Patent Application
20150006082
Hydraulic fracturing or stimulation of rock in earth formations is a technique used to extract hydrocarbons from reservoirs in the formations. Stimulation increases the number of fractures in the rock or opens existing fractures that in turn may increase the production rate of the hydrocarbons. In that production of hydrocarbons can be expensive, it would be well received in oil and gas industries if method and apparatus could be developed to calculate a reservoir volume stimulated by the fracturing that may be used as an input into a reservoir model to estimate the amount and production rate of hydrocarbons that may be produced. Other industries such as the geothermal industry may also benefit from this method and apparatus.
Disclosed is a method for estimating a volume of a stimulated reservoir. The method includes: receiving, using a processor, a seismic signal from each microseismic event in a plurality of microseismic events in an earth formation, the seismic signal being received by an array of seismic receivers; representing each microseismic event by a plurality of markers in the three-dimensional space using the processor, a spatial distribution of the markers representing a volume of rock influenced by a microseismic event, wherein the volume and a location of each event are derived from the seismic signal; calculating a scalar attribute for each marker in the plurality of markers using the processor; dividing the three-dimensional space into a plurality of three-dimensional grid cells using the processor; and summing the scalar attributes for all the markers in each grid cell to provide a total scalar attribute for each grid cell using the processor.
Also disclosed is a method for estimating a volume of a stimulated reservoir. The method includes: stimulating an earth formation using a stimulation apparatus configured to generate a plurality of microseismic events in the formation; receiving, using a processor, a seismic signal from each microseismic event in a plurality of microseismic events in an earth formation, the seismic signal being received by an array of seismic receivers; representing each microseismic event by a plurality of markers in the three-dimensional space using the processor, a spatial distribution of the markers representing a volume of rock influenced by a microseismic event, wherein the volume and a location of each event are derived from the seismic signal; calculating a scalar attribute for each marker in the plurality of markers using the processor; dividing the three-dimensional space into a plurality of three-dimensional grid cells using the processor; and summing the scalar attributes for all the markers in each grid cell to provide a total scalar attribute for each grid cell using the processor.
Further disclosed is a non-transitory computer readable medium having computer executable instructions for estimating a volume of a stimulated reservoir that when executed by a computer implements a method. The method includes: receiving a seismic signal from each microseismic event in a plurality of microseismic events in an earth formation, the seismic signal being received by an array of seismic receivers; representing each microseismic event by a plurality of markers in the three-dimensional space, a spatial distribution of the markers representing a volume of rock influenced by a microseismic event, wherein the volume and a location of each event are derived from the seismic signal; calculating a scalar attribute for each marker in the plurality of markers; dividing the three-dimensional space into a plurality of three-dimensional grid cells; and summing the scalar attributes for all the markers in each grid cell to provide a total scalar attribute for each grid cell.
The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike:
A detailed description of one or more embodiments of the disclosed apparatus and method presented herein by way of exemplification and not limitation with reference to the figures.
Disclosed are method and apparatus for calculating a volume of a stimulated rock in an earth formation. From the stimulated rock volume, a stimulated reservoir volume can be determined. The term “stimulated reservoir volume” relates to a volume in an earth formation reservoir that is effectively stimulated, in which the permeability is increased to allow a sufficient portion of the hydrocarbons to be extracted, thereby increasing the performance (e.g., flow rate) of a well. The stimulated reservoir volume is thus a measure of the efficacy of a stimulus treatment applied to the formation such as fracturing.
Still referring to
A location of each or the seismic receivers 5A, B and C is known and together form an array of seismic receivers. When a rupture occurs (i.e. microseismic event) such as at 8 in
A seismic event occurs on a rupture or fracture plane, which has an associated area. The relative displacement of this area between two sides of the rupture plane in non-zero. When a rupture occurs, seismic waves are radiated from the rupture through the formation and earth.
The rupture process is encoded in the characteristics of the radiated seismic waves. By studying the characteristics of the seismic waves received by the receiver network, the location, magnitude, scalar moment and other seismic characteristics can be obtained. In one or more embodiments, the scalar moment, as defined below, of a microseismic event is derived from the wave magnitude and the wave amplitude spectrum. Other techniques and recorded signals as known in the art may also be used to determine the scalar moment. Further information about each microseismic event may also be obtained from the radiated seismic waves using certain types of receivers, receiver arrays and processing. This further information includes fracture plane size (e.g., area) and orientation along with the fracture displacement. This further information and the scalar moment together may be referred to as the moment tensor. In one or more embodiments, the moment tensor may be represented in three-dimensions by a 3 by 3 matrix. Hence, some seismic data available for processing using the methods disclosed herein may include only location and magnitude or scalar moment while other data may include the moment tensor.
Some of the methods for decoding the radiated seismic waves are based measuring a P-wave and an S-wave of the total seismic wave. For the P-wave, the particles in the solid have vibrations along or parallel to the travel direction of the wave energy. The P-wave has higher speed than the corresponding S-wave in the same medium. Noting that it takes a longer time for a seismic signal to reach a receiver that is farther away from a seismic event than a receiver that is closer, the event location can be derived from the arrival time difference of the received P and/or S signals from all receivers or the separation measured between the recorded P-wave and S-wave for the same receiver. Furthermore, the vibration direction will indicate the direction of wave propagation and, therefore, provide information to determine the source location as well. The rupture magnitude or moment magnitude can be derived from the signal amplitude and its travel distance from the source. Additional information may be derived from a Fourier transform of the received seismic signal. In one example, the corner frequency where the slope of the Fourier transform changes suddenly is used to determine the rupture length and source radius, which is an estimate of the fracture size. Information on the size of the rupture plane can be obtained in many ways. The most common ways are either derived from the corner frequency of a waveform amplitude spectrum analysis, or when this is missing, an approximate power law empirical relationship between magnitude and the rupture plane size. As the techniques for analyzing seismic waves to determine fracture metrics, such as fracture location, fracture area, fracture displacement, and fracture plane orientation, using an array of seismic receivers is well known in the art, these techniques are not discussed in further detail.
A plurality of ruptures may be caused in the formation by stimulation such as by hydraulic fracturing or water injection as non-limiting embodiments. Generally, each rupture in the plurality of ruptures occurs at different times. Hence, the location, size and/or orientation of each fracture plane that has ruptured can be determined by the techniques discussed above. All of the ruptured planes together indicate a stimulated rock volume.
The location error may be defined by the receiver array geometry, the sample rate and which the microseismic events are recorded, and the uncertainty of the velocity model used to calculate the location of each received event using the received seismic signals from each event. Other factors may also contribute to the location error. In one or more embodiments when a detailed error function is not provided, these errors may be considered random in nature and, thus, may be modeled using a probability function such as the Gaussian distribution.
As disclosed herein, the scalar moment, M0=μAD (where μ is shear modulus, D is average displacement of the rupture, and A is the area of the rupture), is used to describe seismic deformation in a rock volume due to rupture of formation rock. The deformed volume caused by a seismic event (i.e., fracture or rupture) with a scalar moment M0 is ΔV=AD and can also be used to describe the stimulated rock volume. The scalar seismic moment is one measure of the microseismic event size and energy released. Alternatively, other microseismic attributes, such as rupture area or seismic energy released, may also be used.
Aspects of block 23 are now discussed in further detail with respect to
Another factor, that influences the spatial spread of the detected microseismic events is location error associated with the location calculated for each microseismic event that is detected. To account for the location error, the spatial spread of the markers are modified (e.g., increased) to include the uncertainty range due to the location error. For example, a cloud associated with an event having a higher level of location error will have a cloud with a greater diameter than a cloud associated with an event having a lower level of location error. The cloud of markers at 4A in
A weight is assigned to each marker with each weight being defined by the probability function. In
Referring back to
Referring back to
Block 26 calls for dividing the three-dimensional space into a plurality of three-dimensional grid cells. It can be appreciated that there may be a tension in determining a size of the grid cells. If the grid cells are made too big, then the accuracy of the calculation of the stimulated reservoir volume may be decreased. If the grid cells are made too small, then there may be many grid cells with no markers inside of them. In one or more embodiments, a length of a side of a grid cell is an average of distances between nearest adjacent microseismic event locations. Alternatively, in one or more other embodiments, a length of a side of a grid cell is a minimum of the distances between adjacent microseismic event locations. Alternatively, in one or more other embodiments, a length of a side of a grid cell can be user defined. It can be appreciated that each grid cell may have the same dimensions as the other grid cells in the plurality of grid cells, although they can also have different dimensions. In one or more embodiments, the grid cells in the plurality are identical cubes. One example of the plurality of grid cells is illustrated in
Block 27 calls for summing all the weighted scalar attributes, such as the weighted scalar moments. This is based on the weighted markers in each grid cell to provide a total scalar attribute for each grid cell. The total scalar attribute for each grid cell is then divided by the grid cell volume to provide a grid cell scalar attribute density.
The grid cell scalar attribute density is then mapped into the grid cells where the magnitude of the grid cell scalar attribute density is represented by shading as illustrated in
Block 28 calls for combining the grid cells with scalar attributes or scalar attribute densities higher than zero to provide the stimulated reservoir volume. The scalar attribute or scalar attribute density of each grid cell provides a measure of the efficacy of stimulation for the corresponding grid cell. The permeability for each grid cell in the stimulated reservoir volume could be made proportional to the scalar attribute density to provide a stimulated reservoir volume with spatial variations based on the microseismic event data.
It can be appreciated that the plurality of markers may be represented “virtually” as calculation points by a computer processing system without being plotted or they may be plotted in a three-dimensional diagram, which may be displayed to a user.
The above techniques provide several advantages. One advantage is that location error for locations of microseismic events derived from seismic signals is accounted for to provide a more accurate estimate of the stimulated reservoir volume. Another advantage is that the stimulated reservoir volume is not limited to any one type of scalar attribute and may be derived from any scalar attribute for which data may be available.
In support of the teachings herein, various analysis components may be used, including a digital and/or an analog system. For example, the data logger 6, the computer processing system 7, the seismic receivers 5, or the stimulation apparatus 10 may include digital and/or analog systems. The system may have components such as a processor, storage media, memory, input, output, communications link (wired, wireless, pulsed mud, optical or other), user interfaces, software programs, signal processors (digital or analog) and other such components (such as resistors, capacitors, inductors and others) to provide for operation and analyses of the apparatus and methods disclosed herein in any of several manners well-appreciated in the art. It is considered that these teachings may be, but need not be, implemented in conjunction with a set of computer executable instructions stored on a non-transitory computer readable medium, including memory (ROMs, RAMs), optical (CD-ROMs), or magnetic (disks, hard drives), or any other type that when executed causes a computer to implement the method of the present invention. These instructions may provide for equipment operation, control, data collection and analysis and other functions deemed relevant by a system designer, owner, user or other such personnel, in addition to the functions described in this disclosure.
Elements of the embodiments have been introduced with either the articles “a” or “an.” The articles are intended to mean that there are one or more of the elements. The terms “including” and “having” are intended to be inclusive such that there may be additional elements other than the elements listed. The conjunction “or” when used with a list of at least two terms is intended to mean any term or combination of terms. The terms “first,” “second” and the like do not denote a particular order, but are used to distinguish different elements.
While one or more embodiments have been shown and described, modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.
It will be recognized that the various components or technologies may provide certain necessary or beneficial functionality or features. Accordingly, these functions and features as may be needed in support of the appended claims and variations thereof, are recognized as being inherently included as a part of the teachings herein and a part of the invention disclosed.
While the invention has been described with reference to exemplary embodiments, it will be understood that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications will be appreciated to adapt a particular instrument, situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
