Abstract:
Degasser assemblies, systems and methods, including an air-driven degasser assembly that includes a degasser having a drive shaft with a seal that inhibits intrusion of ambient air and an air motor having a motor shaft that drives the drive shaft. The motor shaft is distinct from or integral to the drive shaft, and the air motor is positioned to direct air leakage around the motor shaft away from the seal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Provisional U.S. Application Ser. No. 61/825,280, titled “Degasser Air Motor Separated Mounting” and filed May 20, 2013 by Matthew Hay Henderson, Charles Cutler Britton and Neil Patrick Schexnaider, which is incorporated herein by reference. 
    
    
     BACKGROUND 
     As the demand for oil and gas has continued to increase, oil field operators have had to drill increasingly deeper wells to meet this demand. One of the elements of a drilling operation that makes such deep wells feasible is drilling fluid. Drilling fluid or “mud” is typically injected down into a drill pipe, through the drill bit and back up the borehole in the annulus formed between the borehole wall and the exterior of the drill pipe. The fluid provides drill bit lubrication and cooling, controls the borehole pressure, stabilizes the borehole wall and carries drilling cuttings up and away from the bottom of the borehole. The fluid typically flows in a closed loop, wherein the fluid is filtered to remove cuttings and other impurities before it is re-injected into the borehole. 
     As drilling progresses, gasses from the surrounding formation may be released into the drilling fluid, forming bubbles within the fluid. Operators will sometimes separate out the gases from the fluid to measure and analyze the hydrocarbons present in the extracted sample gas. Such separation is performed by a motor-driven drilling fluid degasser. Because of the presence of flammable gases, air motors are generally used to drive the degasser rather than more expensive explosion-proof electric motors. Such motors are typically directly mounted to the degasser in order to reduce the amount of space needed by the degasser/motor assembly, reduce the level of vibration produced by the operation of the assembly and reduce the number of assembly components. 
     As a result of being driven at high pressures and/or high flow rates, air motors typically leak, allowing air to escape from seals around the output shaft. These air leaks can increase if the shaft surface becomes worn due to debris or inadequate lubrication. While such leaks are considered normal for air motors, the inventors have observed that they present a significant drawback when an air motor drives a degasser. Because the motor is mounted directly to the degasser chamber, air can leak from the motor into the degasser chamber where it mixes with the extracted sample gas. This addition of an unknown quantity of air dilutes the extracted sample gas by an undetermined amount. Further, because the air used to drive an air motor is mixed with lubricating oil, additional contamination can occur and add to the error in the measurement of hydrocarbons within the gas stream. To aggravate matters even further, the direct mounting of the motor prevents operation and maintenance personnel from detecting a leak until the degasser is disassembled and the motor shaft is tested for leaks. Although these problems have long been recognized, the inherent limitations of existing degasser designs have prevented the development of viable, practical solutions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accordingly, there is disclosed herein a novel degasser drive configuration that isolates the degasser seals from impinging air motor leaks. In the drawings: 
         FIG. 1  shows an illustrative drilling environment. 
         FIG. 2  shows an illustrative air motor and degasser chamber assembly. 
         FIGS. 3A and 3B  show alternative air-driven degasser assemblies. 
         FIG. 4  shows a flow diagram of an illustrative degassing method. 
     
    
    
     It should be understood that the drawings and corresponding detailed description do not limit the disclosure, but on the contrary, they provide the foundation for understanding all modifications, equivalents, and alternatives falling within the scope of the appended claims. 
     DETAILED DESCRIPTION 
     The disclosed systems are best understood when described in an illustrative usage context. Accordingly,  FIG. 1  shows an illustrative drilling environment. A drilling platform  2  supports a derrick  4  having a traveling block  6  for raising and lowering a drill string  8 . A kelly  10  supports the drill string  8  as it is lowered through a rotary table  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  18 . A pump  20  circulates drilling fluid through a feed pipe  22  to kelly  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 , through degasser assembly  200  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. 
     As shown in  FIG. 1 , drilling fluid exiting borehole  16  flows through degasser assembly  200 , where gases entrained in the drilling fluid are separated out for measurement.  FIG. 2  shows an illustrative embodiment of degasser assembly  200 . Compressed air flows into air motor  202 , which drives the degasser (e.g., driving an impeller of an atmospheric degasser). Air motor  202  is mounted such that it is spaced away from the end of upper degasser chamber  214 , leaving an air gap  201  between the motor and degasser into which leaking air  206  is released. Coupling  208  couples air motor shaft  210  to degasser shaft  216 , which extends into degasser lower chamber  220 . The shaft segments and coupling within the air gap  201  between air motor  202  and upper degasser chamber  214  are surrounded by guard  204  (e.g., a mesh or ventilated cover), which shields the exposed rotating components while also protecting personnel working nearby. In at least some illustrative embodiments, the spacing between the air motor  202  and the degasser assembly  200  is at least 5 centimeters. 
     As can be seen in  FIG. 2 , when leaking air  206  escapes from air motor  202 , the air leaks past a seal  203  around shaft  210 , into the space behind guard  204  and through small holes in the guard. In this manner the leaking air  206  is released into the air surrounding degasser assembly  200  rather than into the degasser. Still, leaking air  206  can under at least some circumstances take the form of a directed air stream that if allowed to impinge on degasser shaft seals  212  could still potentially force air past seals  212  and contaminate the air in the degasser. To help prevent this, in at least some illustrative embodiments a diverter is positioned along shaft  210  and/or shaft  216  to disrupt and/or redirect leaking air  206 . 
     In the example of  FIG. 2 , coupling  208  may further operate as said diverter, directing the leaking air  206  away from degasser shaft seals  212  of degasser assembly  200 . In other illustrative embodiments, a dedicated diverter is attached to at least one of the shafts and is shaped to direct leaking air away from degasser shaft seals  212 . Such a diverter may be formed into any of a number of shapes, including but not limited to a cylinder or a prism, either of which may be tapered. For non-tapered diverters, such as coupling  208  of  FIG. 2 , the flat surface at the end facing the air motor disrupts and/or redirects air  206 . For a tapered diverter, the diameter of the diverter increases with increased distance along shaft from air motor  202 , causing the sides of the diverter to redirect leaking air  206  away from the degasser shaft seals  212  as the air moves along the shaft. In at least some illustrative embodiments, the maximum diameter of the diverter is at least twice that of the shaft. The diverter may be constructed of any of a number of different materials, but preferably materials similar to those of the shaft (e.g., stainless steel) that are resistant to chemical reactions with the fluids (liquid and gas) both introduced from the surface and extracted from downhole during drilling operations. 
     Degasser shaft seals  212  maintain a seal around degasser shaft  216  where it exits upper degasser chamber  214  to prevent gasses from escaping and to inhibit intrusion of ambient air into the degasser. Wiper  218  reduces fluid migration to the sealing surfaces. In at least some illustrative embodiments, sealed bearings are used to retain lubricating grease and provide low pressure difference air sealing (e.g., at or below 5 inches of water or 0.2 psi). By releasing leaked air  206  into the air, redirecting it away from degasser shaft seals  212  and avoiding injecting it into the degasser, the dilution and contamination effects that leaked air  206  would have on the separated gas are avoided, thus ensuring the integrity of the gas samples provided by the degasser. 
     Although the embodiment of  FIG. 2  shows two drive shafts coupled to each other to each other by coupling  208 , other embodiments may use a single shaft where the motor shaft is integral to the drive shaft, or multiple individual shafts coupled to each other using a variety of different coupling mechanisms. For example, in at least one illustrative embodiment a single shaft couples the air motor to the degasser assembly, and a diverter is mounted along the shaft to direct leaking air away from the degasser assembly&#39;s shaft seals. In other illustrative embodiments, the air motor and degasser assembly each have at least one shaft and are positioned side-by-side (i.e., transversally displaced relative to each other) as shown in  FIGS. 3A and 3B . In such embodiments, pulleys  203  are mounted to the drive shafts and belts and/or chains  205  couple the drive shafts of the air motor and degasser to each other. Still other embodiments use one or more gears  207 , wherein the gear(s) couple to each other and/or to splines in the drive shaft(s). Many other configurations that maintain the air motor and degasser assembly spaced away from each other will become apparent to those of ordinary skill in the art, and all such configurations are within the scope of the present disclosure. 
     Referring again to  FIG. 2 , the air motor and degasser shown further enable workers to inspect the air motor shaft for leaks without the need to separate the air motor from the degasser, reducing the overall labor associated with such inspections. In at least some illustrative embodiments, standoffs (not shown) between air motor  202  and upper degasser chamber  214  provide the mounting points for air motor  202 , thus allowing removal of guard  204  as needed to inspect air motor  202  for leaks. This permits the air motor to be inspected without taking the unit out of service. 
       FIG. 4  shows an illustrative method  400  for degassing borehole fluid using the above-described degassers. The method starts by passing borehole fluid through a degasser (block  402 ), e.g., by pumping drilling fluid down a drillstring, back up between the drillstring and casing of a borehole and through the degasser, as shown in  FIG. 1 . The degasser is driven with an air motor (block  404 ), which causes the degasser to separate the gasses entrained in the borehole fluid and allows the gasses to be collected and/or analyzed. During operation of the degasser, air leaking from the air motor is directed away from the degasser drive shaft seal (block  406 ) using any of the previously described configurations and/or diverters, thus completing the method (block  408 ). 
     Numerous other modifications, equivalents, and alternatives, 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 modifications, equivalents, and alternatives where applicable.