Abstract:
In a method and apparatus for estimating a downhole fluid parameter include a body having an elongated cavity defined by an electrically conductive inner surface, an central electrical conductor located coaxially within the cavity, wherein an annular region is defined by the cavity inner surface and an outer surface of the electrical conductor. A fluid is conveyed to the annular region. A first electrode is coupled to the central electrode, a second electrode coupled to the central electrode, and at least one intermediate electrode coupled to the central electrode between the first electrode and the second electrode. An input signal is applied to the central electrode and first and second output signals are emitted from the second electrode intermediate electrode. The output signals are used in part to estimate the fluid property of the fluid existing in the annular region.

Description:
BACKGROUND 
   1. Technical Field 
   The present disclosure generally relates to well bore tools and in particular to methods and apparatus for estimating fluid properties downhole. 
   2. Background Information 
   Oil and gas wells have been drilled at depths ranging from a few thousand feet to as deep as 5 miles. Wireline and drilling tools often incorporate various sensors, instruments and control devices in order to carry out any number of downhole operations. These operations may include formation testing, fluid analysis, and tool monitoring and control. 
   Formation testing may include any number of evaluations and estimations. In some cases, it is desirable to understand fluid content for formation fluids produced from a subterranean formation and/or the content of drilling fluids used during drilling operations. 
   One method and apparatus for estimating fluid properties is a two-port, transverse electromagnetic mode, coaxial waveguide flowline sensor as described in U.S. Pat. No. 5,677,631, which is hereby incorporated by reference. 
   There is a need for determining downhole fluid properties such as capacitance and conductance over an entire range of fluid properties from dry gas to saltwater. 
   SUMMARY 
   The following presents a general summary of several aspects of the disclosure in order to provide a basic understanding of at least some aspects of the disclosure. This summary is not an extensive overview of the disclosure. It is not intended to identify key or critical elements of the disclosure or to delineate the scope of the claims. The following summary merely presents some concepts of the disclosure in a general form as a prelude to the more detailed description that follows. 
   Disclosed is an apparatus for estimating a fluid property downhole. The apparatus includes a body having an elongated cavity defined by an electrically conductive inner surface and a central electrical conductor located coaxially within the cavity, wherein an annular region is defined by the cavity inner surface and an outer surface of the electrical conductor. The apparatus further includes a first electrode coupled to the central electrode, a second electrode coupled to the central electrode, and at least one intermediate electrode coupled to the central electrode between the first electrode and the second electrode, wherein an input signal applied to the central electrode and at least a first output signal emitted from the second electrode and a second output signal emitted from the at least one intermediate electrode are used in part to estimate the fluid property of a fluid existing in the annular region. 
   In one aspect, an apparatus includes a carrier that is transportable to a downhole well borehole location, a sensor body coupled to the carrier, the sensor body having an internal elongated cavity having an electrically conductive inner surface, an inflow fluid passageway for flowing a high-pressure fluid into the internal elongated cavity and an outflow fluid passageway for flowing high-pressure fluid from the internal elongated passageway. The apparatus according to this aspect further includes a central electrical conductor positioned coaxially in the internal elongated cavity such that fluid entering the elongated cavity flows within an annular space between the central electrical conductor and the electrically conductive wall, the central electrical conductor and electrically conductive wall forming a wave guide. A first electrically conductive electrode receives an electrical input signal from a transmitter, the first electrically conductive electrode being connected to a first end of the central electrical conductor. A second electrically conductive electrode emits a first return signal to a receiver, the second electrically conductive electrode being connected to a second end of the central electrical conductor; and at least one intermediate electrode that emits a second return signal to the receiver, the at least one intermediate lead being connected to the central electrical conductor between the first electrically conductive electrode and the second electrically conductive electrode, wherein the first return signal and second return signal are indicative of the fluid property. 
   In another aspect, a method for estimating a fluid property downhole includes carrying a sensor to a downhole location using a carrier, the sensor including a body having an elongated cavity defined by an electrically conductive inner surface, an central electrical conductor located coaxially within the cavity, wherein an annular region is defined by the cavity inner surface and an outer surface of the electrical conductor, a first electrode coupled to the central electrode, a second electrode coupled to the central electrode, and at least one intermediate electrode coupled to the central electrode between the first electrode and the second electrode. The method further includes flowing a fluid into the annular region, transmitting an input signal to the central electrode, emitting a first output signal from the second electrode, emitting a second output signal the at least one intermediate electrode, and using the emitted first output signal and the emitted second output signal in part to estimate the fluid property of the fluid existing in the annular region. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the several non-limiting embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein: 
       FIG. 1  illustrates a non-limiting example of a flowline sensor for estimating properties of a fluid downhole; 
       FIG. 2  is a plot of selected fluid properties with respect to time during a clean-up process; 
       FIG. 3  schematically represents the electrical characteristics of a flowline sensor according to  FIG. 1 ; 
       FIG. 4  is a fluid property plot comparing a two-port flowline sensor to a sensor according to the present disclosure; and 
       FIG. 5  is another fluid property plot comparing a two-port flowline sensor to a sensor according to the present disclosure. 
   

   DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1  illustrates a non-limiting example of a downhole fluid property sensor  100  according to the disclosure. In the example of  FIG. 1 , the sensor  100  is shown mounted on a carrier or tool body  102 . The sensor  100  shown includes a sensor body  104  and an electrically conductive member referred to herein as a central conductor  106  that is disposed inside a sensor body cavity  108  that includes an electrically conductive inner wall. An annular volume is formed between an outer diameter of the central conductor  106  and the electrically conductive inner wall the sensor body cavity  108 . 
   Several electrically conductive electrodes are coupled to the central conductor  106  and extend through the sensor body  104 . In one non-limiting embodiment, the several electrodes include a transmitter electrode  110  electrically coupled to the central conductor  106  at or near one end of the central conductor  106  and a receiver electrode  112  electrically coupled to the central conductor  106  at or near a distal end of the central conductor  106 . In several non-limiting embodiments, one or more intermediate electrodes  114 ,  116  are electrically coupled to the central conductor  106  between the transmitter electrode  110  and the receiver electrode  112 . In several embodiments, an electric insulating material  118  is used to electrically isolate each electrode from the sensor body  104 . Each electrode may be terminated inside a suitable electrical terminating device  120  such as a coaxial contact or other terminating device that allows for the connection of electrical leads that conduct signals to and/or from each electrode. It is not necessary that all terminating devices be of the same type. 
   In the non-limiting example of  FIG. 1 , the sensor body  104  is shown mounted inside an atmospheric cavity  122 . The cavity  122  may be formed by machining a portion of the tool body  102  or by any other suitable method of forming an atmospheric cavity  122 . In a downhole tool as disclosed, the atmospheric cavity  122  may be isolated from the wellbore with a suitable cover  124  and seal  126 . The cavity volume may further be filled using a suitable potting material once all electrical connections to the terminations  120  are made. 
   The carrier  102  may include a fluid channel  128  that leads to a sensor inlet snorkel  130 . Seals  132 ,  134  may be disposed between the inlet snorkel  130  and the sensor body  104  and the carrier  102 , respectively. The inlet snorkel  130  includes an internal channel  136  that interfaces with the fluid channel  128  to provide a continuous channel from the carrier  102  to the sensor body cavity  108 . An outlet snorkel  138  is coupled to the sensor body  104  and includes an inner channel  140  that leads from the sensor body cavity  108  to an exiting fluid channel  142 . Seals  132  and  134  seal the interface between the outlet snorkel  138  and the sensor body  104  and the tool body  102 , respectively. 
   In several non-limiting embodiments, the sensor  100  may be implemented downhole on a wireline or in a while-drilling configuration. The carrier  102  therefore, may be any number of downhole carriers such as a wireline cartridge or sonde, a bottom hole assembly (BHA), a drill-string or any other known carrier that is conveyable into a well borehole to traverse a subterranean formation. In other embodiments, the carrier may be a modular insert that may be detachably coupled to a wireline cartridge or sonde, a bottom hole assembly (BHA), or to a drill-string sub. 
   Still referring to  FIG. 1 , a radio frequency transmitter  144  may be used to generate a sensor signal that is transmitted via a coaxial cable connected to one of the sensor connectors  120   a . In one embodiment, the connector  120   a  provides a standard 50 ohm RF connector to the waveguide flowline sensor  100 . The connector  120   a  may be mounted on a small, rigid, feedthrough conductor  110  that is seated coaxially within a ceramic or other electrical insulator  118  filling a feedthrough tunnel  146  that leads to the sensor body cavity  108  of the sensor  100 . The electrical insulator  118  may be used to form a high pressure seal between the feedthrough conductor and the body of the sensor  100 . The feedthrough conductor and the walls of the tunnel  146  form a coaxial feedthrough waveguide that provides a signal path between the connector  120   a , and the central conductor  106  suspended coaxially within the sensor body cavity  108  by the feedthrough conductor. 
   In the non-limiting example shown, a second rigid coaxial electrical feedthrough conductor  112  may be attached to an opposite end of the central conductor  106 . The second conductor  112  may extend through a feedthrough tunnel  148  to another of the connectors  120   b . The feedthrough conductor  112  is coaxially seated within a ceramic electrical insulator  118  that fills the tunnel  148  to form a feedthrough waveguide which provides a signal path between the waveguide flowline cavity  108  and the output connector  120   b.    
   The central conductor  106  and the walls of the flowline cavity  108  form a TEM mode waveguide, which is referred to herein as the flowline waveguide and is distinguished from the feedthrough waveguides. The output connector  120   b  provides an output signal to a coaxial cable connected to a receiver  150 . The receiver  150  may also receive a calibration signal and a reference signal from the transmitter  144 . In one embodiment, the receiver  150  includes a processor  152  for processing received signals. The processed signals may be an estimate of desired fluid properties or the processed signals may be used to estimate desired properties of fluid in the sensor  100 . In several other non-limiting embodiments, the processor may be located in the transmitter  144 , in another location that is on the carrier  102  or on the wireline or drill string, or the processor  152  may be located at a surface location. 
   In several non-limiting embodiments, intermediate conductors  114 ,  116  are connected to the central conductor  106  and pass through insulated feedthrough tunnels in substantially the same manner as the transmitter conductor  110  and the receiver conductor  112  described above. For brevity, mechanical details and connectors  120  are not shown here for the additional intermediate conductors. These aspects may be implemented by those skilled in the art with the benefit of this disclosure without additional illustration or extended discussion. The output of each intermediate conductor  114 ,  116  is conveyed to the receiver  150  substantially as described with respect to the receiver conductor  112  discussed above. In other particular embodiments, more measuring electrodes may be included and two or more switching transmitters may be used to generate the input signal. In one particular embodiment, at least three intermediate electrodes are used to provide additional measurements along the central conductor. Furthermore, the receiver may include a single multiplexed channel for receiving each sensor output signal, or a multi-channel receiver may be used. 
   Now that several non-limiting structural aspects of the sensor  100  have been described, operational aspects of the disclosure may be discussed.  FIG. 2  illustrates typical output signals from a Resistivity-Capacitance (RC) sensor during a clean-up process for a downhole fluid sample. Dashed plot line  202  illustrates capacitance of a sampled fluid transitioning over time from water to hydrocarbon and solid plot line  204  illustrates resistivity of the fluid transitioning from water to hydrocarbon. As shown, a high water content fluid sample exhibits high capacitance and low resistivity transitioning to a hydrocarbon fluid sample exhibiting low capacitance and high resistivity. There exists a wide range of resistivity of fluids that can be encountered by a formation testing tool. The present disclosure includes a downhole fluid sensor  100  incorporating a high resolution capacitance sensor and a conductivity sensor with a wide dynamic range into a single sensor package. 
   Several sensor embodiments, such as the embodiment described above and shown in  FIG. 1 , may be used to determine the resistivity and capacitance of a fluid by measuring the attenuation and phase shift of a high frequency electromagnetic wave propagating through the fluid. In one non-limiting example, the sensor  100  is constructed in a coaxial arrangement of conductors with a volume between the conductors being filled with the fluid sample being measured. The sensor  100  acts as an electromagnetic signal transmission line with the fluid between the conductors changing the conductance and capacitance between the conductors formed by the central conductor  106  and an inner conductive wall of the sensor body cavity  108 . 
   The conductance per unit length of the sensor coaxial conductors is inversely proportional to the resistivity of the fluid between the coaxial conductors of the sensor. The capacitance per unit length is directly proportional to the dielectric constant of the fluid between the conductors. Both the conductance and capacitance are inversely proportional to the logarithm of the ratio of the inner diameter (I.D) of the sensor body inner wall forming the outer conductor to the outer diameter (O.D.) of the central conductor  106  forming the inner conductor of the coaxial transmission line. As this conductance or capacitance increases, the attenuation and phase shift of signal in the form of an electromagnetic wave propagating through the transmission line increases. Another factor that affects the range and resolution of the sensor is the impedance mismatch between the sensor and cables connecting it to the electronics making the measurement. The effect of these impedance mismatches is covered in detail in U.S. Pat. No. 5,677,631 incorporated herein by reference. To increase the resolution of the capacitance measurement the ratio of the conductor diameters may be minimized and the length of the conductors increased so long as adequate fluid flow through the formation tester sample line is maintained. The measurable conductivity range may be increased by measuring attenuation differentially to negate the attenuation due to the impedance mismatches and by decreasing the length of the conductors. This example may be shown schematically as a transmission line. 
     FIG. 3  illustrates an example of the resulting transmission line  300  having four ports allowing for a single ended capacitance measurement (V 3 /V 0 ) and a differential conductivity measurement (V 2 /V 0 −V 1 /V 0 ). The transmission line  300  is connected to a signal source  302 , and the structural components of the sensor  100  may be electrically represented by a characteristic resistance R of the conductors and a characteristic inductance L of the coaxial conductors. The particular value of R and L are determined by the particular size and materials of construction used for the sensor  100 . Furthermore, R and L may be considered as values per unit length. 
   The electrical characteristics of the annular space between the coaxial conductors may be schematically represented as a characteristic capacitance C and a characteristic conductance G. For each of the intermediate conductors  114 ,  116  a parallel conductance/capacitor circuit  304 ,  306  may be used to represent the effect of the fluid on the input signal Vo at the location of the particular conductor. Likewise, the receiver conductor  112  position may be represented by a similar G-C circuit  308 . Those skilled in the art with the benefit of the present disclosure will understand that additional sensor conductors may be added and may be represented by similar G-C circuits. 
   In operation, referring now to  FIGS. 1 and 3 , fluid being tested flows to the sensor  100  via the fluid inflow channel  128  leading to the sensor inlet snorkel  130 . The fluid flows into the sensor body cavity  108  via the inlet snorkel internal channel  136 . The transmitter generates an RF input signal V 0 , which is transmitted to the signal input (transmitter) lead  110 . The receiver  150  receives a signal V 3  from the end lead  112 , and the receiver receives a separate signal from each on the intermediate leads; in this case V 1  from one intermediate lead  114  and V 2  from the second intermediate lead  116 . A differential voltage signal V diff  is generated by integrated or discrete electronic differential circuits, or V diff  may be generated using the processor  152 . Again, using the processor  152  or other circuits, a single ended capacitance measurement (V 3 /V 0 ) may be used to estimate capacitance of fluid in the sensor  100  and a differential conductivity measurement (V 2 /V 0 −V 1 /V 0 ) may be used to estimate the conductance of the fluid in the sensor. 
   The measurements may be made on single or multi-phase fluids. In several embodiments, the fluid may be drilling fluid, well bore fluid, pristine formation fluid or any combination of fluids. The sensor  100  may be operated with fluid substantially stationary in the sensor cavity or the fluid may be flowing through the sensor  100 . In one embodiment, the input signal is a sweep signal that covers substantially the entire range of downhole fluid properties from dry gas to saltwater. In one example, the entire range of fluid properties is measured using a single input sweep. 
   Fluid may be expelled from the sensor  100  using the outlet snorkel inner channel  140  that leads from the sensor body cavity  108  to the exiting fluid channel  142 . The fluid may be conveyed to a sampling chamber (not shown) for retrieval at the surface, or the fluid may be expelled into the well borehole. 
     FIG. 4  shows attenuation vs. conductivity plots comparing the non-limiting sensor example of  FIG. 1  to a typical two-port sensor. A two port sensor model is represented by dashed line  402  with measured values shown as discrete points  404   a ,  404   b ,  404   c , and  404   d . By comparison, a model of the differential sensor shown as a solid line  406  with discrete measured values shown at  408   a ,  408   b ,  408   c , and  408   d  that show decreased attenuation over the same range of fluid conductivity for the two-port sensor. In conductive fluids the differential measurement has significantly less attenuation of the signal being measured as shown in  FIG. 4 . As a consequence it is possible to measure fluids of higher conductivity. 
     FIG. 5  shows phase shift in degrees vs. dielectric constant plots comparing the non-limiting sensor example of  FIG. 1  to a typical two-port sensor. A two port sensor model is represented by dashed line  502  with measured values shown as discrete points  504   a ,  504   b , and  504   c . By comparison, a model of the differential sensor shown as a solid line  506  with discrete measured values shown at  508   a ,  508   b , and  508   c . The comparison shows an increased phase shift over the same range of fluid dielectric constant. In nonconductive fluids the reduced diameter ratio and increased length of the disclosed sensor causes the phase shift due to the dielectric constant of the fluid to increase as shown in the figure. With a larger phase shift it is possible to resolve smaller changes in dielectric constant. 
   The present disclosure is to be taken as illustrative rather than as limiting the scope or nature of the claims below. Numerous modifications and variations will become apparent to those skilled in the art after studying the disclosure, including use of equivalent functional and/or structural substitutes for elements described herein, use of equivalent functional couplings for couplings described herein, and/or use of equivalent functional actions for actions described herein. Such insubstantial variations are to be considered within the scope of the claims below.