Patent Publication Number: US-9896923-B2

Title: Synchronizing pulses in heterogeneous fracturing placement

Description:
PRIORITY 
     This application claims priority as a nonprovisional patent application of U.S. Provisional Patent Application Ser. No. 61/827,866 filed May 28, 2013 with the same title which is incorporated by reference herein. 
    
    
     BACKGROUND 
     Hydraulic fracturing improves well productivity by creating high-permeability flow passages extending through a reservoir to a wellbore. Hydraulic fracturing includes hydraulically injecting a fracturing fluid, e.g. fracturing slurry, into a wellbore that penetrates a subterranean formation. The fracturing fluid is directed against the formation strata under pressure until the strata is forced to crack and fracture. Proppant is then placed in the fracture to prevent collapse of the fracture and to improve the flow of fluid, e.g. oil, gas or water, through the reservoir to the wellbore. 
     In many fracturing operations, proppant is delivered and mixed with a clean carrier fluid to create the proppant fluid or slurry. The slurry is then pumped by a series of pumps to a common manifold or missile and delivered to a wellhead for injection downhole under pressure. The heterogeneity of the proppant in the proppant fluid can be helpful in improving the conductivity of the fractures once the proppant is injected into the fractures. However, the use of multiple pumps and the design of the overall fracturing system can effectively mix the proppant through the clean fluid and create a substantially homogeneous slurry. 
     SUMMARY 
     In general, a technique is provided for facilitating a fracturing operation by maintaining the heterogeneity of proppant fluid as it is injected into fractures extending through the reservoir. The technique comprises using a blender to deliver proppant material in a pulsating manner to create pulses or slugs of proppant. The pulses or slugs of proppant are mixed with a fluid to create a proppant slurry in which the pulses of proppant material are separated by a second fluid having a lower concentration of proppant. The proppant slurry is then split between a plurality of pumps which are operated to pump the slurry to a well. To maintain heterogeneity, the pump rates of the pumps are individually adjusted to control dispersion of the pulses of proppant downstream of the pumps and to substantially maintain the separated pulses of proppant material and thus the heterogeneity of the proppant slurry. A wide variety of other system adjustments also may be made for enhancing the ability of the overall fracturing system to maintain the separated pulses or slugs of concentrated proppant material. 
     However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and: 
         FIG. 1  is a graphical illustration of a pump schedule for pumping a slurry having pulses of proppant received from a blender, according to an embodiment of the disclosure; 
         FIG. 2  is a schematic illustration of a fracturing system deployed at a well site, according to an embodiment of the disclosure; 
         FIG. 3  is a graphical illustration of proppant slurry having pulses of proppant which moves through a plurality of pumps, according to an embodiment of the disclosure; 
         FIG. 4  is a graphical illustration of proppant concentrations measured by densitometers downstream of the pumps, according to an embodiment of the disclosure; 
         FIG. 5  is a graphical illustration of proppant pulse dispersion prior to pump rate adjustment, according to an embodiment of the disclosure; 
         FIG. 6  is a graphical illustration also showing proppant pulse dispersion, according to an embodiment of the disclosure; 
         FIG. 7  is a graphical illustration of proppant pulse dispersion when pump rates are individually controlled to maintain heterogeneity of the proppant slurry, according to an embodiment of the disclosure; 
         FIG. 8  is an illustration of a graphical user interface which may be used in cooperation with a processor-based control system to adjust fracturing system parameters, according to an embodiment of the disclosure; and 
         FIG. 9  is another illustration of a graphical user interface which may be used in cooperation with a processor-based control system to adjust pumping rates, according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     The present disclosure generally relates to a technique for facilitating a fracturing operation by maintaining the heterogeneity of proppant fluid as it is injected into fractures extending through a reservoir. A blender may be used to deliver proppant material in a pulsating manner to create pulses or slugs of proppant. In this example, the proppant is mixed with a fluid with no proppant and delivered to a missile manifold as a proppant slurry. The proppant slurry is then split between a plurality of pumps which are operated to pump the portions of the proppant slurry to a well. After passing through the plurality of pumps, the portions of the proppant slurry are recombined into a single mixture which may be delivered to a wellhead. To maintain heterogeneity, the pump rates of the pumps are individually adjusted to control dispersion of the pulses of proppant downstream of the pumps and to substantially maintain the separated pulses of proppant material and thus the heterogeneity of the proppant slurry. Other system adjustments also may be made for enhancing the ability of the overall fracturing system to maintain the separated pulses or slugs of concentrated proppant material after the portions of the proppant pulses are passed through the pumps and recombined. 
     In  FIG. 1 , a graph is provided and illustrates the pulses of proppant delivered from the blender to the pumps. In a heterogeneous proppant placement application, the blender may be designed to release proppant, e.g. sand, in a pulsating manner. The pulses of proppant are combined with less proppant fluid pulses such that relatively low proppant concentration fluid pulses  20  are followed by relatively high proppant concentration pulses  22 , as illustrated in  FIG. 1 . 
     In  FIG. 2 , an example of a fracturing system  24  is illustrated as deployed at a well site  26 . It should be noted that fracturing system  24  may comprise a wide variety of other and/or additional components depending on the circumstances including the formation and the design of a given fracturing operation. In the example illustrated, fracturing system  24  comprises a blender  28  which blends proppant and fluid, e.g. clean fluid, to create a fracturing fluid or slurry which is delivered into a manifold  30  of a missile  32 . As described above, the blender  28  may be designed to release the proppant in a pulsating manner to create pulses of proppant separated by pulses of clean fluid having a lower concentration of proppant, as illustrated graphically in  FIG. 1 . 
     Once a pulse of proppant enters the missile manifold  30 , the pulse is split between a plurality of pumps  34 . The plurality of pumps  34  is divided into left side pumps and right side pumps, and the portions of the pulses or slugs of proppant  22  travel through the plurality of pumps  34 . Due to a variety of fracturing system factors, the portions of proppant pulses  22  may exit the manifold  30  at different times which tends to mix the proppant pulses  22  with the clean fluid pulses  20 . For example, due to differences between suction and discharge line diameters of manifold  30 , differences between the way pumps  34  are rigged up, differences in pump rates, and other component differences, the portions of the same proppant pulse  22  can exit the manifold  30  at different times unless manipulated as described in greater detail below. Thus, the initial slug or pulse of concentrated proppant material is not reconstructed at a wellhead  36  and instead of a single highly concentrated pulse of proppant, the pulse becomes dispersed. Injection of this more dispersed proppant slurry into reservoir fractures results in narrower channels as compared to injection of more heterogeneous proppant slurry. 
     In contrast to the dispersion described above, the present design manipulates parameters of the fracturing system  24  to maintain heterogeneity by causing the portions of proppant pulses  22  traveling through the different pumps to meet downstream, e.g. at wellhead  36 , at the same time. In one embodiment, the pumping rates of the high-pressure equipment, e.g. pumps  34 , may be manipulated to cause the proppant pulses  22  to move through the different pumps  34  so that the portions of the proppant pulses are recombined downstream of manifold  30  at the same time. A variety of control schemes may be used to adjust the pumping rates of pumps  34  to achieve the heterogeneous proppant slurry at wellhead  36 . For example, a variety of spreadsheet programs, C language computer programs, processor-based calculations, and/or other calculations utilizing fluid mechanics equations may be used to determine the appropriate manipulation of pump rates. In an embodiment, pump rates are calculated for each pump  34  and those pump rates are manipulated to minimize the dispersion of the proppant pulses  22  as fracturing fluid exits manifold  30  and moves into wellhead  36  after traveling through the various high and low pressure lines. 
     Embodiments described herein comprise a process of adjusting pump rates on surface equipment to cause the pulses of proppant  22  to reach the wellhead  36  at the same time or approximately the same time. This reduces pulse dispersion and increases the effectiveness of the fracturing treatment. The adjustment of pumping rates may be evaluated and selected according to desired control parameters based on, for example, output from spreadsheets, executable computer programs, other processor-based calculations, and/or other types of calculations to determine the flow of particles and thus the flow of portions of the proppant pulses  22  through each of the pumps  34  before reaching the wellhead  36 . The pumping rates may be adjusted automatically by a computer-based control system and/or with input from a field operator. 
     In the embodiment illustrated in  FIG. 2 , the fracturing system  24  comprises six pumps  34  and one missile  32  mounted on a missile trailer  38 . The pumps  34  also may be truck and/or trailer mounted pumps. Depending on the application, other numbers of pumps  34 , missiles  32 , and/or blenders  28  may be employed. The slurry is discharged from missile  32  into high-pressure lines  40 , such as two high-pressure lines  40  having a left high-pressure line and a right high-pressure line, as in the example illustrated in  FIG. 2 . Flow of proppant through the high-pressure lines  40  may be monitored by a downstream densitometer or by a plurality of downstream densitometers  42  prior to delivery of the slurry to wellhead  36 . The high-pressure lines  40  connect the missile  32  with wellhead  36 . 
     Graphs of  FIGS. 3 and 4  illustrate the prevention of dispersion and the maintenance of heterogeneous proppant pulses  22  by both adjustment of the pump rates and by determining a regimen of best practices for maintaining improved heterogeneity even when pump rates are not optimized. In  FIG. 3 , for example, the proppant concentration of the proppant pulses  22  is illustrated at the entrance to missile  32  by a first graph line  44  and at the exit of missile  32  by a second graph line  46  based on data from densitometers  42 . In this example, the pump rates vary between predetermined, optimized rates (see top graphs) and less optimized rates (see bottom graphs). Additionally, the left side and right side of the fracturing system  24  has been represented by the left side graphs in the right side graphs, respectively. The right side of fracturing system  24  has various other system components optimized, as described in greater detail below. 
     As illustrated by the upper left section of the graph, the proppant pulse shape has been reconstructed at the exit of missile  32  to provide substantially recombined or reconstructed proppant pulses, as represented by graph line  46 . However, if the pump rates are not optimized, the heterogeneity of the proppant pulses may be reduced at the exit of missile  32 , as represented in the lower left portion of the graph. If other parameters of fracturing system  24  are optimized, however, the amount of dispersion of the proppant pulses  22  may be reduced even if the pump rates change from optimized rates to less than optimized rates, as represented by the transition between the upper right portion of the graph and the lower right portion of the graph. As illustrated for this example, the proppant pulses or slugs on the left side deteriorate more when the pumping rates move from good (e.g. optimized) rates to less optimized rates at least once other system parameters are not optimized. This result is confirmed by the graphs in  FIG. 4  which show that the left side slugs/proppant pulses are substantially reduced while the right side slugs/proppant pulses maintain a substantial degree of heterogeneity. Consequently, selecting proper pump rate distribution between the plurality of pumps  34  and evaluation of other system parameters may both be used as tools to facilitate reconstruction of the proppant pulses  22  after passage through pumps  34  and missile  32 . 
     If the pump rates of pumps  34  are not adjusted to prevent dispersion, substantial mixing of the proppant and clean fluid can occur, as illustrated graphically in  FIGS. 5 and 6 . In this example, best practices were not followed and the pump rates were not optimized following changes in the circumstances of the treatment operation. Initially, the pulses or slugs of proppant were heterogeneous and separated by clean fluid having a lower concentration of proppant, as represented by graph lines  48 ,  50 , and  52  on the left side of the graph in  FIG. 5 . However, by the end of such a fracturing job, the pulses travelling in different flow lines get to the well head desynchronized (see graph lines  48  and  50  on the right side of the graph in  FIG. 5 ). This scenario mixes all of the pulses  22  and results in a substantially homogenous fracturing fluid (see graph line  52 ). As the surface volume is increased (more lines, pumps, hoses, etc.) the likelihood of this problem increases and it becomes more difficult to control without any adjustment of pump rates and/or without employing best practices in the design of fracturing system  24 . 
       FIG. 6  represents a quick graphical method to quantify the dispersion generated by the lack of synchronization. On the x-axis, we plot sand/proppant concentration at some moment of time as recorded by a densitometer  42  installed in one of the discharge lines  40  of the manifold. On the y-axis we plot sand concentration recorded at the same instant at the densitometer  42  installed in the other line  40 . In this example R 2 =1.0 represents the desired synchronization of the pulses and R 2 =0.0 the worst scenario theoretically possible. For the stage presented in  FIGS. 5 and 6 , a value of R 2 =0.27 was obtained. However,  FIG. 7  represents another stage where the best practices described herein were used to adjust the pumping rates for optimizing recombination and maintenance of the proppant pulses  22  on the downstream side of missile  32 . In this latter example, the synchronization of pulses entering the wellhead  36  was established as R 2 =0.9449. Embodiments of the present technique for maintaining heterogeneous proppant slurry are designed to enable achievement of R 2 &gt;0.90 in most of the cases. The pump rate adjustment technique has been tested on several occasions with consistent results. Additionally, the best practices also may include optimizing the overall design and configuration of fracturing system  24  to further help maintain heterogeneity even if the pumping rates are not fully optimized. 
     The adjustments to pumping rates as well as the enhancement of fracturing system design/configuration may be established with the aid of, for example, a processor-based system  54  having a graphical user interface  56 . As illustrated in  FIG. 8 , graphical user interface  56  may be used to enter a variety of parameters  58  into processor-based system  54  for processing and evaluation of the structure of fracturing system  24 . The processor-based system  54  may be used to automatically control or to provide recommendations regarding adjustments and/or changes with respect to system components and operational parameters. By way of example, processor-based system  54  may utilize a C-language computer program to determine best practices for a given fracturing operation. However, a variety of other computer languages, models, algorithms, programs and other features may be employed to facilitate determination of best practices for the specific fracturing operation. Processor-based system  54  also may be programmed to automatically control the pump rates of the individual pumps  34  in response to specific inputs, such as data received from densitometers  42 . 
     The graphical user interface  56  also may be used to input and output a plurality of pumping rates  60 , as illustrated in  FIG. 9 . By way of example, the graphical user interface  56  may allow an operator to input a variety of pump rates, and a processor-based system  54  may be programmed to analyze those rates and to determine improved rates and/or adjustments to the rates on an ongoing basis during performance of the fracturing operation, thus maintaining heterogeneity of the proppant pulses  22  at wellhead  36 . The graphical user interface  56  also may be used to output a variety of pump rate information from densitometers  42  and other data related to the fracturing operation. 
     The specific procedure for facilitating a given fracturing operation may involve a variety of other and/or additional procedural steps. In some applications, the process for facilitating fracturing involves pre-determining a variety of system parameters in addition to adjusting the pumping rates to maintain synchronization of the proppant pulses/slugs before and after moving through missile  32 . For example, a procedure may involve initially determining the types of low pressure piping or hoses to be employed in fracturing system  24 , including the number, length, and/or placement of those pipes and hoses. Similarly, the procedure may comprise determining the number, length and/or placement of the high pressure piping, e.g. high-pressure lines  40 . 
     Additionally, the procedure for reducing dispersion of proppant material may comprise determining the number of pumps  34  and the type of pumps, e.g. triplex fluid end or quintiplex fluid end pumps. Similarly, the type of blender or blenders  28  may be determined along with the number and type of missiles  32 . The processor-based system  54  also may be employed to help specify a configuration for rigging up the pumps  34 , missiles  32 , and blenders  28 . In some applications, a determination is made as to whether the pumps  34  are restricted with respect to maximum pump rate or minimum pump rate. Additionally, an overall pumping rate for the fracturing job is determined. The processor-based system  54  or another suitable system may then be employed to process the various system parameters and pump parameters to determine an initial, desired pump rate for each of the pumps  34 . 
     By way of example, the processor-based system  54  may be programmed to perform an iterative process for determining the amount of time it takes a particle to leave the blender  28 , travel through the low-pressure side, through the specific pump  34 , and then flow to the wellhead  36 . This calculation is performed for each pump  34  given the length of the low-pressure piping/hoses, the length of the high-pressure lines  40 , and the given pump rate for that specific pump  34 . The pump rate for each pump  34  may then be adjusted so that the time it takes for the particle to travel to the wellhead  36  is the same for each of the pumps  34 . In other applications, the processor-based system  54  may be programmed to adjust the pump rate based on predetermined equations. For example, processor-based system  54  may have multiple sets of flow equations that can be used for each of the pumps  34  and those equations can be solved given the restrictions on minimum rate and maximum rate for each pump  34 . The solutions may be used to adjust the pump rates for each pump  34  to achieve pump rates which match or substantially match the pump rates recommended by the solutions to the equations. 
     In this example, the densitometers  42  may be used to ensure that the proppant concentrations are adequately heterogeneous. In other words, the densitometers  42  may be used to ensure the proppant concentrations moving into missile  32  substantially match the proppant concentrations at wellhead  36 . Such matching indicates that proppant pulse  22  integrity has been maintained. 
     As described herein, the fracturing system  24  may comprise a variety of pumps  34  and other system components depending on the specifics of a given fracturing operation. The design of those components and the overall configuration of the fracturing system  24  may affect the maintenance of fracturing fluid heterogeneity. In many applications, the proppant pulses and thus the heterogeneity of the fracturing fluid may be maintained or improved by adjusting the pump rates. However, additional improvements may be provided by adjusting components and arrangements of components in the overall fracturing system  24 . The adjustments to pumping rates may be calculated according to a variety of manual and automated methods. For example, a processor-based system  54  may be used for processing data according to desired programming and/or equations so as to balance the pump rates of a plurality of pumps  34  in a manner which maintains the proppant pulses at the wellhead, thus facilitating the fracturing operation. 
     Although a few embodiments of the disclosure have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.