Abstract:
Methods and tools for detecting casing position downhole is presented. The method utilizes electromagnetic. (EM) tools with tilted antenna systems to detect casing position. Sometimes titled antenna designs also increase EM tools&#39; sensitivity to formation parameters, which can lead to false signals for casing detection. In addition, it is very difficult to distinguish measured signals between a casing source and a formation source. The methods presented help to distinguish between the two sources more clearly. The methods and tools presented also help to minimize those environmental effects, as well as enhance the signals from a surrounding conductive casing. The methods herein provide ideas of EM tool&#39;s design to precisely determine casing position within a certain distance to casing position.

Description:
BACKGROUND 
       [0001]    The world depends on hydrocarbons to solve many of its energy needs. Consequently, oil field operators strive to produce and sell hydrocarbons as efficiently as possible. Much of the easily obtainable oil has already been produced, so new techniques are being developed to extract less accessible hydrocarbons. These techniques often involve drilling a borehole in close proximity to one or more existing wells. One such technique is steam-assisted gravity drainage (“SAGD”) as described in U.S. Pat. No. 6,257,334, “Steam-Assisted Gravity Drainage Heavy Oil Recovery Process”. SAGD uses a pair of vertically-spaced, horizontal wells less than 10 meters apart, and careful control of the spacing is important to the technique&#39;s effectiveness. Other examples of directed drilling near an existing well include intersection for blowout control, multiple wells drilled from an offshore platform, and closely spaced wells for geothermal energy recovery. 
         [0002]    One way to direct a borehole in close proximity to a cased well is through the use of electromagnetic (EM) logging tools. EM logging tools are capable of measuring a variety of formation parameters including resistivity, bed boundaries, formation anisotropy, and dip angle. Because such tools are typically designed for measuring such parameters, their application to casing detection may be adversely impacted by their sensitivity to such environmental parameters. Specifically, the tool&#39;s response to nearby casing can be hidden by the tool&#39;s response to various environmental. parameters, making it impossible to detect and track a cased well, or conversely making the tool produce false detection signals that could deceive the drilling team into believing they are tracking a nearby cased well when such is not the case. Such difficulties do not appear to have been previously recognized or adequately addressed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    A better understanding of the various disclosed system and method embodiments can be obtained when the following detailed description is considered in conjunction with the drawings, in which: 
           [0004]      FIG. 1  shows an illustrative drilling environment in which electromagnetically-guided drilling may be employed; 
           [0005]      FIG. 2  is an illustrative tilted antenna system with parallel and perpendicular transmitter-receiver pairs; 
           [0006]      FIG. 3  is an illustrative two-layered formation model; 
           [0007]      FIGS. 4A and 4B  are modeled tool responses to formation anisotropy as a function of frequency and dip angle; 
           [0008]      FIGS. 5A and 5B  are modeled tool responses to a nearby boundary as a function of boundary distance and dip angle; 
           [0009]      FIGS. 6A and 6B  are modeled tool responses to a nearby boundary as a function of frequency and dip angle; 
           [0010]      FIGS. 7A and 7B  are experimental 44″ tool responses to a nearby casing as a function of casing distance and frequency; 
           [0011]      FIGS. 8A and 8B  are experimental 52″ tool responses to a nearby casing as a function of casing distance and frequency; 
           [0012]      FIGS. 9A and 9B  are experimental tool responses to a nearby casing as a function of casing distance and antenna spacing; 
           [0013]      FIG. 10  shows a tool model that serves as a basis for a casing sensitivity calculation; 
           [0014]      FIG. 11A  shows tool sensitivity as a function of antenna spacing and frequency; 
           [0015]      FIG. 11B  shows tool signal levels as a function of antenna spacing and frequency; 
           [0016]      FIGS. 12A and 12B  are signal responses of a parallel and perpendicular transmitter-receiver pair, respectively, as a function of antenna spacing and frequency; and 
           [0017]      FIGS. 13A and 13B  are modeled 50′ tool responses as a function of casing distance and dip angle; and 
           [0018]      FIG. 14  is a flow diagram of an illustrative casing detection method. 
       
    
    
       [0019]    While the invention is susceptible to various alternative forms, equivalents, and modifications, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for supporting all alternative forms, equivalents, and modifications falling within the scope of the appended claims. 
       DETAILED DESCRIPTION 
       [0020]    The issues identified in the background are at least in part addressed by the disclosed casing detection tools and methods. At least one disclosed method embodiment includes obtaining formation resistivity measurements from a first borehole. Based at least in part on these measurements, an expected environmental signal level is determined for a second borehole at a specified position relative to the first borehole. At least one of a transmitter-receiver spacing and an operating frequency is then selected to provide a desired detection signal level for the first borehole from the second borehole, such that the desired detection signal level will be greater than the expected environmental signal level, and a bottomhole assembly (BHA) is constructed with a tilted antenna logging tool having the selected spacing and/or operating frequency for use in the second borehole. 
         [0021]    At least one disclosed tool embodiment includes a tilted transmit antenna and two or more tilted receive antennas at least a selected spacing distance from the transmit antenna to detect components of a response to the transmit signal. The transmit signal has a frequency at or below a selected operating frequency, the frequency being selected in conjunction with the spacing to ensure that the expected casing detection signal level is greater than an expected environmental signal level. 
         [0022]    To further assist the reader&#39;s understanding of the disclosed systems and methods, we describe an environment suitable for their use and operation. Accordingly,  FIG. 1  shows an illustrative geosteering environment. A drilling platform  2  supports a derrick  4  having a traveling block  6  for raising and lowering a drill string  8 . A top drive  10  supports and rotates the drill string  8  as it is lowered through the wellhead  12 . A drill bit  14  is driven by a downhole motor and/or rotation of the drill string  8 . As bit  14  rotates, it creates a borehole  16  that passes through various formations. A pump  20  circulates drilling fluid through a feed pipe  22  to top drive  10 , downhole through the interior of drill string  8 , through orifices in drill bit  14 , back to the surface via the annulus around drill string  8 , and into a retention pit  24 . The drilling fluid transports cuttings from the borehole into the pit  24  and aids in maintaining the borehole integrity. 
         [0023]    The drill bit  14  is just one piece of a bottom-hole assembly that includes one or more drill collars (thick-walled steel pipe) to provide weight and rigidity to aid the drilling process. Some of these drill collars include logging instruments to gather measurements of various drilling parameters such as position, orientation, weight-on-bit, borehole diameter, etc. The tool orientation may be specified in terms of a tool face angle (a.k.a. rotational or azimuthal orientation), an inclination angle (the slope), and a compass direction, each of which can be derived from measurements by magnetometers, inclinometers, and/or accelerometers, though other sensor types such as gyroscopes may alternatively be used. In one specific embodiment, the tool includes a 3-axis fluxgate magnetometer and a 3-axis accelerometer. As is known in the art, the combination of those two sensor systems enables the measurement of the tool face angle, inclination angle, and compass direction. In some embodiments, the tool face and hole inclination angles are calculated from the accelerometer sensor output. The magnetometer sensor outputs are used to calculate the compass direction. 
         [0024]    The bottom-hole assembly further includes a ranging tool  26  to induce a current in nearby conductors such as pipes, casing strings, and conductive formations and to collect measurements of the resulting field to determine distance and direction. Using these measurements in combination with the tool orientation measurements, the driller can, for example, steer the drill bit  14  along a desired path  18  relative to the existing well  19  in formation  46  using any one of various suitable directional drilling systems, including steering vanes, a “bent sub”, and a rotary steerable system. For precision steering, the steering vanes may be the most desirable steering mechanism. The steering mechanism can be alternatively controlled downhole, with a downhole controller programmed to follow the existing borehole  19  at a predetermined distance  48  and position (e.g., directly above or below the existing borehole). 
         [0025]    A telemetry sub  28  coupled to the downhole tools (including ranging tool  26 ) can transmit telemetry data to the surface via mud pulse telemetry. A transmitter in the telemetry sub  28  modulates a resistance to drilling fluid flow to generate pressure pulses that propagate along the fluid stream at the speed of sound to the surface. One or more pressure transducers  30 ,  32  convert the pressure signal into electrical signal(s) for a signal digitizer  34 . Note that other forms of telemetry exist and may be used to communicate signals from downhole to the digitizer. Such telemetry may employ acoustic telemetry, electromagnetic telemetry, or telemetry via wired drillpipe. 
         [0026]    The digitizer  34  supplies a digital form of the telemetry signals via a communications link  36  to a computer  38  or some other form of a data processing device. Computer  38  operates in accordance with software (which may be stored on information storage media  40 ) and user input via an input device  42  to process and decode the received signals. The resulting telemetry data may be further analyzed and processed by computer  38  to generate a display of useful information on a computer monitor  44  or some other form of a display device. For example, a driller could employ this system to obtain and monitor drilling parameters, formation properties, and the path of the borehole relative to the existing borehole  19  and any detected formation boundaries. A downlink channel can then be used to transmit steering commands from the surface to the bottom-hole assembly. 
         [0027]      FIG. 2  shows an illustrative antenna configuration for ranging tool  26 . This particular antenna configuration is used below as a specific example for explaining the relative effects of environmental parameters as contrasted with a nearby casing string, but the conclusions are applicable to nearly all electromagnetic logging tools having at least one tilted antenna. Accordingly, the following discussion is not limiting on the scope of the disclosure. The illustrated configuration includes two transmit antennas (labeled Tup and Tdn) and a receive antenna (labeled Rx) midway between the two. Each of the antennas is tilted at 45° from the longitudinal axis of the tool, such that the receive antenna is parallel to one transmit antenna and perpendicular to the other. The centers of the antennas are equally spaced, with d being the distance between the receiver and each transmit antenna. As the tool rotates, the transmitters fire alternately and the receive signals detected by the receiver in response the transmitters Tup and Tdn are V Rx   Tup (β) and V Rx   Tdn (β), respectively, where β is tool&#39;s azimuthal angle, The tool&#39;s responses to a nearby casing string, a nearby fluid interface or bed boundary, or to an anisotropic dipping formation, is expected to take the following form: 
         [0000]        V   Rx   Tup (β)= A   1  cos(2β)+ B   1  cos(β)+ C   1  
 
         [0000]        V   Rx   Tdn (β)= A   2  cos(2β)+ B   2  cos(β)+ C   2  
 
         [0000]    where A i , B i , and C i  are complex coefficients representing the voltage amplitude of azimuthal-dependent double-period sine wave, a single-period sine wave, and a constant value for the receiver&#39;s response to the upper transmitter (i=1) or lower transmitter (i=2). Using a curve fitting function, the three complex voltage amplitudes for each response can be derived from the raw measured signal voltages in a straightforward manner Experiments indicate that when the coefficients for the tool&#39;s response to a nearby casing string are compared to coefficients for the tool&#39;s response to environmental parameters, the A i  coefficient for the casing string response has a larger magnitude than the B i  coefficient, while for responses to environmental parameters the reverse is generally true. Indeed, the B i  coefficient for the casing string response has been found to be relatively small compared to the A i  coefficient. Accordingly, the proposed casing detection tool preferably employs the A i  coefficient for detection and ranging measurements. Temperature compensation and voltage normalization can be accomplished by using the ratio |A i /C i |, and it has been found useful to employ a logarithm of this ratio, e.g., log 10 (|A i /C i |), when modeling the tool&#39;s operation. 
         [0028]    Three representative models will be employed to analyze the tool&#39;s response to (1) formation anisotropy; (2) a nearby boundary; and (3) a casing string.  FIG. 3A  shows a first model in which a tool is positioned in a relatively thick dipping formation having resistive anisotropy. The horizontal resistivity (Rx and Ry) is taken as 1 Ωm, while the vertical resistivity (Rz) is taken as 2 Ωm.  FIG. 313  shows a second model in which the tool is in a resistive formation (R t =200 Ωm) and is approaching a boundary with a more conductive formation (R t =1 Ωm). The tool&#39;s distance to the bed boundary (DTBB) is measured from the receive antenna to the closest point on the boundary.  FIG. 3C  shows a third model in which the tool is positioned at a distance d from a casing string in an otherwise homogeneous formation. 
         [0029]    The tool&#39;s responses to each of these three models are compared, beginning with the anisotropy model.  FIG. 4A  shows the measurements by the parallel transmit-receive antenna pair (hereafter the “parallel response”) with a 52 inch spacing between the antennas, while  FIG. 4B  shows the measurements by the perpendicular transmit-receive antenna pair with the same spacing. In both cases, the measurements are shown as a function of dip angle and transmit signal frequency. The measurements are shown in terms of the logarithm of the coefficient ratio, i.e., log10(|A i /C i |). Generally speaking, a stronger anisotropy response is observed at higher signal frequencies, Moreover, the tool measurements are fairly steady at dips of greater than 10 degrees, but they fall off sharply at smaller dip angles as the model becomes more symmetric about he tool axis. 
         [0030]      FIGS. 5A and 5B  show the tool&#39;s parallel and perpendicular responses to a nearby bed boundary as a function of dip angle and boundary distance. For these graphs, the tool is assumed to have an antenna spacing of 52 inches and a signal frequency of 125 kHz. The tool&#39;s response grows stronger as the distance to bed boundary shrinks, and the signal remains fairly steady so long as the dip angles are greater than about 10 degrees. Below this, the model symmetry increases and the measurements drop sharply. The nearby bed boundary measurements are also shown in  FIGS. 6A and 6B  as a function of signal frequency, confirming again that the tool response increases as a function of frequency, though less dramatically than in the first model. 
         [0031]      FIGS. 7A and 7B  show the tool&#39;s parallel and perpendicular responses to a nearby well casing as a function of casing distance and signal frequency, assuming a 44 inch antenna spacing.  FIGS. 8A and 8B  show the expected responses for a tool having a 52 inch antenna spacing. These responses represent actual measurements obtained via a water tank experiment in which the tank was filled with 1 Ω·m water to represent a homogeneous isotropic formation. The tool was positioned in the center of the tank and a casing tubular was positioned parallel to the tool at a distance that could be varied as desired from 0.85 feet to 6 foot. These figures suggest that signal strength increases as signal frequency decreases. Even though this trend is not monotonic and it reverses slightly at lower signal frequencies (see  FIGS. 12A-12B ), the discrimination between the tool&#39;s response to casing and the tool&#39;s response to other environmental factors is expected to improve as the signal frequency is reduced. Significantly, the use of lower signal frequencies also enables feasible tool operation at increased antenna spacings.  FIGS. 9A and 9B  show the parallel and perpendicular responses of the tool as a function of casing distance for different antenna spacings, assuming a signal frequency of 500 kHz. From this graph it can be observed that the tool&#39;s response to signal strength increases with antenna spacing. A comparison of the tool&#39;s responses to each of the models reveals that a casing detection tool would benefit from using a lower tool operating frequency and/or longer spacing between tool&#39;s transmitter and receiver, as this increases the tool&#39;s sensitivity to nearby casing and simultaneously decreasing the tool&#39;s response to formation anisotropy and nearby shoulder beds. 
         [0032]    On the other hand, reducing frequency also raises a couple of issues. First of all, lower frequency reduces the signal amplitude received at tool&#39;s receiver when other specifications of the tool are consistent (same spacing, same antenna design, etc.). Noise level or signal-to noise ratio will be a challenging issue for very weak signal amplitude. Secondly, the majority of received signal at a receiver is the direct signal transmitted directly from the transmitter to the receiver if operated at low frequency. Processing schemes to determine a casing nearby the tool may fail if direct signal is much stronger than signal from casing. In summary, it would be beneficial to reduce operating frequency for a nearby casing detection, but different formation resistivity and different casing distance to the tool define the optimized operating frequency as well as the optimized spacing between transmitter and receiver. 
         [0033]    To better quantify considerations that may go into an optimization analysis, we take as an example an electromagnetic logging tool located in a homogeneous isotropic formation with resistivity of 50 Ω·m with a parallel casing string at a distance of 10 feet, as indicated in  FIG. 10 . The tool&#39;s sensitivity to the casing can be characterized by measuring the relative strength of the signal attributable to the casing. The casing signal is maximized when the antennas are oriented along the y-axis as shown in  FIG. 10 , as this orientation induces the maximum current flow in the casing and provides the maximum sensitivity to the fields induced by this current flow. The complex amplitude of the signal component measured by this transmitter and receiver orientation is herein referred to as V y   y . The tool sensitivity can then be expressed by comparing the relative strength of the modeled signal (V y   y ) in the presence and absence of the casing: 
         [0000]    
       
         
           
             
               
                 
                   Sensitivity 
                   = 
                   
                     
                        
                       
                         
                           
                             Signal 
                             
                               with 
                                
                               
                                   
                               
                                
                               Casing 
                             
                           
                           - 
                           
                             Signal 
                             
                               no 
                                
                               
                                   
                               
                                
                               Casing 
                             
                           
                         
                         
                           Signal 
                           
                             no 
                              
                             
                                 
                             
                              
                             Casing 
                           
                         
                       
                        
                     
                     × 
                     100 
                      
                     
                       ( 
                       % 
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]      FIG. 11A  shows this sensitivity as a function of antenna spacing and signal frequency. The unsealed signal amplitude with casing (log 10 V y   y ) is shown in  FIG. 11B , again as a function of antenna spacing and signal frequency. The tool designer may employ these figures in conjunction with  FIGS. 12A and 128 , which show modeled responses of log10(A/C) for the parallel Tx-Rx antenna pair and perpendicular Tx-Rx antenna pair shown in  FIG. 2 , for the same range of signal frequencies and antenna spacings of  FIGS. 11A and 11B . Collectively, these figures can be used by the tool designers to select an optimized frequency and antenna spacing to implement an EM tool customized for a nearby casing detection range of 10 feet in a formation having 50 Ω·m resistivity. 
         [0034]    For example,  FIG. 11A  shows that a sensitivity of 100% can be obtained with, e.g., a transmit signal frequency of 100 kHz and an antenna spacing on the order of 35 feet; a transmit signal frequency of 10 kHz and an antenna spacing on the order of 40 feet; and a transmit signal frequency of 1 kHz with an antenna spacing on the order of 50 feet.  FIG. 11B  shows that the amplitude of the signal component attributable to the casing is about −4,2, −5.5, and −6.8, respectively, for these values, which are all acceptably strong enough. Transporting these values (100kHz with 35 feet, 10kHz with 40 feet, and 1 kHz with 50 feet) to  FIGS. 12A and 12B , the designer observes that the scaled tool responses are expected to be in excess of −0.5. 
         [0035]    Since the formation resistivity is assumed to be relatively high (50 Ω·m), formation anisotropy effects will be negligible compared to shoulder bed effects. The designer estimates the shoulder bed response with selected tool parameters.  FIGS. 13A and 13B  show modeled shoulder bed responses where a tool having a 50 foot antenna spacing and a transmit signal frequency of 1 kHz is positioned in a 50 Ω·m at some distance from the boundary with a 1 Ω·m formation. The response is shown as a function of bed boundary distance and dip.  FIGS. 13A  and  13 B indicate that the highest bed boundary signal of log10(A/C) is less than 4, which confirms the tool is able to accurately determine a parallel casing 10 feet away from the tool in 50 Ω·m formation without considerations of other formation effects, such as anisotropy and/or shoulder beds. 
         [0036]      FIG. 14  is a flow diagram of an illustrative casing detection method. The illustrative method begins by obtaining resistivity measurements from a first borehole, as shown in block  1002 . This first borehole is then cased or otherwise made conductive (e.g., by filling it with a conductive fluid). In situations where a cased well already exists and its resistivity logs are unavailable, the resistivity of the formation around the cased well may be estimated based on other information such as remote wells, seismic surveys, and reservoir models. The resistivity data for the formation containing the first borehole may then be employed in block  1004  to predict environmental signals levels that would be encountered by a second borehole drilled near the first. Based on the resistivity measurements, a modeled tool response to environmental effects such as resistive anisotropy and nearby formation bed boundaries or fluid interfaces can be determined along the length of a second borehole path as a function of antenna spacing and transmit signal frequency. 
         [0037]    The resistivity data may be further employed in block  1006  to model the tool&#39;s response signal level to casing as a function of antenna spacing and operating frequency. An upper limit on the desired casing detection range may be used as part of the modeling process. In block  1008 , the casing response may be compared to the environmental signal levels to determine a range of acceptable antenna spacings and a range of suitable operating frequencies. The range may be determined to be a combination of spacing and frequency that provides a casing signal greater than the anticipated environmental signal response, and in some cases at least an order of magnitude greater. Such significant disparity would enable casing ranging measurements to be made while neglecting environmental signal responses. In block  1010  a tilted antenna tool is provided with an antenna spacing and operating frequency from the range of suitable values. The selected values may be based upon available tools or feasible tool configurations. For example, the available tool hardware may require some minimum required receive signal strength to assure adequate receiver response, and this factor may prevent certain combinations of antenna spacing and signal frequency from being chosen. As another example, some tilted antenna tools may have a modular construction in which the transmit module can be spaced at a variable distance from the receive module, thereby providing for a reconfigurable antenna spacing within certain limits. Or the available tilted antenna tools may have a programmable operating frequency range or they may employ multiple operating frequencies including at least one in the designated operating range. 
         [0038]    These and other variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.