Abstract:
A fluid handling system for drilling cuttings utilizes a constant and gravity managed liquid level between a substantially atmospheric separator and a shale shaker to maximize fluid residence time within the separator and ensure substantially all of the gas entrained in the cuttings is evolved and passed to a flare thus preventing evolution of the gas at the shale shaker. Solids from the separator are combined with liquid recirculation from and returning to the shale shaker. Optionally, a vacuum degasser is positioned between the separator and the shale shaker and separated gases are passed from the degasser to the flare. This method and system is particularly applicable to balanced, underbalanced and air drilling operations where the flow of gas is intermittent and unpredictable.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application, filed as a National stage application under 35 USC 371 from PCT/CA2005/000764 having an international filing date of May 20, 2005, is a continuation-in-part application of: U.S. patent application Ser. No. 10/990,523 filed Nov. 18, 2004 now U.S. Pat. No. 7,243,741 which is a CIP of Ser. No. 10/860,097, filed Jun. 4, 2004, now U.S. Pat. No. 7,156,193 the entirety of both of which are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   Embodiments of the invention relate to systems for fluid handling drilling fluids and, more particularly, to the handling of drilling fluids containing intermittent and unpredictable amounts of gaseous hydrocarbons for the prevention of gas release at surface or backflash from a flame used to burn at least a portion of combustible gases from a wellbore, either directly or following separation in a separator. 
   BACKGROUND OF THE INVENTION 
   In the drilling of oil and gas wells and in oil and gas production facilities, flare stacks and/or blooie lines are used, through which combustible gases, off-gassed from the wellbore, are released and burned. The release of gas through the flare stack or blooie line is typically intermittent and has non-predictable rates, including low velocity flow, creating the potential for backflash, which is the advancing of the flame front back through the flow to the source of the gas. 
   During the drilling of oil and gas wells, using a variety of drilling fluids including, but not limited to air, mist, foam, aerated and liquid mud systems, the release of combustible gases is most likely to occur while drilling at balanced or underbalanced phases of well control. Air drilling operations, whether straight air, mist or foam, are particularly at risk for backflash and, particularly so, when stopping and starting the flow of air to the wellbore while making and breaking drillpipe connections. After connection and following commencement of the flow of air in the drillpipe, it takes some time before the air completes the circuit downhole and back to surface, thus leaving a lower gas velocity below the flare igniter and therefore creating the potential for backflash. 
   Generally, backflash is most likely to occur where there is a combination of three factors, namely; a low to zero velocity flow of a combustible air and hydrocarbon gas mixture through the flare stack or blooie line; the combustible gas mixture is contained in a finite structure within the flare stack and/or blooie line or other structure; and there is a means for igniting the combustible gas mixture. One such typical example exists in a flare stack line extending from a separator vessel or a blooie line extending from the wellhead in underbalanced or balanced drilling wherein a combustible gas mixture flows from the wellbore flow tee, diverter or rotating diverter head or the separator to the flare stack and/or blooie line having an outlet to the atmosphere, the flare stack and/or blooie line being equipped with a continuous ignition source. 
   As described in “Flammability and Flashback Prevention (a work in progress)” by Dan Banks, P. E posted on the worldwide web at www.banksengineering.com/about_flame_arrestors_and_detona.htm, flame progresses at a defined rate through a combustible mixture. If the flow velocity of the gas mixture through the flare stack and/or blooie line falls below a minimum gas velocity, the minimum gas velocity being a velocity greater than a flame propagation velocity, the flame is capable of moving upstream from the point of ignition to the source of the gas and igniting the gas therein. For example, in the case of a methane/air mixture, the velocity in the pipe must exceed 1.5 ft/sec to prevent flame propagation upstream to the ignition source. If the gas source of the combustible mixture is at the separator, the separator is at risk of explosion; or if the flame front of the backflash travels down into the wellbore, a downhole fire and possibly an explosion is likely, which could result in the loss of the entire well section. 
   Typically, conventional underbalanced separators utilize backpressure valves during balanced and underbalanced drilling operations to attempt to prevent backflash however, in some circumstances the backflash can still occur through the backpressure valve. Further, pressure maintained in the separator as a result of the backpressure valve retards entrained gas from evolving from the drilling fluids in the separator. As drilling fluids are passed to a shale shaker, entrained gas which did not evolve in the separator can evolve at the shaker, creating a fire potential or the potential for the release of carcinogenic and toxic gases. The backpressure valve may also result in the exertion of a higher bottom hole pressure on the formation which can interfere with underbalanced drilling. In the case of blooie line systems, it is typical that no backflash systems are employed. In either case, it is known in the industry that backflashes to separator vessels and into wellbores have occurred, resulting in compromise to the structural integrity of mud/gas separators and causing underground fires. In Canada, backflashes have been experienced by a number of companies, particularly while air hammer drilling and/or foam drilling. 
   As reported by Susan Eaton in New Technology Magazine, March 2002 “Conquering Foothills Challenges—the air force”, air drilling can be dangerous, risky and costly, and underground fires are a real danger. As suggested, successes have been realized using a combination of air and nitrogen or nitrogen alone to replace combustible mixtures with air, however providing a source of compressed nitrogen suitable for use in the volumes required for air drilling is costly and requires additional specialized equipment at surface. 
   In cases where a large influx of fluids or gas, called a “kick”, is encountered or predicted while drilling, the operator typically shuts the blowout preventer (BOP), weights up the drilling fluid and commences drilling again using a heavier drilling fluid to increase the hydrostatic head in the wellbore which is capable of suppressing or minimizing the fluid influx. Cessation of drilling and weighting up the drilling fluid results in lost drilling time and decreased rates of penetration (ROP). 
   Clearly what is needed is a simple, reliable system for handling drilling fluids, particularly where “kicks” may be anticipated, that permits the evolution of gases from the drilling fluids within a separator for eliminating evolution of gas at the shale shaker preventing backflash, uncontrolled release of gas at the shaker tank or fear of environmental contamination. Further, it is desirable that the system permit continued drilling despite the intermittent influx of combustible hydrocarbons so as to maintain high ROP&#39;s. 
   SUMMARY OF THE INVENTION 
   A liquid handling system for drilling fluids, utilizing a low pressure separator and positioned between a wellhead and a flare, employs fluid level control between the separator and a shale shaker tank for creating a stagnant zone of liquid permitting substantially all of the gas to be evolved from the liquids and solids prior to flowing the liquids and solids to the shale shaker. Thus, evolution of gas at the shale shaker is avoided. Recirculation of substantially solids-free liquid from the shale shaker tank past the solids outlet of the separator conveys the solids from the separator to the shale shaker. 
   In a broad aspect of the invention, the system for handling drilling fluids including drilling cuttings returned from a wellbore during drilling, the fluids further comprising an intermittent and unpredictable flow of gaseous hydrocarbons entrained therein comprises: a vertical separator for receiving drilling fluids from the wellbore further comprising: a liquid volume having a liquid level control, a stagnant zone of liquid for separating the entrained gaseous hydrocarbons from the drilling fluids; a solids outlet for discharging substantially gas free drilling solids therefrom; a liquid outlet for discharging substantially gas free liquids therefrom; and a gas outlet for discharging evolved gaseous hydrocarbons at substantially atmospheric pressure therefrom; a shale shaker for receiving substantially gas free drilling solids and substantially gas free liquids discharged from the separator and for further separating the drilling solids from the liquids; a recirculation line for flowing substantially gas free separated liquids, from the shale shaker, by the solids outlet for conducting substantially gas free liquids and solids to the shale shaker; an ignition source for receiving and combusting evolved hydrocarbons from the separator; and a flame arrestor positioned between the wellbore and the ignition source. 
   Optionally, to handle extremely high volumes of gas return, a high volume line may be connected directly from the BOP with a high pressure line directly to the separator so that liquids and gases can be safely contained and controlled. 
   Preferably, the system further comprises the continuous positive backflash prevention system as set forth in co-pending U.S. application Ser. No. 10/860,097, wherein a method and system for prevention of backflash from an ignition source to a source of combustible gas utilizes a flow of addition fluid, typically air or exhaust gas, introduced into the flow of combustible gas to the ignition source in at least a minimum flame propagation velocity to ensure a continuous positive flow to the ignition source regardless the intermittent and unpredictable nature of the flow of combustible gas. Embodiments of the invention are particularly useful when drilling wellbores in balanced and underbalanced conditions and more particularly, using air/foam/aeration drilling. 
   In the case where a high volume line is used to direct high volume kicks from the wellbore to the separator, an additional continuous backflash prevention system is connected between the BOP or separator on the flow to flare pit/tank to ensure that gas and ignition sources do not reach the separator. 
   In a broad aspect of the invention, a method for prevention of flashback from an ignition source towards a wellbore during drilling of the wellbore comprises injecting a drilling fluid into a wellbore; producing the drilling fluid from the wellbore for removing cuttings from the wellbore, the produced drilling fluid containing combustible gas; flowing the combustible gas to the ignition source for burning of said combustible gas; and continuously providing an addition fluid at a velocity of at least a minimal flame propagation velocity into the flowing combustible gas downstream of the wellbore and upstream of the ignition source for avoiding flashback from the ignition source. 
   In a further broad aspect of the invention, a system for the prevention of flashback from an ignition source connected to a wellbore producing unpredictable and intermittent flows of combustible hydrocarbons during drilling of the wellbore, comprises a source of addition fluid connected to the flow of combustible hydrocarbons between the wellbore and the ignition source; a venturi for accelerating the flow of the addition fluid into the flow of combustible gas for inducing flow of combustible gas to the ignition source; wherein the addition fluid is continuously provided to the flow of combustible hydrocarbons in a velocity in excess of a minimal flame propagation velocity to prevent backflash from the ignition source to the wellbore. 
   The addition fluid is typically air or exhaust gas and in an embodiment of the invention, is provided into the flow between the wellbore and the ignition source using a venturi, which acts to accelerate the flow of the addition fluid causing the combined flow to be accelerated and ensures the combustible gases flows towards the ignition source. The venturi inlet can be positioned anywhere between the wellbore and the ignition source, typically a flare stack or blooie line. 
   In an embodiment of the invention, the venturi is positioned between a separator and the flare stack, the separator acting to provide containment of the off-gas produced with the drilling fluids and cuttings from the wellbore and to direct the gas evolved from the drilling fluids to the flare stack. The use of the separator in combination with the positive flow achieved by the addition fluid, enables drilling to proceed regardless whether “kicks” of combustible gas come from the wellbore, eliminating the need to shut the BOP&#39;s and weight up or otherwise change the drilling fluids and reducing the fear of backflash, while at the same time providing containment of gases within the separator for evolution therein and release to the flare stack without fear of gases remaining entrained and a release to the environment at the shale shaker. The ability to drill without altering the hydrostatic head in the wellbore permits balanced and underbalanced drilling to continue and further results in being able to maintain higher ROP&#39;s. 
   In the case where there is a potential for the release of sour gas from the wellbore, a vacuum degasser can also be introduced between the separator the shale shaker. Liquids exiting the separator are processed through the vacuum degasser to ensure that any gas remaining in the liquid is evolved from the liquid, the evolved gas being flowed to the flare stack and the liquids and solids being directed to the shale shaker. 
   Often drillers overlook the advantages of air drilling due to the time and costs associated with rig up and rig out of conventional air equipment implementation. A further advantage of the system of the present invention is that the system can be installed at the start of well drilling and can be used for all drilling fluid programs which might be employed, including conventional overbalanced, balanced, underbalanced and air drilling and transitions therebetween. Further, implementation of the system of the present invention minimizes drilling interruptions with changes of drilling fluids. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic of a typical mud drilling operation, being an air, mist, foam aerated mud or liquid mud drilling operation, illustrating a conventional wellsite configuration from a wellhead through to a flare or alternatively to a blooie line, a dotted line indicates recycling of drilling mud to the wellbore in the case of a mud drilling operation; 
       FIG. 2  is a schematic illustrating an embodiment of the invention being a system for drilling fluid handling used in a drilling application and incorporating a separator according to an embodiment of the invention, the particular embodiment illustrated being an air drilling operation using air, mist or foam as a drilling fluid, the system however being applicable to all mud drilling systems; 
       FIG. 3  is a schematic illustrating recirculating of fluid from a shale shaker tank past a solids outlet at a bottom of a separator for moving solids from the separator to the shale shaker according to an embodiment of the invention; 
       FIG. 4  is a schematic illustrating an embodiment of the invention having a vacuum degasser and being particularly applicable for drilling operations wherein the off-gas from the wellbore may contain at least a portion being sour gas; 
       FIGS. 5-7  illustrate an alternate embodiment for replacing a conventional poor boy degasser with a secondary high pressure line from a choke manifold to the separator, more specifically: 
       FIG. 5  is a schematic illustrating an embodiment of the invention having a high volume line directed from a BOP choke manifold to the separator, the system having a continuous backflow prevention system installed between the wellhead and the flare; 
       FIG. 6  is a schematic of a wellhead, BOP, flow diverter and choke manifold according to the embodiment of  FIG. 5 ; and 
       FIG. 7  is a separator according to  FIG. 5 , which is capable of receiving high volume flow from the choke manifold and illustrating preferred separation baffles for the separation of solids from drilling fluids. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   With reference to  FIG. 1 , a conventional drilling system comprises a drilling rig  10 , a wellhead  11 , wellbore  12  and a flare  13 . Drilling fluids  14  are injected into the wellbore  12  to aid in extraction of cuttings  15  with the drilling fluids  14  from the wellbore  12 . Suitable drilling fluids  14  include air, mist, foam or aerated mud or non-compressible liquid drilling fluids. The cuttings  15  are separated  16  from the drilling fluids  14  at surface  17 . In the case where aerated mud or non-compressible mud is, the drilling fluid  14  is typically re-circulated to the wellbore  12 , following separation of the cuttings  15 , such as at a shale shaker  16 . In air, mist or foam drilling, air is used to extract cuttings from the wellbore  12 , in place of drilling mud. The cuttings  15  may be lifted as dust or mist should there be an influx of water into the wellbore  12 . Further, agents may be added to the wellbore  12  during drilling to create a foam to aid in lifting the cuttings  15 . Drilling fluids  14  returning to surface  17  often include wellbore gases G including combustible hydrocarbons or off-gas which is burned at the flare  13  or alternatively, directly from a blooie line  18 , which is typically used to discharge returned drilling fluids  14  to a flare pit  19 . The rate of production of off-gases is highly unpredictable and typically intermittent. 
   Having reference to  FIGS. 2 and 3 , a three-phase separator  50  for separating gases from liquids and cuttings produced from the wellbore  12  is provided. The separator  50  is typically positioned between the wellhead  11  and the flare  13  such as a flare stack  20  and, in conventional air drilling operations and underbalanced drilling operations, is at risk for structural damage as a result of explosions caused by backflash from the flare  13 . 
   More particularly, and in a preferred embodiment of the invention, the separator  50  for use in the present system is configured as a vertical separator, adapted for use in mud drilling systems and aerated mud systems, as well as air, mist and foam drilling systems. The separator  50  comprises a tubular, closed body  51  having an inlet  52  formed in a sidewall  53  of the separator  50  adjacent a top end  54  of the separator  50  for receiving a stream of fluids M comprising gases G, liquids L and cuttings  15  from the wellbore  12 . A solids outlet  55  is formed at a bottom  56  for directing solids S, particularly cuttings  15 , out of the separator  50  and a gas outlet  57  is formed at the top  54  of the separator  50  for discharging wellbore off-gases G. 
   Preferably, the bottom  56  is conical and angled at 33° or steeper to ensure that solids S, which are gravity separated from liquids L and gases G therein, do not become trapped in the separator&#39;s bottom  56  and are instead directed for discharge from the solids outlet  55 . 
   Gases G, released from the liquids L and solids S, are contained within a headspace  58  above the liquids L in the separator  50  and are directed from the gas outlet  57  to the flare stack  13 . The gases G flow at substantially atmospheric pressure to the flare stack  20 . Accordingly, in a simple embodiment, the separator  50  can be at substantially atmospheric pressure. 
   A flame arrestor  1  can be positioned at the flare  13  or between the separator  50  and the flare  13  to assist in preventing backflash to the low pressure separator  50 . In another embodiment, a venturi  32  can be located anywhere between the wellhead  11  and the flare  13 . 
   Having reference to  FIG. 2  and in a preferred embodiment the flame arrestor  1  is an embodiment of a flare  13  which can be safely used in flaring wellbore off-gas that comprises a flare stack  20  incorporating the venturi  32  and having an inlet  21  for receiving a flow of wellbore gas G. An ignition source  22  is positioned within an upper end  23  of the flare stack  20  or adjacent an outlet  24 . The ignition source  22  is typically continuous, providing a flame  25  for combusting the combustible wellbore off-gases, and discharging products of said combustion through the outlet  24  to atmosphere. 
   In one embodiment of the invention, a continuous source of addition fluid  30 , typically air or some form of inert gas (nitrogen, membrane nitrogen, CO2) or exhaust gas, is introduced to the flow of off-gases G from the wellhead  11  at a constant velocity equal to or in excess of a minimum flame propagation velocity. The minimum flame propagation velocity is that velocity at which the flame is prevented from traveling upstream through the flow of gases. As shown in  FIGS. 2 and 5 , the addition fluid  30  may be added at any point A in the flow stream downstream of the wellhead  11 , and upstream of the ignition source  22 . 
   Further, in an embodiment shown in  FIGS. 2 and 5 , the addition fluid  30  is introduced through an addition fluid inlet  31  forming the venturi  32 . The venturi  32  may comprise an arrangement wherein the addition gas inlet  31  is located co-axially in the flow stream. The addition fluid  30  is discharged at a velocity higher than the velocity of the wellbore off-gas G and thereby accelerates the wellbore off-gas. Wellbore off-gas is drawn around the addition fluid inlet  31  and into the flow of addition fluid  30  for directing the combined fluid or mixture F to the ignition source  22 . 
   In one embodiment, shown in  FIG. 2 , the addition fluid  30  is introduced into flare stack  20  upstream from the ignition source  22 . An air blower, helical screw or reciprocating compressor  40  or the like, may be used to supply the addition fluid  30  flow to the addition inlet  31 . In the case of a methane/air mixture, the minimum flame propagation velocity is approximately 1.5 ft/s and therefore, the addition fluid  30  must be provided at 1.5 ft/s or greater so that, should there be no flow from the wellbore  12 , the minimum critical velocity is met and the flame  25  will remain at the ignition source  22  and not propagate upstream towards the wellbore  12  or separator  16 . In addition to providing a continuous positive flow of gases from the wellbore  12  to the flare  13  and preventing a backwards propagation of the flame  25  to the wellbore  12 , the venturi  32  creates a suction which can act to draw the produced wellbore off-gases G away from the wellhead  11  and any associated equipment and processes, further increasing the safety of personnel working on site. This may be particularly advantageous in the case of produced sour gas, which if accidentally vented, may present increased hazards to the environment and to personnel on site. 
   As shown in  FIG. 2  and, in greater detail, in  FIG. 3 , largely dewatered solids S, separated from the returned drilling fluids  14  and discharged from the solids outlet  55  at the bottom  56  of the separator  50  are directed to a shale shaker  60  where the solids S can be readily sampled. A level of liquid L in the separator  50  is hydraulically kept constant with a liquid level L in the shale shaker tank  60  resulting in a stagnant sump and causing the solids S to drop from the bottom  56  of the separator  50 . Due to the significant volume of liquid L relative to the solids S in the conical portion of the separator  50 , the residence time within the separator  50  is relatively long, maximizing any gas G evolution therefrom and into the head space  58 . Further, the liquid L forms a liquid barrier preventing gas from venting to the shale shaker tank  60 . 
   Preferably, as shown in  FIG. 3 , to aid in the discharge of solids S from the solids outlet  55 , screened fluids W are re-circulated by pump P, from the shale shaker tank  60  or alternately from a mud tank or spare tank  61 , and past the solids outlet  55  where the fluids W combine with the solids S to carry the solids S onto the shale shaker  60 . The fluids W from the shaker are largely solids free and are continuously re-circulated by the pump P. As there is little remaining solid S in the fluid W following screening on the shale shaker  60 , it is not required that the pump P be a solids pump. 
   A large portion of the liquids L separated in the separator  50  are routed to the shale shaker  60  from a liquid outlet  62  positioned in the sidewall  53  of the separator  50 . 
   In an example, a liquid level volume in the separator  50  is approximately 8-9 m 3 . Screened fluids W are pumped past the solids outlet  55  for a re-circulation rate of about 0.75 to 1.5 m 3  per minute. The art of pumping of screened fluids W is largely based on wellbore diameter, ROP and diameter of the tubing string and is typically calculated to maintain a ratio of cuttings/solids to liquid of about 25%. 
   Advantageously, the vertical separator  50  has a smaller footprint than conventional horizontal separators used in underbalanced drilling and thus requires less space at the wellsite. The system reduces the number of personnel required to operate the site. Depending upon the intended use requirements and reservoir conditions, the separator  50  may or may not be pressure rated. In broader applications as shown in  FIG. 5 , it is advantageous to combine the functions of the separator  50  to include and replace poor boy degasser functions and the separator would be pressure rated. 
   As shown in an embodiment in  FIG. 4 , and for more complete degassing especially for use where the off-gases G produced from the wellbore  12  may contain at least some H 2 S or sour gases, a vacuum degasser  70  is connected to the system at the liquid outlet  62  for increased removal of off-gases G from the drilling fluids  14 . Liquid L transported via the liquid outlet  62  to the vacuum degasser  70  are largely solids-free to avoid plugging of the vacuum degasser  70 . Gas G entrained within the liquid L is removed by the vacuum degasser  70  by differential gas liberation in accordance with conventional technology. The separated gas G is then routed to the flare stack  20  for flaring. 
   With reference to  FIGS. 5 and 6 , in a further embodiment, the wellhead  11  typically comprises blow out preventors (BOP)  120  and a flow diverter  121 . Normally drilling flow passes through the open BOP  120  and through the flow diverter  121  along line  98  to the separator&#39;s inlet  52 . These operations would be conducted substantially atmospheric pressure. 
   In cases of upset operations including higher than normal gas flows, such as a kick, the BOP  120  closed and flow is directed to a choke manifold  122  situated between the wellhead  11  and the separator  50 . A secondary and high pressure capable line  99  extends between the choke manifold  122  and the separator  50 . The choke manifold  122  acts to permit higher backpressure at the wellbore which avoiding applying the same high backpressure to the separator  50 . The relative flow capability into and out of the separator  50  is demonstrated by a typical matching of a 2000 psi, 4 inch incoming line  99  compared to a 12 inch outgoing line from the separator gas outlet  57 . The gas outlet  57  is directed to discharge to a flare pit (not shown) or a flare tank  102  and ignition source such as a flare  13 . 
   Such an arrangement can be used in the case where extremely high volumes of combustible gas are returned to surface with drilling fluids. When such high volumes of gas are detected, the flow of returning fluids can be directed through the choke manifold  122  to the separator  50 . The separator solids outlet  55  and liquid outlet  62  may have to be throttled depending on pressure in the separator  50 . A continuous positive backflow preventer, such as a blower  40  and a venturi  32 , is connected between the wellhead  11  and flare  13  or between the gas outlet  57  and the flare to prevent the backflow of fluids or flame. As shown in  FIG. 5 , the venturi  32  is preferably and optionally positioned before or after the separator  50 . 
   Thus, the separator  50  can continue to receive drilling flow and evolve gases therefrom which it otherwise could not and that would normally be routed to a poor boy degasser in conventional practice. 
   Preferably as shown in  FIGS. 5 and 7 , the separator  50  contains one or more baffles  110 ,  111  for maximizing separation of the solids and liquid form the gas phase. A first angled baffle or baffles  110  adjacent the inlet  52  direct flow downwardly upon entering the separator  50 . An optional and additional baffle or alternating baffles  111  above the first baffle  110  create a serpentine path for the liquid and liberated gases G. Each baffle  110 ,  111  is inclined to shed any liquids and solids above the baffles  110 , 111  for return to the bottom of the separator  50 . An optional bypass inlet  52 ′ branches from the main inlet  52  and discharges higher in the separator  50 . 
   Although preferred embodiments of the invention have been described in some detail herein above, those skilled in the art will recognize that various substitutions and modifications of the invention may be made without departing from the scope of the invention as defined by the claims as defined herein.