Abstract:
A split stream oilfield pumping system is provided which utilizes recycled high Reid vapor pressure production fluids. The oilfield system is made up of two separate fluid streams: a first recycled fluid stream and a second new fluid stream. The recycled fluid stream is enclosed to reduce or eliminate vaporization of the recycled fluid, which typically will have a Reid vapor pressure &gt;14 kPa. Wellbore treatment additives are added to the new fluid stream, and the resultant treatment fluid is mixed with the recycled fluid in a common manifold to provide a final wellbore treatment fluid to be delivered to the wellhead.

Description:
FIELD OF THE INVENTION 
     This invention relates to oilfield pumping operations and more particularly to the pumping of dual fluid streams into a wellbore, one of the fluid streams comprising new or recycled fluids having a high Reid Vapor Pressure. 
     BACKGROUND OF THE INVENTION 
     In certain oilfield applications, pump assemblies are used to pump a treatment fluid from the surface down a wellbore, often at extremely high pressures. Such applications include but are not limited to hydraulic fracturing, cementing, acidizing, and pumping through coiled tubing, among other applications. In the example of hydraulic fracturing operations, a multi-pump assembly is often employed to direct a fracturing fluid into the wellbore and to a selected region(s) of the wellbore. The configuration of the well bore can vary and is subject to the type of completion most effective for the particular situation. The fracturing fluid is pumped from the wellbore into the formation to create “fractures” connecting the native reservoir to the wellbore that intersects the reservoir or the fracture network. To create such fractures, the fracturing fluid is pumped at pressures ranging from 1,000 to 15,000 psi or more. The mass flow rate of the fracturing fluid will vary depending upon what is required for the wellbore conditions. In addition, the fracturing fluid may or may not contain a propping agent, hereinafter called “proppant”. The proppant is used to keep the fracture “propped” open after the creation of the fracture as well as aiding in other fracturing mechanisms. These fractures provide communication pathways to the reservoir and allow formation deposits to flow into the wellbore and to the surface of the well. These additional pathways serve therefore to enhance the production of the well. 
     A power pump is typically employed for conveying the fracturing fluid into the wellbore during fracturing operations. A power pump is a positive displacement pump consisting of one or more cylinders each containing a piston or plunger. Such pumps are sometimes also referred to as positive displacement pumps, intermittent duty pumps, triplex pumps, or quintuplex pumps. This style of pump translates rotating motion to a linear actuation by means of a crankshaft—slider mechanism. The plunger is moved in two directions along a single axis. This motion moves the plunger in and out of a chamber in a pressure housing (typically referred to as a fluid end) in order to change the fluid volume of the chamber. Fluid enters the chamber through a one way valve as the plunger is sliding out of the chamber and the chamber volume increases. As the plunger slides into the chamber and decreases chamber volume, the fluid is displaced out of the chamber through a one way valve. This pumping action occurs in each of the fluid chambers of the fluid end and the summation of the individual compartments combines for a total output flow from the fluid end. 
     Multiple 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 caused by the presence of proppant tends to wear out the internal components of the plunger pumps and associated piping components that are used to pump it. The repair, replacement and/or maintenance expenses for the internal components are extremely high, and the overall life expectancy is low for components used to convey fracturing fluids to the well bore. 
     To combat this state of affairs, pumping systems have been developed wherein a “dirty” stream of fracturing fluid (containing the abrasive proppant) and a “clean” stream of fracturing fluid (without proppant) is mixed in a common manifold at or in close proximity to the wellhead and delivered down hole to the zone to be fractured adjacent the wellbore. Each stream is supplied to the common wellhead manifold via a separate bank of positive displacement pumps. In such “split-stream” pumping systems, the excessive wear caused by entrained proppant is completely eliminated in the bank of pumps handling the “clean” fluids. Therefore, the extra maintenance is limited to the “dirty” bank of pumps. 
     An example of a split stream oilfield pumping system is disclosed in U.S. patent application Ser. No. 11/759,776 published under No. 2007/0277982 A1 on Dec. 6, 2007 to Shampine et al. Shampine however makes no provision for the use of recycled treatment fluid. Both of Shampine&#39;s fluid streams make use of new treatment fluid not previously recovered from the wellbore. Shampine moreover makes no provision for the use of fluids with a high Reid Vapor Pressure (RVP) that may or may not have been previously recovered from a wellbore. 
     Recycled treatment fluids demonstrate particular advantages for use in connection with fracturing operations. The recycling of fluid reduces the amount of new fluid required and the amount of fluid to be disposed of when the same fluid is used for a plurality of fracture treatments. Recycled treatment fluids present issues of their own however, which can limit their economic advantages. One of these issues is the Reid Vapor Pressure. 
     Under the ASTM Method D 323, Reid Vapor Pressure (“RVP”) is the absolute vapor pressure exerted by a liquid at 100° F. (37.8° C.). The higher this value, the more volatile the sample and the more readily it will evaporate. Unlike distillation data, RVP provides a single value that reflects the combined effect of the individual vapor pressure of the different petroleum fractions in a fluid sample in accordance with their mole ratios. It is thus possible for two wholly different products to exhibit the same vapor pressure at the same temperature—provided the cumulative pressures exerted by the fractions are the same. RVP plays a role in the prediction of hydrocarbon performance. 
     Recycled fracturing fluids typically have high RVP due to entrained hydrocarbons ingested when the fluids are pumped into and then recovered from oil and gas bearing formations. However, it is also contemplated that new (i.e. non-recycled) fluids may also have high RVP values. For this reason, the term “recycled” when used in reference to fluids will hereinafter refer generally to fracturing fluids having a high RVP value, regardless of whether the fluids are recycled or new. Some jurisdictions have regulations stipulating that such high RVP fluids for safety and environmental reasons are to be handled in a closed system to reduce or eliminate vaporization of the volatile hydrocarbon fluids, or the escape of the vapors to atmosphere. In the alternative, these high RVP fluids can undergo a re-conditioning process to remove the volatile hydrocarbon fractions as well as other substances. The need either to employ large scale containment systems or recondition the fracturing fluid prior to reuse, reduces the economic incentive to use them at all. Particularly, if a recycled fluid with a higher than acceptable RVP is to be blended with proppants, the size and expense of the containment system needed to enclose the proppant, the proppant auger and the blender is a significant disadvantage. 
     Accordingly, there is need for an oil field pumping system that can economically employ un-reconditioned recycled treatment fluids having high RVP without also requiring the use of large scale containment systems which would otherwise be necessary if the recycled fluid had to be blended with proppants. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the present invention to provide an oilfield pumping system wherein recycled treatment fluids with a high RVP can be economically and safely reused in pumping operations. 
     Accordingly, in at least one embodiment, the present invention provides a system wherein at least two streams of fluid are pumped to a common mixing manifold in close proximity to or at the wellhead. The first stream is preferably comprised of a new fluid having a RVP lower than 14 kPa. This fluid can further contain wellbore treatment additives such as wellbore treatment chemicals and/or energizing fluid. This new fluid typically will meet fluid classifications suitable for either open or closed mixing systems. The base fluid for the new fluid can include but is not limited to hydrocarbons, acids, carbon dioxide, nitrogen or water or mixtures thereof. 
     The second stream comprises a recycled fluid that may or may not have a RVP lower than 14 kPa. This recycled fluid is typically a hydrocarbon fluid that may contain treatment chemicals and/or energizing fluid. In cases where the RVP of the recycled fluid is high (i.e. greater than 14 kPa), the recycled fluid is stored and delivered to the wellhead by a closed recycled fluid system. 
     The ratio of new fluid to recycled fluid in the mixture delivered to the wellhead will vary depending on the application. 
     Each fluid is stored in a separate containment device and is delivered to the wellhead by a separate pumping system, the choice of pumps for the systems depending on the particular application at hand. Pump types contemplated for use in connection with the present invention include but are not limited to progressive cavity, centrifugal or positive displacement pumps. Various pump arrangements are contemplated to deliver each stream to the common mixing manifold. These include but are not limited to multiple pumps connected in parallel or in series. 
     The benefits derived from the present invention will be readily apparent to those skilled in the art. Particularly, in at least one embodiment, the present invention can enable safe and economic recycling of high RVP fluids. The economic savings result from the fact that the recycled fluid does not require costly conditioning and less new fluids are required for the pumping operation. The present invention enables a continuous mixing of proppant and additives by integrating open and closed mixing systems. It is therefore unhindered by the constraints and added expense that would come with a closed system for mixing high RVP fluids and proppants. 
     Furthermore, in at least one embodiment, the present invention can maintain the high level of safety required in the petroleum production industry as the high RVP recycled fluid remains in a closed system, however the physical size of the closed system is reduced as only the recycled fluid streams need to be enclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a process diagram demonstrating an oilfield pumping system employing high RVP recycled production fluids. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be described for the purposes of illustration only in connection with certain embodiments; however, it is to be understood that other objects and advantages of the present invention will be made apparent by the following description of the drawings according to the present invention. While a preferred embodiment is disclosed, this is not intended to be limiting. Rather, the general principles set forth herein are considered to be merely illustrative of the scope of the present invention and it is to be further understood that numerous changes may be made without straying from the scope of the present invention. 
       FIG. 1  shows one embodiment of the present split stream oilfield pumping system  100 . In this example, the oilfield pumping system is preferably comprised of a recycled fluid subsystem  200 , a new fluid subsystem  300 , a wellhead  102 , and a common mixing manifold  104 . The present oilfield pumping system  100  could be designed to service a single wellhead or multiple wellheads located in the same general vicinity. Furthermore, it is contemplated that the oilfield pumping system  100  could be designed such that it is easily mobile and can be transported from wellhead to wellhead via tractor-trailer, truck, or in any other manner known to the skilled person in the art. It is also contemplated that the pumping system  100  could be integrated into one or more moveable trailers, tractor-trailer combinations, or truck mounts capable of being installed on a truck(s) or other vehicle. 
     Recycled treatment fluid is preferably recovered from the wellhead  102 , and stored in recycled fluid storage tanks  202 . However, the recycled fluid may be obtained from any number of sources and may be transported to the well site by any means, including by tanker truck. Entrained proppants and residual dirt and debris are removed from the recycled fluid by filtration, centrifuging, settling or any other method known in the art. Recycled fluid storage tanks  202  are supplied with recycled fluid from wellhead  102  by recovery pump  204  or other means. The pump can be any type of pump suited to the application. Storage tanks  202  are enclosed to provide a contained storage environment for the recycled fluid, which can possibly have a Reid Vapor Pressure (“RVP”) of higher than 14 kPa and in such instances is accordingly volatile. 
     Recycled fluid is pumped from storage tanks  202  by way of recycled fluid supply pump  206 , which typically is a centrifugal pump but can be any pump properly chosen by a person skilled in the art. Supply pump  206  provides recycled fluid to high pressure pumps  208 , which are typically positive displacement plunger pumps arranged in parallel between supply pump  206  and common mixing manifold  104 . However, other pump types and arrangements are contemplated. Because recycled fluid system  200  provides the “clean” stream to manifold  104 , it is the more compact of the two fluid subsystems which reduces the size of the containment field for high RVP fluids at substantial cost savings. 
     New fluid is stored in fluid storage tanks  302  or any other suitable receptacles know to those in the art. New fluid may be manufactured on site or transported to the worksite via typical tanker trucks. Storage tanks  302  can be enclosed or remain open to atmosphere, depending on the type of new fluid employed in the application, which would be chosen by the person skilled in the art. It is contemplated that new fluid could possibly be comprised of (but not limited to) hydrocarbons, acids, carbon dioxide, nitrogen and water. 
     New fluid is pumped from storage tanks  302  by way of new fluid supply pump  306 , which typically is a centrifugal pump but can be any pump properly chosen by a person skilled in the art. Supply pump  306  provides new fluid to blending subsystem  400 . 
     Blending subsystem  400  is preferably comprised of a delivery system  402 , such as an auger, to provide proppant from a bulk source to blender mixing chamber  404 . New fluid from supply pump  306  is blended with proppant from delivery system  402  at mixing chamber  404 , which produces a fracturing fluid with entrained proppant. It is contemplated that other wellbore additives chosen by the skilled person in the art could be added to the fracturing fluid at this time. Blending system  400  could further comprise a chemical additive injector to facilitate the addition of any additives. Assuming the new fluid has a RVP of less than 14 kPa, blending system  400  does not have to be closed. 
     The density of the fracturing fluid can be determined using a means  406 , such as a fluid density meter or a mass flow meter, located downstream from mixing chamber  404 , to determine the percentage of proppant entrained in the resultant fracturing fluid. Instrumentation measuring other characteristics of the resultant fracturing fluid is also contemplated at this point in the process. A bypass  408  located between the supply pump  306  and downstream of mixing chamber  404  allows new fluid to be directly pumped from new fluid subsystem  300  to a point downstream of the mixing chamber in situations where proppant is not required, such as when pumping a fluid pad to initiate a fracture in the treatment zone, or there is a need to isolate the mixing chamber. 
     Pumping system  100  preferably comprises a blending system discharge pump  410  to pump resultant fracturing fluid to high pressure pumps  308 , which are typically positive displacement plunger pumps arranged in parallel between discharge pump  410  and common mixing manifold  104 . However, other pump types and arrangements are contemplated. 
     As will be understood by a person skilled in the art, all pumps and piping components that are exposed to abrasive proppant entrained in the fracturing fluid are subject to accelerated wear. Accordingly, maintenance efforts and costs are reduced for the recycled fluid-side pumps, namely the centrifugal pumps and the positive displacement pumps which are not exposed. Furthermore, new fluid supply pump  306  also does not pump proppant-entrained fluid. 
     The resultant fracturing fluid from high pressure pumps  308  is mixed with the recycled fluid from high pressure pumps  208  in common mixing manifold  104 , typically located near the wellhead  102 . Blended fluid from the two streams is then delivered to wellhead  102  where it is directed down hole to the wellbore for use in fracturing operations. It is contemplated that common mixing manifold  104  and wellhead  102  can be integral with one another. Alternatively, the two fluid streams from pumps  208  and  308  can be mixed on the low pressure side downstream of mixing manifold  104 . The combined fluid streams can then be pumped into a single bank of plunger pumps. 
     It will be understood that the preferred embodiments mentioned here are merely illustrative of the present invention. Numerous variations in design and use of the present invention may be contemplated in view of the following claims without straying from the intended scope and field of the invention herein disclosed.