Abstract:
A technique is designed to facilitate obtaining of acoustic data. The technique comprises traversing a tool through a subterranean formation from a first depth to subsequent depths. The tool receives a seismic signal during pre-determined time windows. The seismic signal is generated by a seismic source which is activated at varying times relative to the predetermined time windows based on the depth of the tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present document is based on and claims priority to U.S. Provisional Application Serial No. 61/230747, filed Aug. 3, 2009, the contents of which are hereby incorporated by reference for all purposes and intents. 
     
    
     BACKGROUND OF THE DISCLOSURE 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure relates to techniques for acquiring acoustic data. More particularly, the present disclosure relates to obtaining improved seismic signal data by controlling an activation time of the seismic source. 
         [0004]    2. Background of the Related Art 
         [0005]    The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section. 
         [0006]    Measurement-while-drilling involves the sensing of one or more downhole parameters during the drilling process. Sensors, typically mounted within drill collars located above the drill bit, are used to obtain information regarding the drilling process or subsurface conditions. A subset of these measurements may be transmitted to the surface, often using an acoustic or “mud pulse” telemetry system. Other measurements may be stored in recording devices located within the drill collars. This data can be retrieved when the drill bit is raised (also called “tripped”) to the surface. 
         [0007]    Seismic measurement-while-drilling data is acquired using seismic. sensors, such as geophones or hydrophones that are typically located within a drill collar positioned above the drill bit. Since the drill bit generates a tremendous amount of noise, it is typical to collect seismic data, which may be generated by an uphole source, only during the time the drill bit or drill string is not moving or drilling. This usually equates to the time during which the drill string or drilling is temporarily stopped in order to add or remove pieces of drill pipe to the drill string at the surface. 
         [0008]    As mentioned above, communication between the tool and the surface is typically accomplished with a “mud pulse” or other low data telemetry system, making it impractical for the operator to send commands to the tool during drill stoppage to regulate the time sequence or activity related to the seismic data acquisition. Furthermore, due to the vast amount of data that is acquired from the seismic signal, it is also impractical to collect seismic signal data for large periods of time, such as from the time the drilling ceases to the time it commences. To overcome these limitations, the seismic recording system in the tool is programmed to start and stop recording for specific time periods and at specific intervals. In conjunction, the source is activated relative to the programmed time periods to ensure that the seismic signal arrives at the tool during the recording period. 
         [0009]    It has been found that there are preferred locations within the recording period where the signal should arrive. This has mostly to do with the processing of the seismic signal once received. It has also been found that as the tool traverses through the earth formation (as the tool is drilling for example), the distance between the tool and the source, and the velocity profile of the formation there between, changes causing the seismic signals to arrive at the tool at undesired times within the period and, more drastically, to miss the period altogether. 
       SUMMARY 
       [0010]    The present invention generally relates to a method and system for obtaining acoustic data by improving the receipt of seismic signals within specified periods. The technique comprises traversing a tool through a subterranean formation from a first depth to subsequent depths. The tool receives a seismic signal during predetermined time windows. The seismic signal is generated by a seismic source which is activated at varying times relative to the predetermined time windows based on the depth of the tool. 
         [0011]    Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Certain embodiments of the invention will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying drawings illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein. The drawings are as follows: 
           [0013]      FIG. 1  illustrates an example of a wellsite system in which the method for obtaining seismic data may be employed, according to an embodiment of the present invention; 
           [0014]      FIG. 2  illustrates an example of a tool used in cooperation with a seismic source, according to embodiment of the present invention; 
           [0015]      FIG. 3  illustrates an example of a tool used in cooperation with a plurality of seismic sources, according to an embodiment of the present invention; 
           [0016]      FIG. 4  illustrates an example of a tool having a plurality of receivers used in cooperation with a seismic source, according to an embodiment of the present invention; 
           [0017]      FIG. 5  illustrates an example of a tool having a plurality of receivers used in cooperation with a plurality of seismic sources, according to an embodiment of the present invention; 
           [0018]      FIG. 6  illustrates an example of a tool receiving signals from a seismic source as the tool is moved progressively deeper into a subterranean formation, according to an embodiment of the present invention; and 
           [0019]      FIG. 7  provides a graphical representation of an embodiment of a methodology for changing the timing of the seismic source relative to predetermined time windows for receiving the seismic signal at the tool, according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    So that the above recited features and advantages of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the accompanied drawings and graphs. In the following description, numerous details are set forth to provide an understanding of the present invention. However, it will be understood by those of ordinary skill in the art that the present invention may .be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
         [0021]    In the specification and appended claims: the terms “connect”, “connection”, “connected”, “in connection with”, “connecting”, “couple”, “coupled”, “coupled with”, and “coupling” are used to mean “in direct connection with” or “in connection with via another element”; and the term “set” is used to mean “one element” or “more than one element”. As used herein, the terms “up” and “down”, “upper” and “lower”, “upwardly” and downwardly“, “upstream” and “downstream”; “above” and “below”; 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. 
         [0022]      FIG. 1  illustrates a wellsite system in which the present invention can be employed. The wellsite can be onshore or offshore. In this exemplary system, a borehole  11  is formed in subsurface formations by rotary drilling in a manner that is well known. Embodiments of the invention can also use directional drilling, as will be described hereinafter. 
         [0023]    A drill string  12  is suspended within the borehole  11  and has a bottom hole assembly  100  which includes a drill bit  105  at its lower end. The surface system includes platform and derrick assembly  10  positioned over the borehole  11 . The assembly  10  may include a rotary table  16 , kelly  17 , hook  18  and rotary swivel  19 . The drill string  12  is rotated by the rotary table  16 , energized by means not shown, which engages the kelly  17  at the upper end of the drill string. The drill string  12  is suspended from a hook  18 , attached to a traveling block (also not shown), through the kelly  17  and a rotary swivel  19  which permits rotation of the drill string relative to the hook. As is well known, a top drive system could alternatively be used. 
         [0024]    In the example of this embodiment, the surface system further includes drilling fluid or mud  26  stored in a pit  27  formed at the well site. A pump  29  delivers the drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , causing the drilling fluid to flow downwardly through the drill string  12  as indicated by the directional arrow  8 . The drilling fluid exits the drill string  12  via ports in the drill bit  105 , and then circulates upwardly through the annulus region between the outside of the drill string and the wall of the borehole, as indicated by the directional arrows  9 . In this well known manner, the drilling fluid lubricates the drill bit  105  and carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation. 
         [0025]    The bottom hole assembly  100  of the illustrated embodiment may comprise a logging-while-drilling (LWD) module  120 , a measuring-while-drilling (MWD) module  130 , a roto-steerable system and motor, and drill bit  105 . 
         [0026]    The LWD module  120  is housed in a special type of drill collar, as is known in the art, and can contain one or a plurality of known types of logging tools. It will also be understood that more than one LWD and/or MWD module can be employed, e.g. as represented at  120 A. (References, throughout, to a module at the position of  120  can alternatively mean a module at the position of  120 A as well.) The LWD module includes capabilities for measuring, processing, and storing information, as well as for communicating with the surface equipment. In the present embodiment, the LWD module includes a seismic measuring device as described in greater detail below. 
         [0027]    The MWD module  130  also is housed in a special type of drill collar, as is known in the art, and can contain one or more devices for measuring characteristics of the drill string and drill bit. The MWD tool further includes an apparatus (not shown) for generating electrical power to the downhole system. This may typically include a mud turbine generator powered by the flow of the drilling fluid, it being understood that other power and/or battery systems may be employed. In the present embodiment, the MWD module includes one or more of the following types of measuring devices: a weight-on-bit measuring device, a torque, measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, a seismic measuring device, and an inclination measuring device. 
         [0028]      FIGS. 2-5  illustrate a seismic-while-drilling tool which can be the LWD tool  120 , or can be a part of an LWD tool suite  120 A of the type disclosed in P. Breton et al., “Well Positioned Seismic Measurements,” Oilfield Review, pp. 32-45, Spring, 2002, incorporated herein by reference. 
         [0029]    The downhole LWD tool can have a single receiver (as depicted in  FIGS. 2 and 3 ), or plural receivers (as depicted in  FIGS. 4 and 5 ), and can be employed in conjunction with a single seismic source at the surface (as depicted in  FIGS. 2 and 4 ) or plural seismic sources at the surface (as depicted in  FIGS. 3 and 5 ). 
         [0030]    Accordingly,  FIG. 2 , which includes reflection off a bed boundary, and is called a “zero-offset” vertical seismic profile arrangement, uses a single source and a single receiver.  FIG. 3 , which includes reflections off a bed boundary, and is called a “walkaway” vertical seismic profile arrangement, uses plural sources and a single receiver.  FIG. 4 , which includes refraction through salt dome boundaries, and is called a “salt proximity” vertical seismic profile, uses a single source and plural receivers.  FIG. 5 , which includes some reflections off a bed boundary, and is called a “walk above” vertical seismic profile, uses plural sources and plural receivers. 
         [0031]    As described in the background, to overcome the obstacles created by a low data telemetry system and/or by the vast amounts of data acquired from seismic signals, a system  200  for obtaining seismic data is provided. A schematic embodiment of one example of the system  200  is illustrated in  FIG. 6 . System  200  may comprise a tool  202  which is traversed along the borehole  11  through, for example, a series of depths  204 ,  206  and  208  at which seismic signals are received and recorded. The number of depths at which seismic signals are recorded may vary from one application to another, and in some applications, the number of different depths at which recordings are made can be substantial. 
         [0032]    System  200  further comprises at least one seismic source  210  which may be located at or near a surface location  212 . In some applications, system  200  may comprise a plurality of seismic sources  210 , as illustrated in the embodiment of  FIG. 3  or  FIG. 5 . Similarly, the type of tool  202  may vary from one application to another and may comprise individual components or a bottom hole assembly, such as bottom hole assembly  100  (see  FIG. 1 ). However, the tool  202  comprises a seismic receiver  214  which is part of a seismic recording system  216 . By way of additional example, the tool  202  may comprise drill bit  105  combined with a traversing while drilling system, such as logging while drilling module  120  and/or measuring while drilling module  130 . The traversing while drilling system may be designed to incorporate the seismic receiver  214  and seismic recording system  216 . It should be noted that in other embodiments, tool  202  may comprise tubing or other drilling components. In some cases, a wireline operation may be performed using aspects of the embodiments described herein. 
         [0033]    In the example illustrated, seismic recording system  216  of tool  202  is programmed to automatically start recording at specified times and for specified periods in conjunction with the timely activation of the source  210 . Accordingly, in one example of a system and methodology for obtaining seismic data, the tool  202  (e.g. MWD module, LWD mitral, Coiled tubing, Wireline, or other tools or tool components) is traversed through a subterranean formation to a first depth  204 , as illustrated in  FIG. 6 , and as further illustrated graphically in  FIG. 7 . Once at the first depth, the seismic signal recording mechanism  216  is activated in the tool  202 . The recording mechanism  216  is programmed to record a seismic signal during a plurality of predetermined recording periods which may be referred to as time windows. 
         [0034]    In parallel, the seismic source  210  is activated at or near surface  212  a plurality of times while the tool  202  is at the first depth  204 . The seismic source  210  is activated such that the seismic signal produced by the source  210  is received at the tool  202  and its seismic receiver  214  at a desired instant in each recording period/time window  218  (see  FIG. 7 ). The tool  22  is then traversed to a second depth, e.g. depth  206 , deeper than the first depth  204 , and the seismic signal recording mechanism  216  in the tool  202  is activated again. When the tool  202  is at the second depth  206 , the seismic source  210  is activated earlier, relative to each predetermined recording period than when at the first depth  204 , as graphically illustrated by timing difference  220  in  FIG. 7 . By activating the seismic source  210  earlier relative to the predetermined time windows of the recording mechanism  216  at the deeper depth  206 , the seismic signal produced by the source  210  is received at the tool  202  at the same desired instant in the recording periods/time windows  218 , as illustrated by timing markers  222 . 
         [0035]    As the tool  22  is then traversed to a third depth, e.g. depth  208 , deeper than the second depth  206 , the seismic signal recording mechanism  216  in the tool  202  is activated again. When the tool  202  is at the third depth  208 , the seismic source  210  is activated still earlier (relative to each predetermined recording period) than when at the second depth  206 , as graphically illustrated by the next subsequent timing difference  220  in  FIG. 7 . By activating the seismic source  210  earlier at the deeper depth  208  relative to the predetermined time windows of the recording mechanism  216 , the seismic signal produced by source  210  is received at tool  202  at the same desired instant in the recording periods/time windows  218 , as illustrated by timing markers  222 . This process may be repeated at each subsequent depth to facilitate recording of the seismic signal at the desired point within the recording period during which seismic recording system  216  is activated. As illustrated in  FIG. 7 , the timing of seismic source  210  may be progressively earlier relative to the timing of the recording period  218  during activation of seismic recording system  216 . Thus, there is a timing difference  220  between initiation of the seismic source  210  at a subsequent depth versus initiation of seismic source  210  at the previous depth relative to initiation of the predetermined recording period for seismic recording system  216  of tool  202 . 
         [0036]    According to one embodiment, the plurality of predetermined time windows  218  may be about 3 seconds in duration, separated by a duration of about 12 seconds of non-recording. In a specific example, the desired instant at which the seismic signal is received within the time windows  218  may be toward the beginning of the time windows. However, the length of the predetermined recording periods/time windows  218 , as well as the duration by which the timing windows are separated at each depth, may be changed according to the specifics of a given application and environment. Additionally, the changes in timing of the seismic source  210  relative to the recording periods  218  may be the same or different as tool  202  is moved to different depths. 
         [0037]    For example, the timing differences  220  may be adjusted to achieve a variety of goals with respect to recording seismic data and with respect to environmental considerations. Velocity models are sometimes prepared to address specific characteristics of a given subterranean environment. In these applications, the seismic source  210  may be activated relatively earlier or the timing differences  220  may be otherwise selected based at least partially on variation predicted by the velocity model between one depth and subsequent depths. 
         [0038]    Generally,  FIG. 7  depicts one example of the correlation of a time delay in activation of source  210  with the signal being received by tool  202 . The right side of  FIG. 7  shows that regardless of the depth of the tool  202 , the seismic signal is received at approximately the same time in the acquisition time window  218 . This is accomplished by activating the seismic source  210  earlier with respect to the seismic recording system time window  218 , as the tool  202  runs deeper and as illustrated on the left side of  FIG. 7 . 
         [0039]    As discussed above, the system  200  for obtaining seismic data may be combined with many types of downhole tools, such as drilling related tools, to improve the receipt of seismic signals within specified periods of seismic recording mechanism activation. The seismic receiver  214  and a seismic recording system  216  may be mounted on or with an MWD system, a LWD system, various other seismic while drilling tools, or on or with entirely different types of tools. The system  200  also may be used with tools in the form of various types of tubing strings and bottom hole assemblies in applications where accumulation of seismic data is desired. Furthermore, the number of seismic sources and seismic recording systems/seismic receivers may vary from one application to another. The changes in activation timing of the seismic source  210  relative to the activation time window/recording period of the seismic recording system also may be adjusted linearly or non-linearly depending on the environment, the distance between recording depths, the length of the recording period, the characteristics of the tool, and on other factors related to specific seismic applications. 
         [0040]    Accordingly, although only a few embodiments of the present invention have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this invention. Such modifications are intended to be included within the scope of this invention as defined in the claims.