Patent Grant
8840298
The present invention relates to mixers and, more particularly, in certain embodiments, to mixers for blending particulates, or fluid into a fluid stream.
Traditional oil field fracturing blenders are open top mixing systems that require sophisticated fluid control systems to maintain a nominal level of fluid in a mixing tub. The typical open tub fracturing blender in oil field services utilizes an atmospheric pressure open top blending vessel to blend particulates with carrier fluid (usually a viscous polymer fluid system). The level of the fluid in the blending vessel is controlled by various control valves and level sensors through proprietary computer software control systems. Although advancements have been made in providing a rugged, tough, responsive fluid level system, the system is still a major cause of critical equipment failures on the fracturing blenders. In order to eliminate these components and systems, centrifugal type, closed system blenders have been used.
The typical centrifugal blending system utilizes a minimal volume mixer case to collect particulates and carrier fluid and redirect them to the mixer discharge. These systems typically use a combination centrifugal force impeller to inject the particulates and provide carrier fluid under pressure to the mixer. In addition to creating pressure, the centrifugal force on the carrier fluid in the mixer prevents the carrier fluid from exiting the mixer. The particulates enter the mixer at an eye of a rotating impeller, which provides motive force to move the particulates into the mixer and prevent the pressurized carrier fluid from escaping to the atmosphere. The carrier fluid section or the mixer impeller must provide sufficient flow at the pressure required by high-pressure downhole pumps (typically 50 to 75 psi). The particulates section of the pump impeller must be able to inject particulates into the pressurized mixer and keep the carrier fluid contained. In some cases, an external boost pump (such as a low pressure, high volume axial flow pump) is used to provide efficient suction characteristics to keep the carrier fluid section of the mixer impeller primed. However, these high mix pressures, which require a high mixer rpm, may cause severe erosion on mixer rotating components due to the high velocities of abrasive fluids.
Generally, the centrifugal mixer volume is kept small to minimize required wall thickness (required by the typical operating pressure range of 50-70 psi.), along with associated weight and cost. For example, for 50-70 psi operating pressure, the volume of the mixer is typically less than two barrels. This small volume prevents significant dwell times. For example, at 50 barrels per minute, the dwell time of a 2 barrel volume is less than 2.5 seconds. Thus, when abrupt changes occur in the carrier fluid (e.g. slurry or water) supply or particulate delivery rate, (i.e., sand-off, empty frac tank, etc), the concentration of particulates in the mixer can become extremely high or low before the control system can properly respond to the abrupt change. Thus, fluctuations in the carrier fluid delivery system (e.g., the slurry delivery system and/or the water supply system), or the particulate delivery system can be catastrophic, even causing the entire fracturing job to fail, requiring extensive rework.
Further, when throughput is slowed, and the fluid velocity drops below the minimum particle carrying velocity, there is a tendency for the particulates to “fall out” of the carrier fluid. When downhole rate stops, the mixer may deadhead under mixing pressure, and any slurry in the mixer will tend to separate. This necessitates a flush of the mixer before mixing is stopped, so that there is a clean fluid plug when mixing resumes. Additionally, getting particulates into the mixer vanes may be very difficult. Particulates are directed from vertical to horizontal and accelerated to enter the vanes. Thus, the vanes are either very deep or inducer vanes are used. Finally, this design lacks an atmospheric pressure tub to provide for removal of entrained air in the downhole pressure piping, necessitating a connection to an external holding tank to allow the high pressure pumping units to “prime-up” or recirculate fluid to remove entrapped air.
The present invention relates to mixers and, more particularly, in certain embodiments, to mixers for blending particulates, or fluid into a fluid stream.
In some embodiments, a mixing system may comprise a closed mixer having an inlet, a discharge and an inlet/discharge, and a recirculation line in fluid communication with the inlet and the inlet/discharge.
In some embodiments, a mixing system may comprise a closed mixer, and an averaging volume attached to the closed mixer.
The features and advantages of the present invention will be readily apparent to those skilled in the art. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.
The present invention relates to mixers and, more particularly, in certain embodiments, to mixers for blending particulates, or fluid into a fluid stream.
Referring to
Also illustrated in
Depending on the application, all of the slurry may enter the recirculation line 126, or all of the slurry may enter the discharge line 128. For instance, at no-thru-put conditions, the pressure exerted by mixer 112 will overcome the set pressure provided by suction pump 130 and mixer 112 will recirculate the slurry. When thru-put occurs, fluid pressure at inlet/discharge 117 is reduced, and suction pump pressure will dominate and provide carrier fluid to inlet line 118 to keep the dynamic loop full. Inlet/discharge 117 may function as an inlet when inlet 114 does not pass enough fluid at a set pressure of suction pump 130. At job start up, high pressure pumping equipment may use the mixing system to prime-up by circulating fluid through prime-up line 138 to mixer 112 where entrained air can be allowed to escape. This mixing system 110 may allow mixing at low rates, even with large diameter piping (low downhole rates) due to the recirculating feature. The recirculation flow allows the mixer volume to remain active and avoid stagnation of the slurry. In some embodiments, when optional booster pump 132 is used, mixer 112 may operate at low mixing pressure and/or have a lower mixer speed, allowing for decreased mixer wear.
Referring now to
Also illustrated in
Depending on the application, all of the slurry may enter the recirculation line 226, or all of the slurry may enter the discharge line 228. For instance, at no-thru-put conditions, the pressure exerted by mixer 212 will overcome the set pressure provided by suction pump 230 and mixer 212 will recirculate the slurry. When thru-put occurs, fluid pressure at inlet/discharge 217 is reduced, and suction pump pressure will dominate and provide carrier fluid to inlet 215 to keep the dynamic loop full. At job start up, high pressure pumping equipment may be used to prime-up the system by introducing pressure to prime-up line 238, which in turn may introduce pressure to recirculation line 226.
As illustrated in
This mixing system 210 may allow mixing at low rates, even with large diameter piping (low downhole rates) due to the recirculating feature. The recirculation flow allows the mixer volume to remain active and avoid stagnation of the slurry. In some embodiments, when optional booster pump 232 is used, mixer 212 may operate at low mixing pressure and/or have a low mixer speed, allowing for decreased mixer wear.
Referring now to
Depending on the application, all of the slurry may enter the recirculation line 326, or all of the slurry may enter the discharge line 328. For instance, at no-thru-put conditions, the pressure exerted by mixer 312 will overcome the set pressure provided by pressurized line 320 and mixer 312 will recirculate the slurry. When thru-put occurs, fluid pressure at recirculation line 326 is reduced, and pressurized line 320 will dominate and provide carrier fluid to inlet 314 to keep the dynamic loop full. At job start up, high pressure pumping equipment may use the mixing system to prime-up by circulating fluid through prime-up line 338 to mixer 312 where entrained air can be allowed to escape.
Additionally, the embodiment illustrated in
For example, at a 50 barrel per minute mixing rate, the dwell time of a 2 barrel mixer is less than 2.5 seconds. If the averaging volume 342 were 10 barrels, it would provide an additional dwell time of 12 seconds. Various sizes of averaging volumes 342 may be appropriate. In some embodiments, the total mixer volume, including the averaging volume, may be 50% larger than the volume of a mixer without an averaging volume. In other embodiments, the total mixer volume may be double the volume of the mixer without an averaging volume. In still other embodiments, the total mixer volume may increase by a factor of about 3 or 4 times over the volume of the mixer without an averaging volume. In alternate embodiments, the total mixer volume may be about 5 times the volume of the mixer without an averaging volume. In some embodiments, the averaging volume may be up to 10 barrels or larger. In other embodiments, the total mixer volume may increase as much as tenfold over the volume of the mixer without an averaging volume. In some embodiments, when optional booster pump 332 is used, mixer 312 may operate at low mixing pressure and/or have a low mixer speed, allowing for decreased mixer wear.
The advantages of the “top drive” configuration discussed with respect to the embodiment of
In the illustrated embodiments, recirculation line 126/226/326 may provide particulate concentration averaging, helping to reduce effects of system disruptions. The recirculation line 126/226/326 may also provide the ability to dead head, or stop downhole rate, while keeping the mixer fluid stream active. Additionally, recirculation line 126/226/326 may help reduce the effects of mixer upset, and allow for prime up on location. Further, the carrier fluid may be injected into an atmospheric pressure area of impeller 136/236/336 rather than into the pressurized volute as is typical with typical centrifugal mixer designs, thus allowing the use of a low pressure/low power carrier fluid supply pump. Additionally, the design of impeller 136/236/336 may expose the carrier fluid stream to the particulates, providing motive force to convey particulates into the impeller vanes. Finally, exposing the carrier fluid and/or the slurry to atmospheric pressure may assist in de-aeration.
As illustrated in the various figures, drive 124 is a bottom drive, and drives 224 and 324 are top drives. However, any of a number of drives may be suitable, as will be appreciated by a person skilled in the art. Likewise, mixers 112, 212, and 312 are illustrated as centrifugal mixers having impeller(s) 136, 236, 336 connected to respective drives 124, 224, 324 via drive shaft. However, this is not intended to be limiting on the invention, and mixers 112, 212, 312 may be progressive cavity pumps or other positive displacement pumps with or without impellers, so long as mixers 112, 212, and 312 are closed (e.g., have fixed volumes and are not at atmospheric pressure). Impellers 136, 236, 336 may likewise be replaced by another source of recirculation or agitation. Similarly, inlets 114, 214, 314, as illustrated, are situated at the eye of a centrifugal mixer. More particularly, the carrier fluid is shown directed onto a nose cone on impellers 136, 236, 336 that divert the fluid velocity from a vertical to a horizontal direction. In these embodiments, as the carrier fluid is converted to a horizontal velocity, the particulates impinge on the carrier fluid stream and are induced into the impeller vanes for expulsion into the mixer case. However, inlets 114, 214, 314, and 215 may be readily modified by one skilled in the art.
Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. All numbers and ranges disclosed above may vary by any amount (e.g., 1 percent, 2 percent, 5 percent, or, sometimes, 10 to 20 percent). Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Moreover, the indefinite articles “a” or “an”, as used in the claims, are defined herein to mean one or more than one of the element that it introduces. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee.
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