Abstract:
A separator apparatus for separating flush effluent from an oil and gas well following a fracture stimulation process, the separator having a vessel that maintains the effluent at a selected liquid level, an inlet is connected to the vessel at a location below the liquid level so that the effluent enters the vessel at a location therebelow. A plurality of baffles within the vessel slow the effluent so that solids settle out to a drain which is periodically opened to transfer the solids to an appropriate disposal. An upper outlet permits the venting of gases that percolate from the liquid constituent, a backpressure valve disposed in communication therewith to maintain a positive pressure within the vessel. A liquid drain withdraws the liquid constituent from the vessel at a rate determined by a level assembly.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the field of oil well servicing and workovers and more particularly, but not by way of limitation, to a separating apparatus for the safe and environmentally sound recovery of stimulation materials and subterranean debris following an oil well stimulation. 
     BACKGROUND 
     Oil and gas wells are often stimulated in order to enhance production. Stimulation techniques are employed to make marginal wells economically feasible for continuing production. Most stripper wells would not be financially viable absent the advances in stimulation techniques available in the art today. Although there are numerous stimulation methods that have gained credence in the art, the most widely used methods involve the use of explosives, acidizing, and fracturing. 
     One explosive approach is nitro-shooting, which involves the use of explosive detonations at select subterranean locations to shatter and fracture a geological formation. The fractured formation is thusly provided increased permeability, resulting in increased production capability. The use of subterranean explosives is almost as old as the oil industry itself. More recently, the use of explosives has given way to the use of guns that fire bullets into the formation. This enhances the ability to direct the fracturing relative to a desired fault line, the results being more predictable than the relatively random effects of underground explosives. 
     Acidizing has 19th century origins and was fully recognized as a viable well stimulation method by the 1930s. Acidizing involves subterranean injection of acid into an acid-soluble formation where the dissolving action enlarges openings in the formation to increase permeability. Many techniques are employed to localize and direct the dissolving reaction, such as packing or cementing portions of the casing to isolate the treatment region. Acidizing has more recently been widely replaced with hydraulic fracturing. 
     First introduced in the late 1940s, hydraulic fracturing sparked immediate and widespread acceptance as a viable method of increasing the permeability of a well. Unlike acidizing, fracturing is effective in any type of formation and can be combined with acidizing for even better results in certain formations. This method consists generally of injecting a fracturing fluid and propping agent mixture into the formation and applying pressure, typically a hydraulic pressure, to reopen existing fractures and create new ones. The propping agent commonly is an appropriately sized silica sand. 
     After the hydraulic pressure has forced open the fractures in the formation, a portion of the propping agent remains wedged in the fractures to hold the fractures open. The oil and gas subsequently produced thereby flows through the embedded propping agents that remain wedged in the fracture. 
     All of these stimulation methods have in common the need to clean the stimulated area, drill string, and casing before resuming oil and gas production. Debris from explosives—residue from acidizing and propping agent particles from fracturing—similarly must be flushed from the well prior to resuming production. The well formation pressure is used to expel the debris and residue; that is, prior to returning to service, the well is first operated to produce a flush stream to flush the debris from the well. The mixture of oil and gas products with debris, otherwise referred to as a flush effluent, must be disposed in an appropriate manner. In the past, the flush effluent was commonly discharged into open earthen pits where the gases and liquids evaporated and the solids were then buried or hauled away for disposal. 
     Concerns about pollution leaching into the soil and jeopardizing potable water tables have stimulated state and federal legislation to effectively eliminate the use of earthen pits for storing the flush effluent. Today, in most cases, the flush effluent must be handled as a hazardous waste material. The current common practice is to collect the flush effluent as a whole, that is, as a mixture of subterranean fluids and debris. The flush effluent is typically delivered into a storage container, such as a frac tank. There are, however, generally recognized difficulties associated with collecting and hauling off the flush effluent as a whole. 
     One problem is associated with the erosive nature of the flush effluent. The formation pressure that delivers the flush effluent is typically high, and the propping agent sufficiently abrasive, so that the flush effluent stream can quickly cut away the steel floor of a frac tank. Extensive monitoring and frequent repair of the tanks is necessary to prevent catastrophic leaks. Should a tank fail while in transit, waste spills can result in public areas, thereby creating unacceptable environmental hazards. 
     Another problem is associated with the relative expense involved with collecting the effluent as a mixture rather than as separated constituent parts. By separating the effluent into constituent parts before disposal, significant improvements in operating efficiency are realized. It would be desirable to perform an on-site separation, but commercial separators available in the art, those typically used in other operations such as the recovery of drilling fluid, are inherently incapable of separating the effluent at hand given the characteristic throughput requirements. 
     For example, U.S. Pat. No. 5,718,298, issued to Rusnak, teaches a separator for separating the constituent parts of gases and solids in a drilling fluid where air is the primary constituent. Since the separator of Rusnak &#39;298 separates two constituents, it is commonly referred to as a two-stage separator. Three-stage separators, capable of separating solids, liquids and gases, are similarly employed to dispose of drilling fluids, such as those taught by U.S. Pat. No. 5,415,776 issued to Homan and U.S. Pat. No. 4,247,312 issued to Thakur et al. 
     One skilled in the art will recognize that these and other commercial separators available in the prior art are not suited for the characteristics of an effluent flow like that of concern in the present invention. The velocity and abrasiveness of the effluent resulting from a fracturing procedure is far too aggressive to be operatively controlled by the structural components of these devices. Particularly, the involute of Homan &#39;776 and the deflector plates of Rusnak &#39;298 would be quickly eroded and thus rendered ineffective under the conditions of recovering fracturing materials from a well. Thakur &#39;312 teaches a separator having a relatively low velocity inlet stream wherein the solids settle out directly beneath the inlet. This is an unworkable solution because the relatively high velocity of the effluent stream of present concern is too aggressive to succumb to the settling action of solids and weiring action of liquids provided by the separator of Thakur &#39;312. 
     Although U.S. Pat. No. 5,513,704 issued to Sander makes provision for the relatively aggressive characteristics of a flush effluent of present concern, there is lacking the capability for three-stage separation of the flush effluent. The gas constituent is separated from the flush effluent, but the solids and liquids are removed from the collection vessel as a mixture and must therefore be separated remotely from the collection vessel. As such, there is lacking in the prior art any solution that provides a self-contained apparatus for the separating of all constituents, that is, solid, liquid, and gas constituents, before removal thereof from the receiving vessel. 
     A more serious problem lies in the flammable propensity associated with methods of collecting the flush effluent. The flush effluent typically contains flammable hydrocarbons, both liquid and gaseous, and also contains subterranean fragments and fracturing materials that are delivered at a high velocity into the storage tank. The impacting fragments against the steel tank often create sparks, which in the presence of combustion air and the hydrocarbons can result in a fire or explosion. Today it is a common precaution to keep a ready supply of drilling mud on hand for the sole purpose of extinguishing a fire resulting from these conditions. This reactive approach to a known dangerous condition has tragically harmed numerous oil well personnel. Even assuming that injuries could be prevented, the property losses stemming from fire damage to wells and equipment is significant. The health, safety, and equipment costs associated with meeting the environmental concerns at hand are widely recognized as significant. These costs justify the use of a separator that eliminates the possibility of fire or explosion in recovering the effluent. 
     The Sander &#39;704 separator fails to contemplate the flammable nature of recovering the effluent of present concern. Sander &#39;704 teaches the use of an impact deformable member, such as a green hardwood log, to divert the effluent flow in a receiver vessel. The propants and the debris in the inrushing effluent embed in the deformable member, providing an erosion resistant diverter surface. Although this may solve the erosion problem associated with recovering the effluent at hand, the propant and fragments that subsequently strike the embedded propant and fragments will create sparks, and the sparks are in the presence of combustion air and fuel. 
     Hence Sander &#39;704 fails to resolve the inherent safety concerns associated with recovery of the effluent at hand. 
     There is, therefore, a need in the industry for an improved separating apparatus to collect the flush effluent from a well-site stimulation, such as the fracturing material used in hydraulically fracturing a formation to increase permeability. The apparatus would advantageously capture the effluent as delivered at the formation pressure and process the effluent to separate the constituent parts and safely dispose of the constituents in a self-contained manner. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus for collecting and separating a flush effluent that is delivered by the formation pressure of a subterranean oil and gas well after a well stimulation. The flush effluent is a mixture of constituents such as solids, liquids, and gases made up of subterranean fluids and debris as well as propants that remain subsequent to a well stimulation, such as from a fracturing procedure. 
     The apparatus of the present invention has a vessel adapted to support a liquid bed and to receive the effluent from the well into the liquid bed. The liquid bed can be initially provided by pumping an appropriate liquid into the vessel which is subsequently removed proportionately to the rate that effluent enters the vessel so as to maintain a preselected level of liquid in the vessel. That is, as effluent is admitted to the vessel the rate at which liquid is withdrawn from the vessel is determined by the rate at which a liquid constituent of the effluent enters the vessel so as to maintain the preselected level of liquid, or liquid bed. 
     An inlet is provided for admitting flush effluent to the vessel at a location below the liquid bed. A choke valve is supported by the inlet to meter the flow of flush effluent into the vessel, thus reducing the flow rate of the flush effluent to a preselected flow rate. 
     The flush effluent flows longitudinally within the vessel and encounters a plurality of baffles which consecutively slow the flush effluent velocity, allowing solids in the flush effluent to settle to the bottom of the vessel and the gases of the flush effluent to percolate to the top of the vessel where a vent is provided for removing the gases. Separate drains are provided to permit removal of accumulated solids and liquids. 
     A backpressure manifold is connected to the vent, having a backpressure valve for controlling the flow of gas exiting the vent so as to maintain a preselected internal pressure in the vessel. The backpressure manifold further has an emergency valve fluidly in parallel with the backpressure valve in the event the vessel pressure exceeds a preselected pressure. 
     The liquid constituent of the effluent is withdrawn at a forward end of the vessel, at a location downstream of where the solids settle out and the gases percolate out. The liquid constituent is withdrawn from the vessel at a rate to maintain a preselected level of the liquid bed above the inlet. A portion of the liquid can be recirculated into the choke valve to provide a continuous flow through the choke valve and vessel to prevent freezing during certain operating conditions. 
    
    
     An object of the present invention is to provide a well-site separator capable of separating the gaseous, liquid and solid constituents of a flush effluent and the like. Other objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the drawings and appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a separating apparatus constructed in accordance with a preferred embodiment of the present invention. 
     FIG. 2 is a semi-detailed, top view of the separating apparatus of FIG.  1 . 
     FIG. 3 is a top view of the choke valve of FIG.  2 . 
     FIG. 4 is a side view of the choke valve of FIG.  3 . 
     FIG. 5 is a sectional view taken at  5 — 5  in FIG.  4 . 
     FIG. 6 is a diagrammatical sectional view of the separating apparatus of FIG. 1 illustrating a preferred arrangement of internal baffles and the separation of the effluent constituents. 
     FIG. 7 is an elevational view of an internal baffle of the separating apparatus of FIG. 
     FIG. 8 is a sectional view of the baffle of FIG. 7 taken along the section line  8 — 8 .  1 . 
     FIG. 9 is an elevational view of another internal baffle of the separating apparatus of FIG.  1 . 
     FIG. 10 is a sectional view of the baffle of FIG. 9 taken along the section line  10 — 10 . 
     FIG. 11 is an elevational view of yet another internal baffle of the separating apparatus of FIG.  1 . 
     FIG. 12 is a semi detailed schematic of a backpressure manifold assembly. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides an apparatus for separating a flush effluent, sometimes referred to herein simply as “effluent,” that is flushed from an oil and gas well after a stimulation procedure. The effluent of present concern is commonly a mixture of subterranean fluids including oil, gas and water, along with debris from the stimulation such as propping agents (or propants) and subterranean fragments. The separator apparatus of the present invention receivingly processes the flush effluent to separate the gaseous, solid, and liquid constituents for safe and environmentally sound disposal thereof. 
     FIG. 1 shows a perspective view of a separator apparatus  100  constructed in accordance with a preferred embodiment of the present invention. A vessel  102  is supported by a wheel assembly  104  and a hitch assembly  106  for transport in a conventional manner by a towing vehicle bearing an appropriate hitch receptacle (not shown). In this manner the separator apparatus  100  is transported to the well site in the manner of a trailer, the towing vehicle capable of disconnecting from the separator apparatus  100  which is supportable upon a pair of telescoping support legs  108  which serve to support and level the vessel  102 . Although the vessel  102  illustrated in FIG. 1 is a horizontally disposed, straight barrel tank, a commonly used snub-nose tank could be equivalently used in construction of the present invention to benefit from towing with a smaller towing vehicle. Also, a skid mounted vessel is within the contemplation of the present invention, as is commonly employed in transporting and using portable oilfield equipment. 
     At an entry end  112  the vessel  102  has an inlet  103  supporting a choke valve  114  which is connectable to a production line  116 . The production line  116  provides a conduit for transferring the effluent from an oil and gas well (not shown) to the vessel  102 . The choke valve  114  restricts an inlet flow rate of the effluent below that of a preselected flow rate. Alternatively the choke valve can be located at the well head of the oil and gas well, at the opposite end of the production line  116 . 
     It will be noted that a separator built in accordance with a preferred embodiment of the present invention admits effluent into the vessel  102  at an inlet  103  that is below the surface of a liquid bed inside the vessel  102 . The liquid bed serves to cushion the inrushing effluent to slow the flowing effluent to a workable velocity. The liquid bed furthermore serves to prevent inrushing fragments or propants from striking against internal components within the vessel  102  in the presence of combustion air. This prevents the flammable condition that otherwise occurs wherein sparks and heat are created in the presence of combustion air and fuel. 
     When the separator apparatus  100  is operational, the liquid bed provides a static fluid head above the inlet  103 . As will be evident by the following discussion, as effluent is recovered the liquid bed is maintained by removing liquid from the vessel  102  at a rate to balance the incoming flow of liquid constituent so as to maintain a desired liquid level above the inlet  103 . It is advantageous to provide the liquid bed before beginning recovery operations, so that the first effluent admitted to the vessel  102  enters the liquid bed. An initial liquid bed can therefore be provided by initially admitting a fill liquid, such as a brine solution, to the vessel  102  to the preselected level. Alternatively, such as when an appropriate fill liquid is not available, the effluent can be trickled into the vessel until a sufficient liquid bed accumulates. 
     As noted, the liquid bed is provided initially either by admitting the effluent under controlled conditions or by admitting a fill liquid, and the liquid bed is thereafter supplemented by the liquid constituent of the effluent. For purposes of the following discussion, therefore, the term liquid applies equivalently to the fill liquid and to the liquid constituent of the flush effluent. 
     A number of sight glasses  118  provide visual indication of the liquid level inside the vessel  102  at selected locations. A number of temperature gauges  120  indicate the temperature of the effluent at select locations. The locations selected for each sight glass  118  and temperature gauge  120  correspond to the longitudinal position of particular separating chambers inside the vessel  102 , as described below. Monitoring the temperature of the effluent can be advantageous in preventing ice from accumulating within the vessel  102  which can restrict fluid flow. Freezing conditions can arise under certain conditions, such as operating in sub-freezing ambient conditions, and from recovering an effluent containing certain constituents such as carbon dioxide. 
     A pair of conventional floats  121 ,  122  monitor the level of the liquid in a forward end  123  of the vessel  102  to control the rate at which the liquid is withdrawn from the vessel  102 . As shown in FIG. 1, the floats  121 ,  122  are each linked to a corresponding mechanical dump valve  125 ,  126  so that when a selected level is reached the floats  121 ,  122  open the respective dump valves  125 ,  126  to remove liquid. In this manner there is provided a balance of the outflowing liquid and the inflowing liquid so as to maintain the liquid bed above the inlet  103 . Alternatively, in another embodiment the floats  121 ,  122  can each provide an input signal in a conventional manner to a corresponding motorized valve (in place of dump valves  125 ,  126 ) to automatically control and throttle, if desired, the rate at which liquid is withdrawn from the vessel  102 . 
     On top of the vessel  102  a man way access  127  is provided for access by an operator into the vessel  102 , such as for inspection or maintenance. A pair of smaller clean out caps  128  cover openings in the vessel  102  that accommodate the use of a power washer to clean the vessel  102  at a location where the solid constituent of the effluent settles out, as discussed below. The man way access  127  and the clean out caps  128  are constructed in a conventional manner of a hinged cover having an open position for access and a closed position where a sealing engagement within the vessel  102  is provided. A pressure relief valve  129  is provided to relieve the vessel  102  internal pressure in the event a preselected limit is exceeded. An outlet  130  is connected to a discharge line  132  for removing the gaseous constituent of the effluent from the vessel  102 . 
     Mounted below the vessel  102  is a pair of drains  134  wherein the solid constituent is collected. The drains  134  can be connected to a common header  136 , and a valve  138  on the header  136  can be periodically opened in order to purge an accumulation of solids from the vessel  102 . It is advantageous to provide a sufficiently large vessel  102  and drains  134  in order to provide a capacity to retain all the solid constituent from the clean up procedure. In this way the solids can be hauled off in the vessel  102  for disposal at an appropriate remote site. Alternatively, the header  136  can serve as a conduit to transfer the solid constituent to a storage container at the well site. 
     FIG. 2 is a top view of the separator apparatus  100  of FIG. 1 further showing a recirculation line  150  that fluidly connects the forward end  123  of the vessel  102  with the choke valve  114 . The opening of a valve  154  provides a continuous flow of liquid through the choke valve  114  and into the inlet  103  as necessary to prevent freezing. This recirculation of liquid flow is provided both by the positive pressure in the forward end  123  of the vessel  102  and by the venturi effect provided by the inrushing effluent relative to where the recirculation line  150  fluidly connects to the choke valve  114 . The recirculation of fluid is particularly advantageous under certain operating conditions which tend to induce freezing of the effluent as it expands downstream of the choke valve  114 . As noted, one such condition is where sub-freezing ambient conditions chill the inlet  103 . Another condition occurs when the effluent contains certain constituents such as carbon dioxide. 
     FIG. 3 shows a top view of the choke valve  114 , formed by an attachment of a conventional choke valve housing  155  to a valve  156 . FIG. 4 is a side view of the choke value  114  and FIG. 5 is a sectional view taken along the line  5 — 5  of FIG.  4 . 
     The choke valve housing  155  has a threaded end  158  for threading engagement with the production line  116  (see FIG.  1 ). FIG. 5 shows a positive pressure choke bean  159  is supported by a choke bean holder  160  having an annular tailpipe portion  161  receivingly disposed within the choke valve housing  155 . The choke bean  159  is sized in a manner well know to a skilled artisan for controlling the effluent flow rate into the vessel  102 . 
     FIGS. 3 through 5 show the manner in which the recirculation line  150  is fluidly connected to the inrushing effluent flow. The choke valve housing  155  supports a nipple  162  and a tee  164  attached at a distal end thereof. The valve  154  is shown in the recirculation line  150  connected to the tee  164 . The nipple  162  fluidly connects the recirculation line  150  with the tailpipe  161  so that the inrushing effluent creates a venturi effect that draws fluids from the recirculation line  150  into the effluent stream. Also connected to the tee  164  is a valve  168  used for the injection of fluids into the vessel  102 . For example, in some instances it is advantageous to inject methanol to enhance the freeze-protection. Also, fluids such as diesel fuel can be injected as a defoaming agent. 
     In the choke valve  114  shown in FIG. 3, opening the valve  154  provides a flow passage for liquid through the recirculation line  150  and through the valve  156  into the inlet  103  of the vessel  102 . This provides for a continuous liquid flow through the vessel  102  even when the effluent being recovered contains no liquid constituent. As noted previously, this is particularly advantageous when operating under conditions conducive to freezing, such as low temperature and carbon dioxide recovery. Alternatively, as noted above, closing valve  154  and opening the valve  168  allows injection of fluids during operation of the separating apparatus  100 . It is possible that under certain conditions both valves  154 ,  168  could be open to provide simultaneous recirculation and injection. 
     An opposing nipple  170  on the choke valve housing  155  supports a valve  172  that is connectable to a fill liquid supply line (not shown) used to fill the vessel with the fill liquid to provide the initial liquid bed. FIG. 4 illustrates this start up condition wherein the vessel  102  is filled with a liquid, such as a brine solution, to a liquid level  173  above the inlet  103 . 
     Also shown in FIG. 4 is a sampling assembly  174  that is used to withdraw a sample of the effluent during processing. The sample is advantageous in determining the status of a well clean up procedure, as in determining the amount of solids contained in a sample. When the solids content of the flush effluent drops below a threshold amount the recovery procedure can be declared completed. 
     The sampling assembly  174  has an upper nipple  176  which is connected to the inlet  103  and from which an upper valve  178  and a lower valve  180  depend, separated by a lower nipple  182 . A sample is collected by closing the lower valve  180  and opening the upper valve  178  at a time when the effluent is passing through the inlet  103  into the vessel  102 . The valve  180  can be cracked open to bleed air trapped between the valves  178 ,  180 . A sample of the effluent will be collected in the lower nipple  182 , and is retrieved by closing the upper valve  178  and opening the lower valve  180  after placing a collection vessel, such as a beaker, below the lower valve  180 . 
     Turning now to FIG. 6, a diagrammatic sectional view illustrates generally the manner in which the solid, liquid, and gaseous constituents of the effluent are separated by the separator apparatus  100 . The flush effluent enters the vessel  102  through the inlet  103  and under the static head of the fluid bed provided by the liquid level  173 . It will be noted the effluent therein passes generally in a directed flow toward the forward end  123  of the vessel  102 , the flow induced by the momentum with which the effluent enters the vessel  102 . 
     In flowing from the inlet  103  toward the forward end  123 , the effluent encounters first, second and third baffles  177 . Thereafter the effluent encounters a fourth baffle  183 , and a fifth baffle  184 . The baffles  177 ,  183 ,  184  cooperate with interior portions of the vessel  102  to form a plurality of sequential separating chambers designated as: a first chamber  181 A; a second chamber  181 B; a third chamber  181 C; a fourth chamber  181 D; a fifth chamber  181 E; and a sixth chamber  181 F. The baffles  177 ,  183  and  184  will be described below. 
     The greatest turbulence is found in the first separating chamber  181 A, where the inrushing effluent encounters the restriction provided by the first baffle  177 . The longitudinal flow of the effluent in combination with a downward force of gravity urges the solids  187  toward the drains  134 . In this manner the solids  187  accumulate in the drains  134  for removal by opening the valve  138  (see FIG.  1 ). 
     Similarly, the gaseous constituent  185  percolates upwardly out of the flush effluent and rises toward the top of the vessel  102 . By controlling the internal pressure of the vessel  102  below a predetermined value by means of a backpressure valve in the conduit  132 , discussed herein below, the gaseous constituent  185  can be collected via the outlet  130 . 
     At the forward end  123  of the vessel  102 , downstream of the baffles  177  an effectively filtered liquid  186  is collected in the fourth, fifth and sixth separating chambers  181 D,  181 E,  181 F. The liquid  186  in the fourth separating chamber  181 D is withdrawn by cooperation of the float  121  and the dump valve  125  (see FIG.  1 ). The liquid  186  in the sixth separating chamber  181 F is withdrawn by cooperation of the float  122  and the dump valve  126  (see FIG.  1 ). 
     A conduit  188  provides passage for liquid from the fourth chamber  181 D at a time when the dump valve  125  alone is unable to remove liquid at a sufficient rate to maintain the preselected liquid bed level. It will be noted the conduit  188  transfers the liquid from the fourth chamber  181 D to the sixth chamber  181 F, thus bypassing the fifth chamber  181 E. In this manner, the cleanest liquid is maintained in the fifth chamber  181 E for use in the recirculation line  150 . 
     FIG. 7 is an elevational view of one of the baffles  177 , and FIG. 8 is a sectional view thereof taken along the line  8 — 8  in FIG.  7 . Each of the baffles  177  has a planar plate portion  189  with an upper opening  190  and a lower opening  192 . The upper opening  190  provides free passage for the gaseous constituent  185  along the top of the vessel  102  toward the outlet  130 . The lower opening  192  provides restricted passage for the liquid constituent of the flush effluent. A pair of openings  194  provide a flow path for liquid from a downstream chamber to an upstream chamber. For example, the relatively high velocity of the inrushing effluent would force the liquid bed out of the first chamber  181 A were it not for a return path for the liquid to flow back from the chamber  181 B into the chamber  181 A. 
     Each baffle  177  forms a trailing deflector  196  which is generally downwardly sloping from the planar plate portion  189 . As shown in FIG. 8, the deflector  196  has a substantially vertical portion  198  that imparts a downwardly directed force on the effluent, which is a mixture of the liquid and the solid constituent  187 . The deflector  196  thereby urges the solid constituent  187  toward the bottom of the vessel  102  where settling ultimately occurs. 
     FIG. 9 is an elevational view of the fourth baffle  183 , and FIG. 10 is a sectional view thereof taken along the line  10 — 10  in FIG.  9 . Similar to the first, second and third baffles  177 , the fourth baffle  183  has a planar plate portion  200  with an opening  202  for passage of the gaseous constituent  185 , an opening  204  for passage of liquid  186 , and an opening  206  in fluid communication with the conduct  188  for overflow passage of liquid from the fourth chamber  181 D to the sixth chamber  181 F as described above. It will be noted that the opening  204  is relatively small in comparison to the openings  192  of the baffles  177 , because the solids  187  having already been settled out, only a marginal opening is necessary to provide passage of the liquid  186  into the fifth chamber  181 E. FIG. 10 shows the size of the opening  204  being minimized by a tab  203  bent slightly out of parallel relationship to the planar plate portion  200 . 
     FIG. 11 shows an elevational view of the fifth baffle  184  which provides a solid partition weir across a lower portion of the vessel  102  as an arcuate surface  207  is contoured to match a characteristic inner surface of the vessel  102 , the fifth baffle  184  being sealingly attached thereto as in by a welded attachment therebetween. It will be noted from FIG. 6 that the conduit  188  is supported by the baffle  184 , and that otherwise liquid from the fifth chamber  181 E can spill over the baffle  184  into the sixth chamber  181 F. 
     Returning with FIG. 6, liquid mist particles are carried with the gaseous constituent  185  percolating out and rising upwardly toward the outlet  130 , thereby “wetting” the gaseous constituent  185 . The amount of wetting is dependent on the velocity of the gaseous constituent  185  exiting the vessel  102 . As depicted in FIG. 6, a mist eliminator or demister  208  is disposed in the vessel  102  adjacent to the outlet  130  to provide a mechanical barrier to the exiting gas. The mist eliminator  208  is of conventional design and causes the liquid particles to be slowed by the physical obstruction of the mist eliminator  208 . The liquid particles coalesce on the surfaces and flow downwardly, dripping off the mist eliminator  208  into the sixth separating chamber  181 F. 
     Preferably, the mist eliminator  208  is constructed of a perforated steel sheet. Alternatively, and without deviating from the scope of the present invention, the mist eliminator  208  can be formed of a knitted-wire mesh pad, or other equivalent material suitable for use as a demister media. 
     The magnitude of pressure from the well formation that delivers the effluent to the vessel  102  is typically less than 10,000 psi in low pressure wells and 10,000 to 20,000 psi in high pressure wells. Even after choking the flow through the choke valve  114  the fluid pressure is significantly high. As mentioned above, backpressure is maintained in the vessel  102  by a backpressure manifold  210 , shown in FIG. 12, that is connected to the discharge line  132  between the vessel  102  and a conventional flare (not shown) used to combust the gaseous constituent  185 . 
     The backpressure manifold  210  can be provided as a unit operating within the discharge line  132 , having a flange  212  for connection to the mating flange  213  of a segmented discharge line  132 . The backpressure manifold  210  has an inlet header  216  and a outlet header  218  that are fluidly joined by a valve  220 , a first backpressure valve  222  and a second backpressure valve  224 . In this manner, a selected backpressure can be provided by setting the first backpressure valve  222  to a nominal desired working pressure in the vessel  102 . The second backpressure valve  224  can be set at a maximum desired working pressure. The valve  220  provides a redundant safety pressure release in addition to the pressure relief valve  129  in the event the vessel  102  pressure continues to climb above the maximum desired working pressure. 
     Finally, returning to FIG. 2 it will be noted that a line  226  and a line  228  provide a flow path when valves  156 ,  168 ,  172 , and  229  are closed for pressure from the vessel  102  to be communicated to a header  230  (partially shown in FIG. 1 for clarity) which joins the dump valves  125 ,  126  to a common dump line  232 . In this manner the vessel  102  internal pressure can be used to purge the header  230  and dump line  232 , such as when the recovery is complete and the recovery apparatus  100  is being readied for removal from the well site. 
     The present invention provides a separating apparatus (such as  100 ) for the recovery of an effluent following a stimulation procedure on an oil and gas well. The separating apparatus has a vessel (such as  102 ) with a choke valve (such as  114 ) connected at an inlet (such as  103 ) and a backpressure manifold (such as  210 ) connected to an outlet (such as  130 ) in order to maintain a selected internal pressure in the vessel. 
     The flush effluent enters the vessel under a liquid static head provided by a liquid bed maintained at a desired liquid level (such as  173 ). In starting up the separating apparatus, the fluid bed is initially provided by partially filling the vessel with liquid through a fill line connected to the choke valve. The fluid bed is maintained at the desired level so that effluent entering the vessel enters under the static head of the fluid, thereby providing a non-compressible medium for cushioning the inrushing effluent, and for preventing high velocity impacting of solids against vessel internals in the presence of combustion air. 
     The effluent flowingly encounters a series of baffles (such as  177 ,  183 ,  184 ). As the effluent flows, a solid constituent (such as  187 ) settle out to the bottom of the vessel and is collected within drains (such as  134 ) that can be periodically opened to withdraw the collected solids from the vessel. Furthermore, the gaseous constituent (such as  185 ) percolates upwardly out of the liquid and ultimately exits the vessel by way of an outlet. The exiting gaseous constituent passes through a mist eliminator (such as  208 ). Floats (such as  121 ,  122 ) monitor the liquid level and control dump valves (such as  125 ,  126 ) to balance the outflowing liquid with respect to the inflowing effluent so as to maintain the desired liquid level above the inlet. 
     A recirculation line (such as  150 ) can provide a continuous flow of liquid through the choke valve to prevent freezing, such as that which occurs at low ambient temperature or when recovering effluent containing carbon dioxide. 
     A backpressure manifold (such as  210 ) is connected to the outlet of the vessel in order to control the internal pressure of the vessel. The backpressure manifold provides a valve (such as  220 ) in parallel with one or more backpressure valves (such as  222 ,  224 ) which control the pressure in the vessel and provide pressure relief in the event of an abrupt or unanticipated pressure increase. A flare is connected downstream of the backpressure manifold to combust the gaseous constituent. 
     The foregoing description of a preferred embodiment of the present invention is exemplary and not itself limiting of the contemplated scope of the invention, wherein an effluent is received into a vessel and under the static head provided by a liquid bed and thereafter slowed by a baffle to induce the settling out separation of the solids and gaseous constituent of the effluent. 
     The separator apparatus of the present invention provides a safe and self-contained apparatus, permitting an associated method for the collection of effluent from an oil and gas well following a stimulation procedure. The receiver apparatus allows the user to remain in full compliance with modern legislation that prohibits the spilling of, or open pit storage of, the effluent. The collection is done in a safe manner by preventing conditions that are conducive to fire or explosive hazards in the collection vessel. 
     In carrying out the present invention with the separator apparatus  100  described above, an operator first connects the production line  116  from the oil and gas well to the choke valve  114  at the threaded connector  158 . The oil and gas well has a shut off valve on the well head that remains closed while the valve  156  and valve  172  are opened to admit a fill fluid (usually a brine solution) into the vessel  102  to provide an initial liquid bed. The production line  116  is likewise filled with the fill fluid so that when the well head valve is opened the inrushing effluent is immediately dampened by the liquid bed. 
     As the flush effluent enters the vessel  102 , the floats  121 ,  122  open and close the respective dump valves  125 ,  126  to maintain the desired liquid bed in the vessel  102 . As needed, the recirculation line  150  is used by opening valve  154 . Also, chemicals may be injected into the tank by opening the valve  168 . The backpressure valves  222 ,  224  are set to provide a preselected internal pressure range on the vessel  102 . As necessary, the valve  138  is opened to purge an accumulation of solids from the vessel  102 . 
     It is clear that the present invention is well adapted to carry out the objects and to attain the ends and advantages mentioned as well as those inherent therein. While embodiments of the invention have been described for purposes of this disclosure, numerous changes can be made which will readily suggest themselves to those skilled in the art and which are encompassed within the spirit of the invention disclosed and as defined in the appended claims.