Active seismic monitoring of fracturing operations

CROSS-REFERENCE TO RELATED APPLICATIONS

0001This application is a continuation-in-part of co-pending of U.S. patent application Ser. No. 13/112780 filed May 20, 2011, which is a continuation of U.S. patent application Ser. No. 12/256285 filed Oct. 22, 2008; both of which are incorporated herein by reference in their entireties.

BACKGROUND

00021. Field of the Invention

0003Implementations of various technologies described herein generally relate to methods and systems for hydraulic fracturing operations.

00042. Description of the Related Art

0005The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.

0006In the recovery of hydrocarbons from subterranean formations it is common practice, particularly in formations of low permeability, to fracture the hydrocarbon-bearing formation to provide flow channels. These flow channels facilitate movement of the hydrocarbons to the well bore so that the hydrocarbons may be pumped from the well.

0007In such fracturing operations, a fracturing fluid is hydraulically injected into a well bore penetrating the subterranean formation and is forced against the formation strata by pressure. The formation strata or rock is forced to crack and fracture, and a proppant is placed in the fracture by movement of a viscous-fluid containing proppant into the crack in the rock. The resulting fracture, with proppant in place, provides improved flow of the recoverable fluid, i.e., oil, gas or water, into the well bore.

0008Fracturing fluids customarily comprise a thickened or gelled aqueous solution which has suspended therein “proppant” particles that are substantially insoluble in the fluids of the formation. Proppant particles carried by the fracturing fluid remain in the fracture created, thus propping open the fracture when the fracturing pressure is released and the well is put into production. Suitable proppant materials include sand, walnut shells, sintered bauxite or similar materials. The “propped” fracture provides a larger flow channel to the well bore through which an increased quantity of hydrocarbons can flow, thereby increasing the production rate of a well.

0009A problem common to many hydraulic fracturing operations is the loss of fracturing fluid into the porous matrix of the formation. Fracturing fluid loss is a major problem. Hundreds of thousands (or even millions) of gallons of fracturing fluid must be pumped down the well bore to fracture such wells, and pumping such large quantities of fluid is very costly. The lost fluid also causes problems with the fracturing operation. For example, the undesirable loss of fluid into the formation limits the fracture size and geometry which can be created during the hydraulic fracturing pressure pumping operation. Thus, the total volume of the fracture, or crack, is limited by the lost fluid volume that is lost into the rock, because such lost fluid is unavailable to apply volume and pressure to the rock face.

SUMMARY

0010Described herein are implementations of various technologies for a method for managing a fracturing operation. In one implementation, the method may include positioning a seismic source and at least one seismic receiver near a hydrocarbon reservoir; pumping a fracturing fluid into a well bore of the hydrocarbon reservoir such that the fracturing fluid may contain an additive that enhances acoustic impedance between the fracturing fluid and subsurface formations in which the hydrocarbon reservoir is located and such that the additive produces a foam; performing a seismic survey with the seismic source and the at least one seismic receiver during the fracturing operation; and identifying locations of the fracturing fluid within subsurface formations in which the hydrocarbon reservoir is located.

0011In another implementation, the method may include positioning a seismic source and at least one seismic receiver below a surface of the Earth near a hydrocarbon reservoir;

0012pumping a fracturing fluid into a well bore of the hydrocarbon reservoir, such that the fracturing fluid may contain an additive that enhances acoustic impedance between the fracturing fluid and subsurface formations in which the hydrocarbon reservoir is located;

0013performing a seismic survey with the seismic source and the at least one seismic receiver during the fracturing operation; and identifying locations of the fracturing fluid within the subsurface formations in which the hydrocarbon reservoir is located. The seismic source, the at least one seismic receiver, or both may be disposed inside the well bore. Alternatively, the seismic source, the at least one seismic receiver, or both may be disposed inside a borehole proximate the hydrocarbon reservoir.

0014In yet another implementation, the method may include positioning at least one seismic source and at least one seismic receiver near a hydrocarbon reservoir; performing a baseline seismic survey; performing the fracturing operation by pumping a fracturing fluid into a well bore of the hydrocarbon reservoir; performing a closing seismic survey at the end of the fracturing operation; analyzing one or more differences between an image generated by the baseline seismic survey and an image generated by the closing seismic survey; and identifying one or more locations of the fracturing fluid within a subsurface formation in which the hydrocarbon reservoir is located.

0015In yet another implementation, the method may include positioning at least one seismic source and at least one seismic receiver near a hydrocarbon reservoir; performing the fracturing operation by pumping a fracturing fluid into a well bore of the hydrocarbon reservoir; performing a first seismic survey during the fracturing operation; performing a second seismic survey at the end of the fracturing operation; analyzing one or more differences between an image generated by the first seismic survey and an image generated by the second seismic survey; and identifying one or more locations of the fracturing fluid within a subsurface formation in which the hydrocarbon reservoir is located.

0016The claimed subject matter is not limited to implementations that solve any or all of the noted disadvantages. Further, the summary section is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description section. The summary section is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

0017Implementations of various techniques will hereafter be described with reference to the accompanying drawings. It should be understood, however, that the accompanying drawings illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein.

0018<figref idref="DRAWINGS">FIG. 1</figref> illustrates a system for monitoring a hydraulic fracturing operation, in accordance with one or more implementations of various techniques described herein.

0019<figref idref="DRAWINGS">FIG. 2</figref> illustrates a flowchart of a method for managing hydraulic fracturing operations, according to implementations described herein.

0020<figref idref="DRAWINGS">FIG. 3</figref> illustrates a computer network, into which implementations of various technologies described herein may be implemented.

DETAILED DESCRIPTION

0021The discussion below is directed to certain specific implementations. It is to be understood that the discussion below is only for the purpose of enabling a person with ordinary skill in the art to make and use any subject matter defined now or later by the patent “claims” found in any issued patent herein.

0022The following paragraph provides a brief summary of various techniques described herein. In general, various techniques described herein are directed to determining the location of fractures and fracturing fluid in formations surrounding a hydrocarbon reservoir. Rather than passively monitoring for fractures created by the fracturing operation, active seismic monitoring of fracturing operation may be used to provide stronger signaling for fracture detection. Further, pumping fracturing fluid with a high acoustic impedance contrast to the surrounding subsurface formations may increase the visibility of the fracturing fluid on the seismic survey. In one implementation, the fracturing fluid may contain an additive that provides the high acoustic impedance contrast. One or more implementations of various techniques for determining the location of fractures and fracturing fluid in formations surrounding a hydrocarbon reservoir will now be described in more detail with reference to <figref idref="DRAWINGS">FIGS. 1-3</figref> in the following paragraphs.

0023<figref idref="DRAWINGS">FIG. 1</figref> illustrates a system <b>100</b> for monitoring a hydraulic fracturing operation in accordance with one or more implementations of various techniques described herein. The hydraulic fracturing operation may be also referred to herein as the fracturing operation. In the system <b>100</b>, the fracturing operation may be conducted in concert with an active seismic survey in order to improve the effectiveness of the fracturing operation. The system <b>100</b> may include a pumping mechanism <b>102</b>, a well bore <b>104</b>, a hydrocarbon reservoir <b>108</b>, a seismic array <b>112</b> and a seismic source <b>114</b>.

0024In performing the fracturing operation, the pumping mechanism <b>102</b> may pump a fracturing fluid into the well bore <b>104</b> of the hydrocarbon reservoir <b>108</b>. The hydrocarbon reservoir <b>108</b> may be disposed within a subsurface formation <b>110</b>, such as a sandstone, carbonate, shale, or chalk formation. The pressure resulting from the pumping of fracturing fluid may create fractures <b>106</b> in the formation <b>110</b>. The fractures <b>106</b> may improve the flow of hydrocarbons to the well bore <b>104</b>.

0025In a typical fracturing operation, the well bore <b>104</b> may be perforated such that the fracturing fluid enters the hydrocarbon reservoir <b>108</b> at a specified location. The location of the perforations may influence where the fractures <b>106</b> are induced in the formation.

0026The seismic array <b>112</b> may be a standard seismic receiver array used in seismic surveying, and may include geophones, receivers, or other seismic sensing equipment. The seismic array <b>112</b> may be positioned on the surface or in a borehole. The seismic source <b>114</b> may be a standard seismic source used in seismic surveying, such as a vibroseis, or dynamite. Like the seismic array <b>112</b>, the seismic source <b>114</b> may be located on the surface or in a borehole. The seismic source <b>114</b> and seismic array <b>112</b> may be used to perform a seismic survey during the fracturing operation.

0027In one implementation, the seismic survey may be used to improve the effectiveness of the fracturing operation. For example, by performing a seismic survey during the fracturing operation, it may be possible to identify where in the formation <b>110</b> the fractures <b>106</b> are induced.

0028Sometimes, the fractures <b>106</b> that are induced by the fracturing operation may be disposed such that the fractures <b>106</b> do not improve the flow of hydrocarbons to the well bore <b>104</b>. In such a scenario, the perforations in the well bore <b>104</b> may be plugged. The well bore <b>104</b> may then be re-perforated to change the location within the hydrocarbon reservoir <b>108</b> where the fracturing fluid enters. After re-perforating the well bore <b>104</b>, the fracturing operation may resume.

0029<figref idref="DRAWINGS">FIG. 2</figref> illustrates a flow chart of a method <b>200</b> for managing a fracturing operation according to implementations described herein. It should be understood that while method <b>200</b> indicates a particular order of execution of the operations, in some implementations, certain portions of the operations might be executed in a different order.

0030At step <b>210</b>, the seismic array <b>112</b> and the seismic source <b>114</b> may be positioned near the hydrocarbon reservoir <b>108</b>. In one implementation, the seismic array <b>112</b> and/or the seismic source <b>114</b> may be above the Earth's surface, i.e., above the hydrocarbon reservoir <b>108</b>. In another implementation, the seismic array <b>112</b> and/or the seismic source <b>114</b> may be positioned below the Earth's surface, e.g., inside a well bore. Surface or subsurface referenced systems may be positioned to record reflections and refractions from the fracturing fluid and the fractures that contain the fracturing fluid. This positioning can be determined through well known techniques involving seismic modeling methods, such as ray tracing or full wavefield propagation. The seismic source <b>114</b> and receivers <b>112</b> may include devices for generating and recording pressure waves, shear waves or any combinations thereof and may encompass cabled, wireless, autonomous systems or combinations thereof

0031A typical fracturing operation passively listens for acoustic signals that result from the creation of the fractures <b>106</b> induced by the fracturing operation. Because these acoustic signals may be weak, a vertical seismic profile (VSP) may be created. The VSP may be used to improve the reliability of the seismic data collected.

0032To create a VSP, a secondary well bore may be dug as an observational well. Seismic receivers may then be positioned in the observational well, or may be positioned in the observational well in addition to the surface receivers in the seismic array <b>112</b>. The acoustic signals recorded by the receivers in the observational well may then be correlated with the signals recorded at the surface.

0033Using method <b>200</b>, it may not be necessary to dig an observational well because the seismic source <b>114</b> is used to actively survey for fractures during the fracturing operation. The seismic source <b>114</b> may provide a stronger signal than the signals generated in creating the fractures, such as acoustic signals generated by the breaking of rocks. The seismic source <b>114</b> and/or the seismic array <b>112</b> may be positioned in the observational well.

0034At step <b>220</b>, the pumping mechanism <b>102</b> may pump fracturing fluid into the well bore of the hydrocarbon reservoir <b>108</b>. As stated previously, pumping the fracturing fluid into the well bore <b>104</b> may induce fracturing of the formation <b>110</b> of the hydrocarbon reservoir <b>108</b>.

0035At step <b>230</b>, the seismic source <b>114</b> and the seismic array <b>112</b> may be used to perform the seismic survey. The pumping mechanism <b>102</b> may produce acoustic signals that introduce noise into the seismic survey. As such, the fracturing operation may be coordinated with the seismic survey such that the pumping mechanism <b>102</b> is halted while the seismic survey is being performed.

0036In one implementation, the acoustic signals produced by the pumping mechanism <b>102</b> may be used as an additional seismic source for the seismic survey. In another implementation, the pumping mechanism <b>102</b> may be used as the seismic source <b>114</b>.

0037In another implementation, a baseline seismic survey may be performed before the fracturing operation. The baseline seismic survey may then be compared to the seismic survey performed during the fracturing operation to determine changes in amplitude, structural deformation and changes in rock properties, such as formation pressure, and to relate these changes to fracture fluid movement and fracture locations.

0038In a further implementation, a closing seismic survey may be performed after the fracturing operation has concluded. The closing survey may then be compared to the seismic survey performed during the fracturing operation, and/or to the baseline seismic survey, to determine changes in amplitude, structural deformation and changes in rock properties, such as formation pressure, and to relate these changes to fracture fluid movement and fracture locations.

0039In another implementation, in addition to the seismic source <b>114</b>, and as illustrated in <figref idref="DRAWINGS">FIG. 1</figref>, a second seismic source <b>116</b> may be used to perform the seismic survey, the baseline seismic survey, the closing seismic survey or combinations thereof. The second seismic source <b>116</b> may be a standard seismic source used in seismic surveying, such as a vibroseis or dynamite, and it may be located on the surface or in a borehole. Similar to seismic source <b>114</b>, the second seismic source <b>116</b> may be positioned near the hydrocarbon reservoir <b>108</b>. For example, the second seismic source <b>116</b> may be above the Earth's surface, i.e., above the hydrocarbon reservoir <b>108</b>, or it may be positioned below the Earth's surface, e.g., inside a well bore.

0040The energy that is sensed by the seismic array <b>112</b> due to a given firing, or “shot,” of a seismic source typically is recorded from a time interval that spans from the time at which the shot occurred and ends slightly before the time at which the next shot occurs. Techniques, such as the one disclosed in U.S. Pat. No. 5,924,049, entitled “METHODS FOR ACQUIRING AND PROCESSING SEISMIC DATA,” which issued on Jul. 13, 1999 and is hereby incorporated by reference in its entirety, allow multiple seismic sources to fire simultaneously and essentially allow more than one record to be recorded at the same time.

0041As further described in U.S. Pat. No. 7,916,576, entitled “OPTIMIZING A SEISMIC SURVEY FOR SOURCE SEPARATION,” which issued on Mar. 29, 2011 and is hereby incorporated by reference in its entirety, the seismic source <b>114</b> and the second seismic source <b>116</b> may be fired in a sequence such that the two seismic sources may be fired simultaneously or near simultaneously in a short interval of time so that a composite energy signal that is sensed by the seismic array <b>112</b> contains a significant amount of energy from more than one seismic source. In other words, the seismic source <b>114</b> and the second seismic source <b>116</b> interfere with each other such that the composite energy signal is not easily separable into signals that are attributed to the specific seismic sources.

0042However, source separation techniques may be applied to process the acquired seismic data to form datasets that are each associated with one of the seismic sources so that each dataset ideally indicates the component of the composite seismic energy signal that is attributable to an associated seismic source. In performing the source separation, though, some of the sensed energy (called the “residual energy”) may not ultimately be attributed to any of the seismic sources, and some of the sensed energy (called the “leakage energy”) may be attributed to the wrong seismic source by the separation process.

0043The seismic survey has a number of characterizing parameters which affect the quality of the source separation. More specifically, these parameters may include the timing sequence that governs the seismic source firings, the source geometry (crossline and inline separations of air guns, for example), and the receiver geometry (the type of spread and the crossline and inline separations of the seismic sensors <b>58</b>, as examples). Therefore, survey parameters, such as firing times, source geometry, receiver geometry (i.e., the geometry of the seismic sensors), etc., may be randomly or pseudo randomly varied within predefined ranges for purposes of determining the optimal survey parameters for source separation.

0044In one implementation, optimization of the seismic survey may occur when the seismic survey is characterized by a set of parameters that minimizes the residual and leakage energies or at least produces residual and leakage energies that are below selected thresholds.

0045In another implementation, survey parameters may be optimized for source energy separation based on a linear system that characterizes the seismic survey. For example, a technique to optimize survey parameters may include modeling seismic datasets as being determinable from a linear system that is characterized by a survey geology and survey parameters. Each dataset is associated with the sensed energy uniquely attributable to a different seismic source. The linear system may be inverted for the seismic datasets. The survey parameters are optimized for source separation by maximizing the accuracy of the inversion for the recovery of the datasets.

0046As a more specific example, source dithering (i.e., using a source firing timing sequence in which the source firing times are slightly offset for each other) may be used for purposes of enabling the separation of interfering seismic sources as described in U.S. patent application No. 11/964,402, entitled, “SEPARATING SEISMIC SIGNALS PRODUCED BY INTERFERING SEISMIC SOURCES,” which was filed on Dec. 26, 2007, and is hereby incorporated by reference in its entirety. Linear operator transforms may be used in a model of the survey system for purposes of decomposing the sensed composite energy signal into signals that are each uniquely associated with a particular seismic source. The invertability of a matrix of this system is maximized for purposes of determining an optimum survey design from the standpoint of source separation.

0047The source separation techniques described herein may be applied to the seismic survey, the baseline seismic survey, the closing seismic survey or combinations thereof. In other implementations, the source separation techniques described herein may be applied to more than two seismic sources fired simultaneously or near simultaneously in a short interval of time.

0048At step <b>240</b>, an image of the hydrocarbon reservoir <b>108</b> may be generated. The receivers of the seismic array <b>112</b> may record acoustic signals from the seismic source <b>114</b> during the seismic survey. Using the recorded acoustic signals, a computing system (not shown) may generate an image of the hydrocarbon reservoir <b>108</b>. In the implementation where the baseline seismic survey is performed, an image may also be generated from the acoustic signals recorded during the baseline seismic survey. In the implementation where the closing seismic survey is performed, an image may also be generated from the acoustic signals recorded during the closing seismic survey.

0049At step <b>250</b>, the fractures <b>106</b> and/or the fracturing fluid may be identified on the generated image. In the implementation that includes the baseline seismic survey, the fractures <b>106</b> and the fracturing fluid may be identified by analyzing differences between the image generated by the baseline seismic survey and the image generated by the seismic survey performed during the fracturing operation. In the implementation that includes the closing seismic survey, the fractures <b>106</b> and the fracturing fluid may be identified by analyzing differences between the image generated by the closing seismic survey and the image generated by the seismic survey performed during the fracturing operation. In another implementation, the fractures <b>106</b> and the fracturing fluid may be identified by analyzing differences between the image generated by the closing seismic survey and the image generated by the baseline seismic survey.

0050In some implementations, the fracturing operation may be modified. The modification to the fracturing operation may be based on the identified fracturing fluid or the differences between the baseline image and the image obtained during the fracturing operation. For example, if the identified fracturing fluid is disposed within the formation <b>110</b> such that fractures are not being produced, the fracturing operation may be modified to direct the fracturing fluid towards another location in the formation <b>110</b>.

0051In one implementation, the fracturing fluid may contain an additive that enhances the acoustic impedance contrast between the fracturing fluid and the formation <b>110</b> of the hydrocarbon reservoir <b>108</b>. Depending on the signal to noise ratio achieved in the seismic survey, even small changes on the order of several percent can be detected. Giving the fracturing fluid a larger acoustic impedance contrast with the formation <b>110</b> helps to distinguish the fracturing fluid from the formation <b>110</b> in the generated image.

0052For example, a fracturing fluid, such as water, may not have a large acoustic impedance contrast with carbonate and chalk formations. As such, methane may be dissolved in the fracturing fluid, producing a fizz gas or foam. The fizz gas or foam may appear as bright spots in the generated image, thereby distinguishing the fracturing fluid from the formation <b>110</b>.

0053<figref idref="DRAWINGS">FIG. 3</figref> illustrates a computing system <b>300</b>, into which implementations of various technologies described herein may be implemented. The computing system <b>300</b> may include one or more system computers <b>330</b>, which may be implemented as any conventional personal computer or server. However, those skilled in the art will appreciate that implementations of various technologies described herein may be practiced in other computer system configurations, including hypertext transfer protocol (HTTP) servers, hand-held devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, and the like.

0054The system computer <b>330</b> may be in communication with disk storage devices <b>329</b>, <b>331</b> and <b>333</b>, which may be external hard disk storage devices. It is contemplated that disk storage devices <b>329</b>, <b>331</b> and <b>333</b> are conventional hard disk drives, and as such, will be implemented by way of a local area network or by remote access. Of course, while disk storage devices <b>329</b>, <b>331</b> and <b>333</b> are illustrated as separate devices, a single disk storage device may be used to store any and all of the program instructions, measurement data, and results as desired.

0055In one implementation, seismic data from the receivers may be stored in disk storage device <b>331</b>. The system computer <b>330</b> may retrieve the appropriate data from the disk storage device <b>331</b> to process seismic data according to program instructions that correspond to implementations of various technologies described herein. The program instructions may be written in a computer programming language, such as C++, Java and the like. The program instructions may be stored in a computer-readable medium, such as program disk storage device <b>333</b>. Such computer-readable media may include computer storage media and communication media. Computer storage media may include volatile and non-volatile, and removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Computer storage media may further include RAM, ROM, erasable programmable read-only memory (EPROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other solid state memory technology, CD-ROM, digital versatile disks (DVD), or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the system computer <b>330</b>.

0056Communication media may embody computer readable instructions, data structures, program modules or other data in a modulated data signal, such as a carrier wave or other transport mechanism and may include any information delivery media. The term “modulated data signal” may mean a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media may include wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, RF, infrared and other wireless media. Combinations of any of the above may also be included within the scope of computer readable media.

0057In one implementation, the system computer <b>330</b> may present output primarily onto graphics display <b>327</b>, or alternatively via printer <b>328</b>. The system computer <b>330</b> may store the results of the methods described above on disk storage <b>329</b>, for later use and further analysis. The keyboard <b>326</b> and the pointing device (e.g., a mouse, trackball, or the like) <b>325</b> may be provided with the system computer <b>330</b> to enable interactive operation.

0058The system computer <b>330</b> may be located at a data center remote from the survey region. The system computer <b>330</b> may be in communication with the receivers (either directly or via a recording unit, not shown), to receive signals indicative of the reflected seismic energy. These signals, after conventional formatting and other initial processing, may be stored by the system computer <b>330</b> as digital data in the disk storage <b>331</b> for subsequent retrieval and processing in the manner described above. While <figref idref="DRAWINGS">FIG. 3</figref> illustrates the disk storage <b>331</b> as directly connected to the system computer <b>330</b>, it is also contemplated that the disk storage device <b>331</b> may be accessible through a local area network or by remote access. Furthermore, while disk storage devices <b>329</b>, <b>331</b> are illustrated as separate devices for storing input seismic data and analysis results, the disk storage devices <b>329</b>, <b>331</b> may be implemented within a single disk drive (either together with or separately from program disk storage device <b>333</b>), or in any other conventional manner as will be fully understood by one of skill in the art having reference to this specification.

0059While the foregoing is directed to implementations of various technologies described herein, other and further implementations may be devised without departing from the basic scope thereof, which may be determined by the claims that follow. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.