Patent Publication Number: US-2018030968-A1

Title: Methods and systems for pressurizing harsh fluids

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefits from U.S. Provisional Patent Application No. 62/119,392 filed Feb. 23, 2015, the contents of which are hereby incorporated herein by reference. 
    
    
     FIELD 
     The subject disclosure relates generally to methods and systems for pumping a fluid from a surface of a well to a wellbore at high pressure. More particularly, the subject disclosure relates to a pressure exchanger which exchanges pressure energy from a high pressure flowing fluid system to a relatively low pressure flowing fluid system. 
     BACKGROUND 
     In special oilfield applications, pump assemblies are used to pump a fluid from the surface of the well to a wellbore at extremely high pressure. Such applications include hydraulic fracturing, cementing, and pumping through a coiled tubing, among other applications. In the example of a hydraulic fracturing operation, a multi-pump assembly is often employed to direct an abrasive containing fluid, or fracturing fluid through a wellbore and into targeted regions of the wellbore to create side “fractures” in the wellbore. To create such fractures, the fracturing fluid is pumped at extremely high pressures, sometimes in the range of 10,000 to 15,000 psi or more. In addition, the fracturing fluids contain an abrasive proppant which both facilitates an initial creation of the fracture and serves to keep the fracture “propped” open after the creation of the fracture. These fractures provide additional pathways for underground oil and gas deposits to flow from underground formations to the surface of the well. These additional pathways serve to enhance the production of the well. 
     Plunger pumps are typically employed for high pressure oilfield pumping applications, such as hydraulic fracturing operations. Such plunger pumps are sometimes also referred to as positive displacement pumps, intermittent duty pumps, triplex pumps or quintuplex pumps. Plunger pumps typically include one or more plungers driven by a crankshaft toward and away from a chamber in a pressure housing (typically referred to as a “fluid end”) in order to create pressure oscillations of high and low pressures in the chamber. These pressure oscillations allow the pump to receive a fluid at a low pressure and discharge it at a high pressure via one-way valves (also called check valves). 
     Multiple plunger pumps are often employed simultaneously in large-scale hydraulic fracturing operations. These pumps may be linked to one another through a common manifold, which mechanically collects and distributes the combined output of the individual pumps. For example, hydraulic fracturing operations often proceed in this manner with perhaps as many as twenty plunger pumps or more coupled together through a common manifold. A centralized computer system may be employed to direct the entire system for the duration of the operation. 
     However, the abrasive nature of fracturing fluids is not only effective in breaking up underground rock formations to create fractures therein, it also tends to wear out the internal components of the plunger pumps that are used to pump it. Thus, when plunger pumps are used to pump fracturing fluids, the repair, replacement and/or maintenance expenses for the internal components of the pumps are extremely high, and the overall life expectancy of the pumps is low. 
     For example, when a plunger pump is used to pump a fracturing fluid, the pump fluid end, valves, valve seats, packings, and plungers require frequent maintenance and/or replacement. Such a replacement of the fluid end is extremely expensive, not only because the fluid end itself is expensive, but also due to the difficulty and timeliness required to perform the replacement. A large percentage of plunger pump maintenance expenses may be spent on valve replacement. In addition, when a valve fails, the valve seat is often damaged as well, and seats are much more difficult to replace than valves due to the very large forces required to pull them out of the fluid end. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     In an embodiment, a method of pumping an oilfield fluid from a well surface to a wellbore is disclosed. The method comprises operating at least one low pressure pump to pump a harsh fluid; operating at least one high pressure pump to pump a clean fluid; using a piston which is in contact with clean fluid in its direction of movement and which pushes the clean fluid using the pressure from the high pressure pump in order to provide pressure to the harsh fluid, thereby pumping the harsh fluid into the wellbore. In one aspect, the harsh fluid is not in contact with the piston assembly when pressurized by the high pressure pump. 
     In one embodiment, the harsh fluid is not in contact with either side of the piston. 
     In a further embodiment, a system for pumping an oilfield fluid from a well surface to a wellbore is disclosed. The system comprises at least one low pressure pump in communication with a supply of harsh fluid; at least one high pressure pump in communication with a clean fluid; and a tubular comprising a piston assembly, wherein the piston assembly is in contact with clean fluid in its direction of movement and the piston assembly pushes the clean fluid using the pressure from the high pressure pump in order to provide pressure to the harsh fluid, thereby pumping the harsh fluid into the wellbore. In one aspect, with the provided arrangement, harsh fluid under high pressure is not in contact with the piston. 
     In one embodiment, a system for uninterrupted high pressure pumping of an oilfield fluid from a well surface to a wellbore includes at least one pair of tubulars, each comprising a piston assembly, with one tubular of a pair in a high pressure cycle phase while another tubular of the pair is in a low pressure cycle phase. 
     Further features and advantages of the subject disclosure will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject disclosure is further described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of the subject disclosure, in which like reference numerals represent similar parts throughout the several views of the drawings, and wherein: 
         FIGS. 1-5  are schematics depicting an embodiment of the subject disclosure at multiple stages of a half cycle; 
         FIGS. 6-10  are schematics depicting another embodiment of the subject disclosure at multiple stages of a cycle where the embodiment includes a “stop” element toward the end of the tubular and a check valve in the piston; 
         FIG. 11  depicts one design of a piston for embodiments of the subject disclosure; and 
         FIGS. 12 a -12 f    depict multiple stages of a sequence of operation of an embodiment of the subject disclosure using check valves. 
     
    
    
     DETAILED DESCRIPTION 
     The particulars shown herein are by way of example and for purposes of illustrative discussion of the examples of the subject disclosure only and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details in more detail than is necessary, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Furthermore, like reference numbers and designations in the various drawings indicate like elements. 
     Pumping slurry laden erosive, corrosive or abrasive fluids (also called “harsh” fluids) leads to high cost for manufacture and maintenance of pumps. Embodiments of the subject disclosure generally relate to a pumping system for pumping a fluid from a surface of a well to a wellbore at high pressure and more particularly to such a system that includes using clean fluids to transfer pressure to the harsh fluids. This provides a low cost of operation for these systems. 
       FIGS. 1-5  depict an embodiment of the subject disclosure operating at multiple points of a half-cycle. System  10  includes two storage tanks  20 ,  30 , three pumps  42 ,  44 ,  46 , two (complementary) tubulars  50 ,  60  with respective first ends  50   a ,  60   a  and second ends  50   b ,  60   b  having respective pistons  55 ,  65 , ten solenoid valves SV- 1  through SV- 10 , and a series of pipes  71 - 78  that contain clean fluid  95 , e.g., water, and a series of pipes  81 - 85  often containing harsh fluids  96 . The clean fluid  95  is shown in the Figures with a lighter color and the harsh fluid  96  is shown with a darker color. Storage tank  20  is provided for the clean fluid while storage tank  30  is provided for the harsh fluid. Pump  42  is a high pressure pump for the clean fluid. For purposes herein, “high” pressure is to be understand as a relative term relative to “low” pressure, and in various embodiments can generate 5,000 psi, or 10,000 psi, or 15,000 psi, or more, or pressures therebetween. In non-limiting examples, the high pressure pump may be a triplex or a quintuplex pumps. Pumps  44  and  46  are low pressure pumps for the clean and harsh fluids respectively. For purposes herein, “low” pressure is to be understood as a relative term relative to “high” pressure, and in various embodiments can generate 20 psia, or 60 psia, or 100 psia or pressures therebetween or other lower or higher pressures that are lower than the high pressure pump pressure. In a non-limiting example, the low pressure pumps may be C Pumps. 
     As seen in  FIGS. 1-5 , storage tank  20  is coupled to low pressure pump  44  via pipe  71 . The output of low pressure pump  44  is coupled to a second end  50   b  of the tubular  50  via pipe  72  and valve SV- 5  and to a second end  60   b  of the tubular  60  via pipe  73  and valve SV- 6 . Storage tank  20  is also coupled to high pressure pump  42  via pipe  74 . The output of high pressure pump  42  is coupled to a first end  50   a  of the tubular  50  via pipe  75  and valve SV- 1  and to the first end  60   a  of tubular  60  via pipe  76  and valve SV- 2 . Storage tank  20  is further coupled to the first end  60   a  of tubular  60  via pipe  77  and valve SV- 4  and to the first end  50   a  of tubular  50  via pipe  78  and valve SV- 3 . 
     Storage tank  30  is coupled to low pressure pump  46  via pipe  81 . Low pressure pump  46  pumps harsh fluid from storage tank  30  to the second end  50   b  of tubular  50  via pipe  82  and valve SV- 7  and to the second end  60   b  of tubular  60  via pipe  83  and valve SV- 8 . The second ends of tubulars  50  and  60  are also coupled to a high pressure manifold (not shown) via valves SV- 9  and SV- 10  respectively. 
     In order to initialize the system  10 , the high pressure pump  42  and low pressure pump  44  are started, and valves SV- 2  and SV- 10  are opened in order to fill tubular  60  with clean fluid (thereby pushing piston  65  to the second end  60   b  of tubular  60 ). Similarly, SV- 1  and SV- 9  are opened in order to fill tubular  50  with clean fluid (thereby pushing piston  55  to the second end  50   b  of tubular  50 ). After the tubulars  50 ,  60  are filled with clean fluid, valves SV- 1  and SV- 9  are closed. Valves SV- 6  and SV- 3  are then opened in order to introduce a predetermined amount of clean fluid  95   a  to the second-end side of piston  55 . Embodiments for selecting the predetermined amount of clean fluid (also called a buffer) to be introduced are discussed hereinafter. In any event, valve SV- 5  is then closed. With valve SV- 5  closed, pump  46  is started, and valves SV- 7  and SV- 3  are opened. Valve SV- 7  permits the injection of harsh fluid on the second end side of the piston  55  into tubular  50 , and valve SV- 3  permits clean fluid ejected from the front end  50   a  of tubular  50  to be directed back to storage tank  20 . Harsh fluid is injected into tubular  50  until piston  55  travels to the first end  50   a  of tubular  50 . 
     After the system  10  is initialized, the pumping of harsh fluid at high pressures may start. At the start of the cycle as shown in  FIG. 1 , valves SV- 1 , SV- 4 , SV- 6  and SV- 9  are open and valves SV- 2 , SV- 3 , SV- 5 , SV- 7 , SV- 8  and SV- 10  are closed. In this configuration, clean fluid from tank  20  may be pumped under high pressure via pump  42  and valve SV- 1  into the first end of tubular  50  in order to push piston  55  forward. Piston  55 , in turn, displaces the clean fluid buffer  95   a  directly in front of it and the harsh fluid  96  in front of the clean fluid buffer toward the second end  50   b  of tubular  50  and out through valve SV- 9  and pipe  85  to the high pressure manifold which is connected to the wellhead. At the same time, clean fluid  95  from tank  20 , pipe  71 , low pressure pump  44 , pipe  73  and valve SV- 6  is introduced to the second end side of piston  65  in tubular  60 . Thus, as the cycle starts, under high pressure, piston  55  starts moving to the right (toward the second end of tubular  50 ), while under low pressure, piston  65  starts moving to the left (toward the first end of tubular  60 ) with clean fluid  95  exiting tubular  60  and being sent via pipe  77  and valve SV- 4  back to the storage tank  20 . As will be appreciated, with this arrangement, both pistons  55 ,  65  have clean fluid  95 ,  95   a  on both sides of the pistons. The arrangement of having valves SV- 1 , SV- 4 , SV- 6  and SV- 9  open and the other valves closed continues until, as shown in  FIG. 2 , a predetermined amount of clean fluid is introduced to the second end side of piston  65 . At that time, valve SV- 6  is closed and valve SV- 8  is opened so that harsh fluid may be injected from tank  30 , via low pressure pump  46 , pipe  83  and valve SV- 8  into the second end side of tubular  60   b , as shown in  FIG. 3 . 
     With valves SV- 1 , SV- 4 , SV- 8  and SV- 9  open and the other valves closed, harsh fluid  96  continues to be ejected from tubular  50  under high pressure via pipe  85  and valve SV- 9  toward the wellbore, while clean fluid  95  continues to be ejected from tubular  60  under low pressure via pipe  77  and valve SV- 4  to the storage tank until the pistons  55  and  65  nearly reach the respective first and second ends  50   b ,  60   a  of their respective tubulars  50 ,  60  as seen in  FIG. 4  with all harsh fluid having been ejected out of tubular  50 . At that point, if desired, and in one embodiment, valves SV- 1 , SV- 4  SV- 8 , and SV- 9  are closed, and valves SV- 7 , SV- 3 , SV- 2  and SV- 10  are opened to reverse the directions of the pistons  55 ,  65 , and start the second half of the cycle. With that arrangement, clean fluid under high pressure is injected into tubular  60  via valve SV- 2  to cause harsh fluid  96  to be ejected from tubular  60  to the high pressure manifold via valve SV- 10 . At the same time, harsh fluid  96  from storage tank  30  is injected via valve SV- 7  under low pressure into the second end  50   b  of tubular  50 , and clean fluid  95  is ejected from the first end  50   a  of tubular  50  back to the clean fluid storage tank  20  via valve SV- 3 . This arrangement could continue until piston  55  is pushed back to nearly the first end  50   a  of tubular  50  and piston  65  is pushed back to nearly the second end  60   b  of tubular  60 . At that point, the fluids in the tubulars would assume the arrangement as shown in  FIG. 2  and a complete cycle could be completed. In addition, at that point, valves SV- 1 , SV- 4 , SV- 8  and SV- 9  would be re-opened and valves SV- 2 , SV- 3 , SV- 5 , SV- 6 , SV- 7  and SV- 10  would be closed and a new cycle would start. 
     In another embodiment, valves SV- 1 , SV- 4  SV- 8 , and SV- 9  may remain open until the pistons reach the ends of their respective tubulars as seen in  FIG. 5  such that the entire contents of the tubulars are discharged. The amount of the clean fluid  95   a  ejected from the second end  50   b  of tubular  50  may be sufficient to flush the harsh fluid from the segment of pipe  85  from the second end  50   b  of tubular  50  and valve SV- 9 , thereby prolonging the operating life of this valve. 
     If the respective valves are held in position to reach the arrangement of  FIG. 5 , at that point, valves SV- 1 , SV- 4  SV- 8 , and SV- 9  would be closed, and valves SV 2 , SV- 10 , SV- 5  and SV- 3  would be opened to reverse the directions of the pistons  55 ,  65 , and start the second half of the cycle. At the beginning of the second half of the cycle, clean fluid under high pressure is injected into the first end  60   a  of tubular  60  in order to cause harsh fluid to be ejected from the second end  60   b  of tubular  60  via valve SV- 10  toward the wellbore. At the same time, clean fluid is injected under low pressure via valve SV- 5  at the second end side of piston  55 , and clean fluid is forwarded from the first end  50   a  of tubular  50  via valve SV- 3  back to storage tank  20 . After a predetermined amount of clean fluid  95   a  fills the second end  50   b  of tubular  50 , valve SV- 5  is closed and valve SV- 7  is opened in order to permit harsh fluid  96  to fill tubular  50  “behind” the clean fluid buffer  95   a  adjacent the piston. As with a previously described embodiment, this arrangement could continue until pistons  55  and  65  are pushed back to the arrangement of  FIG. 2 , or to the arrangement of  FIG. 1 , such that (in either case), a complete cycle would be completed. If the pistons are pushed back to the arrangement of  FIG. 1 , with piston  55  at the first end  50   a  of tubular  50  and piston  65  pushed back to the second end  60   b  of tubular  60 , clean fluid  95   a  from tubular  60  will clean a segment of pipe  84  and valve SV- 10 . At that point, valves SV- 1 , SV- 4 , SV- 6  and SV- 9  would be re-opened and valves SV- 2 , SV- 3 , SV- 5 , SV- 7 , SV- 8  and SV- 10  would be closed and a new cycle would start. If the pistons are pushed back to the arrangement of  FIG. 2 , with piston  55  near the first end  50   a  of tubular  50  and piston  65  near the second end  60   b  of tubular  60 , valves SV- 1 , SV- 9 , SV- 8  and SV- 4  are re-opened and the remainder of the valves are closed and a new cycle starts. 
     In one aspect, the cycles of the above-described embodiments are repeated between the two tubulars to maintain a constant discharge rate of high pressure harsh fluid. The cycles may alternate between the arrangement shown in  FIG. 1  to the arrangement shown in  FIG. 4 , or the arrangement shown in  FIG. 1  to the arrangement shown in  FIG. 5 , or the arrangement shown in  FIG. 2  and the arrangement shown in  FIG. 4 , or the arrangement shown in  FIG. 2  to the arrangement shown in  FIG. 5 . If it is desired to “wash” valves SV- 9  and SV- 10  only intermittently, the cycles may be changed such that the standard cycle is  FIG. 1  to  FIG. 4 , but occasionally the cycle might extend to the arrangement of  FIG. 5 . 
     In the above-described embodiments of the subject disclosure two tubulars are used, but this method is applicable for any number of tubulars. 
     In the above-described embodiments of the subject disclosure a low pressure pump for the harsh fluid and a storage tank for the harsh fluid are provided, but this method is applicable where harsh fluid is supplied by upstream operations without a storage tank for the harsh fluid or for a harsh fluid pressure pump. 
     In one embodiment, the tubulars are between two and six inches in diameter and between ten and forty feet in length. In other embodiments, the tubulars have smaller or larger diameters and shorter or longer lengths. 
     In one embodiment, some or all of the valves are check valves. 
     In an embodiment, the internal diameters of the tubulars and/or pipes  84  and  85  are coated with a hard, abrasion resistant coating to withstand pumping of harsh slurries under high pressures. 
     In one embodiment, the valves are electrically powered and are under the control of a processor. If desired, sensors may be provided to detect the position of one or both pistons  55 ,  65 , and the sensors may be coupled to the processor so that the processor may open and close the valves accordingly. 
     In one embodiment, the tubulars are positioned to be horizontal (i.e., perpendicular to gravitational forces). 
     In one aspect, the lifetime of the piston assemblies  55 ,  65  (and the discharge valves SV- 9  and SV- 10 ) are increased as a result of the injection of a small amount of clean fluid as a protective (buffer front) layer between the piston assemblies and the harsh fluid. 
     The amount of clean fluid utilized as the protective layer may be predetermined, and in one embodiment is chosen to be in excess of the dispersion length l D  of the harsh fluid. The dispersion length may be calculated as follows. 
     Consider a pipe of diameter d and length l and a flow rate is fixed at q. A fluid of tracer concentration C when introduced into a pipe of length l, undergoes dispersion. A step profile in C is smeared over a length scale l D , which over a sufficiently large  1  becomes Gaussian. For a sufficiently large Reynolds number, Re, the calculation relies on turbulent flow friction that provides an estimate for velocity profile. For laminar flow, dispersion is induced by shear and is non-Gaussian. It becomes Gaussian when 2( t) 1/2 &gt;&gt;d, where   is the diffusion coefficient. In the following calculations, only Gaussian dispersion is considered, and when this is inapplicable, the pipe length required for acceptable dispersion is so large that the concept of a buffer front becomes impractical. It is also assumed that the clean and harsh fluids have a similar viscosity, and therefore no viscous instability occurs during displacement. Similar viscosity may be justified, since the clean fluid is expected to be similar to the harsh fluid, but without the proppant. The calculation is provided below, along with a table of dispersion length. 
     Density ρ, viscosity μ, flow rate q, and diffusion coefficient   are given. The objective is to estimate the dispersion length l D  or a function thereof. Results for two different pipe diameters, approximately 10 cm and 7.5 cm corresponding to nominal sizes of 4 inches and 3 inches respectively are provided. The average velocity is 
     
       
         
           
             
               
                 
                   
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     This allows a delineation of the flow regime into laminar, transitional, and turbulent categories, so that an appropriate dispersion length may be calculated. The transitional part is ignored, and a cut-off is assumed between turbulence and laminar regime at Reynolds number Re=2400 since most fracturing applications have sufficient finite-amplitude noise to induce turbulence. 
     For turbulent flow, a friction factor f is calculated, either from charts or from one of the known functions. For simplicity, a smooth pipe is assumed, and thus Nikuradse&#39;s form gives: 
     
       
         
           
             
               
                 
                   
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     For Taylor dispersion, the friction velocity may be obtained from f, which in turn may be used to infer the dispersion coefficient D according to 
     
       
         
           
             
               
                 
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     The dispersion length l D  may be obtained from the dispersion coefficient D according to 
     
       
         
           
             
               
                 
                   
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     where L is the length of the tubular. A buffer length l b  may be chosen to be a multiple of the dispersion length l D . By way of example, for a certainty of three sigma (i.e., 99.7% confidence) that the buffer length will be sufficient to prevent harsh fluid from reaching the piston, the buffer length l b  may be chosen according to l b =3l D . In other embodiments, the buffer length may be chosen to be equal or greater than the dispersion length. In another embodiment, the buffer length may be twice the dispersion length. In another embodiment, the buffer length is approximately (defined herein to be plus or minus 20%) three times the dispersion length. With a chosen buffer length, and knowing the inner diameter of the tubular, the predetermined amount of clean fluid injected into the tubular at the piston face in front of the harsh liquid is easily calculated as equal to l b πd/4. 
     For laminar flow, the calculations are different. Dispersion is Gaussian for a very long tube, i.e., for those situations where radial diffusion renders the concentration to be a function of axial distance. For such cases the dispersion coefficient D is given by the Taylor-Aris theory according to 
         D=     { 1+ 1/192( d     v   / ) 2 }  (6)
 
     It should be appreciated that the dispersion coefficient D of eq. (6) is the longitudinal dispersion coefficient which indicates how much mixing occurs between two types of fluid parallel to the direction of motion. With eq. (6) as the longitudinal dispersion coefficient for laminar flow, the characteristic dispersion length is equal to √{square root over (D convection time)} where the convection time equals the tubular length divided by the average velocity. This dispersion length is often much longer than the tubular length as may be seen from Table 1 below. 
                     TABLE 1                  Dispersion length for a variety of conditions.       The density is kept at 1100 kg m −3  and the Schmidt number       is    1000. Schedule no. is 80. The pipe internal       diameter is calculated accordingly. All units are SI: d in       m, μ in Pas, l D  in m, q in m 3 s −1 .                                 μ   d   q         l D                                           0.1   0.09718   0.265   38189   1.08       1.0   0.09718   0.265   3819   1.25       0.1   0.07366   0.265   50383   0.92       1.0   0.07366   0.265   5038   1.07       0.1   0.09718   0.265   38189   1.08       0.1   0.09718   0.053   7638   1.19       1.0   0.09718   0.053   764   5       0.1   0.07366   0.053   10077   1.02       1.0   0.07366   0.053   1008   5       0.1   0.09718   0.053   7638   1.19       0.1   0.09718   0.265   38189   1.53       1.0   0.09718   0.265   3818   1.77       0.1   0.07366   0.265   50383   1.31       1.0   0.07366   0.265   5038   1.51       0.1   0.09718   0.265   38189   1.53       0.1   0.09718   0.053   7638   1.69       1.0   0.09718   0.053   764   10       0.1   0.07366   0.053   10077   1.44       1.0   0.07366   0.053   1078   10       0.1   0.09718   0.053   7638   1.69       0.01   0.07366   0.008   15115   1.40       0.001   0.07637   0.008   151148   1.23       0.001   0.07637   0.008   151148   1.23                    
Thus, the dispersion length result becomes irrelevant and may be ignored for cases where the radial diffusion length given by ( *convection time) 5  is very small compared to the tubular radius. In most cases this condition is met, and hence the longitudinal dispersion length is limited by the tubular length. In other words, where the radial diffusion length, is very small compared to the tubular radius, the dispersion length is taken to be equal to the longitudinal dispersion length which will often be larger than the tubular length (and therefore ineffective for buffering).
 
     Based on the analysis of the laminar flow situations set forth above, according to one embodiment, flow within tubulars that are used to inject harsh fluid under high pressures toward a wellbore is purposely maintained in turbulent flow in order to prevent the harsh fluid from coming into contact with the forward moving faces of the pistons in those tubulars. 
     Turning now to  FIGS. 6-10 , another embodiment is seen System  110  is similar to system  10  of  FIGS. 1-5 , and where elements are the same or substantially the same, the same notation number is utilized. Thus, system  110  is shown to include two storage tanks  20 ,  30 , two pumps  42 ,  46 , two (complementary) tubulars  50 ,  60  with respective first ends  50   a ,  60   a  and second ends  50   b ,  60   b  having respective check valve pistons  155 ,  165  and stop elements  158 ,  168 , eight solenoid valves SV- 1 , SV- 2 , SV- 3 , SV- 4 , SV- 7 , SV- 8 , SV- 9  and SV- 10 , and a series of pipes  74 - 78  that contain clean fluid, e.g., water, and a series of pipes  81 - 85  often containing harsh fluids. The clean fluid is shown in the Figures with dashed shading and the harsh fluid is shown with darker shading. Storage tank  20  is provided for the clean fluid while storage tank  30  is provided for the harsh fluid. Pump  42  is a high pressure pump for the clean fluid, and pump  46  is a low pressure pumps for the harsh fluid. 
     As seen in  FIGS. 6-10 , storage tank is coupled to high pressure pump  42  via pipe  74 . The output of high pressure pump  42  is coupled to a first end  50   a  of the tubular  50  via pipe  75  and valve SV- 1  and to the first end  60   a  of tubular  60  via pipe  76  and valve SV- 2 . Storage tank  20  is further coupled to the first end  60   a  of tubular  60  via pipe  77  and valve SV- 4  and to the first end  50   a  of tubular  50  via pipe  78  and valve SV- 3 . 
     Storage tank  30  is coupled to low pressure pump  46  via pipe  81 . Low pressure pump  46  pumps harsh fluid from storage tank  2  to the second end  50   b  of tubular  50  via pipe  82  and valve SV- 7  and to the second end  60   b  of tubular  60  via pipe  83  and valve SV- 8 . The second ends of tubulars  50  and  60  are also coupled to a high pressure manifold (not shown) via valves SV- 9  and SV- 10  respectively. 
     Tubular  50  is provided with a check valve piston  155  and a stop  158 , while tubular  60  is provided with a check valve piston  165  and a stop  168 . The stops limit the movement of the check valve pistons in the tubulars as further described below, and may be designed in a variety of ways. In non-limiting examples, the stop elements may be inner rings, discrete parts of rings, strainers, or anything that will impede the movement of the piston. The check valve pistons allow for clean fluid to flow through the check valve at the end of the harsh fluid discharge cycle and flush the check valve, tubular, downstream piping and valves with clean fluid as further described below. 
     In order to initialize the system  110 , the high pressure pump  42  is started, and valves SV- 1  and SV- 2  (and SV- 9  and SV- 10 ) are opened in order to fill tubulars  50  and  60  with clean fluid  95  (thereby pushing pistons  155  and  165  toward the second ends  50   b  and  60   b  of the tubulars  50  and  60 ). All other valves are in the closed position. When the check valve pistons  155  and  165  reach stops  158  and  168  inside the tubulars, the check valves in the pistons open at a cracking pressure to permit the remainder of the tubulars to fill with clean fluid. The clean fluid provided to the right of the stops act as the buffers  95   a . Thus, the stops may be chosen to be at a location which will provide the desired buffer size. When the tubulars are completely filled with clean fluid, valves. SV- 2 , SV- 9  and SV- 10  are closed, and valves SV- 4  and SV- 8  are opened, and pump  46  is started. As a result, harsh fluid is directed into end  60   b  of tubular  60  and pushes piston  165  back toward end  60   a  of the tubular. When piston  165  reaches end  60   a  of the tubular, the harsh fluid  96  fills the tubular  60  except for a buffer  95   a  as seen in  FIG. 6 . 
     After the system  10  is initialized, the pumping of harsh fluid  96  at high pressures toward the wellbore may start. In particular, with both pumps  42  and  46  running, valves SV- 2 , SV- 10 , SV- 7  and SV- 3  are opened and all other valves are closed. Tubular  60 , which was previously filled with harsh fluid  96  (except for the buffer  95   a ) is now receiving clean fluid  95  via valve SV- 2  and harsh fluid  96  is being discharged at high pressure through SV- 10 , with the pressure supplied by the clean fluid through SV- 2 . The pressure difference between the clean fluid side and the harsh fluid side in tubular  60  is smaller than the cracking pressure for the check valve in piston  165  so that the check valve stays closed. Similarly, tubular  50  which was previously filled with clean fluid  95  is now receiving harsh fluid  96  via valve SV- 7  and clean fluid  95  is being discharged back to storage tank  20  via valve SV- 3 . Again, the pressure of the harsh fluid  96  is higher than the pressure of the clean fluid  95  so the check valve stays closed. The check valves in both pistons are designed to open when the pressure on the clean fluid side is higher than the pressure on the harsh fluid side by a preset amount, referred to as the cracking pressure. 
       FIG. 7  shows the same valve arrangement as  FIG. 6 , with the two pistons  155 ,  165  having advanced to the middle of the tubulars  50 ,  60  after a period of time. Piston  155  is moving to the left (toward end  50   a  of tubular  50 ) and piston  165  is moving to the right (toward end  60   b  of tubular  60 ), and the check valves in both pistons are closed. 
     After some further time, and as seen in  FIG. 8 , the piston  165  reaches the stop  168  along the inner diameter of the tubular  60  before piston  155  reaches first end  50   a  of tubular  50  (as it is moving faster in this embodiment). Once piston  165  reaches stop  168 , the piston  165  will be unable to move despite the pressure exerted by the high pressure clean fluid  95 . Because piston  165  is unable to move, pressure builds on the clean fluid side, and this leads to the opening of the check valve in piston  165  which permits high pressure clean fluid to move past the piston  165  and discharge into pipe  84  and valve SV- 10 , thereby cleaning that pipe and valve. In addition, the movement of the clean fluid past the piston  165  effectively recharges the buffer  95   a ; i.e., removing elements of the harsh fluid that may have dispersed into the buffer fluid. 
     Sometime later, and as seen in  FIG. 9 , piston  155  will reach tubular end  50   a , thereby completing a half cycle. After the half cycle, valves SV- 2 , SV- 3 , SV- 7  and SV- 10  may be closed and valves SV- 1 , SV- 9 , SV- 8 , and SV- 4  may be opened. The high pressure clean fluid  95  will then start to flow into tubular  50  via SV- 1 , pushing piston  155  and the clean fluid buffer  95   a  toward tubular end  50   b , thereby pressurizing the harsh fluid  96  which is discharged through SV- 9  toward the high pressure manifold. Similarly, harsh fluid  96  will be provided to the tubular end  60   b , thereby pushing the clean fluid buffer  95 ; piston  165 , and clean fluid  95  in the tubular  60  toward tubular end  60   a.    
     The system configuration a short time later is shown in  FIG. 10  with piston  155  pushing the buffer  95   a  and the harsh fluid  96  toward tubular end  50   b , and piston  165  pushing clean fluid  95  toward tubular end  60   a . This configuration is substantially the same as the one shown in  FIG. 7 , except the tubulars are switched; i.e., tubular  50  is providing the harsh fluid % for output to the wellbore, and tubular  60  is being filled with harsh fluid  96  with clean fluid  95  draining back to storage tank  20 . This process is continued and the tubular that is providing harsh fluid for the wellbore are switched at regular intervals to provide a steady flow rate of high pressure harsh fluid  96 . 
     According to another embodiment described in more detail with reference to  FIGS. 11 and 12   a - 121  the system  110  shown in  FIGS. 6-10  is utilized except that it is modified so that no stops are provided in the tubulars, and so that each of the pistons utilizes a check valve having a low cracking pressure so that the piston will continually discharge clean fluid to the harsh fluid side during the harsh fluid discharge cycle. In one embodiment, the flow rate of clean fluid through the check valve will be determined by the machined geometry through the piston and the friction on the piston rings. The check valve, downstream flow geometry on the piston, and the friction drag on the piston, are designed so that the volume of clean fluid on the back of the piston grows at the desired rate so that the final volume is enough to “flush” the downstream check valves with the clean fluid. In one embodiment, the fluid discharge through to the harsh fluid side is tangential in nature and may cause the piston to rotate in reaction thereto. 
     One piston design for this embodiment (and the embodiment described in  FIGS. 6-10 ) is shown in  FIG. 11 . Piston (assembly)  175  comprises a cylindrical block  1100  having at least one circumferential piston ring  1101  extending around the block, and defining a fluid passageway  1103  which houses a spring loaded check valve  1105 . Downstream of the check valve  1103  the fluid passageway  1103  splits into a plurality of streams that flush harsh fluid away from the piston rings  1101 . 
     Using check valve piston  175  of  FIG. 11  in the system of  FIGS. 6-10  and without stops in the tubulars, a sequence of operation is shown in  FIGS. 12 a -12 f   . The start of the cycle is shown in  FIG. 12 a   , with tubular  50  completely filled with the harsh fluid  96  and tubular  60  at the end of the discharge cycle, filled with clean fluid  95 . As tubular  50  is provided with high pressure clean fluid, the piston  175   a  in tubular  50  starts moving to the right such that harsh fluid  96  is discharged from tubular end  50   b  at high pressure. Concurrently, and immediately upon high pressure clean fluid being provided to tubular  50 , based on the piston design, the check valve in the piston will open, and a small amount of clean fluid will be discharged through the check valve to the harsh side of piston  175   a , thereby building a layer of clean fluid  95   a  adjacent to the face piston assembly  175 . The rate of fluid leakage is determined such that despite dispersion, a clean fluid layer is maintained close to the piston assembly. Thus, whenever piston  175   a  provides high pressure for the discharge of harsh fluid  96 , a layer (or buffer) of clean fluid  95   a  is provided ahead of the piston in the direction of motion of the piston. As seen in  FIGS. 12 b , 12 c  and 12 d   , the size of the clean fluid buffer  95   a  increases as the piston  175   a  moves toward tubular end  50   b . Thus, before piston  175   a  reaches tubular end  50   b , enough clean fluid has moved past the piston  175   a  to ensure that when piston  175   a  does reach tubular end  50   b  (in  FIG. 12 e   ), harsh fluid  96  will have been flushed from the exit pipe and valve (SV- 9 ). 
     As tubular  50  ejects harsh fluid  96  to the high pressure manifold, tubular  60  is filled up with the harsh fluid  96  under low pressure, and clean fluid  95  contained in tubular  60  is discharged via tubular end  60   a  at low pressure back to the storage tank. The process continues as shown in  FIGS. 12 b , 12 c , 12 d  and 12 e   . It is noted that in  FIGS. 12 a -12 e   , the harsh fluid  96  is in contact with piston  175   b , albeit not during the high-pressure discharge cycle, and only under low pressure conditions. However, because piston  175   b  has a check valve  1105 , the harsh fluid  96  will not move to the clean fluid side of the check valve and tubular. In addition, because tubular  60  is not under high pressure in this portion of the cycle, the harsh fluid is unlikely to cause damage to the piston. The pistons  175   a  and  175   b  push on clean fluid, thereby avoiding damage from the harsh fluid. 
     Once all of the clean fluid  95  has been ejected from tubular  50 , and tubular  60  is totally filled with harsh fluid (as seen in  FIG. 12 e   ), appropriate valves are opened and closed and the cycle is continued by switching the tubular operation, as shown in  FIG. 12 f   . Harsh fluid  96  is now pushed out of tubular  60  as a layer of clean fluid  96   a  pushes past the valve and protects the piston assembly  175   b . Simultaneously, harsh fluid is injected into end  50   b  of tubular  50  under low pressure. 
     In an embodiment, a high viscosity protective layer between the piston and the harsh fluid will help to extend the operating life of the piston assembly. 
     In an embodiment, the low pressure pump  46 , and/or storage tank  30  may not be desirable if there is a continuous supply of the harsh fluid at low pressure available from upstream operations. 
     In one aspect, different types of high pressure generation devices may be utilized, including, without limitation, reciprocating pumps, centrifugal pumps, rotary screw compressors, and lobe pumps. 
     In one embodiment the clean fluid may comprise a gas. 
     In one aspect, the embodiments effectively provide pressure exchangers, which exchange pressure energy from high pressure clean fluid systems to relatively low pressure harsh fluid systems for use in pressurizing the harsh fluids and directing them to a wellbore as high pressure harsh fluids without the harsh fluid contacting identified portions of the pressure exchangers. 
     Some of the methods, processes and systems described above can be performed by and/or utilize a processor. The term “processor” should not be construed to limit the embodiments disclosed herein to any particular device type or system. The processor may include a computer system. The computer system may also include a computer processor (e.g., a microprocessor, microcontroller, digital signal processor, or general purpose computer) for executing any of the methods and processes described above. 
     The computer system may further include a memory such as a semiconductor memory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-Programmable RAM), a magnetic memory device (e.g., a diskette or fixed disk), an optical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card), or other memory device. 
     Some of the methods and processes described above, can be implemented as computer program logic for use with a computer processor. The computer program logic may be embodied in various forms, including a source code form or a computer executable form. Source code may include a series of computer program instructions in a variety of programming languages (e.g., an object code, an assembly language, or a high-level language such as C, or JAVA). Such computer instructions can be stored in a non-transitory computer readable medium (e.g., memory) and executed by the computer processor. The computer instructions may be distributed in any form as a removable storage medium with accompanying printed or electronic documentation (e.g., shrink wrapped software), preloaded with a computer system (e.g., on system ROM or fixed disk), or distributed from a server or electronic bulletin board over a communication system (e.g., the Internet or World Wide Web). 
     Alternatively or additionally, the processor may include discrete electronic components coupled to a printed circuit board, integrated circuitry (e.g., Application Specific Integrated Circuits (ASIC)), and/or programmable logic devices (e.g., a Field Programmable Gate Arrays (FPGA)). Any of the methods and processes described above can be implemented using such logic devices. 
     Although only a few examples have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the examples without materially departing from this subject disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. §112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.