Patent Publication Number: US-2020291773-A1

Title: Dual rotor pulser for transmitting information in a drilling system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of U.S. application Ser. No. 16/402,842, filed May 3, 2019, and entitled “Dual Rotor Pulser For Transmitting Information In A Drilling System,” which is a divisional application of U.S. application Ser. No. 15/433,412, filed Feb. 15, 2017, now U.S. Pat. No. 10,323,511, and entitled “Dual Rotor Pulser For Transmitting Information In A Drilling System,” the entire contents of which are incorporated by reference into the present application. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed to an improved dual rotor pulser for transmitting information in a drilling system, such as a rotator pulser used in a mud pulse telemetry system employed in a drill string for drilling an oil well. 
     BACKGROUND 
     In underground drilling, such as gas, oil or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of long pipe, referred to as a “drill pipe,” so as to form an assembly commonly referred to as a “drill string” that extends from the surface to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string and/or the drill bit. In order to lubricate the drill bit and flush cuttings from its path, pumps on the surface pump a high pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore. 
     Depending on the drilling operation, the pressure of the drilling mud flowing through the drill string will typically be between 1,000 and 25,000 psi. In addition, there is a large pressure drop at the drill bit so that the pressure of the drilling mud flowing outside the drill string is considerably less than that flowing inside the drill string. Thus, the components within the drill string are subject to large pressure forces. In addition, the components of the drill string are also subjected to wear and abrasion from drilling mud, as well as the vibration of the drill string. 
     The distal end of a drill string, which includes the drill bit, is referred to as the “bottom hole assembly.” In “measurement while drilling” (MWD) applications, sensing modules in the bottom hole assembly provide information concerning the direction of the drilling. This information can be used, for example, to control the direction in which the drill bit advances in a steerable drill string. Such sensors may include a magnetometer to sense azimuth and accelerometers to sense inclination and tool face. 
     Historically, information concerning the conditions in the well, such as information about the formation being drill through, was obtained by stopping drilling, removing the drill string, and lowering sensors into the bore using a wire line cable, which were then retrieved after the measurements had been taken. This approach was known as wire line logging. More recently, sensing modules have been incorporated into the bottom hole assembly to provide the drill operator with essentially real time information concerning one or more aspects of the drilling operation as the drilling progresses. In “logging while drilling” (LWD) applications, the drilling aspects about which information is supplied comprise characteristics of the formation being drilled through. For example, resistivity sensors may be used to transmit, and then receive, high frequency wavelength signals (e.g., electromagnetic waves) that travel through the formation surrounding the sensor. By comparing the transmitted and received signals, information can be determined concerning the nature of the formation through which the signal traveled, such as whether it contains water or hydrocarbons. Other sensors are used in conjunction with magnetic resonance imaging (MRI). Still other sensors include gamma scintillators, which are used to determine the natural radioactivity of the formation, and nuclear detectors, which are used to determine the porosity and density of the formation. 
     In both LWD and MWD systems, the information collected by the sensors must be transmitted to the surface, where it can be analyzed. Such data transmission is typically accomplished using a technique referred to as “mud pulse telemetry.” In a mud pulse telemetry system, signals from the sensor modules are typically received and processed in a microprocessor-based data encoder of the bottom hole assembly, which digitally encodes the sensor data. A controller in the control module then actuates a pulser, also incorporated into the bottom hole assembly, that generates pressure pulses within the flow of drilling mud that contain the encoded information. The pressure pulses are defined by a variety of characteristics, including amplitude (the difference between the maximum and minimum values of the pressure), duration (the time interval during which the pressure is increased), shape, and frequency (the number of pulses per unit time). Various encoding systems have been developed using one or more pressure pulse characteristics to represent binary data (i.e., bit 1 or 0)—for example, a pressure pulse of 0.5 second duration represents binary 1, while a pressure pulse of 1.0 second duration represents binary 0. Transmitting information via pressure pulses, including schemes for encoding pressure pulses, are described in U.S. Published Application No. 2006/0215491 (Hall), hereby incorporated by reference in its entirety. The pressure pulses travel up the column of drilling mud flowing down to the drill bit, where they are sensed by a strain gage based pressure transducer. The data from the pressure transducers are then decoded and analyzed by the drill rig operating personnel. 
     Various techniques have been attempted for generating the pressure pulses in the drilling mud. One technique involves incorporating a pulser into the drill string in which the drilling mud flows through passages formed by a stator. In one type of pulser, referred to as a mud siren, a rotor, which is typically disposed adjacent the stator, is rotated continuously, thereby generating pulses in the drilling fluid. In another type of pulser, the rotor is oscillated or rotated incrementally in one direction, so that the rotor blades alternately increase and decrease the amount by which they obstruct the stator passages, thereby generating pulses in the drilling fluid. An oscillating type pulser is disclosed in U.S. Pat. No. 6,714,138 (Turner et al.) and U.S. Pat. No. 7,327,634 (Perry et al.), each of which is hereby incorporated by reference in its entirety. 
     Unfortunately, such rotary pulsers have limited flexibility in terms of their ability to vary their operating mode as drilling conditions change or the quantity or type of data to be transmitted changes. For example, while continuous rotation in a mud siren mode might be optimal in some situations, oscillatory rotation might be optimal in other situations. Different operating modes might be needed if the pulser jambs and/or debris has to be cleared frequently. The ability to change data transmission wavelength in a siren may move the data band to a frequency where there is less noise. 
     Further, such rotary pulsers are prone to plugging. In order to ensure that oil and gas in the formation do not enter the borehole during drilling (which is environmentally undesirable), the pressure of drilling mud in the borehole is kept high. However, this can cause the drilling mud to flow into the formation at a rate that is greater than the rate at which the mud is pumped down into the hole. As a result, no mud returns to the surface, a condition referred to as lost circulation. When circulation of drilling mud is lost, drilling chips and debris from the formation are not flushed away from the drill bit. To prevent the loss of drilling mud, various types of debris and trash—referred to as lost circulation material—are pumped down the drill string along with the drilling mud so that the debris will plug the passages in the formation and prevent the loss of drilling mud. However, this lost circulation material can plug the passages in the stator of the pulser. Further, long strands of lost circulation material can become wrapped around the pulser&#39;s rotor, essentially plugging the passages between rotor blades, especially if the rotor is rotated continuously in one direction. 
     SUMMARY 
     It would be desirable to provide a mud pulse telemetry system and a pulser in which the operating mode of the pulser could be varied to allow higher amplitude pulse signals to be generated downhole and observed at the surface. In addition, it would be desirable to have a pulser that is less prone to plugging than traditional continuous or oscillating pulsers. 
     In one embodiment, the invention comprises a pulser configured to transmit information from a portion of a drill string operating at a down hole location in a well bore toward a location proximate the surface of an earthen formation. The pulser includes an outer housing assembly. The pulser further includes a first stator mounted to the outer housing assembly, the first stator including a first stator passage through which drilling fluid can flow. The pulser further includes a first rotor adjacent to the first stator, the first rotor including a first passage through which the drilling fluid can flow. The first rotor is rotatable with respect to the first stator and the outer housing assembly to at least partially block the first stator passage. The pulser further includes a second stator mounted to the outer housing assembly, the second stator including a second stator passage through which drilling fluid can flow. The pulser further includes a second rotor adjacent to the second stator, the second rotor including a second passage through which the drilling fluid can flow. The second rotor is rotatable with respect to second stator and the outer housing assembly to at least partially block the second stator passage. The pulser further includes a motor assembly coupled to the first rotor and the second rotor to drive rotation of the first rotor and the second rotor, such that,  1 ) rotation of the first rotor relative to the first stator creates first pressure pulses in the drilling fluid when the drilling fluid is flowing through the first passage and the first stator passage, and  2 ) rotation of the second rotor relative to the second stator creates second pressure pulses in the drilling fluid when the drilling fluid is flowing through the second passage and the second stator passage. The information is encoded in the first and second pressure pulses. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side schematic diagram of a drilling system including a dual rotor pulser according to an embodiment of the present disclosure. 
         FIG. 2  is a schematic diagram of a dual rotor pulser according to an embodiment. 
         FIG. 3A  is a perspective view of a first pulser rotor. 
         FIG. 3B  is a perspective view of a second pulser rotor. 
         FIG. 4  is a cross sectional view through the second pulser rotor taken through line IV-IV shown in  FIG. 3 . 
         FIG. 5A  is a cross-sectional view of the pulser taken along line V-V shown in  FIG. 2  with the rotors in a maximum obstruction configuration. 
         FIG. 5B  is a cross-sectional view of the pulser taken along line V-V shown in  FIG. 2  with the rotors in an intermediate obstruction configuration. 
         FIG. 5C  is a cross-sectional view of the pulser taken along line V-V shown in  FIG. 2  with the rotors in a minimum obstruction configuration. 
         FIG. 6  is a schematic diagram of another embodiment of a dual rotor pulser according to an embodiment. 
         FIG. 7  is a schematic diagram of another dual rotor pulser according to an embodiment. 
         FIG. 8  is a longitudinal cross-section through a portion of the downstream pulser half of the dual rotor pulser shown in  FIG. 7 . 
         FIG. 9  is a schematic diagram a dual rotor pulser according to another embodiment. 
         FIG. 10  is a schematic diagram of a dual rotor pulser according to another embodiment. 
         FIG. 11  is a schematic diagram of a dual rotor pulser according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Embodiments of the present disclosure include a dual rotor pulser configured to transmit information along a drill string through a drilling fluid during a drilling operation where a bore is formed in an earthen formation. Dual rotor pulsers as described herein may include at least two rotors which are rotatable with respect to other and/or a stator to create pressure pulses in the drilling fluid. As such, at least two rotors may be used with or without stators to generate pressure pulses. The dual rotor pulsers as described herein may form part of a mud-pulse telemetry of a drilling system  1 . 
     Referring to  FIG. 1 , a drilling system  1  includes a rig or derrick  5  that supports a drill string  6 . The drill string  6  includes a bottomhole (BHA) assembly  11  coupled to a drill bit  15 . The drill bit  15  is configured to drill a borehole or well  2  into the earthen formation  3  along a vertical direction V and an offset direction O that is offset from or deviated from the vertical direction V. The drilling system  1  can include a surface motor (not shown) located at the surface  4  that applies torque to the drill string  6  via a rotary table or top drive (not shown), and a downhole motor (not shown) disposed along the drill string  6  that is operably coupled to the drill bit  15 . The drilling system  1  is configured to operate in a rotary steering mode, where the drill string  6  and the drill bit  15  rotate, or a sliding mode where the drill string  6  does not rotate but the drill bit does. Operation of the downhole motor causes the drill bit  15  to rotate along with or without rotation of the drill string  6 . Accordingly, both the surface motor and the downhole motor can operate during the drilling operation to define the well  2 . During the drilling operation, a pump  17  pumps drilling fluid downhole through an internal passage  180  (see  FIG. 7 ) of the drill string  6  out of the drill bit  15  and back to the surface  4  through an annular passage  13  defined between the drill string  6  and well wall. The drilling system  1  can include a casing  19  that extends from the surface  4  and into the well  2 . The casing  19  can be used to stabilize the formation near the surface. One or more blowout preventers can be disposed at the surface  4  at or near the casing  19 . 
     Continuing with  FIG. 1 , the drill string  6  is elongate along a longitudinal central axis  27  that is aligned with a well axis E. The drill string  6  further includes an upstream end  8  and a downstream end  9  spaced from the upstream end  8  along the longitudinal central axis  27 . A downhole or downstream direction D refers to a direction from the surface  4  toward the downstream end  9  of the drill string  6 . Uphole or upstream direction U is opposite to the downhole direction D. Thus, “downhole” and “downstream” refers to a location that is closer to the drill string downstream end  9  than the surface  4 , relative to a point of reference. “Uphole” and “upstream” refers to a location that is closer to the surface  4  than the drill sting downstream end  9 , relative to a point of reference. The drilling system  1  can include one or more telemetry systems  100 , one or more computing devices  200 , and one or more downhole tools used to obtain data concerning the drilling operation during drilling. The telemetry system  100  facilitates communication among the surface control system components and downhole control system. For instance, in a drilling operation, the drill bit  15  drills a bore hole into an earthen formation. A mud pump pumps drilling fluid downward through the drill string  6  and into the drill bit  15 . The drilling fluid flows upward to the surface through the annular passage  13  between the bore hole and the drill string  6 , where, after cleaning, it is recirculated back down the drill string  6  by the mud pump. As is also conventional in MWD and LWD systems, sensors, such as those of the types discussed above, are located in the bottom hole assembly portion of the drill string. The pulser  10  located in the drill collar of the bottom hole assembly  11  so that drilling fluid flows through the pulser  10 . By generating encoded pressure pulses, the pulser transmits information, such as information from the sensors, to the surface. 
       FIG. 2  illustrates a dual rotor pulser  10  according to an embodiment of the present disclosure. The dual rotor pulser may include an outer housing assembly (not shown in  FIG. 2 ) which is mounted to the drill collar or a second of drill pipe. In some embodiments, the outer housing assembly may be a portion of the drill collar or drill pipe. The pulser  10  has first and second motors  16  and  18 , respectively, mounted on a shaft  56 . The motors  16  and  18  are preferably brushed reversible DC motors supplied with power from a power source, such as a battery or a turbine alternator driven by the flow of drilling fluid. The first motor  16  drives a rotatable inner housing  14 . The inner housing  14  drives an inner shaft  42  via a first magnetic coupling  20 . An inner portion  22  of the magnetic coupling  20  is mounted on the inner shaft  42  and disposed radially inboard of a pressure housing  26 , while an outer portion  24  is mounted on the inner housing  14  and disposed radially outboard of the pressure housing. This allows the magnetic coupling  20  to transmit torque across the pressure housing  26 . As discussed in U.S. Pat. No. 6,714,138 (Turner et al.) and U.S. Pat. No. 7,327,634 (Perry et al.), incorporated by reference above and providing mechanical details concerning the construction of a pulser, on one side of the pressure housing  26  is a gas-filled chamber in which the motors  16  and  18  are located, whereas an oil-filed chamber is formed on the other side of the pressure housing. The inner shaft  42  is supported on bearings  44  and  46  and drives rotation of a first rotor  50 . 
     As shown in  FIGS. 3 and 4 , according to one embodiment of the invention, the first rotor  50  comprises a hub  57  mounted on the inner shaft  42  and a rim  58 . A series of blades  184  extending between the hub  57  and the rim  58  form generally axially extending passages  186  therebetween through which the drilling mud  182  flows. As shown in  FIG. 4 , at least one of the walls of the passages  186  may, but need not, be oriented at an angle to the axial direction so as to impart swirl to the flow of drilling fluid  182  in additional to swirl created by the rotation of the rotor  50 . 
     Continuing with  FIG. 2 , the second motor  18 , which is disposed adjacent the first motor, drives a rotatable outer housing  12 . The outer housing  12  drives an outer shaft  40 , arranged coaxially with respect to the inner shaft  42 , via a second magnetic coupling  30 . An inner portion  34  of the second magnetic coupling  30  is mounted on the outer shaft  42  and disposed radially inboard of the pressure housing  26 , while an outer portion  32  is mounted on the outer housing  12  and disposed radially outboard of the pressure housing. This allows the second magnetic coupling  30  to transmit torque across the pressure housing  26  to the outer shaft  40 , which drives rotation of a second rotor  52 . 
     As shown in  FIG. 2 , the second rotor  52  is preferably disposed immediately downstream from the first rotor  50 . The second rotor  52  comprises a hub  171 , which is mounted on the outer shaft  40 . A plurality of rotor blades  170  extending radially outward from a hub so as to form passages  172  therebetween through which the drilling fluid  182  flows. In the illustrated embodiment, the rotors  50  and  52  have radially extending blades that form passages therebetween. In alternative embodiments of the present disclosure, other types of rotors in which a portion of one rotor was capable of at least partially blocking the flow of drilling fluid through the other rotor, such as rotors formed by discs in which holes were formed, may be used. 
     The pulsers according to an embodiment of present disclosure need not utilize a stationary stator. Specifically, the first and second rotors  50  and  52  are arranged adjacent to each other so that the blades of each rotor can at least partially, and in some cases almost fully, block the flow of drilling fluid through the passages in the adjacent rotor when the blades are circumferentially aligned with the passages. Furthermore, the pulser  10  could include at least two rotors that are similar to each other. For instance, the first and second rotor could be similar to rotor  50  illustrated in  FIG. 3A . In another embodiment, the first and second rotors can be configured similar to rotor  52  illustrated in  FIG. 3B . In still the embodiment illustrated, the first rotor is similar to rotor  50  in  FIG. 3A  and the second rotor is similar to rotor  52  in  FIG. 3B . Accordingly, a “rotor” as used throughout the present disclosure includes a rotatable structure that includes a plurality of passages through which drilling fluid can flow. A “stator” is a structure that is fixed, or held stationary, and that includes at least one passage through which drilling fluid can flow. 
     The first and second motors  16  and  18  are separately controlled by a controller, such as by the controller (not shown) shown in  FIG. 6 , so that the two rotors  50  and  52  need not be rotated in the same manner. Based on the digital code from a data encoder, the controller directs control signals to drivers for the motors  16  and  18 . In a preferred embodiment, the motor driver receives power from the power source and directs power to a switching device. The switching device transmits power to the appropriate windings of the motors so as to effect rotation of the rotors in either a first (e.g., clockwise) or opposite (e.g., counterclockwise) direction so as to generate pressure pulses that are transmitted through the drilling mud. The pressure pulses are sensed by a sensor at the surface and the information is decoded and directed to a data acquisition system for further processing, as is conventional. 
     According to an embodiment, a pressure pulse is created in the drilling fluid whenever the one or both of the rotors rotate from a relative circumferential orientation in which the rotor blades of one rotor are not aligned with the passages in the other rotor and, therefore, do not obstruct the passages in the other rotor as shown in  FIG. 5C , or are only partially aligned with the passages as shown in  FIG. 5B , to a circumferential orientation in which the blades are fully aligned with the passages in the other rotor as shown in  FIG. 4  and  FIG. 5A  so as to provide the maximum obstruction to the flow of drilling fluid. A pressure pulse is also created in the drilling fluid whenever the blades of one rotor rotate from a circumferential orientation in which they are partially aligned with the passages of the other rotor and, therefore, partially obstruct the flow of drilling fluid as shown in  FIG. 5B , to a circumferential orientation in which the blades are not aligned with the passages in the other rotor as shown in  FIG. 5C . 
     The rotary pulser as described herein provides flexibility in terms of the operating mode of the pulser. In operation, one or both of the rotors  50  and  52  can be rotated continuously in the same or opposite directions, or both of the rotors can be oscillated, or one of the rotors can oscillate while the other rotates continuously in one direction. Further, one rotor can be rotated while the other rotor remains stationary, so that the stationary rotor acts as a stator. Alternatively, one rotor can be operated at a constant rotary speed, thereby generating a carrier wave within the drilling fluid, while the other rotor can rotate at a different constant rotary speed in the same direction so as to impart a phase shift in the carrier wave that is used to transmit information. In general, the rotors can be rotated in the same direction or in opposite directions. The pulser has one or more clearing operating modes when debris jams or plugs the pulser  10  such that one or both rotors  50  and  52  can be rotated as necessary to clear the debris. For example, one clearing operating mode is where one rotor rotates in a first direction while the other rotor remains stationary. In another example of a clearing operating mode is where a first rotor rotates in a first direction while the second rotor rotates in a second direction that is opposite to the first direction. In yet another example of a clearing operating mode, the first rotor remains stationary and the second rotor rotates. 
     The pulser  10  may include a control system (not shown) used to control operation of the pulser. The control system includes at least one controller and at least one position sensor. The controller may include one or more processors, a memory, and a communications link. The position sensor(s) may be mounted in air, compensated oil, or drilling mud environment within the downhole tool. In the embodiment shown in  FIG. 6 , for example, the position sensor  275  is mounted on the inner housing  214 . There may be a position sensor associated with each rotor so that each position sensor can determine the rotational position of the rotors. The sensor data obtained from the position sensors can be transmitted to the controller. The controller, in turn, can initiate an operating mode based on the position of the sensors and/or instructions from the rig operator or instructions stored in the controller. However, in alternative embodiments, the position of the rotors can be determined by monitoring the pressure wave generated by rotor motion. The control system described in this paragraph may be implemented in each of the other embodiments of the dual rotor pulsers described further below. 
     Another embodiment of a pulser  210  is shown in  FIG. 6 . In this embodiment, a first motor  216 , mounted on a shaft  270 , drives a first reduction gear  246  via shaft  280 . The reduction gear drives a rotatable inner housing  214  supported on bearings  268 . The inner housing  214  drives an inner shaft  242  via a first magnetic coupling  260 . An inner portion  262  of the magnetic coupling  260  is mounted on the inner shaft  242  and disposed radially inboard of a pressure housing  226 , while an outer portion  264  is mounted on the inner housing  214  and disposed radially outboard of the pressure housing. This allows the magnetic coupling  260  to transmit torque across the pressure housing  226 . The inner shaft  242  drives rotation of the first rotor  50 . In the embodiment shown in  FIG. 6 , the position sensor  275  is mounted to the inner housing. 
     Continuing with  FIG. 6 , a second motor  218  drives a rotatable outer housing  212 . The outer housing  212  drives a second reduction gear  248  via a second magnetic coupling  250 . An inner portion  252  of the second magnetic coupling  250  is mounted on the second reduction gear  248  and is disposed radially inboard of the pressure housing  226 , while an outer portion  254  is disposed radially outboard of the pressure housing. This allows the second magnetic coupling  250  to transmit torque across the pressure housing  226  to the second reduction gear  248 , which drives rotation of an outer shaft  240 . The outer shaft  240  drives rotation of the second rotor  52 . Element  290  includes is point of fixity where by shafts of the first and second motors are fixed. 
     Another embodiment of a pulser  310  is shown in  FIGS. 7 and 8 . In this embodiment, the pulser  310  comprises two pulser portions  302  and  302 ′, which may be identical. The pulser portions  302  and  302 ′ are mounted in passage  180  (not shown) formed within the drill collar  307 , and through which the drilling fluid  182  flows, so that their rotors  352  and  352 ′ are adjacent each other. Whereas in the embodiments discussed above, both motors were disposed on the same side, relative to the direction of flow of the drilling fluid, of the rotors  352  and  352 ′, in the embodiment shown in  FIGS. 7 and 8 , the motors  304  and  304 ′ are disposed on opposite sides of the rotors  352  and  352 ′. A controller  320  separately controls the motors  304  and  304 ′. As shown in  FIG. 7 , each pulser portion comprises a rotor  352  mounted within an outer housing assembly  338 . The outer housing assembly  338  is mounted within the drill collar  307  or section of drill pipe. The outer housing assembly  338  may include an annular shroud housing  339 , a first housing  366  and a second housing  368 . The rotor  352  is driven by a shaft  334 , which is driven by a reduction gear  346 . An electric motor (not shown in  FIG. 7 ) drives the reduction gear  346 . As previously discussed, rotation of one rotor relative to the other rotor generates pressure pulses within the drilling fluid without the need for a stationary stator. Rotation of the rotors also allows debris to be cleared from the pulser. 
     Another embodiment of a pulser  410  is shown in  FIG. 9 . As illustrated in  FIG. 9 , the pulser includes an outer housing assembly  438  for mounting in a passage  180  (not shown) of the drill string. A stator  430  is mounted to the outer housing assembly  438 . The stator  430  includes at least one stator passage  431  through which the drilling fluid can flow. The pulser  410  also includes a first rotor  450  that includes a first passage through which the drilling fluid  182  can flow, and a first motor  416  coupled to the first rotor  450  so as to drive rotation of the first rotor  450 . The pulser  410  includes a second rotor  452  having a second passage through which the drilling fluid  182  can flow, and a second motor  418  to drive rotation of the second rotor  452 . In the embodiment shown in  FIG. 9 , the stator  430  is disposed adjacent to and between the first rotor  450  and the second rotor  452 . The pulser  410  can operate similar to the pulsers described above. For instance, the pulser is designed so that at least one of the first rotor  450  and the second rotor  452  are rotatable so as to at least partially block the stator passage. Thus, rotation of one or both of the first and second rotors  450 ,  452  relative to the stator  430  creates pressure pulses. A controller can operate the motors  416  and  418  and encode the information into the pressure pulses created by rotation of the first and second rotors. The pulser  410  may include drive shafts, couplings and other components, similar to that described above and shown in the figures with respect to pulser  10 ,  210  and  310 . 
     Another embodiment of a pulser  510  is shown in  FIG. 10 . The pulser  510  shown in  FIG. 10  is also configured to transmit information from a location downhole toward a location proximate the surface of the earthen formation. The pulser  510  includes an outer housing assembly  538  for mounting in a passage of the drill string. The outer housing assembly  538  may form part of the drill string or it may be separate component attached to the drill string. A first stator  530  is mounted to the outer housing assembly  538 . The first stator  530  includes a first stator passage  531  through which drilling fluid can flow. A first rotor  550  is positioned adjacent to the first stator  530 . The first rotor  550  includes a first passage (not shown) through which the drilling fluid  182  can flow. The first rotor  550  is rotatable with respect to the first stator  530  and the outer housing assembly  538  to at least partially block the first stator passage  531 . 
     Continuing with  FIG. 10 , the pulser  510  includes a second stator  532  mounted to the outer housing assembly  538 . The second stator  532  includes a second stator passage  533  through which drilling fluid  182  can flow. A second rotor  552  is positioned adjacent to the second stator  532 . The second rotor includes a second passage through which the drilling fluid can flow. The second rotor is also rotatable with respect to second stator  532  and the outer housing assembly  538  to at least partially block the second stator passage  533 . The pulser  510  also includes a motor assembly  516  may be coupled to the first rotor  550  and the second rotor  552  to drive rotation of the first rotor  550  and the second rotor  552 . In operation, rotation of the first rotor  550  relative to the first stator  530  creates first pressure pulses in the drilling fluid  182  when the drilling fluid is flowing through the first passage and the first stator passage  531 . Furthermore, rotation of the second rotor  552  relative to the second stator  532  creates second pressure pulses in the drilling fluid  182  when the drilling fluid is flowing through the second passage and the second stator passage  533 . Rotation of the rotors according to control signal from a controller also encode the information in the first and second pressure pulses. The pulser  510  may include drive shafts, couplings and other components, similar to that described above and shown in the figures with respect to pulser  10 ,  210  and  310 . 
     Another embodiment of a pulser  610  is shown in  FIG. 11 . The pulser  610  is configured to transmit information through the drilling fluid, similar to embodiments described above. In the pulser  610  includes dual pulser portions  602  and  604 , one of which is disposed downhole with respect to the other. The pulser  610  includes an outer housing assembly  638  for mounting in a passage of the drill string. The outer housing assembly  638  may form part of the drill string, such as a portion of the drill collar or drill pipe. Alternatively, the outer housing assembly  638  may be attached to the drill string. The dual pulser portions may be a first pulser portion  602  and a second pulser portion  604  that is similar to, and is mounted downhole with respect to, the first pulser portion  602 . The first pulser portion  602  includes a first rotor  650 , a first stator  630 , and a first motor  616 . The first stator  630  may be mounted to the outer housing assembly  638 . The first stator  630  may also include a first stator passage  631  through which drilling fluid  182  can flow. The first rotor  650  is adjacent to the first stator  630  and also includes a first passage through which the drilling fluid can flow. The first rotor  650  is rotatable with respect to the first stator  630  and the outer housing assembly  638  to at least partially block the first stator passage  631 . 
     Continuing with  FIG. 11 , the second pulser portion  604  includes a second rotor  652 , a second stator  632 , and a second motor  618 . The second stator  632  may be mounted to the outer housing assembly  638 . The second stator  632  may also include a second stator passage  633  through which drilling fluid  182  can flow. The second rotor  652  is adjacent to the second stator  632  and also includes a second passage through which the drilling fluid can flow. The second rotor  652  is rotatable with respect to the second stator  632  and the outer housing assembly  638  to at least partially block the first stator passage  633 . The pulser  610  may include drive shafts, couplings and other components, similar to that described above and shown in the figures with respect to pulser  10 ,  210  and  310 . In operation, rotation of the first rotor  650  relative to the first stator  630  creates first pressure pulses in the drilling fluid when the drilling fluid is flowing through the first passage and the first stator passage. Furthermore, rotation of the second rotor  652  relative to the second stator  632  creates second pressure pulses in the drilling fluid when the drilling fluid is flowing through the second passage and the second stator passage. Rotation of the rotors according to control signal from a controller also encode the information in the first and second pressure pulses. 
     The embodiments of each pulser  10 ,  210 ,  310 ,  410 ,  510  and  610  each include a control system that controls operation of the pulser. The control system includes a controller that operates the motors (or motor assembly) to cause rotation of the first and second rotors, one or more position sensors, a power source. The controller may include one or more processors, a memory, and a communications link that can be used to transmit control signals to the motors (or motor assembly). A variety of operation modes may be used to control rotor operation. In one example, the controller is configured to operate the first and second motors so as to selectively rotate one of the first motor and the second motor while inhibiting rotation of the other of the first motor and the second motor. This may be useful in cleaning modes to remove debris. In another example, the controller is configured to cause the first motor and the second motor to continuously rotate the first rotor and the second rotor, respectively, in a similar rotational direction. In other words, the first rotor and the second rotor both rotate counterclockwise (or clockwise). In this case, the controller can cause the motor (or motor assembly) to rotate at different rotational speeds. This mode of operation may be used to adjust the data signal, in particular, the waveform of the created pressure pulsers may be adjusted. In an alternative embodiment, the controller can be set to continuously rotate the first rotor and second rotor, respectively, in different rotational directions. For example, the first rotor may rotate clockwise and the second rotor may rotate counter clockwise (or vice versa). In yet another example, the controller may be configured to the motors (or motor assembly) to oscillate the first rotor and second rotors. Furthermore, the controller may be configured to oscillate the first and second rotors at different oscillation speeds. 
     It should be appreciated that each pulser of the present disclosure may utilize a number of different rotor configurations. For instance, the pulsers with adjacent rotors may utilize rotors that are similar to each other. For instance, the first and second rotor could be similar to rotor  50  illustrated in  FIG. 3A . In another embodiment, the first and second rotors can be configured similar to rotor  52  illustrated in  FIG. 3B . In still another embodiment, the first rotor is similar to rotor  50  in  FIG. 3A  and the second rotor is similar to rotor  52  in  FIG. 3B . 
     In another embodiment, a method of transmitting information from a portion of a drill string operating at a down hole location in a well bore to a location proximate the surface of the earth may use one or more of pulsers as described herein. The pulser may include at least first and second rotors. The method includes flowing drilling fluid through the drill string passage. The method may also include rotating the first and second rotors so that each of the rotors at least partially blocks the passage in the other of the rotors so as to create pressure pulses in the drilling fluid. The rotation of rotors is selected to encode information into the pressure pulsers being transmitted to the surface through the drilling fluid. 
     Typical rotary pulsers have drawbacks and the dual rotor pulsers as described in the present disclosure may address those drawbacks. For example, conventional rotary pulsers have limited flexibility in terms of their ability to vary their operating mode as drilling conditions change or the quantity or type of data to be transmitted changes. For example, while continuous rotation in a mud siren mode might be optimal in some situations, oscillatory rotation might be optimal in other situations. Different operating modes might be needed if the pulser jambs and/or debris has to be cleared frequently. The ability to change data transmission wavelength in a siren may move the data band to a frequency where there is less noise. It would be desirable to provide a mud-pulse telemetry system and a dual rotor pulser in which the operating mode of the pulser could be varied to allow higher amplitude pulse signals to be generated downhole and observed at the surface. In addition, the dual rotor pulsers have a number of different operating modes that allow the operator (or control system) to adjust or change the data transmission wavelength during the drilling operation. 
     Furthermore, typical rotary pulsers are prone to plugging. In order to ensure that oil and gas in the formation do not enter the borehole during drilling (which is environmentally undesirable), the pressure of drilling mud in borehole is kept high. However, this can cause the drilling mud to flow into the formation at a rate that is greater than the rate at which the mud is pumped down into the hole. As a result, no mud returns to the surface, a condition referred to as lost circulation. When circulation of drilling mud is lost, drilling chips and debris from the formation are not flushed away from the drill bit. To prevent the loss of drilling mud, various types of debris and trash—referred to as lost circulation material—are pumped down the drill string along with the drilling mud so that the debris will plug the passages in the formation and prevent the loss of drilling mud. However, this lost circulation material can plug the passages in the stator of the pulser. Further, long strands of lost circulation material can become wrapped around the pulser&#39;s rotor, essentially plugging the passages between rotor blades, especially if the rotor is rotated continuously in one direction. In addition, it would be desirable to have a pulser that is less prone to plugging than traditional continuous or oscillating pulsers. The dual rotors as described in the present disclosure have several different cleaning modes that aid in removing debris downhole. This has the advantage of avoiding to have to remove the tools to remove the debris manually. This can also improve tool reliability of and minimize the possibility of catastrophic failures. 
     Thus, although embodiments described above have been illustrated by reference to certain specific embodiments, those skilled in the art, armed with the foregoing disclosure, will appreciate that many variations could be employed. Therefore, it should be appreciated that the embodiment may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.