A downhole cementing system employing a cement choke within the casing to reduce the downward velocity of cement through the casing and thereby inhibit formation fracturing caused by vibration of the casing. The cement choke includes a tubular body defining a fluid passageway, a seat coupled to the tubular body and defining a seat orifice, a choke element coupled to the seat and restricting flow through the seat orifice, and a check valve coupled to the seat and operable to substantially block fluid flow through the seat orifice.

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems for cementing casing in a wellbore. In a further aspect, this invention relates to a system for reducing formation fracturing when cementing casing in an air drilled wellbore. 
     2. Discussion of Prior Art 
     During the construction of oil and gas wells a borehole is drilled to a certain depth. The drill string is then removed and casing is inserted into the borehole. After insertion of the casing into the borehole, cement slurry is pumped down through the casing and up into the space, or annulus, between the outside of the casing and the wall of the borehole. The cement slurry, upon setting, stabilizes the casing in the wellbore, prevents fluid exchange between or among formation layers through which the wellbore passes, and prevents gas from rising up the wellbore. 
     Casing which is lowered into the borehole is typically equipped with a check valve mounted on or adjacent to the bottom of the casing. The check valve is incorporated into a device commonly known as either a float collar or a float shoe. If the device is located on the end of the casing string it is generally referred to as a float shoe. If the device is located between adjacent joints of casing it is generally referred to as a float collar. During cementing of the casing, the check valve permits cement to flow downward through the casing and out into the annulus, but prevents back flow of cement from the annulus into the casing. 
     During lowering of the casing into the borehole, it is frequently necessary to open the check valve in order to allow fluid to flow upwardly therethrough. The need for opening the check valve during lowering of the casing into the borehole is caused by the presence of liquid-phase fluids in the borehole which exert an upward buoyancy force on the casing that is sufficient to float the casing in the borehole. Such liquid-phase fluids may include drilling mud and/or other wellbore fluids which are typically present in a borehole drilled using liquid-based drilling fluids. 
     In an air-drilled wellbore, however, the borehole is typically devoid of liquid-phase fluids which would be sufficient to float the casing. Rather, an air-drilled borehole typically contains primarily gas-phase fluids. Thus, when casing equipped with a check valve is lowered into an air-drilled borehole, it is not necessary to open the check valve and permit upward fluid flow into the casing in order prevent floating of the casing. In fact, in a air-drilled borehole it is undesirable to allow such upward fluid flow through the casing because the upward flow of gas-phase fluids through the casing may present a fire hazard at the top of the casing. 
     One problem encountered when cementing casing in an air-drilled wellbore is that the cement charged to the top of the casing free-falls downward through the gas-phase fluids in the casing. Because these gas-phase fluids provide only minimal resistance to the downward flow of the cement through the casing, the velocity of the cement falling through the casing can reach excessively high levels. When the high velocity cement reaches the bottom of the casing, it can cause large pressure surges which are transferred to the rock matrix. Pressure surge is undesirable because it can cause fracturing of the subterranean formation. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a wellbore cementing method is provided. The cementing method comprises the steps of: (a) lowering a casing into a borehole which contains fluids that are insufficient to float the casing; (b) charging cement to an upper end of the casing; and (c) restricting the downward flow of the cement through the casing with a cement choke. 
     In accordance with another embodiment of the present invention, a wellbore cementing method is provided. The wellbore cementing method comprises the steps of: (a) coupling a choke element to a float collar; (b) coupling the float collar between two adjacent joints of casing; (c) lowering the casing and the float collar into a borehole; (d) at least substantially blocking upper fluid flow through the float collar; (e) charging cement to the upper end of the casing so that the cement falls downward towards the float collar; and (f) contacting the cement with the choke element so that the velocity of the cement exiting the float collar is less than it would have been had step (a) not been performed. 
     In accordance with a further embodiment of the present invention a downhole choke couplable between two adjacent joints of wellbore casing is provided. The downhole choke comprises a tubular body, a seat, a choke element, and a check valve. The tubular body defines a fluid passageway. The seat is coupled to the tubular body and defines a seat orifice. The seat orifice is in fluid communication with the fluid passageway. The choke element is coupled to the seat and defines a choke orifice. The choke element is operable to at least partially inhibit fluid flow through the seat orifice in a first flow direction. The check valve is coupled to the seat and operable to at least substantially block fluid flow through the seat orifice in a second flow direction which is generally opposite the first flow direction. 
     In accordance with a still further embodiment of the present invention, a wellbore which has been readied for cementing is provided. The wellbore comprises a generally downwardly extending borehole, a casing string, and a cement choke. The casing string presents upper and lower ends and defines a fluid passageway therebetween. The casing string is disposed in the borehole and is at least substantially fixed relative to the borehole. The cement choke is coupled to the casing string below the upper end of the casing. The cement choke presents a flow restricting surface operable to at least partially inhibit the downward flow of cement through the fluid passageway and dampening pressure surges. The fluid passageway above the cement choke primarily contains gas-phase fluids. 
     In accordance with another embodiment of the present invention a method of making a downhole cement choke is provided. The downhole cement choke is made by modifying a conventional float collar which includes a seat presenting a seat opening and a check valve coupled to the seat and operable to provide one-way flow through the seat orifice. The seat defines a surface into which a conventional auto-fill valve can be mounted. The method of making the downhole cement choke comprises the steps of: (a) forming a choke element which defines a choke orifice having a flow area which is less than the flow area of the seat orifice; and (b) placing the choke element in registry with the surface which could hold the conventional auto-fill sleeve so that the choke element is spaced from the check valve. 
     The present invention provides a system for inhibiting the fracturing of subterranean formations when cementing casing in a wellbore. Other aspects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments and the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     Preferred embodiments of the invention are described in detail below with reference to the attached drawing figures, wherein: 
     FIG. 1 is a side view showing a drilling rig lowering casing into a borehole; 
     FIG. 2 is an assembly view of a downhole cement choke; 
     FIG. 3 is an isometric view of a choke element with certain sections being cut away; 
     FIG. 4 is a top view of a downhole cement choke; 
     FIG. 5 is a cross-sectional view of a downhole cement choke taken along lines  5 — 5  in FIG. 4; and 
     FIG. 6 is a cross-sectional view of a downhole cement choke showing cement flowing therethrough. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a drilling rig  10  lowering a length of uncemented casing  12  into a wellbore  14 . Wellbore  14  includes a surface casing  16  extending generally downward from aground surface  18  and presenting a casing head  20  located proximate ground surface  18 . Wellbore  14  is also shown as including an intermediate casing  22  located below surface casing  16 . In FIG. 1, surface casing  16  and intermediate casing  22  are shown as having already been cemented in wellbore  14 . 
     Positioned below intermediate casing  22  is a borehole  24  which has been drilled into a subterranean formation  26 . 
     Casing  12  is lowered into borehole  24  via drilling rig  10  and a pipe  26 . Casing  12  presents an upper end  28 , a lower end  30 , and a fluid passageway  32  extending therebetween. A cement choke  34  is coupled between an upper joint  36  of casing  12  and a lower joint  38  of casing  12 . Casing  12  further includes a shoe  40  coupled to lower end  30  for guiding casing  12  through borehole  24 . An annulus  42  is formed between the outside of casing  12  and a borehole wall  44 . 
     When casing  12  is lowered to its desired depth in borehole  24 , cement pump  46  can be actuated to pump cement slurry from a cement source  48  into wellbore  14 . In wellbore  14 , the cement travels downwardly through fluid passageway  32 , out of casing  12  through shoe  40 , and up into annulus  42 . 
     In accordance with the present invention, prior to lowering casing  12  into borehole  24 , borehole  24  preferably contains fluids which are insufficient to float casing  12 . More preferably, borehole  44  contains primarily gas-phase fluids. Most preferably, borehole  24  contains substantially only gas-phase fluids. In order to obtain a borehole having the above-described properties, borehole  24  may be drilled using under balanced drilling techniques which employ low density circulating fluids. The circulating fluid used during drilling of borehole  24  preferably has a density of less than two pounds per gallon, more preferably less than one pound per gallon. Examples of suitable low density circulating fluids include air, nitrogen, natural gas, carbon dioxide, foams, mists, stiff foams, and aerated drilling fluids. Most preferably, bore hole  24  is air drilled with a primarily gas-phase drilling fluid such as, for example, air, natural gas, and/or nitrogen. 
     After drilling borehole  24  in accordance with the above described techniques, the fluids contained in borehole  24  are insufficient to float casing  12 . Thus, because there is little resistance to the downward travel of casing  12  through borehole  24 , there is no need to permit the fluids in borehole  24  to pass upwardly through fluid passageway  32  of casing  12 . Further, because the fluids contained in borehole  24  may be combustible, it is preferred that the fluid is at least substantially blocked from upward flow through fluid passageway  32  when casing  12  is being lowered into borehole  24 . If upward fluid flow is not blocked, a fire hazard may be created at the base of drilling rig  10 . 
     Blocking upward flow through fluid passageway  32  during the lowering of casing  12  in borehole  24  results in fluid passageway  32  containing primarily gas-phase fluids when casing  12  is positioned for cementing. In such an arrangement, cement charged to upper end  28  of casing  12  is subjected to substantially free-fall conditions above cement choke  34 . In accordance with the present invention, cement choke  34  is operable to reduce the velocity of the cement falling through fluid passageway  32  and thereby reduce pressure being transferred external to the casing. 
     FIG. 2 shows the components and construction of cement choke  34  in detail. Choke  34  generally comprises a float collar  50 , a choke element  52 , and a resilient ring  54  for coupling choke element  52  to float collar  50 . 
     Float collar  50  includes a tubular body  56  supporting a seat  58  which is coupled to a check valve  60 . Tubular body  56  includes an upper end  62  presenting an upper opening  64  and a lower end  66  presenting a lower opening  68 . Tubular body  56  defines a flow passageway  70  extending between upper opening  64  and lower opening  68 . Tubular body  56  is couplable between two adjacent joints of casing via internal threads  72  on upper end  62  and external threads  74  on lower end  66 . Tubular body  56  is composed of any suitably strong material, such as, for example, steel. 
     Seat  58  is fixedly coupled to tubular body  56 . Seat  58  can be formed within tubular body  56  or can be manufactured separate from tubular body  56  and then threaded into tubular body  56  via internal threads  72 . Seat  58  is generally disposed in flow passageway  70  and presents an inner seat wall  76 . Inner seat wall  76  defines a seat orifice  78  which is in fluid communication with flow passageway  70 . Seat orifice  78  has a flow area which is generally less than the flow area of flow passageway  70 . As used herein, the term “flow area” shall mean the cross-sectional area of an opening through which fluid may flow, with the cross-section being taken along a plane which is generally perpendicular to the direction of flow through the opening. Preferably, seat orifice  78  has a flow area which is less than fifty-percent of the flow area of flow passageway  70 . Most preferably. seat orifice  78  has a flow area which is less than twenty-five percent of the flow area of flow passageway  70 . Seat  58  can be made of any suitable strong material, such as, for example, aluminum or fiber-reinforced cement. Seat  58  includes an upper portion  80  to which choke element  52  may be coupled and a lower portion  82  to which check valve  60  may be coupled. 
     Upper portion  80  presents a mounting recess  84  located adjacent inner seat wall  76 . Mounting recess  84  includes a generally horizontal surface  86  and a generally vertical surface  88 . Vertical surface  88  is interrupted by a slot  90  formed therein. Slot  90  is adapted to receive resilient ring  54  when choke element  52  is mounted on seat  58 . 
     Check valve  60  is operable to at least substantially block upward fluid flow through seat orifice  78  while permitting downward fluid flow through seat orifice  78 . Check valve  60  is shiftable between an open position during which fluid flow through seat orifice  78  is permitted and a closed position during which fluid flow through seat orifice  78  is at least substantially blocked. Check valve  60  is preferably a flapper-type valve including a flapper body  92  which is pivotally coupled to lower portion  82  of seat  58  by a hinge  94 . Check valve  60  is biased towards the closed position in which flapper body  92  substantially covers seat orifice  78 . In the closed position, flapper body  92  substantially sealingly contacts lower portion  82  of seat  58  with an  0 -ring seal  95 . A spring  96  located proximate hinge  94  urges check valve  60  toward the closed position. Float collar  50  can be a commercially available flapper float collar, such as, for example, a Model 1406 Auto-fill Flapper Float Collar available from Weatherford Inc., Houma, La. Choke element  52 , described in detail below, can be mounted on seat  58  in place of a conventional auto-fill sleeve. The conventional auto-fill sleeve is replaced by choke element  52  because the auto-fill sleeve undesirably holds check valve  60  in the open position while the casing is being lowered into the borehole. Further, the conventional auto-fill sleeve is likely to be incapable of acting as a cement choke because its flanges which mount it to the seat may not be durable enough to withstand the impact of cement free-falling through a substantial length of casing. 
     As perhaps best illustrated in FIG. 3, choke element  52  includes a generally hollow body  96  presenting an upper flow restricting surface  98  and an inner cylindrical surface  100  which defines a choke orifice  102 . Choke orifice  102  has a flow area which is generally less than the flow area of seat orifice  78 . Preferably, choke orifice  102  has a flow area which is less than twenty-five percent of the flow area of flow passageway  70 . Most preferably, choke orifice  102  has a flow area which is less than fifteen percent of the flow area of flow passageway  70 . Body  96  includes an upper annular portion  104  and a lower annular portion  106 . Upper annular portion  104  presents lower circumferential surface  108  and lower annular portion  106  presents upper circumferential surface  110 . The outside diameter of upper annular portion  104  is greater than the outside diameter of lower annular portion  106  to thereby form a mounting flange  112 . Mounting flange  112  presents a lower mounting surface  114  extending between upper circumferential surface  108  and lower circumferential surface  110 . Choke element  52  can be made of any suitable material which is strong enough to withstand the impact of falling cement without breaking mounting flange  112 . Preferably, choke element  52  is formed of aluminum. 
     As perhaps best seen in FIG. 2, choke element  52  can be mounted on seat  58  by positioning mounting flange  112  in registry with mounting recess  84  and then inserting resilient ring  54  into slot  90 . FIG. 4 shows that a portion of ring  54  extends over flow restricting surface  98  to thereby restrain movement of choke element  52  relative to seat  58 . Ring  54  has a generally C-shape and includes a pair of openings  116  at its ends for inserting and removing ring  54  from slot  90 . Ring  54  can be made of any suitably strong and resilient material such as, for example, steel. 
     FIG. 5 shows choke element  52  mounted on seat  58  and restrained from movement by ring  54 . FIG. 5 illustrates that choke element  52  is spaced from check valve  60  by a gap  118  and therefore does not interfere with the operation of check valve  60 . 
     FIG. 6 shows check valve  60  in the open position with cement  120  flowing through choke orifice  102 . As can be seen in FIG. 6, all cement  120  passing through cement choke  34  must pass through choke orifice  102 . 
     The preferred forms of the invention described above are to be used as illustration only, and should not be utilized in a limiting sense in interpreting the scope of the present invention. Obvious modifications to the exemplary embodiments, as hereinabove set forth, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
     The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.