower density mixtures than the previously mentioned hydrocarbons. 
         [0039]    Those skilled in the art will appreciate that the disclosed embodiments described herein are by way of example only, and that numerous variations will exist. The invention is limited only by the claims, which encompass the embodiments described herein as well as variants apparent to those skilled in the art.