Patent Publication Number: US-2017350233-A1

Title: Detecting a structure in a well

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 61/075,913, entitled “Casing Collar/Centralizer Identification Logging in Receiver Well,” filed Jun. 26, 2008 (having Attorney Docket No. 23.0699), which is hereby incorporated by reference. It is also a continuation of application No. 2001-0204896 (filed Jun. 10, 2009 under the U.S. Pat. No. 12/996,524) also entitled “Detecting a structure in a Well”, which is hereby incorporated by reference. 
    
    
     BACKGROUND 
     Geological formations forming a reservoir for the accumulation of hydrocarbons or other fluids in the subsurface of the earth contain a network of interconnected paths in which fluids are disposed whereby the fluids may ingress or egress from the reservoir. To determine the behavior of the fluids in this network, knowledge of both the porosity and permeability of the geological formations is desired. From this information, efficient development and management of hydrocarbon reservoirs may be achieved. For example, the resistivity of geological formations is a function of both porosity and permeability. Considering that hydrocarbons are electrically insulating and most water contain salts, which are highly conductive, resistivity measurements are a valuable tool in determining the presence of a hydrocarbon reservoir in the formations. 
     One technique to measure formation resistivity involves the use of electromagnetic induction via transmitters of low frequency magnetic fields that induce electrical currents in the formation. These induced electrical currents in turn produce secondary magnetic fields that can be measured by a magnetic field receiver. 
     The performance of a magnetic field receiver positioned within a wellbore may be disrupted by the presence of certain electrically conductive and/or magnetic structures such as parts of the well casing assembly, such as casing collars or casing centralizers, patches, or perforated casing segments. Casing collars are used to connect different sections of a casing, while casing centralizers are used to generally center the casing within a well. Distortion of the magnetic field detected by a magnetic field receiver due to the presence of such structures may cause inaccurate results to be obtained from the electromagnetic induction survey data. 
     SUMMARY 
     The present disclosure relates generally to detecting a structure within a well casing assembly in a well. The present disclosure also relates to a method to minimize casing imprints on induction survey data and improve the resolution of the inversion images and results for electromagnetic induction survey, such as cross-well, surface to borehole, and single-well EM surveys. 
     In general, according to an embodiment, a tool for detecting a structure in a well includes a receiver coil having a first winding (main winding) and a second winding (feedback winding) wound on a magnetic core, and a circuit to apply an input signal to the second winding. The tool further includes a detection circuit to detect a response of the first winding to the input signal applied to the second winding, or the trans-impedance between the feedback winding and the main winding, where the response of the first winding indicates the presence of the structure in the well if the receiver coil is positioned proximate to the structure. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative arrangement that includes a tool according to an embodiment of the invention; 
         FIG. 2  is a schematic diagram of components in the tool for detecting electrically conductive and/or magnetic structures within a well casing assembly in a well, according to an embodiment; 
         FIG. 3  is a flow diagram of a process of detecting a structure within a well casing assembly in a well using a tool according to an embodiment; 
         FIG. 4  is a schematic diagram of an illustrative arrangement that includes a tool having receivers, where the tool is positioned to avoid interference by electrically conductive and/or magnetic structures within a well casing assembly in a well, according to an embodiment; and 
         FIG. 5  is an example of CCID log in a well,  5 A showing a Receiver CCID log in Cr steel cased well section, and  5 B showing a Receiver CCID log in Carbon steel cased well section 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these details and that numerous variations or modifications from the described embodiments are possible. 
     As used here, the terms “above” and “below”; “up” and “down”; “upper” and “lower”; “upwardly” and “downwardly”; and other like terms indicating relative positions above or below a given point or element are used in this description to more clearly describe some embodiments of the invention. However, when applied to equipment and methods for use in wells that are deviated or horizontal, such terms may refer to a left to right, right to left, or diagonal relationship as appropriate. 
     In accordance with some embodiments, a mechanism or technique is provided to allow for detection of structures within a well casing assembly in a well that may interfere with an electromagnetic (EM) induction survey used for acquiring information about a subterranean formation surrounding the well. The EM induction survey can comprise a cross-well survey, a surface-to-wellbore survey, or a single wellbore survey. To implement a cross-well survey, one or more EM transmitters are placed in a first wellbore, while one or more EM receivers are placed in a second wellbore to detect EM signals transmitted by the EM transmitter(s) and affected by the subterranean formation between the first and second wellbores. To implement a surface-to-wellbore survey, one or more EM transmitters are placed at or near the earth surface (e.g., land surface or sea floor) or towed in a body of water (marine), or towed in air above the surface (air-borne), and one or more EM receivers are placed in a wellbore to detect EM signals transmitted by the EM transmitter(s) and affected by the subterranean formation between the earth surface and the wellbore. To implement a single-wellbore survey, both EM transmitter(s) and EM receiver(s) are placed in the same wellbore. In the first two of the survey techniques discussed above, the EM transmitter is positioned relatively far away from the EM receiver, and in the third survey techniques (single well survey), the transmitter is placed away from the receivers such that receivers are in the far-field region of the transmitter, and thus, is considered a remote EM transmitter. 
     The structures within a well casing assembly in a well that can be detected using a mechanism or technique according to some embodiments include casing collars, casing centralizers, or any other electrically conductive and/or magnetic structure that can interfere with EM induction surveying. Electrically conductive and/or magnetic structures such as casing collars and casing centralizers add relatively strong imprints to cross-well, surface-to-wellbore, or single wellbore EM measurements made by an EM receiver positioned proximate such a structure. Typically, attempting to remove such imprints to obtain high-resolution images (through data inversion) of a surrounding subterranean formation is challenging. Therefore, receivers can be placed in the well positioned so as to avoid or limit effects from these structures during EM induction surveys. 
     In accordance with some embodiments, the mechanism or technique of detecting electrically conductive and/or magnetic structures within a well casing assembly in a well involves using a tool that has a detection mechanism that includes a receiver coil having both a main winding and a secondary winding (referred to herein as a “feedback” winding). The main winding and feedback winding are wound around a core, which can be a magnetic core or an air core. The detection mechanism according to some embodiments also includes an application circuit to apply an input signal to the feedback winding, and a detection circuit to detect a response of the main winding to the input signal applied to the feedback winding. The response of the main winding can be processed to identify the presence of an electrically conductive and/or magnetic structure proximate the receiver coil. If the receiver coil is positioned proximate an electrically conductive and/or magnetic structure, then the response of the main winding will indicate the presence of such structure. A receiver coil is considered to be “proximate” the electrically conductive and/or magnetic structure if the receiver coil is close enough such that the structure affects the electromagnetic behavior of the receiver coil. 
     Note that the electrically conductive and/or magnetic structures that are detected by the detection mechanism according to some embodiments are structures in addition to any casing that may be present in the well. A “casing” refers to any structure that lines a wellbore. The casing provides a relatively smaller effect on EM measurements made by an EM receiver in the wellbore, as shown in  FIG. 5  as an example. The structures that are detected by the detection mechanism according to some embodiments are “intermittent” structures that are not continuously provided within sections of the wellbore. These intermittent electrically conductive and/or magnetic structures are distinguished from the casing that extends continuously along at least a portion of the wellbore. 
       FIG. 1  illustrates a tool  102  that has been lowered into a wellbore  104  by a carrier structure  106 . The carrier structure  106  can be a wireline, coiled tubing, or any other carrier structure that extends from a wellhead  107  of the wellbore  104 . The carrier structure  106  includes a communications medium (e.g., electrical communications medium, optical communications medium, etc.) to allow for communication between the tool  102  and surface equipment  108 . 
     The surface equipment  108  includes a computer  110  that has a processor  112  and storage media  114 . Software  116  is executable on the processor to perform predefined tasks. In accordance with some embodiments, the software  116  can process measurement data received from the tool  102  to determine presence and locations of intermittent electrically conductive and/or magnetic structures within the well casing assembly in the wellbore  104  that can interfere with an EM induction survey. 
     The measurement data that can be received by the computer  110  includes measurement data collected by receivers R 1 , R 2 , R 3 , and R 4 . Although four receivers are shown in  FIG. 1 , it is noted that in alternative implementations, different numbers of receivers can be employed, from one to more than one. One or more of the receivers R 1 -R 4  include the detection mechanism according to some embodiments that can be used for detecting intermittent electrically conductive and/or magnetic structures in the within the well casing assembly in the wellbore  104 . As shown in  FIG. 2 , each of the receivers R 1 -R 4  includes the same detection mechanism. In other implementations, the detection mechanism can be omitted in some of the receivers R 1 -R 4 . 
     As shown in  FIG. 2 , such detection mechanism in each receiver includes a receiver coil  200  that has a main winding  202  and a feedback winding  204  both wound on the core  206 . An application circuit  208  is used to apply an input signal  210  to the feedback winding  204 . The application circuit  208  for applying the input signal  210  to the feedback winding  204  can be driven by a local signal generator provided in the tool  102 . Alternatively, the application circuit  208  can include conductive lines that are driven by a signal generator provided in the surface equipment  108 . 
     The input signal  210  provided to the feedback winding  204  induces a response in the main winding  202 . The induced response includes an electrical voltage across the main winding  202  that can be detected by the detection circuit  208 . The detection circuit  208  provides an output voltage V out  that represents the response of the main winding  202  to the input signal  210  applied to the feedback winding  204 . 
     The input signal  210  provided to the feedback winding  204  includes either an oscillating (periodic) signal having a predetermined frequency, or an input pulse that induces a transient response in the main winding  202 . 
     If the input signal  210  is an oscillating signal having a predetermined frequency, then the response at the main winding  202  measured by the detection circuit  208  can be a first harmonic response. In accordance with some embodiments, the frequency of the input signal  210  can be varied, and the corresponding responses of the different frequencies can be measured. 
     The drive current (of the input signal  210 ) applied to the feedback winding  204  can also be monitored, such that the trans-impedance, i.e., the ratio between the measured voltage on the main winding  202  and the current in the feedback winding  204  can be measured. 
     When the receiver coil  200  is positioned proximate an intermittent electrically conductive and/or magnetic structure within the well casing assembly in the wellbore  104 , the response of the receiver coil is different than the response of the receiver coil positioned at a larger distance away from the intermittent electrically conductive and/or magnetic structure. Different types of such intermittent structures, such as casing collars and casing centralizers, can cause different responses in the receiver coil  200 . By measuring the responses of the receiver coil  200  at multiple different frequencies, it is possible to identify and distinguish between the different types of intermittent structures. 
       FIG. 3  shows a process of performing detection of intermittent electrically conductive and/or magnetic structures within the well casing assembly in the wellbore  104 . A tool that includes a detection mechanism according to some embodiments is lowered (at  302 ) into the wellbore  104  ( FIG. 1 ). As the tool is lowered in the wellbore  104 , an excitation can be applied (at  304 ) to cause the input signal  210  ( FIG. 2 ) to be applied to the feedback winding  204  of the receiver coil  200 . The excitation that is applied can be produced at a local signal generator provided in the tool, or a signal generator located at the surface equipment  108  in  FIG. 1 . 
     If there are multiple detection mechanisms in the corresponding receivers (such as receivers R 1 -R 4 ) in the tool, then the applied excitation input signal  210  is applied to each of the feedback windings in the corresponding detection mechanism. 
     The voltage across the main winding  202  (that is responsive to the input signal  210  applied to the feedback winding  204 ) is then measured (at  306 ). The computer  110  in the surface equipment  108  then receives (at  308 ) the measured voltage of the main winding of each receiver coil  200 . The computer  110  further receives the voltage and/or current in the feedback winding  204  induced by the input signal  210 . If there are multiple detection mechanisms, then multiple measured voltages and currents of main windings and feedback windings are received at the computer  110 . Note that the applied excitation can cause the frequency of the input signal  210  provided to the feedback winding  204  of each detection mechanism to be varied, such that responses of the main winding of each detection mechanism at corresponding different frequencies are received. 
     Trans-impedance values are then calculated (at  310 ) based on the received measured voltages of the main winding(s) and the voltages and/or currents of the feedback winding(s). If the input signal  210  applied to a feedback winding  204  has been varied across multiple frequencies, then the trans-impedances at different frequencies can be determined. Based on the calculated trans-impedance values, the computer  110  determines (at  312 ) whether any intermittent electrically conductive and/or magnetic structure has been detected. 
     Note that the process including tasks  302 - 312  can be continually performed as the tool is lowered, or up-logged in the wellbore  104 . The trans-impedance values are continually monitored to detect intermittent electrically conductive and/or magnetic structures. Once such an intermittent structure is detected, then the corresponding position of the intermittent structure can be recorded. 
     The procedure of  FIG. 3  can be performed in the context of a depth log. Effectively, measurements at different depths of the tool are collected. The measurements are used to identify intermittent electrically conductive and/or magnetic structures in the wellbore. These identified intermittent structures can be provided in the depth log. Based on locations (depths) of the detected intermittent structures, a well-log operator can position the tool  102  of  FIG. 1  such that receivers R 1 -R 4  are positioned away from the intermittent electrically conductive and/or magnetic structures during survey measurements. Such an arrangement is shown in  FIG. 4 , where each receiver R 1 -R 4  is positioned between a pair of intermittent structures (either casing collars or casing centralizers). In this manner, when the receivers R 1 -R 4  are used to perform EM induction surveying, interference caused by the intermittent electrically conducted and/or magnetic structures with EM measurements collected by receivers R 1 -R 4  can be avoided. 
     Embodiments of the invention can be performed in wellbores that are lined with either magnetic or non-magnetic casings. With magnetic casings, however, it is noted that the frequencies used for exciting the feedback windings should be set at lower frequencies. 
     The detection mechanism according to some embodiments can also be used to correlate depths of the tool in the wellbore. If the depths of casing collar locators and/or casing centralizers have been previously determined, then the detection mechanism can be used to detect presence of such casing collar locators and/or casing centralizers such that the depth of a tool including the detection mechanism can be determined. 
     As yet another embodiment, the detection mechanism can be used to detect sections of a casing that have abnormalities, detect missing casing sections (e.g., sections that have been removed), detect casing patches, or detect sections that have been perforated. 
     Tasks  308 ,  310  and  312  depicted in  FIG. 3 , along with tasks for determining a depth log and identifying locations of intermittent electrically conductive and/or magnetic structures, can be performed by the software  116  executed in the computer  110  shown in  FIG. 1 . Instructions of the software  116  are loaded for execution on a processor (such as processor  112  in  FIG. 1 ). The processor includes microprocessors, microcontrollers, processor modules or subsystems (including one or more microprocessors or microcontrollers), or other control or computing devices. As used here, a “processor” can refer to a single component or to plural components (e.g., one CPU or multiple CPUs). Alternatively, various of the determining and location identifying steps could be performed by analogous software executed on a processor in a downhole tool. 
     Data and instructions (of the software) are stored in respective storage devices, which are implemented as one or more computer-readable or computer-usable storage media. The storage media include different forms of memory including semiconductor memory devices such as dynamic or static random access memories (DRAMs or SRAMs), erasable and programmable read-only memories (EPROMs), electrically erasable and programmable read-only memories (EEPROMs) and flash memories; magnetic disks such as fixed, floppy and removable disks; other magnetic media including tape; and optical media such as compact disks (CDs) or digital video disks (DVDs). 
     While the invention has been disclosed with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover such modifications and variations as fall within the true spirit and scope of the invention.