METHODS AND SYSTEMS FOR PRESSURIZING HARSH FLUIDS

Abstract

Methods and systems relate to a pressure exchanger which exchanges pressure energy from a high pressure clean fluid system to a relatively low pressure harsh fluid system for use in pressurizing the harsh fluid and directing the fluid to a wellbore as high pressure harsh fluid without the harsh fluid contacting identified portions of the pressure exchanger.

Description

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. 12a-12f 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 50a, 60a and second ends 50b, 60b 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 50b of the tubular 50 via pipe 72 and valve SV-5 and to a second end 60b 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 50a of the tubular 50 via pipe 75 and valve SV-1 and to the first end 60a of tubular 60 via pipe 76 and valve SV-2. Storage tank 20 is further coupled to the first end 60a of tubular 60 via pipe 77 and valve SV-4 and to the first end 50a 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 50b of tubular 50 via pipe 82 and valve SV-7 and to the second end 60b 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 60b 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 50b 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 95a 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 50a 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 50a 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 95a directly in front of it and the harsh fluid 96 in front of the clean fluid buffer toward the second end 50b 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, 95a 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 60b, 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 50b, 60a 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 50b of tubular 50, and clean fluid 95 is ejected from the first end 50a 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 50a of tubular 50 and piston 65 is pushed back to nearly the second end 60b 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 95a ejected from the second end 50b of tubular 50 may be sufficient to flush the harsh fluid from the segment of pipe 85 from the second end 50b 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 SV2, 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 60a of tubular 60 in order to cause harsh fluid to be ejected from the second end 60b 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 50a of tubular 50 via valve SV-3 back to storage tank 20. After a predetermined amount of clean fluid 95a fills the second end 50b 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 95a 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 50a of tubular 50 and piston 65 pushed back to the second end 60b of tubular 60, clean fluid 95a 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 50a of tubular 50 and piston 65 near the second end 60b 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>>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

$\begin{array}{cc}\stackrel{\_}{v}=\frac{4q}{\pi d^2}& \left(1\right)\end{array}$

with the corresponding Reynolds number being

$\begin{array}{cc}Re=\frac{d\stackrel{\_}{v}\rho }{\mu }.& \left(2\right)\end{array}$

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's form gives:

$\begin{array}{cc}\frac{1}{f}=4.0\log Re\sqrt{f}-0.4.& \left(3\right)\end{array}$

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

$\begin{array}{cc}D=10.1\frac{d\stackrel{\_}{v}}{2}\sqrt{\frac{f}{2}}.& \left(4\right)\end{array}$

The dispersion length l_D may be obtained from the dispersion coefficient D according to

$\begin{array}{cc}{l}_D=\sqrt{D\frac{l}{\stackrel{\_}{v}}}& \left(5\right)\end{array}$

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_bπd/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, lD in m, q in m3s−1.
μ	d	q		lD
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 50a, 60a and second ends 50b, 60b 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 50a of the tubular 50 via pipe 75 and valve SV-1 and to the first end 60a of tubular 60 via pipe 76 and valve SV-2. Storage tank 20 is further coupled to the first end 60a of tubular 60 via pipe 77 and valve SV-4 and to the first end 50a 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 50b of tubular 50 via pipe 82 and valve SV-7 and to the second end 60b 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 50b and 60b 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 95a. 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 60b of tubular 60 and pushes piston 165 back toward end 60a of the tubular. When piston 165 reaches end 60a of the tubular, the harsh fluid 96 fills the tubular 60 except for a buffer 95a 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 95a) 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 50a of tubular 50) and piston 165 is moving to the right (toward end 60b 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 50a 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 95a; 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 50a, 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 95a toward tubular end 50b, 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 60b, thereby pushing the clean fluid buffer 95; piston 165, and clean fluid 95 in the tubular 60 toward tubular end 60a.

The system configuration a short time later is shown in FIG. 10 with piston 155 pushing the buffer 95a and the harsh fluid 96 toward tubular end 50b, and piston 165 pushing clean fluid 95 toward tubular end 60a. 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 12a-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. 12a-12f. The start of the cycle is shown in FIG. 12a, 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 175a in tubular 50 starts moving to the right such that harsh fluid 96 is discharged from tubular end 50b 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 175a, thereby building a layer of clean fluid 95a 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 175a provides high pressure for the discharge of harsh fluid 96, a layer (or buffer) of clean fluid 95a is provided ahead of the piston in the direction of motion of the piston. As seen in FIGS. 12b, 12c and 12d, the size of the clean fluid buffer 95a increases as the piston 175a moves toward tubular end 50b. Thus, before piston 175a reaches tubular end 50b, enough clean fluid has moved past the piston 175a to ensure that when piston 175a does reach tubular end 50b (in FIG. 12e), 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 60a at low pressure back to the storage tank. The process continues as shown in FIGS. 12b, 12c, 12d and 12e. It is noted that in FIGS. 12a-12e, the harsh fluid 96 is in contact with piston 175b, albeit not during the high-pressure discharge cycle, and only under low pressure conditions. However, because piston 175b 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 175a and 175b 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. 12e), appropriate valves are opened and closed and the cycle is continued by switching the tubular operation, as shown in FIG. 12f. Harsh fluid 96 is now pushed out of tubular 60 as a layer of clean fluid 96a pushes past the valve and protects the piston assembly 175b. Simultaneously, harsh fluid is injected into end 50b 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.

Claims

1. A method of pumping an oilfield fluid from a well surface to a wellbore, comprising: providing a low pressure source of harsh fluid;operating at least one high pressure pump to pump a clean fluid; andusing a piston assembly which is in contact with clean fluid in a direction of movement and which pushes the clean fluid using the pressure from the at least one high pressure pump in order to provide high pressure to the harsh fluid, thereby pushing the harsh fluid under high pressure toward the wellbore.

2. The method of claim 1, wherein a predetermined amount of clean fluid is used to buffer the piston assembly from the harsh fluid.

3. The method of claim 2, wherein the predetermined amount is a function of a dispersion length of the harsh fluid in the clean fluid when the piston assembly causes movement of the clean fluid and harsh fluid under high pressure.

4. The method of claim 3, wherein the function of a dispersion length is approximately three times the dispersion length.

5. The method of claim 1, wherein using a piston assembly comprises utilizing a system having at least two tubulars each containing a respective piston assembly, the piston assemblies working in opposite phases to continuously pump the harsh fluid into the wellbore, such that in a first half cycle, a first of the respective piston assemblies is causing harsh fluid to be ejected from a first of the two tubulars under high pressure while a second of the tubulars is being filled with harsh fluid under low pressure, and in a second half cycle, a second of the respective piston assemblies is causing harsh fluid to be ejected from a second of the two tubulars under high pressure while the first of the tubulars is being filled with harsh fluid under low pressure.

6. The method of claim 5, wherein the respective piston assemblies are surrounded by clean fluid in both the first half cycle and the second half cycle.

7. The method of claim 6, wherein a predetermined amount of clean fluid is used to buffer each of the respective piston assemblies from the harsh fluid in both the first half cycle and the second half cycle.

8. The method of claim 5, wherein in the first half cycle, an increasing amount of clean fluid is used to buffer the first of the respective piston assemblies as the first of the respective piston assemblies moves under high pressure from a first end to a second end of the first of the two tubulars.

9. The method of claim 8, wherein in the first half cycle, the harsh fluid is in low pressure contact with the second respective piston on a side opposite the direction of movement of the piston assembly.

10. The method of claim 1, wherein the clean fluid is water.

11. The method of claim 1, wherein the piston assembly comprises a check valve.

12. The method of claim 1, further comprising: impeding movement of the piston assembly using a stop element.

13. The method of claim 1, wherein the harsh fluid is a formation fracturing fluid.

14. A system for pumping an oilfield fluid from a well surface to a wellbore comprising: a low pressure supply of harsh fluid;a high pressure pump in communication with a clean fluid; andat least two tubulars each comprising a piston assembly wherein each piston assembly is in contact with the clean fluid in the direction of movement of that piston assembly and pushes the clean fluid using the pressure from the high pressure pump in order to provide pressure to and push the harsh fluid, thereby pushing the harsh fluid under pressure toward the wellbore.

15. The system of claim 14, further comprising: a plurality of valves, wherein the high pressure pump is in fluid communication with the at least two tubulars at respective first ends of the tubulars via respective first and second valves of the plurality of valves, and the harsh fluid at low pressure is in fluid communication with the tubulars at respective second ends of the tubulars via respective third and fourth valves of the plurality of valves.

16. The system of claim 15, further comprising: a first storage tank for the clean fluid, wherein the respective first ends of the tubulars are in fluid communication with the first storage tank via fifth and sixth valves of the plurality of valves, andthe plurality of valves comprise seventh and eight valves respectively coupled between the respective second ends of the tubulars and the wellbore.

17. The system of claim 16, further comprising a low pressure pump for the harsh fluid and a second storage tank for the harsh fluid.

18. The system of claim 14, wherein each piston assembly is in contact with a predetermined amount of the clean fluid in the direction of movement of that piston assembly.

19. The system of claim 18, wherein the predetermined amount is a function of a dispersion length of the harsh fluid in the clean fluid when the piston assembly causes movement of the clean fluid and harsh fluid under high pressure.

20. The system of claim 19, wherein the function of a dispersion length is approximately three times the dispersion length.

21. The system of claim 14, wherein the respective piston assemblies comprise check valves.

22. The system of claim 21, wherein the check valves permit flow of fluid past the check valves at a defined cracking pressure.

23. The system of claim 22, further comprising: a stop element located in each of the at least two tubulars, the stop elements located toward the respective second ends of the at least two tubulars, the stop elements arranged to stop movement of the piston assemblies beyond the stop elements toward the respective second ends.

24. The system of claim 21, wherein the piston assemblies work in opposite phases to continuously pump the harsh fluid into the wellbore, such that in a first half cycle, a first of the respective piston assemblies is causing harsh fluid to be ejected from a first of the two tubulars under high pressure while a second of the tubulars is being filled with harsh fluid under low pressure, and in a second half cycle, a second of the respective piston assemblies is causing harsh fluid to be ejected from a second of the two tubulars under high pressure while the first of the tubulars is being filled with harsh fluid under low pressure.

25. The system of claim 24, wherein in the first half cycle, clean fluid is continuously pushed past the check valve such that an increasing amount of clean fluid is used to buffer the first of the respective piston assemblies as the first of the respective piston assemblies moves under high pressure from a first end to a second end of the first of the two tubulars.

26. The system of claim 19, wherein in the first half cycle, the harsh fluid is in low pressure contact with the second respective piston on a side opposite the direction of movement of the piston assembly.