Patent Application
20100238765
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1. Field of the Invention
The invention relates generally to the field of imaging the Earth's subsurface using passive seismic detection techniques. More specifically, the invention relates to processing methods for passive seismic signals to improve the ability to detect subsurface seismic events from such signals and characterize reservoir.
2. Background Art
Passive seismic techniques are used to image and to characterize petrophysical properties of subsurface rock formations. Techniques known in the art for passive seismic data acquisition and processing are described, for example in U.S. Patent Application Publication No. 2008/0068928 filed by Duncan et al., the patent application for which is assigned to the assignee of the present invention.
One of the physical properties that is desired to be determined for characterization of subsurface rock formations is seismic anisotropy. Seismic anisotropy (i.e. dependence of the seismic velocities on direction of the propagation) is commonly observed both in large-scale studies (upper mantle, core-mantle boundary and inner core of the Earth) as well as in small-scale crustal imaging of seismic data used for mineral and petroleum exploration. In mineral and petroleum exploration, anisotropy is used for seismic reservoir imaging, obtaining rock type information, identification of fracture or stress orientations, monitoring of time-lapse changes in elastic properties, etc. Understanding the velocity anisotropy is important for precise and accurate location of induced microseismic events. However, inversion for seismic anisotropy remains difficult mainly due to a multiparameter nature of the problem and inability of conventional (controlled source) seismic reflection data to constrain the relevant parameters. Those issues are usually dealt with by making assumptions about the type of seismic anisotropy, and fitting acquired seismic data to the assumed model.
The foregoing difficulty in inversion for anisotropy can be alleviated if more measurements of seismic anisotropy are available. One such measurement of anisotropy is known as “shear-wave splitting”, which does not occur in isotropic media.
It is desirable therefore to have a method for measuring shear-wave splitting in addition to compressional-wave times with passive seismic measurement techniques that can be used in connection with petroleum and mineral exploration.
A method for determining the effective seismic anisotropy of subsurface rock formations includes measuring passive seismic signals at a plurality of locations above an area of the Earth's subsurface to be surveyed. The compressional- and shear-wave arrival times of at least one seismic event occurring in the subsurface are determined from the measured seismic signals. The arrival times are inverted to estimate seismic anisotropy.
A method for monitoring a formation fracturing procedure according to another aspect of the invention includes recording seismic signals at selected positions near the Earth's surface proximate a wellbore during pumping of fracturing fluid into the wellbore. From the measured signals, shear wave arrival times and compressional wave arrival times are selected from at least one origin location of a seismic event occurring in the subsurface. The shear wave and compressional wave arrival times are inverted to obtain values of seismic anisotropy parameters. A position of a fluid front is determined from the inverted shear wave and compressional wave arrivals by determining a location of origin of at least one seismic event.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
Each of a plurality of seismic sensors, shown generally at 12, is deployed at a selected position proximate the Earth's surface 14. In marine applications, the seismic sensors would typically be deployed on the water bottom in a device known as an “ocean bottom cable.” Additionally, the seismic sensors could be deployed in multiple shallow or deep boreholes in an arrangement known as a “buried array” The seismic sensors 12 in the present embodiment may be so called “three component” sensors. Three component sensors include three or more, typically mutually orthogonally oriented particle motion sensing elements. The sensing elements may be geophones, but may also be accelerometers or any other sensing device known in the art that is responsive to velocity, acceleration or motion of the particles of the Earth proximate the sensor. The seismic sensors 12 generate electrical or optical signals in response to the particle motion or acceleration along each of the three directionally sensitive components, and such signals are ultimately transmitted to a recording unit 10 for making a time-indexed recording of the signals from each sensor 12 for later interpretation by a method according to the invention.
In some examples, the seismic sensors 12 may be arranged in sub-groups having spacing therebetween less than about one-half the expected wavelength of seismic energy from the Earth's subsurface that is intended to be detected. Signals from all the sensors in one or more of the sub-groups may be added or summed to reduce the effects of noise in the detected signals. The adding or summing may be performed within the group itself, such as by connecting the sensors in electrical series, or may be performed, e.g., digitally in the recording unit 10.
A wellbore 22 is shown drilled through various subsurface rock formations 16, 18, through a hydrocarbon producing formation 20. A wellbore tubing 24 having perforations 26 formed therein corresponding to the depth of the hydrocarbon producing formation 20 is connected to a valve set known as a wellhead 30 disposed at the Earth's surface. The wellhead may be hydraulically connected to a pump 34 in a “fracture pumping unit 32.” The fracture-pumping unit 32 is used in the process of pumping a fluid, which in some instances includes selected size solid particles, collectively called “proppant”. Pumping such fluid, whether propped or otherwise, is known as hydraulic fracturing. The movement of the fluid is shown schematically at the fluid front 28 in
The fracturing of the formation 20 by the fluid pressure creates seismic energy that is detectable by the seismic sensors 12. The time at which the seismic energy is detected by each of the sensors 12 with respect to the time-dependent position in the subsurface of the formation fracture created by the injected fluid is related to the acoustic velocity of each of the formations 16, 18, 20, and the position of each of the seismic sensors 12.
A particular arrangement of seismic sensors that may be used advantageously in some implementations is an array of seismic sensors such as shown in
The arrangement of the seismic sensors shown in
The recording system is disposed proximate the array and may include equipment (not shown separately) to be used to record the signals generated by the seismic sensors in each of the sensor lines 111-120. As explained above, the signals may be recorded individually for each sensor 122, or in some examples, selected numbers of adjacent seismic sensors in each line 111-120 may have their signals combined or summed by electrical series connection or other electrical configuration, or the signals may be equivalently summed in the recording system 10. The recording system 10 may include a general purpose, programmable computer (not shown separately) for processing the recorded signals, and storing or displaying the signals on, for example, a computer display according to the invention. Processing signal recordings according to the invention may be also performed at any other location.
Recording seismic signals generated by the sensors 122 in the array may be performed continuously over a selected period of time, for example from several minutes to several weeks in duration. In other examples, signal recording may take place over a time period extending as long as several years in duration. Thus, for each sensor (or selected groups of sensors) a signal recording will include signal amplitude with respect to time for the entire selected recording time interval, for each of the three components of motion detected by each seismic sensor.
The recorded seismic signals can be scanned to determine the presence of one or more events that may be reasonably inferred to be of seismic origin. Such scanning may include, for example, identifying signal amplitudes in the recorded signals that exceed a selected threshold (amplitude peaks). When one or more of such events are detected in a plurality of the recorded signals, the time of arrival of each such event in each recorded signal is determined in order to establish that the events are possibly of seismic origin or by other detection criteria such as those discussed in Duncan et al. patent application publication referred to above. In the present invention, it is believed possible to determine seismic anisotropy only by measuring times of arrival of mutually perpendicular quasi S-waves (S1 and S2), or in other words, to measure the spatial distribution of shear-wave splitting, if in addition compressional-wave arrival times are known the full type and strength of anisotropy can be inverted.
Shear-wave splitting is the difference between the arrival times of S1 and S2 waves. In the present example, it can be obtained by rotating a portion of three-component trace containing both S1- and S2-waves in such a way to place them on different components. Then, the shear-wave splitting is found from the location of maximum of the cross-correlation of the rotated traces. Another possible technique might be to pick the time of the first maximum or minimum (amplitude peaks) in the recorded seismic signals. To pick the amplitudes automatically, a two-step procedure may be used. First, all peak amplitudes within a predetermined time interval (the interval being predetermined from the identified seismic event, using, e.g., signal to noise ratio detection) on each signal recording (“trace”) along each receiver line are identified. Next, is to manually (e.g., visually) select a particular event arrival, typically a trace where the event arrival has the apparent best signal-to-noise ratio, and then determine arrival times for each sensor in the sensor array from each of the corresponding amplitude peaks.
A similar arrival time selection procedure may also be applied to determine compressional (P) wave arrivals in the recorded signals. Typically, the sensor signal components in the vertical plane are used to determine P wave arrivals. One technique for determining anisotropy parameters from the detected arrivals is to perform inversion with respect to the arrival times of all three detected arrival times of compressional-, S1- and S2-waves. Such an inversion is nonlinear and consists of searching (e.g., grid search, gradient, etc.) of an optimal model, source location, and origin times. This sort of inversion generally cannot be applied to conventional active-source reflection seismic data because S-waves are typically either not excited or are not recorded at sufficiently large distances from the seismic energy sources to be able to estimate anisotropy in a unique fashion. In passive seismic data, however, shear-waves are excited naturally and the acquisition geometry such as shown in
To illustrate that the foregoing inversion is feasible, a model was constructed of a vertically transversely isotropic (VTI) subsurface, and 200 microseismic events were simulated within the modeled formations. The arrival times from each event to a plurality of receivers located at the Earth's surface were calculated. The calculated travel times were then contaminated with random noise that has a standard deviation of 10 milliseconds. The noisy times were then used as an input for the inversion. The inversion searches for the unknown event origin time, event location, and the effective VTI parameters. The VTI parameters include the vertical compressional- and S-wave velocities and Thomsen coefficients ε, δ, and γ (see, e.g., U.S. Pat. No. 4,888,743 issued to Thomsen).
The anisotropic coefficients ε and δ estimated for each of 200 events are presented in
An example procedure using compressional-wave and shear-wave splitting times determined as explained above will be explained with reference to
At 58, if the passive detecting of seismic energy is performed as explained with reference to
At 60, if during fracture pumping (
At 62, for each case in which repeated determination of anisotropy parameters is performed, e.g., for fracture front monitoring as shown at 58 or for multiple formations being fractured, at 60, an inversion procedure may be performed in which the results are the density of the fractures, the orientation of the fractures, or the maximum and minimum horizontal stresses in the formation and the orientations thereof.
Methods according to the invention may provide for better determination of formation seismic anisotropy and better characterization of subsurface rock formations than methods known in the art prior to the present invention. The invention provides an inversion for effective anisotropy using only seismic event arrival times from passive seismic data, and does not require the use of polarization as is required with techniques known in the art. The invention provides inversion of general anisotropy as a result of using a dense array of sensors having multiple offsets and multiple azimuths. Previous inversion techniques used with active source seismic acquisition have relatively limited offsets or apertures and as a result therefore have to make assumptions about the type of anisotropy in order to achieve acceptable inversion results.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
