Abstract:
Used water-in-oil emulsified drilling muds are prepared for reuse by removing excess water. The excess water is removed by passing the used mud through a cavitation device, under controlled conditions, along with injected air. While the emulsion is heated in the cavitation device, the air is finely dispersed in it, so is able to absorb water evaporated from the discrete, discontinuous phase aqueous droplets in the emulsion. A mixture of incoming and aerated emulsion is maintained at an optimal temperature in a process tank while a steady state continuous process is controlled to utilize the heat input, after compensating for radiant heat losses and other system losses, substantially exclusively for the evaporation of water in the emulsion. The process is applicable to any water-in-oil emulsion.

Description:
TECHNICAL FIELD 
       [0001]    Used water-in-oil emulsified drilling muds are prepared for reuse by removing excess water. The excess water is removed by passing the used mud through a cavitation device, under controlled conditions, along with injected air. While the emulsion is heated in the cavitation device, the air is finely dispersed in it, so is able to absorb water evaporated from the discrete, discontinuous phase aqueous droplets in the emulsion. A mixture of incoming and aerated emulsion is maintained at an optimal temperature in a process tank while a steady state continuous process is controlled to utilize the heat input, after compensating for radiant heat losses and other system losses, substantially exclusively for the evaporation of water in the emulsion. The process is applicable to any water-in-oil emulsion. 
       BACKGROUND OF THE INVENTION 
       [0002]    Used water-in-oil drilling muds, as they are returned from the well, are virtually unfit for reuse without rejuvenation. They contain drill cuttings and other solids, which can be removed in a known manner, but the most difficult problem with the remaining fluid is that it contains too much water. Drilling fluids are commonly in the form of water-in-oil emulsions, typically having an original ratio of 70-90% oil to 10-30% water (by volume), apart from the solids content. The oil/water ratio influences the rheology of the fluid and is an important factor in maintaining the drilling mud properties. For example, when a 90/10 ratio emulsion returns to the surface after use, it might have been diluted to perhaps 70/30 v/v in addition to entraining significant amounts of solids such as drill cuttings. The used mud will also commonly contain other materials picked up from the formation and may have changed chemically and physically in other respects; in addition it is likely to have reduced contents of one or more beneficial additives, but the major difficulty is that, to restore its usefulness, the ratio of oil to water must be restored to the desired figure. One way to do this is to add more oil and other ingredients, which requires more storage and increases the cost of the fluid by the amount of excess oil and ingredients required. In some cases it takes literally two times the original volume to correct the oil/water ratio. 
         [0003]    For example, if the original volume/volume ratio was 9.0 oil/1.0 water (10% water by volume), and the returned material is 8.0 oil/2.0 water (20% water), restoring it to the desired ratio of 9:1 by adding oil would require ten (10) volumes of oil, doubling the overall quantity and creating problems in both mixing and storing. Ratios lower than 8:2 are common in the returned fluid. The problem is even more severe for a ratio of, for example, 7:3—to restore this mixture to 9:1 with oil alone would require about 3.85 volumes of oil. 
         [0004]    Both environmentally and economically, it is very desirable to recycle used drilling mud. Doing so greatly reduces the need for water and additional drilling chemicals, but also drastically alleviates the waste disposal problems of the operators. 
         [0005]    The desired ratio of oil to water can be restored by removing excess water rather than adding oil and the other drilling fluid ingredients. The original emulsions, however, were carefully constructed using excellent emulsifiers, and are difficult or expensive, or both, to break by chemical means or by simple boiling even when additional water has upset the original emulsification balance. On the other hand, used drilling fluids have been subjected to great stress, and the original even dispersion is seldom preserved—that is, the discontinuous phase will have been disrupted and reconstituted by those excellent emulsifiers, now having additional water to work with, to a range of sizes not conducive to good purpose for reuse in a well. The process of reducing the high water content of a used mud should include, if possible, an even redistribution of the discontinuous phase in the desired dimensions. Another common characteristic of drilling fluids is that they contain significant amounts of calcium chloride, deliberately added to “hold” the water in the emulsion as it is subjected to turbulence and stress. The presence of the calcium chloride makes the task of removing water from the emulsion more difficult. A practical method of restoring the original ratio should conserve time and expense. The ability to process the used mud on return to the surface would help to reduce storage of the mud and inventory of various ingredients. A practical method of removing water from water-in-oil emulsions, particularly one which can lead to ready-to-use drilling muds, is needed. 
       SUMMARY OF THE INVENTION 
       [0006]    Our invention employs a cavitation device to process water-in-oil emulsions, such as drilling mud, with air. The cavitation device has several highly useful effects, namely (1) it heats the emulsion, (2) it causes water in the emulsified water droplets to vaporize (3) it finely divides the air, generating very high air surface areas in the simultaneously heated emulsions, facilitating intimate contact with the vaporized water, in turn enabling the air to entrain the vaporized water, and (4) it creates an evenly distributed, smooth emulsion from one that, as is typically the case, exhibits noticeable stratification of oil and solids settling. The cavitation device can do all of this while handling a fluid having a significant solids content. However, there are many practical constraints on the mud restoration process that require much more than simply passing the emulsion through the cavitation device. Without additional steps and proper process control, the cavitation device alone cannot produce the desired emulsion from an undesirable one. 
         [0007]    For example, we have observed that a careful control of temperature in the process is important for efficient energy usage. An object of our process is to evaporate water from the emulsion; therefore heating the oil in the emulsion to temperatures higher than the boiling point of water is not only unnecessary but inefficient. Because the oil in the mud has a boiling point higher than that of water, and is present in much greater amounts than the water, it can act more or less as a heat sink, taking up heat energy without a direct benefit. Moreover, because our process includes recycling of the process mixture through the cavitation device, maintaining a temperature in the oil higher than 212° F. or the boiling point of the water phase of the emulsion would not only be unnecessarily time-consuming but also waste a portion of the power consumed in recycling the mud. Ideally, the heat energy input to the mud will continuously equal the heat of evaporation of the water removed. We are able to maintain such an equilibrium at temperatures lower than 212° F. 
         [0008]    Also, a continuous process requires that the air and water vapor mixture must be able to escape the system; otherwise the heat energy needed for vaporization of the water will remain in the system and cause the oil to increase in temperature until it reaches its boiling point. “Super” heating the oil will then only contribute to a superheating of the water vapor, which is not necessary, and wasteful, for the process. Ideally, our process introduces only the amount of heat energy required to vaporize the portion of water we need or want to remove. Our process takes into account the fact that the mud cools as water vapor escapes, taking with it the heat of vaporization. We maintain a base temperature in the process tank of at least 180° F., preferably not lower than 188° F., and most preferably 190 to 200° F. so that the continuous injection of heat energy into the tank, while air and water vapor are continuously removed, is employed to evaporate water rather than to heat the oil to any higher temperature. 
         [0009]    Another factor is that the vapor captured by the air should be disposed of in a manner that does not unduly waste heat. Otherwise for every pound of water vapor released to the atmosphere, approximately 1000 BTU of energy is lost. 
         [0010]    The air is also heated in the process and, if vented, carries away heat energy. Thus, ideally, the captured water vapor will be separated from the air and the warm dry air (or its heat energy) somehow recycled. There are a number of conventional methods used to recycle heat in a gas or liquid, including mechanical vapor recompression, heat pipes, or using the hot vapor to preheat the fluid coming into the process. All of these known methods essentially convert the heat of vaporization back into the heat of condensation so it can be reused as heat to further drive the process. Another advantage of condensing the water vapor is that it provides a convenient way to measure the amount of water evaporated, since the ultimate goal is to produce a drilling fluid with the proper ratio of oil and water; evaporating too much water is to be avoided as unnecessarily expensive and time-consuming. 
         [0011]    Yet another problem is controlling the process to make an emulsion having ingredients essentially ready for reuse. All of this must be considered while acting on an emulsion for which one may have only an imperfect idea of its composition other than the oil/water ratio. 
         [0012]    And overall, an efficient continuous process requires maintaining steady-state conditions throughout, or at least at several points in the system. Typically the water, oil and solids in a drilling mud are determined by a retort test done that heats the fluid to very high temperatures such that both the water and oil boil out of the fluid and are collected in a graduated cylinder to determine the volume ratio of oil to water. The test works, but takes time to complete. It is advantageous to have a mathematical model that predicts the amount of water evaporated with time in the particular system. Such a model must account for the specific heats of the oil, water and solids. Furthermore air volumes, flow rates, mud volumes, accurate temperatures should be known. 
         [0013]    Our invention removes water by a form of evaporation, without the need to reach the normal boiling point of water, and without “super” heating the oil such that most of the energy goes into evaporation. 
         [0014]    Because we use injected air to absorb water vapor, and because the vapor is absorbed by the air in the partial vacuum of the cavitation device, the aerated emulsion in the cavitation device is well below 212° F.; we maintain it no lower than 180° F. and preferably no higher than 200° F. 
         [0015]    Since the system is dynamic, it requires controls which are further discussed below. Also, we have developed a startup procedure to implement the continuous process. Our process will be explained further with reference to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a flow sheet showing the basic invention. 
           [0017]      FIG. 2  illustrates a cavitation device such as may be used as cavitation device  4  in  FIG. 1 . 
           [0018]      FIG. 3  is a cavitation device equipped with an integral disc pump; this modified cavitation device may replace cavitation device  4  in  FIG. 1  or pump  18  in  FIG. 3 , as will be explained. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]    Referring to  FIG. 1 , the used emulsion (hereafter sometimes called a drilling mud or simply “mud”) in conduit  1 , designated “Feed,” is pumped through a heat exchanger  2  to a pump  3 . The heat exchanger  2  heats the used emulsion to at least 140° F., transferring heat from at least one of two or more different sources in conduits  14 ,  15 , and optionally conduit  23 , which can be directed to run through heat exchanger  2 , as will be further explained. Pump  3  sends the heated used emulsion through conduit  5  to cavitation device  4 . Air is added to the heated mud in conduit  5  through compressed air line  6 . The air may be dried in any convenient manner, before or after compressing. In the cavitation device  4 , the mud and air are further heated and intimately mixed as will be explained further below. From the cavitation device  4 , the heated, now aerated mud is sent through conduit  7  to a valve  8 . A portion of the heated, aerated mud may be recycled through conduit  15  to the heat exchanger  2 ; this will join the feed in conduit  1  and pick up additional heat energy while passing through the heat exchanger  2  before being sent again to pump  3 . Most of the material in conduit  7  is sent by valve  8  through conduit  9 , which passes through the top of process tank  10 , terminating in the air space above liquid level  11  which defines the top of the process mixture in tank  10 . Situated directly in the path of the mud emitted from line  9  is a splash plate  12 . Splash plate  12  is a metal plate oriented to receive the heated, aerated mud and cause it to splash and break up on entering process tank  10 . 
         [0020]    The action of the cavitation device  4  has caused a significant portion of the water in the emulsion to evaporate, and be absorbed into the air which is dispersed in the mud. As the heated, aerated mud strikes the splash plate  12 , a portion of the air, with its thus entrained water vapor, is liberated from the mud and released into the air space in tank  10  over liquid level  11 . Air and water vapor are removed from process tank  10  either through a vent  13  or by a vacuum drawn through conduit  14 . Vent  13  simply releases the vapor-laden air to the atmosphere; because we normally condense as much vapor as possible to conserve its heat energy and to measure the condensed water volume, vent  13  is normally closed so that air and water vapor may escape through conduit  14 . Conduit  14  directs the vapor-laden air to heat exchanger  2 , where its heat is transferred to the incoming feed while also condensing the vapor; a vacuum may be applied by vacuum pump  16 , and condensation of the water vapor may be completed in condenser  24 . Liquid water in conduit  25  may be collected for measurement and possibly for other uses. Additional air may be injected into the system in a loop comprising a pump  18  for removing liquid from tank  10  through conduit  17  and returning it through conduit  19  to tank  10  after injecting more air from compressed air source  21 . Conduit  19  has a terminus  20  within tank  10  preferably above liquid level  11 . A second recycle loop may be provided as illustrated by conduit  22 , which returns mud (process mixture) from tank  10  to pump  3 , where it is passed through cavitation device  4  again after mixing with feed from lines  1  and  5  and after receiving more air from compressed air line  6 . The finished, reprocessed mud, having a (now higher) desired ratio of oil to water, is removed in conduit  23  and can be stored or sent directly to a drilling site for use. Conduit  23  may be directed to pass through heat exchanger  2  for additional heat energy savings before storage or use. Its makeup may be adjusted for a general or particular use by adding various chemicals such as fluid loss agents or weighting agents in line or during storage. 
         [0021]    Thus our basic process is a method of removing water from a water-in-oil emulsion comprising (a) injecting air into the emulsion, (b) passing the emulsion containing injected air through a cavitation device, thereby forming bubbles of air and entraining water vapor in the bubbles of air, and (c) separating at least some of the bubbles of air containing water vapor from the emulsion. The emulsion may be any water-in-oil emulsion from which it is desired to remove water. As a practical matter, most used oil field emulsions will have a ratio of at least 70:30 oil to water, by volume. 
         [0022]    While any satisfactory method may be used to separate the air containing water vapor from the emulsion as in step (c), we find that splash plate  12  is especially efficient. Drilling fluids use chemical emulsifiers to hold water in the emulsion. Also, drilling fluids are typically weighted and have a base level of viscosity. Viscous drag and physical tie up of the water molecules with the hydrates of calcium chloride and chemical emulsifiers inhibit even the air bubbles that have positive pressure from getting out of the fluid with the water vapor. The splash plate obviates the element of viscous drag, first by impact and then by spreading out the emulsion in a thin layer that cascades into the process tank. The splash plate is superior to most jets for our purposes because their orifices plug quickly with fluid loss additives and solids in the fluid. 
         [0023]    In spite of the inhibiting action of emulsifiers on release of the air bubbles, we have found that, in many cases, it is beneficial to add emulsifiers if there are not enough emulsifiers in the emulsion, as the excellent mixing action of the cavitation device will assure an improved distribution and emulsifying effect. The addition may be made anywhere in the system, but is particularly effective when added upstream of a cavitation device, as before the injection of air in line  6 , or after it; it may also be added simultaneously with the air. 
         [0024]    An efficient process will entail controls to (a) maintain a temperature of at least 140° F. in the mixture of mud and air entering the cavitation device from conduit  5 , (b) and maintain a temperature of at least 188° F., preferably at least 190° F., in the liquid in tank  10 . The mud entering the cavitation device is desirably between 140° F. and 150° F., and the temperature in the tank  10  is desirably 190° F. to 200° F. Controlled variables which may be used to maintain those temperatures include (a) the feed rate of original used mud in line  1  and its original temperature, (b) the temperature and flow rate (as determined by the position of valve  8 ) of the recycled aerated mud in conduit  15 , as well as the heat exchange efficiency in heat exchanger  14 , (c) the flow rate and temperature of material removed for recycle in conduit  17  (adjusting for the injection of air in line  21 ), (d) the flow rate and temperature of aerated mud entering tank  10  through conduit  9 , (e) the flow rate and temperature of finished reprocessed mud being removed from tank  10  in conduit  23 , (f) the flow rate and temperature of vented air and water vapor in vent  13 , if any, (g) the flow rate and temperature of mud recycled to cavitation device  4  through conduits  22  and  5 , (h) the flow rate and temperature of air injected into the system, (i) the flow rate and temperature of water vapor laden air removed from the system through conduit  14 , and (j) the RPM&#39;s of the cavitation device and/or its power input. Instead of the flow rate of vapor-laden air in variable (i), one may measure the amount of water condensed, or may periodically perform a retort test as explained elsewhere herein. A particular liquid level in tank  10  is not critical, but maintenance of steady-state conditions will be simplified if a level control is implemented. A mathematical model may be constructed for installation in a computer control system to implement the process in equipment of particular specifications, using all or most of the above criteria. 
         [0025]    It is known that the presence of small air bubbles in a fluid will reduce turbulence when it is flowing in a conduit. Accordingly, an increase of injected air will tend to reduce the pressure drop from point to point, tending in turn to increase the flow or reduce power requirements for the pumps. This phenomenon may also be taken into account in constructing the model and also in choosing pumps, conduit sizes, valves, and other equipment. Also, over a wide range of air content in the fluid, the amount of water vapor removed will increase with the amount of air present even if the temperature is reduced slightly. But, as indicated above, the dynamic nature of the system means the user must be aware of other effects of a high air content, as it permeates the entire system. For example, a high air content may increase the flow rate in a given conduit, which in turn may affect radiant heat loss. 
         [0026]    Heat exchanger  2  may be of any conventional construction. Its main purpose is to elevate the temperature of the incoming mud prior to entering cavitation device  4  while conserving heat energy from the product water vapor in conduit  14 . While the cavitation device  4  will increase the temperature even more, its plural function of dispersing the air, homogenizing the emulsion, and heating the emulsion is performed more efficiently if the incoming temperature of the mud is elevated somewhat. The air/vapor mixture in conduit  14  passes directly through heat exchanger  2 , contacting a heat transfer surface, not shown, for heating the incoming mud feed. The recycled mud in conduit  15  merges with the incoming mud; it may contribute heat also to the incoming mud through a heat transfer surface, or not. A supplemental heater may be used to provide additional heat to the incoming mud. 
         [0027]    Fluid flow rates, heat inputs, recycles, and air injection rates are preferably controlled to maintain a desired level  11  in process tank  10  and the temperature in process tank  10  no lower than 180° F., preferably no lower than 188° F. Any appropriate automatic control devices may be used. The heat exchanger  2  should be capable of elevating the temperature of incoming “feed” used mud in conduit  1  to 150° F. 
         [0028]    Referring now to  FIG. 2 , a cavitation device useful as the cavitation device  4  of  FIG. 1  is shown in outline form. The cavitation device comprises a housing  30 , which is cut away for illustration to expose cylinder  31  mounted on motorized shaft  32 . Shaft  32  passes through housing  30  with suitable seals and bearings not shown and is fixed centrally to cylinder  31 . Housing  30  has a cylindrical internal surface substantially concentric with the cylindrical surface of cylinder  31 . On the cylindrical surface of cylinder  31  are a plurality of cavities  33  which may be bored or otherwise formed on the surface. Preferably they are distributed evenly as shown or otherwise. Fluid to be processed, in this case the used mud which contains added air as explained with reference to  FIG. 1 , enters housing  30  from conduit  5  and is immediately dispersed, as shown by the arrows, to the space  34  between the face of cylinder  31  and housing  30 , flowing then into the confined space  35  between housing  30  and the cylindrical surface of cylinder  31 . Aerated mud entering the confined space  35  between housing  30  and the rapidly rotating cylinder  31  tends to enter cavities  33  but is immediately ejected by centrifugal force, creating a mini-vacuum which is immediately violently filled. This cavitation effect imparts heat energy to the fluid while evenly and finely dispersing the constituents of the emulsion. In the process, fine bubbles are formed of the air, which is able to entrain water vapor as it is freed from the emulsion. The mud thus processed exits the cavitation device through an exit port  36  leading to, in  FIG. 1 , conduit  7 . By regulating the revolutions per minute (RPM) of the cylinder  31 , temperatures within the cavitation device should be maintained no lower than 180° F. 
         [0029]    The cavitation device  4  need not be designed exactly as depicted in  FIG. 2 . Devices which create similar cavitation effects are known, as seen in U.S. Patents to Griggs (U.S. Pat. Nos. 5,188,090, 5,385,298, and 5,957,122), Hudson et al U.S. Pat. No. 6,627,784, Wyszomerski U.S. Pat. No. 3,198,191, Selivanov U.S. Pat. No. 6,016,798, Thoma U.S. Pat. Nos. 7,089,886, 6,976,486, 6,959,669, 6,910,448, and 6,823,820, Costa et al U.S. Pat. No. 6,595,759, Giebeler et al U.S. Pat. Nos. 5,931,153 and 6,164,274, and Archibald et al U.S. Pat. No. 6,596,178, all of which are hereby specifically incorporated herein by reference in their entireties. A cavitation device, as the term is used herein, includes any of the devices depicted in the above incorporated patents as well as cavitation device  4  of  FIG. 1 . As the term is used herein, a cavitation device is any device which will impart thermal energy to flowing liquid by causing bubbles or pockets of partial vacuum to form within the liquid it processes, the bubbles or pockets of partial vacuum being quickly imploded and filled by the flowing liquid. The bubbles or pockets of partial vacuum have also been described as areas within the liquid which have reached the vapor pressure of the liquid. The turbulence and/or impact, which may be called a shock wave, caused by the implosion imparts thermal energy to the liquid, which, in the case of water, may readily reach topical boiling temperatures. The bubbles or pockets of partial vacuum are typically created by flowing the liquid through narrow passages which present side depressions, cavities, pockets, apertures, or dead-end holes to the flowing liquid; hence the term “cavitation effect” is frequently applied. Commonly there is a shearing effect in the narrow passages between two close surfaces, at least one of which is moving. 
         [0030]    It should be understood that the capability of forming bubbles or pockets in the above description relates to the processing of a liquid which has not been injected with air or other gas. The addition of air as called for by the present invention does not alter the cavitation effect on the liquid, but facilitates the evaporation of water from it; the water vapor is captured by the injected air, the surface area of which has been greatly multiplied by the action of the cavitation device. 
         [0031]    A modified cavitation device is shown in  FIG. 3 . As in  FIG. 2 , cylinder  31  mounted on motorized shaft  32  fits concentrically into cylindrical housing  30  and has a number of cavities  33  on its cylindrical surface. The device is modified by including three discs  40 ,  41 , and  42  which are mounted by disc supports  43  and  44  in planes substantially parallel to cylinder  31 . Discs  40 ,  41 , and  42  each have a central orifice  45 . Operation of the modified cavitation device is as follows. Material to be processed, in this case for example the process mixture to be recycled from tank  10  through conduit  17  ( FIG. 1 ), enters the housing  30  as indicated by arrow  46  and passes through the orifices  45  of discs  40 ,  41 , and  42  while also being distributed between them, as indicated by the arrows. Additional air may be injected into conduit  17  from a source such as source  21 . Since the discs  40 ,  41 , and  42  are attached to cylinder  31  by supports  43  and  44 , they turn with it as the motor (not shown) turns shaft  32 . Discs  40 ,  41 , and  42  thus act as a disc pump (and may be called a “disc pump fixture”), obviating the need for an external pump. The process mixture from process tank  10  is passed through confined space  35  in a manner similar to that described for  FIG. 2 , and continues through exit  36 , in this case to go by way of a conduit such as conduit  19  ( FIG. 1 ) directly to process tank  10 . Although air has already been added, more air can be injected as from source  21  in the configuration of  FIG. 1  if it is deemed desirable. 
         [0032]    The number of discs in the modified cavitation device of  FIG. 3  need not be limited to three. Any number of discs which will provide the desired pumping effect is satisfactory. Faces of the discs need not be smooth—slight projections may assist the centrifugal force effect of the discs, as is known in the pump art. 
         [0033]    The modified cavitation device of  FIG. 3  can simplify the model and control requirements of the system by replacing both pump  3  and cavitation device  4 . As indicated above, discs  40 ,  41 , and  42  act as a pump. Since they are integral with the cavitation device housing  30  and rotor  31 , we are able to eliminate pump  3  as a controlled variable in our model—controlling the rotation of shaft  32  by regulating its motor (not shown) will control both the pump action of the discs and the cavitation action of cavitation rotor  31 . 
         [0034]    Similarly, the modified cavitation device of  FIG. 3  may be used in place of pump  18 , receiving fluid from tank  10  and recirculating it to tank  10 . In this case, air injection source  21  may remain as shown or may be placed on line  17  to inject air upstream of the modified cavitation device. The rotation of rotor  31  would also operate the disc pump action as explained with reference to  FIG. 3 . Emulsifiers and/or other fluid additives may be added in this recycle loop also where needed or desired. 
         [0035]    The compressed air may be dried, and is desirably typical of industrial plant air that includes a mechanical separation of water from the compressed air to prevent corrosion in plant air lines and associated equipment. The air can be further dried with various mechanical devices and chemical methods including desiccants and membranes to yield an ultra-low dew point; however, generally ordinary plant air (air having a dew point lower than 0° F.) is sufficient. Otherwise, larger quantities of air may be required than are necessary. 
         [0036]    The continuous process described above with respect to  FIG. 1  may be varied somewhat. For example, one or more additional auxiliary heaters may supplement heat exchanger  2 ; a heater may be placed inside process tank  10  or an external source of supplemental heat may be used to compensate for radiant heat losses in the system. A recycle line may pass directly from the downstream end of the cavitation device back to the upstream end, with provision for recycling as much as 99 percent or none at all. 
         [0037]    Our invention includes a startup procedure for processing a used drilling mud having a known ratio of oil to water as determined by retort or otherwise. Our startup procedure has three basic steps. First, a known amount of the used emulsion of known oil/water ratio (by volume) is placed in process tank  10 . It is heated to a desired temperature within the range 180-200° F. either in the process tank  10  or in a holding tank not shown, or otherwise, but is to have such a temperature in tank  10 . This composition is passed, by way of conduit  22 , and without replenishment in the tank, through the cavitation device by a pump such as pump  3  with the addition of air from a source  6  injected between the pump and the cavitation device, recycling it to the process tank  10  by striking splash plate  12 , and venting the tank or drawing a vacuum until enough water has been removed to provide the desired volume/volume ratio of oil to water. The oil/water ratio is monitored while maintaining the desired temperature in the tank. The total volume of emulsion in the tank has now been reduced a measured amount in a measured time while the cavitation device is operating at a certain rate; these factors are used to calculate the flow of additional used fluid to the tank. The calculated inlet flow is part of a mass balance that must equal the amount of water vapor to be removed plus the amount of mud to be exported at the proper concentration. The heat balance must also be considered such that both heat and a set process tank level are maintained. 
         [0038]    So the second basic step is to initiate the continuous process by starting the continuous addition of more used emulsion to tank  10  at a rate calculated to maintain a desired level. It is not necessary to restore the original level in tank  10 , but this is an option; the mass balance and heat balance should be adjusted accordingly, and tank level is useful as a controlled variable. 
         [0039]    For the third basic startup step, air is added to the emulsion as from source  6  prior to passing through cavitation device  4 . Cavitation device  4  is controlled to maintain a desired temperature in tank  10 . 
         [0040]    Another method to maintain the temperature in the tank is to use mechanical vapor recompression to push hot air vapor mix through internal or external coils that transfer the heat of vaporization back to the process. That method requires enough compression to raise the enthalpy of the vapor. However, any convenient method of supplementing heat input to the tank  10  will be useful. 
         [0041]    A series of tests was conducted to help define the invention. A measured amount (“batch”) of used oil field emulsion was placed in a tank similar to process tank  10 , equipped with means for recycling it through a cavitation device as in  FIG. 1 . The emulsion was heated to 190° F. Air injection and circulation of the emulsion were then begun. In Table  1 , the oil/water ratio (by volume) is reported for the start and at two-hour checkpoints. For each batch, the total water removed is also reported. 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Batch 
                   
                 Start 
                 Check 1 
                 Check 2 
                 Check3 
                 Check 4 
                   
               
               
                   
               
             
             
               
                 1 
                 Solids 
                 29.0% 
                 33.0% 
                 33.0% 
                 31.5% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 57.05 
                 57.0% 
                 58.5% 
                   62% 
                   
                 Removed 
               
               
                   
                 Water 
                 14.0% 
                 10.0% 
                  8.5% 
                  6.5% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.80/0.20 
                 0.85/0.15 
                 0.87/0.13 
                 0.91/0/09 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 2.4  
                 1.10 
                 1.22 
                   
                 56.28% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   29% 
                   18% 
                   25% 
               
               
                 2 
                 Solids 
                 29.5% 
                 30.5% 
                   34% 
                 32.5% 
                 32.5% 
                 Total Water 
               
               
                   
                 Oil 
                 55.5% 
                 56.5% 
                 55.5% 
                 58.5% 
                 61.0% 
                 Removed 
               
               
                   
                 Water 
                   15% 
                 13.0% 
                 10.5% 
                  9.0% 
                  6.5% 
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.79/0.21 
                 0.81/0.19 
                 0.84/0.10 
                 0.87/0.13 
                 0.90/0.10 
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.20 
                 1.63 
                 1.03 
                 1.50 
                 59.45% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   13% 
                   21% 
                   17% 
                   29% 
               
               
                 3 
                 Solids 
                 31.0% 
                 33.0% 
                   33% 
                 35.5% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 56.5% 
                 57.5% 
                 60.0% 
                 60.5% 
                   
                 Removed 
               
               
                   
                 Water 
                 12.5% 
                  9.5% 
                  7.0% 
                  4.0% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.82/0.18 
                 0.86/0.14 
                 0.90/0.10 
                 0.94/0.06 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.80 
                 1.63 
                 1.81 
                   
                 69.83 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   24% 
                   29% 
                   44% 
               
               
                 4 
                 Solids 
                 31.0% 
                 32.5% 
                 34.5% 
                 34.5% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 56.5% 
                 58.0% 
                 59.0% 
                 63.0% 
                   
                 Removed 
               
               
                   
                 Water 
                 12.5% 
                  9.5% 
                  6.5% 
                  2.5% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.82.0.18 
                 0.86/0.14 
                 0.90/0.10 
                 0.96/0.04 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.80 
                 1.92 
                 2.38 
                   
                 81.24% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   24% 
                   34% 
                   63% 
               
               
                 5 
                 Solids 
                 30.0% 
                 30.5% 
                 31.5% 
                 33.0% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 57.0% 
                 59.5% 
                 61.0% 
                 62.0% 
                   
                 Removed 
               
               
                   
                 Water 
                 13.0% 
                 10.0% 
                  7.5% 
                  5.0% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.81/0.19 
                 0.86/0.14 
                 0.89/0.11 
                 0.93/0.07 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.80 
                 1.64 
                 1.54 
                   
                 63.74% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   23% 
                   27% 
                   35% 
               
               
                 6 
                 Solids 
                 29.5% 
                 29.5% 
                 32.5% 
                 33.0% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 58.0% 
                 61.0% 
                 61.5% 
                 64.0% 
                   
                 Removed 
               
               
                   
                 Water 
                 12.5% 
                  9.5% 
                  6.0% 
                  3.0% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.82/0.18 
                 0.87/0.13 
                 0.91/0.09 
                 0.96/0.04 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.80 
                 2.21 
                 1.81 
                   
                 77.60% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   24% 
                   39% 
                   52% 
               
               
                 7 
                 Solids 
                 34.5% 
                 34.5% 
                 34.0% 
                   
                   
                 Total Water 
               
               
                   
                 Oil 
                 57.5% 
                 60.0% 
                 62.0% 
                   
                   
                 Removed 
               
               
                   
                 Water 
                  8.0% 
                  5.5% 
                  3.5% 
                   
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.88/0.12 
                 0.92/0.08 
                 0.95/0.05 
                   
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 1.50 
                 1.25 
                   
                   
                 57.34% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   31% 
                   38% 
               
               
                 8 
                 Solids 
                 33.5% 
                 34.5% 
                 34.5% 
                 35.0% 
                   
                 Total Water 
               
               
                   
                 Oil 
                 56.5% 
                 59.5% 
                 61.0% 
                 61.0% 
                   
                 Removed 
               
               
                   
                 Water 
                 10.0% 
                  6.0% 
                  4.5% 
                  4.0% 
                   
                 (as vol. % of 
               
               
                   
                 o/w (v/v) 
                 0.85/0.15 
                 0.91/0.09 
                 0.93/0.07 
                 0.94/0.96 
                   
                 Start): 
               
               
                   
                 Water rmvd (bbl) 
                 — 
                 2.40 
                 1.01 
                 0.33 
                   
                 62.27% 
               
               
                   
                 Water rmvd (v. %) 
                 — 
                   40.% 
                  28.% 
                   13% 
               
               
                   
               
             
          
         
       
     
         [0042]    It will be noted that most of the batches were taken to a ratio of oil to water of greater than 9/1. Efficient practice of our method includes maintaining a substantially steady temperature in tank  10  of at least 180° F. but no higher than the boiling point of the aqueous composition in the tank. Preferably the temperature in the tank will be maintained in the range of 188-200° F. The continuous input of heat energy from cavitation device  4  and any supplemental heating means should be equivalent to the total system heat loss including heat energy in the continuously removed reconstituted product in conduit  23  and the heat of evaporation of the water removed. Although the oil boils at a temperature higher than water, there is no advantage to heating it beyond the optimal temperature for transferring heat from the oil to the aqueous phase. But because the far greater volume of the emulsion is oil, and because elevating its temperature is time-consuming, it is advantageous to continue evaporating water, while the oil is hot, beyond the exact amount needed to make a 90/10 composition, for example. A 95/5 emulsion may be made, even if some of the salts come out of solution, for storing and use as a base for reconstituting a new 90/10 emulsion at a later time. We find that such “over-evaporated” used muds are readily adapted for further use. For example, if the water phase of an 80/20 o/w emulsion contains 20,000 parts chlorides, it will still contain 20,000 parts chloride at the end of our process, but if half the water is removed, the concentration will be doubled in terms of parts per million. Thus it is part of the invention to cause the aqueous phase of the reconstituted mud to be supersaturated with at least one inorganic salt. Such a reconstituted mud can be stored and adjusted for reuse with the addition of water and other constituents at a later time. 
         [0043]    On occasion, the operator may wish to recycle a used mud on short notice. He may have a mud having an 85:15 ratio of oil to water, for example, that he wants to use without waiting for dewatering. In this case, it is convenient to have on hand a super-concentrated used mud having, say, a 95:5 ratio, having been processed by our method. An equal amount of such a stored mud mixed with the 85:15 mud will yield the desired 90:10 emulsion. Our invention therefore includes mixing the stored super-concentrated used mud (one having at least one inorganic salt in an amount greater than saturation) with a separate used drilling mud emulsion including inorganic salts in amounts lower than saturation amounts, to make an emulsion having a desired oil to water ratio. 
         [0044]    Therefore it is seen that our invention includes a method of removing water from a water-in-oil emulsion comprising (a) injecting air into the emulsion, (b) passing the emulsion containing injected air through a cavitation device, thereby heating the emulsion, forming bubbles of air in the emulsion, and entraining water vapor in the bubbles of air, and (c) separating at least some of the bubbles of air containing water vapor from the emulsion. 
         [0045]    The invention also includes a continuous method of removing water from a used water-in-oil drilling mud emulsion comprising (a) heating the drilling mud emulsion to at least 140° F., (b) injecting air into the drilling mud emulsion, to form an aerated emulsion, (c) directing the aerated emulsion into a cavitation device, whereby the aerated emulsion is heated to at least 180° F. and finely dispersed, (d) directing the aerated emulsion onto a splash plate in a process tank, thereby (i) forming a process mixture in the process tank and (ii) releasing air containing water vapor from the emulsion, (e) directing the air containing water vapor to a condenser and condensing the water vapor to form water, and (f) removing process mixture from the process tank for use as a reconstituted drilling fluid. Additional emulsifier may be injected into said emulsion before, during or after step (b). 
         [0046]    In addition, our invention includes a method of starting up a continuous method of removing water from a used water-in-oil emulsion comprising (a) providing a known volume of the used emulsion in a tank at a temperature of 180-200° F., (b) passing the emulsion through a cavitation device together with air, thereby entraining water vapor in air within the emulsion, (c) recycling the emulsion to the tank by causing it to strike a splash plate in the tank, (d) withdrawing air and water vapor from the tank until a desired ratio of oil to water is attained in the tank, (e) calculating mass and energy balances from the time, volume of emulsion in the tank, temperatures, flow rates of the emulsion, and air input and heat energy inputs in steps (a), (b), (c), and (d), and (f) implementing the mass and energy balances to start processing the emulsion to remove water therefrom continuously.