Patent Publication Number: US-2013250725-A1

Title: Retrievable vertical geophone cable and method

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the subject matter disclosed herein generally relate to methods and systems for collecting seismic data using a vertical geophone cable and, more particularly, to mechanisms and techniques for increasing a coupling of the geophones from a vertical geophone cable to the ground. 
     2. Discussion of the Background 
     Land seismic data acquisition and processing may be used to generate a profile (image) of the geophysical structure under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where oil and gas reservoirs are located. 
     Traditionally, as illustrated in  FIG. 1 , a land seismic survey  100  that uses vertical geophone cables is performed in the following way. Plural geophones  102  are electrically connected to a recorder along a cable  104 . A well  106  is dug into the ground  108  to accommodate the plural geophones. 
     After all the geophones have been deployed, one or more seismic sources are brought into the field and actuated to generate the seismic waves. The seismic waves propagate through the ground until they are reflected by various reflectors. The reflected waves propagate to the geophones, where a movement of the earth is recorded. However, if the coupling between the geophone and the dirt around the geophone is not good, the recorded data is of low quality. 
     A geophone typically has a cylindrical shape and a small size, e.g., around 3 cm long and 2 cm in diameter. Thus, a coupling between the geophone and the well might be a problem when their diameters are very different. The coupling is improved if the diameter of the geophone is close to the diameter of the well. However, the coupling between the ground and geophone is not well understood. The geophone-ground coupling may be defined as the difference between the velocity measured by the geophone and the velocity of the ground without the geophone. This definition is appropriate for designing a geophone. 
     However, once the geophone is designed and needs to be deployed, the practicing geophysicist has to deal with the fact that the geophone may not be appropriately deployed. For example, the geophone may not be “well” coupled to its surroundings. In this situation, the above definition might not be appropriate. For this situation, those skilled in the art would consider that a “bad” geophone coupling refers to a difference between the velocity as measured by the badly-planted geophone and the velocity as measured by the well-planted geophone. 
     Irrespective of the definition to be used, the ground-geophone coupling is a persistent problem in the art because it is problematic to make the casing of the geophone to tightly contact the well and, at the same time, to ensure that the geophones are easily retrievable from the well when desired. One method known in the industry is to attach a cable  110  with a high mechanical resistance to the casing of each geophone and, when the time arrives to remove the geophones, to pull this cable up to retrieve the geophones. However, if a portion of the well has collapsed at the location of one geophone, that geophone may be stuck at that position and even pulling the cable  110  may not retrieve that geophone. 
     Therefore, there is a need to improve the coupling of the geophone to the ground and at the same time to make easier and safer the process of retrieving the geophones. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment, there is a retrievable vertical geophone cable for collecting seismic data underground. The retrievable vertical geophone cable includes an envelope having a first end at which a connector mechanism is provided to close the envelope, plural geophones distributed inside the envelope at predetermined positions, and a first expansion mechanism attached to a geophone of the plural geophones and configured to expand the envelope when actuated with a first fluid under pressure. 
     According to another exemplary embodiment, there is a retrievable vertical geophone cable for collecting seismic data underground. The retrievable vertical geophone cable includes an envelope, a geophone provided inside the envelope at a predetermined position, and an expansion mechanism attached to the geophone and configured to expand the envelope when actuated with a first fluid under pressure. 
     According to still another exemplary embodiment, there is a method for deploying a retrievable vertical geophone cable for collecting seismic data. The method includes a step of deploying the retrievable vertical geophone cable inside a well that has an inner diameter (d 2 ) larger than an outer diameter (d 1 ) of an envelope of the retrievable vertical geophone cable; a step of actuating an expansion mechanism, attached to a geophone of the retrievable vertical geophone cable, so that the envelope is pressed against the well; and a step of collecting the seismic data with the geophone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  illustrates a vertical arrangement of geophones deployed in a well; 
         FIG. 2  is a schematic diagram of a retrievable vertical geophone cable according to an exemplary embodiment; 
         FIG. 3  is a schematic diagram of an expansion mechanism according to an exemplary embodiment; 
         FIG. 4  is a longitudinal cross-section of a geophone and associated expansion mechanism according to an exemplary embodiment; 
         FIG. 5  is a transversal cross-section of a geophone and associated expansion mechanism according to an exemplary embodiment; 
         FIG. 6  illustrates a retrievable vertical geophone cable deployed in a well according to an exemplary embodiment; 
         FIG. 7  is a flowchart of a method for deploying a retrievable vertical geophone cable in a well according to an exemplary embodiment; 
         FIG. 8  is a schematic diagram of horizontally deployed geophones; and 
         FIG. 9  is a schematic diagram of vertically deployed geophones according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a retrievable vertical geophone cable. However, the embodiments to be discussed next are not limited to a vertical geophone cable but may be applied to slanted and/or horizontal cables. 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. 
     According to an exemplary embodiment, there is a retrievable vertical geophone cable for collecting seismic data underground. The retrievable vertical geophone cable includes an envelope, a geophone provided inside the envelope at a predetermined position, and an expansion mechanism attached to the geophone and configured to expand the envelope to increase the coupling between the envelope and the well. 
     According to an exemplary embodiment illustrated in  FIG. 2 , a retrievable vertical geophone cable  200  includes an envelope  202  that has, at one end  202 A, a connector mechanism  204  and, at the other end  202 B, a cap  206  so that a fluid  208  provided inside the envelope  202  does not leak to the ambient. Further, the connector mechanism  204  and the cap  206  are so configured that the fluid does not leak outside the vertical geophone cable when the fluid is pressurized. In the following, it is noted that the term “vertical” means that an angle formed between the geophone cable  200  and gravity is smaller than a few degrees, e.g., smaller than 10 degrees. 
     The envelope  202  further includes a geophone  220  and, preferably, a plurality of them. A hydraulic hose  230  connects the connector mechanism  204  to each of the geophones  220 . A geophone  220  includes a casing  222 , inside which is provided a geophone sensor  224 . Further, the geophone  220  may include an expansion mechanism  226  connected to the hydraulic hose  230 , and the expansion mechanism  226  is configured to expand inside the envelope  202  so that the envelope  202  increases its diameter (i.e., its volume). Thus, the envelope  202  is made of a flexible material, for example, polyurethane. 
     The expansion mechanism  226  may be a jack having a casing  226 A and two or more pistons  226 B as illustrated in  FIG. 3 . A fluid  228  under pressure may be provided through the hydraulic hose  230  to the casing  226 A to activate the pistons  226 B. The fluid  228  may be identical or different from the fluid  208  that is present among the geophones. The fluid  228  may be a bio-degradable oil, a mineral oil, water, etc. The expansion mechanism  226  may be provided at one end of the geophone sensor  224  as shown in  FIG. 2  or at both ends of the geophone sensor  224 . 
     Closer views of a single geophone  220  of the retrievable vertical geophone cable  200  are shown in  FIGS. 4 and 5 .  FIG. 4  shows a longitudinal cross-section through the envelope  202 , the geophone  220  and two expansion mechanisms  226  and  240 . The first expansion mechanism is provided at the top of the geophone  220 , and the second expansion mechanism  240  is provided at the bottom of the geophone  220 .  FIG. 5  is a transversal cross-section of the above devices and shows that the pistons  226 B of the first expansion mechanism  226  are offset (at 90 degrees in  FIG. 5 ) relative to pistons  240 B of the second expansion mechanism  240 . In one application, pistons of different expansion mechanisms maybe aligned or provided at other angles than 90 degrees. 
     One purpose of the expansion mechanisms is to ensure better contact between the envelope (and consequently the geophone) and the walls of the well. This is explained next while also explaining how the retrievable vertical geophone cable is deployed and retrieved from a well. 
     Consider, as shown in  FIG. 6 , that a well  600  has been formed in the ground  602 . The retrievable vertical geophone cable  200  is then inserted into the well  600 . The external diameter d 1  of the retrievable vertical geophone cable  200 , i.e., an external diameter of the envelope  202 , is slightly smaller than the internal diameter d 2  of the well  600 . This is so for easily deploying the retrievable vertical geophone cable  200  inside the well  600 . Once the retrievable vertical geophone cable  200  is in place, the gap between the envelope  202  and the walls of the well  600  are reduced by providing the fluid  228  under pressure to the expansion mechanisms  226  and  240 . As a result of this action, the pistons  226 B and  240 B expand, pressing the envelope  202  against the well  600 . 
     Further, the fluid  208  may be pressurized (with a pressure smaller than a pressure of the fluid  228 ) to maintain the increased volume of the envelope  202 . In this way (i.e., having a pressure difference between the fluid  228  and  208 ), the coupling between the geophone sensor  224  and the well  600  is improved. The pressurized fluid  228  may be provided from a pump  604  connected to the connector mechanism  204 . The same pump or another pump may provide the extra pressure to the fluid  208 . The connector mechanism  204  may be directly connected to the pump  604  or together with similar connector mechanisms from other retrievable vertical geophone cables. 
     It is noted that, when in use, the fluid  228  is under a pressure higher than the pressure of the fluid  208 . In one application, the fluid  208  does not communicate with the fluid  228 . The fluid  208  is trapped inside the envelope, and it is supposed to not escape into the ambient of the envelope. In the event that the integrity of the envelope is compromised, if the fluid  208  is a bio-degradable oil or water, there is minimal impact to the environment. 
     Seismic data from the geophones is collected through an electrical cable  620 . This cable connects each geophone to the connector mechanism  204 . Thus, the connector mechanism  204  is an electric and hydraulic connector. A distance h 1  between the connector mechanism  204  and the first geophone may be about 2 to 4 m, and a distance h 2  between the geophones may be about 1.5 to 3 m. Other distances may be used, depending on the application. The retrievable vertical geophone cable  200  may include any number of geophones. 
     After the seismic survey has been completed, to remove the retrievable vertical geophone cable  200  from the well, the fluid  228  is released or its pressure is decreased (in one application, the pressure of fluid  208  needs to be also decreased. In still another application, e.g., marsh, water or air needs to be flush around the cable to retrieve it), so that the pistons of the expansion mechanisms  226  and  240  are retracted and the envelope is slightly deflated (i.e., its volume is reduced) to not be under tight contact with the walls of the well  600 . Further, part of the fluid  208  is released from the envelope to accommodate its volume change. In this way, the retrievable vertical geophone cable  200  can easily be retrieved from the well. 
     The above process may be summarized based on the flowchart shown in  FIG. 7  as follows.  FIG. 7  illustrates a method for deploying a retrievable vertical geophone cable for collecting seismic data. The method includes a step  700  of deploying the retrievable vertical geophone cable inside a well that has an inner diameter (d 2 ) larger than the outer diameter (d 1 ) of an envelope of the retrievable vertical geophone cable, and a step  702  of actuating an expansion mechanism, attached to a geophone of the retrievable vertical geophone cable, so that the envelope is pressed against the well. The method may optionally include a step of pumping a second fluid inside the envelope, among the geophones, for increasing the volume of the envelope. Further, the method may also include a step of collecting the seismic data, a step of reducing the volume of the envelope and a step of removing the vertical geophone cable from the well. 
     It is noted that the retrievable vertical geophone cable is intended to replace traditional geophones  800  that are deployed, in a horizontal manner, above or below ground  802  as illustrated in  FIG. 8 . In this way, instead of having a single geophone  800  at a given X and Y position, a string of geophones (cable  200 ) are deployed at the same X and Y position, as shown in  FIG. 9 , each geophone  220  having a different depth Z. In this way, it is expected, besides a better coupling, to eliminate horizontal filtering which currently reduces the noise but damages the signal, and/or to record the seismic data in a quieter environment, and/or to take profit of some Rayleigh wave properties to separate these waves from the recorded signal (body waves). 
     The disclosed exemplary embodiments provide a method and a retrievable vertical geophone cable. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. 
     Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein. 
     This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.