Passive Booster for Pumping Liquified Gases

It is common practice in the petroleum industry to employ gases such as CO.sub.2 or N.sub.2 in stimulation and treatment of oil and gas wells, such as in acidizing, fracturing, well cleanout or CO.sub.2 flooding. In addition, such a gas may be employed in foaming cement to be employed in cementing operations in a well bore. The gas is transported in low temperature liquified form to the well site in insulated tank trailers, where it is introduced into the suctions of one or more positive displacement pumps (generally referred to as the "primary" pumps) in order to increase the pressure of the liquified gas prior to mixing with cement or a primary treating fluid which may carry various additives. If well treatment involves fracturing the producing formations in the well, the treating fluid may also carry proppants to prevent formation closure after fracturing. The CO.sub.2 or N.sub.2 provides a gaseous phase in the treating fluid upon increase in temperature and decrease in pressure in the formation, which gas is highly beneficial to the treatment in that it reduces the amount of treating fluid and additives required, provides a light weight carrier medium for proppants, and places less stress on the producing formation than a heavier, unfoamed treating fluid. In a similar manner, foamed cement is employed when a heavier cement may be deleterious to the producing formations. This latter effect is of particular concern in gas wells, where the formations may be physically weak and susceptible to collapse under the weight of a column of unfoamed treating fluid or cement.

Recently, methods have been developed to stimulate wells employing CO.sub.2 as the primary treating fluid, with a relatively small proportion of another liquid or gel employed to transport additives or support proppants.

The prior art layout of equipment employed in operations such as are described above requires the use of a centrifugal or vane type booster pump and preferably a liquid/gas separator between the liquified gas tank and a primary pump. This equipment is required due to the heat gain in the line leading to the primary pump, which heat gain induces vapor lock in the line and prevents proper liquid intake into the primary pump, causing cavitation in the fluid end thereof and possible destruction of the pump itself. The vaporization problem increases as the gas is emptied from the tank, as the tank pressure drops with a consequential tendency toward vapor lock. The aforementioned booster pump and separator necessitates at least one additional trailer on site, as well as constant monitoring of the booster pump and a fairly high level of maintenance between jobs. On large jobs, several booster pump trailers may be required. In addition, the prior art booster pump becomes less effective at high tank pressures due to increased tendency of the fluid to form vapor.

In contrast to the booster pumps of the prior art, the passive booster of the present invention provides a relatively simple, compact apparatus and method of use thereof for reducing the temperature of liquified gas employed in a well treating fluid, thereby maintaining the gas in a liquid state while avoiding the need for a boost in liquified gas pressure in the feed line to the primary pump. The passive booster of the present invention comprises a heat exchanger, the tube side of which communicates with the main liquified gas feed line, and the shell side of which is supplied with liquified gas from the tube side through a variable orifice valve. In the case of CO.sub.2, a back pressure valve is employed on the gas outlet vent from the shell side of the booster, to maintain pressure on the shell side at a high enough level to prevent solidification of the low temperature CO.sub.2. An automatic control system to regulate liquified gas flow to the shell side of the booster may be employed, or flow may be manually regulated. In certain instances it may be desirable to employ a passive booster in series with a conventional booster pump to feed a primary pump, thus providing not only a temperature reduction but also a pressure increase to accommodate long feed lines, high flow rates, high ambient temperatures or a combination thereof.

The passive booster of the present invention, its theory of operation and method in which it is used may be more fully understood by one of ordinary skill in the art by reference to the following detailed description of the preferred embodiments and their operations, taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic of a prior art booster pump system for conveying a liquified gas from a source to a primary pump and to inject the gas in the stream of a treating fluid to be injected into an oil or gas well.

FIG. 2 is a schematic similar to FIG. 1 showing the preferred embodiment of the passive booster system of the present invention employed in lieu of the booster pump system of the prior art.

FIG. 3 is a schematic similar to FIG. 1 showing an alternative embodiment wherein the passive booster system of the present invention is utilized with the booster pump of the prior art.

FIG. 4 is a pressure versus temperature chart for carbon dioxide.

FIG. 1 of the drawings shows a prior art system of conveying a liquified gas to a primary pump used to inject the gas into a fluid stream used to treat an oil or gas well. Insulated tank trailer 10 is driven to the well site, and feed line 12 is attached thereto, to carry the liquid gas (CO.sub.2 is used as an example and not by way of limitation) to a liquid/gas separator 14, then to booster pump 16, which is generally a centrifugal or vane type pump which may have an hydraulic or a direct drive. From booster pump 16, intake line 18 carries the liquified CO.sub.2 to the fluid end 22 of a high pressure positive displacement type pump 20 (hereinafter referred to as the "primary" pump), generally a triplex pump driven by a diesel engine.

The liquified CO.sub.2 enters the suction 24 of the fluid end 22 of the primary pump 20 on the intake stroke of pump plunger 26. On the compression stroke of plunger 26, suction 24 closes and outlet 28 opens to permit flow of the liquified gas into output line 30 having CO.sub.2 vent 32 thereon. Flowmeter 34, is incorporated in output line 30 so that the flow of the liquified CO.sub.2 may be monitored and adjusted relative to the flow of the treatment fluid in injection line 36, which output line 30 joins at tee 38. Flow of the treatment fluid is shown by arrows in conjunction with injection line 36. The term "treatment fluid" should be understood to encompass any fluid, gel or slurry with which the liquified gas is mixed for injection into the well. Of course, the injection line runs to the wellhead, to a wellhead isolation tool or to other manifolding which transmits the gas-laden treatment fluid to tubing in the well bore, all of which is well known in the art and therefore will not be further described herein. Also shown in FIG. 1 is return line 40 which runs from liquid/gas separator to tee 42 on output line 30, valve 44 being used to shut off back flow output line 30 when pump 20 is operating. Strainer 50 prevents contaminants from entering liquid/gas separator 14, and check valve 52 on output line 30 prevents back flow of liquified gas and/or treatment fluid from injection line 36. Valve 54 is employed to close off output line 30 when desired.

It should be understood that tank 10 may comprise a plurality of tanks, feed line 12 may be included in a manifold system leading to one or more booster pumps 16 and one or more primary pumps 20. However, for purposes of simplicity, all operations are described herein with reference to single components. Prior to the treating operation, the discharge valve of tank 10 is opened to bleed liquid CO.sub.2 into feed line 12, liquid/gas separator 14 and booster pump 16. CO.sub.2 vent 32 and valve 44 are opened to fill and cool primary pump 20. When carbon dioxide "snow" appears at vent 32, the lines are filled. If a bleedoff valve is included in fluid end 22, it is also opened to ensure complete filling of the pump cylinders. Booster pump 16 is started, and primary pump 20 put into operation gradually. Other primary pumps associated with the treatment fluid in injection line 36 are also on line, valve 32 is closed and valve 54 is opened to permit CO.sub.2 flow into injection line 36.

In the treatment operation, liquid CO.sub.2 is transported to the site in insulated tank 10 under a pressure of 250 to 350 psig and a temperature of -10.degree. F. to 7.degree. F.; as the liquid CO.sub.2 is drawn from tank 10 and liquid CO.sub.2 in the tank vaporizes, the temperature and pressure in the tank drop. Feed line 12 takes CO.sub.2 from tank 10 to booster pump 16, the CO.sub.2 acquiring heat from the environment therebetween. Since the acquired heat may cause a portion of the CO.sub.2 to vaporize if the well site is at a high ambient temperature, liquid/gas separator 14 should be placed in feed line 12 before booster pump to remove some of the CO.sub.2 vapor phase. Separator 14 essentially comprises a pressure vessel with return line 40 leading to output line 30. Relief valve 56 relieves CO.sub.2 into the atmosphere from separator 14 if pressure therein exceeds a predetermined level.

Liquid CO.sub.2 with some of the vapor removed therefrom is fed into booster pump 16, which increases the pressure thereof 50 to 125 psi above that in feed line 12 to counter the heat input from the environment. The pressure increase raises the temperature at which the CO.sub.2 forms a vapor phase, and thus tends to inhibit vapor lock at suction 24 of fluid end 22. Fluid end 22 of pump 20 then takes the CO.sub.2 feed from inlet line 18, raises its pressure, 2,000 psi to 10,000 psi or more, and outlet line 30 takes the high pressure liquified CO.sub.2 to injection line 36, where it mixes with the treatment fluid therein and is subsequently injected into the well.

The prior art booster pump system disclosed in FIG. 1 works with a self-defeating phenomenon, e.g. it raises the pressure of the liquified gas in order to maintain it in a liquid state while adding heat thereto which causes the needed pressure increase to become even greater.

Referring now to FIG. 2 of the drawings, the preferred embodiment of the passive booster system of the present invention is shown. As in the prior art system, liquified CO.sub.2 is brought to the well site in tank trailer 10. However, in lieu of liquid/gas separator 14 and booster pump 16, passive booster 100 is placed in gas feed line 12.

Passive booster 100 comprises a tube and shell type heat exchanger including pressure vessel 102 surrounded by insulating material 104, and a plurality of tubes 106 running through pressure vessel 102. The interiors of tubes 106 extend between inlet chamber 108 and outlet chamber 110 of the passive booster 100. Inlet chamber 108, outlet chamber 110 and the interiors of tubes 106 are isolated from cooling chamber 112 extending between and surrounding tubes 106, except as noted below. A plurality of baffles 114 support tubes 106 and disperse flow in cooling chamber 112 as will be further described hereinafter. Passive booster 100 is an extremely compact device, which may be eight feet or less in length and from substantially ten inches to substantially twenty inches in diameter, depending upon desired flow capacity and cooling capability.

Variable orifice throttling valve 116 is positioned at the mouth of inlet passage 118 to cooling chamber 112, which mouth opens on outlet chamber 110. Throttling valve 116 controls CO.sub.2 flow to cooling chamber 112, and through the Joule-Thomson Effect associated with the flow of a fluid from a higher pressure region to a lower pressure region through a constricted passage, the throttled CO.sub.2 entering cooling chamber 112 is reduced in temperature. This cooled CO.sub.2, which is in a mixed liquid and vapor phase, acts through the walls of tubes 106 to cool the main CO.sub.2 flow through passive booster 100. Baffles 114, half of which extend downward from the top of pressure vessel 102 and half of which extend upward from the bottom of pressure vessel 102, ensure a serpentine CO.sub.2 flow pattern from inlet passage 118 through cooling chamber 112 to outlet 120, which terminates in back pressure valve 122. Back pressure valve 122 should be set to ensure a back pressure of at least 70 psig, to prevent solidification of CO.sub.2 in the cooling chamber. Suitable back pressure valves are commercially available and well known in the art.

An automatic throttling valve control 124 may be incorporated in passive booster 100 if desired. The control may work in several ways. For example, probes 126 and 128 may be employed to measure the temperature at the inlet and outlet ends of passive booster 102, so that throttling valve control 124 modulates CO.sub.2 flow into cooling chamber 112 in response to a temperature differential between the readings of probes 126 and 128, in order to provide sufficient cooling for the CO.sub.2 in passive booster 100 to prevent vapor lock at pump 20, while ensuring that the CO.sub.2 flow through the cooling chamber 112 is not excessive.

An alternative monitoring approach to throttling valve control 124 involves the use of probe 126 to measure CO.sub.2 pressure at the passive booster inlet end instead of temperature and probe 128 to measure CO.sub.2 temperature at its outlet end. In this instance, CO.sub.2 pressure in feed line 12 is measured by probe 126, and CO.sub.2 flow into cooling chamber 112 is modulated by throttling valve 118 to reduce the measured temperature at probe 128 to ensure sufficient cooling of the CO.sub.2 in passive booster 100 to avoid vapor lock at the measured pressure (allowing for further heat gain in inlet line 18 leading to primary pump 20), while avoiding excessive CO.sub.2 flow through cooling chamber 112.

At the well site, passive booster 100 is employed in lieu of a booster pump. The tank 10 discharge valves are opened and CO.sub.2 bled into feed line 12 through passive booster 100 and into inlet line 18 as CO.sub.2 vent 32 on the downstream side of primary pump 20 is opened to allow liquid CO.sub.2 to completely fill feed line 12, inlet line 18 and passive booster 100. If a bleedoff valve is incorporated in fluid end 22, it is also opened to ensure complete filling of the pump cylinders.

In the treating operation, flow is begun from tank to primary pump 20, the temperature of the CO.sub.2 passing through passive booster 100 from feed line 12 to intake line 18 being maintained at a low enough level to avoid vapor lock at pump suction 24. The CO.sub.2 is then raised in pressure by primary pump 20 and conveyed to injection line 36 by output line 30 as heretofore described with respect to FIG. 1.

By way of example, a field test was conducted at Duncan, Okla. to test the characteristics of an eight foot long by twelve inch diameter passive booster. At a CO.sub.2 flow rate of 1.5 barrels per minute, the passive booster lowered CO.sub.2 temperature in the line 20.degree. F., which is equivalent to boosting pressure 96 psi, or approximately 77 to 192 percent of the boost available with prior art booster pumps as heretofore described. Approximately thirteen percent of the CO.sub.2 flow was consumed by the passing booster in cooling the remainder. During another part of the aforementioned test, the passive booster lowered CO.sub.2 temperature in the line 28.degree. F., equivalent to a 135 psi boost, or approximately 110 to 270 percent of prior art booster pump capability.

It should be noted that the passive booster of the present invention will, unlike the booster pumps of the prior art, effect a certain change in the enthalpy of the CO.sub.2 regardless of the ambient temperature at the well site. This is in contrast to the booster pump, which became less effective as the ambient temperature and correspondingly the tank pressure increase, due to the heat input to the CO.sub.2.

Referring now to FIG. 3 of the drawings, passive booster 100 is depicted in series with a booster pump 16 of the prior art. As all the components depicted in FIG. 3, and their operation, have been previously described, a detailed description thereof will not be repeated. However, the absence of liquid/gas separator 14 and its associated plumbing should be noted.

In operation, liquid CO.sub.2 from tank 10 is first raised in pressure in booster pump 16, and then cooled in passive booster 100. This procedure provides a notable advantage, as not only is the CO.sub.2 pressure in the lines raised, but the associated cooling negates the heat input of the booster pump 16 as well as providing additional equivalent boost.

Referring now to FIG. 4 of the drawings, a pressure versus temperature chart for carbon dioxide, the principle of operation of the passive booster of the present invention may be graphically illustrated. Assume that liquid CO.sub.2 in a supply tank is at approximately 300 psig and -4.degree. F., noted at point 1 on FIG. 4 on the liquid/vapor phase change line. The operator may employ a prior art booster pump to increase pressure 100 psi to 400 psig (point 2). As can easily be seen on FIG. 4, a 100 psi pressure increase removes the CO.sub.2 18.degree. F. from the liquid/vapor phase change line (point 3), or an equivalent of an 18.degree. F. temperature reduction which could be effected by the passive booster of the present invention. Point 4 illustrates an alternative 18.degree. F. passive booster temperature reduction, which removes the CO.sub.2 100 psi from the liquid/vapor phase change line (point 5) or an equivalent of a 100 psi pressure boost.

FIG. 4 also illustrates the maximum temperature reduction which may be effected by the passive booster at a given pressure; this, of course is limited by the solidification temperature of CO.sub.2 at a particular pressure.

As shown in FIG. 4, one may achieve a maximum temperature drop of 73.degree. F. in the CO.sub.2 with the CO.sub.2 supply at 350 psi before the CO.sub.2 solidifies (point 6). This would be equivalent to a pressure boost of 280 psi, a surprising and unexpected result. The maximum possible equivalent boost is reduced as the pressure in the liquid CO.sub.2 tank diminishes as it is drawn off and the temperature in the tank decreases as the vapor state CO.sub.2 expands. Of course, the maximum temperature reduction effected at a given pressure in the tube side of passive booster is dependent upon the length, diameter, design and materials employed in the device, as well as CO.sub.2 flow rate therethrough, all of which affect the tube side to shell side heat transfer.

Thus there has been described a novel and unobvious apparatus and method for conditioning carbon dioxide and other gases used in treatment of oil and gas wells. Because a passive booster of adequate capacity may be carried on a primary pump trailer, the advantage of using the apparatus of the present invention in lieu of a booster pump is quite obvious. In addition, the low required maintenance level and automatic operation in comparison to the prior art booster pumps constitute additional advantages. Numerous additions, deletions and modifications to the preferred embodiments of the method and apparatus of the present invention will be readily apparent to one of ordinary skill in the art. For example, heat exchanger designs other than tube and shell may be employed as a passive booster, the passive booster may be placed in the line to the primary pump before a booster pump, or a single temperature could be monitored at the passive booster to simply maintain outlet gas temperature below a given level. In addition, gas may be fed into the shell or low temperature side of the heat exchanger directly from the gas source, rather than from another part of the heat exchanger. One gas could be employed in the high temperature (tube side) of the heat exchanger as a well treatment fluid and a second, different gas employed in the shell side to cool the first. Furthermore, while the foregoing specification refers to "liquid" gas, and to "vapor" gas, one of ordinary skill in the art will appreciate that liquid gas may have some vapor within, being only substantially liquified and vapor may have liquid particles therein, being only substantially vaporized. There also may be many other combinations of partial liquid and partial vapor within systems such as have been described in the specification at any given time, and the specification has not attempted to exclude their existence by failure to comment thereon, nor imply that the method and apparatus of the present invention is workable only with completely liquid and completely vapor states of a gas.