Seismic data analysis including modelling slippage planes
A method of analyzing measured microseismic events obtained from monitoring induced hydraulic fracturing of underground geological formations, the method involving (a) postulate a geomechanical model for the region bounding the microseismic events, the model including the parameters vertical stress, reservoir pore pressure, minimum horizontal stress and the orthogonal horizontal stress, (b) select a microseismic event and (c) for the selected microseismic event assume an associated slippage plane with a postulated orientation, (d) apply the geomechanical model to the postulated orientation to determine the resulting shear stress and normal stress applied to the postulated orientation, (e) repeat steps (c) and (d) to produce a number of postulated slippage planes each with their own shear stress and normal stress attributable to them, (f) select the fracture plane having the highest ratio of shear stress to normal stress as being the fracture plane most likely to be representative of a real slippage plane consistent with the geomechanical model, (g) repeat steps (b) to (f) to analyze a number of microseismic events to generate a slippage plane most likely to be representative of a real slippage plane for each microseismic event is provided.
1. A method of analyzing measured microseismic events, the method comprising:
measuring primary and secondary sound waves from the microseismic events obtained from monitoring induced hydraulic fracturing of a subterranean geological formation; and
determining one or more most likely slippage planes associated with the microseismic events using the measured primary and secondary sound waves, comprising:
(a) postulating a geomechanical model for a subterranean region within the subterranean geological formation bounding locations where the microseismic events occurred, the model including at least one of the parameters of vertical stress, reservoir pore pressure, minimum horizontal stress or orthogonal horizontal stress;
(b) selecting a location where one of the microseismic events occurred;
(c) for the selected location, assuming an associated slippage plane with a postulated orientation;
(d) applying the geomechanical model to the postulated orientation to determine the resulting shear stress and normal stress to the postulated orientation;
(e) repeating steps (c) and (d) to produce a probability distribution of possible combinations of shear stress and normal stress attributable to a number of assumed slippage planes associated with the one of the microseismic events;
(f) comparing the assumed slippage planes of the microseismic event location established in steps (a) to (e) with the slippage planes that are consistent with one of a combination with and an alternative to: amplitudes of the primary and secondary sound waves for the one of the microseismic events to obtain a fit;
(g) comparing the fit obtained in step (f) to a statistical baseline where plane orientation information in the measured primary and secondary sound waves for the microseismic events is destroyed, wherein the statistical baseline is generated by repeating step (f) wherein the one of the combination with or the alternative to the amplitudes of the measured primary and secondary sound waves for the microseismic events are shuffled randomly and is a distribution of random fit probability for the number of assumed slippage planes associated with the microseismic events; and
(h) selecting one or more most likely slippage planes associated with the microseismic events from the number of assumed slippage planes using the probability distribution of possible combinations of shear stress and normal stress, wherein the most likely slippage planes have the highest ratios of shear stress to normal stress, and wherein the probability that each of the one or more most likely slippage planes is representative of a real slippage plane is found using the distribution of random fit probability.
2. The method according to claim 1 , wherein the geomechanical model employs an arbitrarily assigned value to at least one of the parameters in the model.
3. The method according to claim 1 , wherein in step (c), the postulated slippage plane has a randomly assigned orientation within bounds.
4. The method according to claim 1 , wherein in step (e), the repeated steps start from a new randomly selected postulated slippage plane.
5. The method according to claim 1 , wherein in step (e), steps (c) to (d) are repeated at least 100 times.
6. The method according to claim 1 , wherein steps (a) to (f) are performed many times wherein the geomechanical model employs an arbitrarily assigned value and the arbitrarily assigned value is different than in previous geomechanical models.
7. The method according to claim 6 , wherein steps (a) to (f) are performed many times and the parameters in the geomechanical model varied or evolved until a geomechanical model which produces the most likely plane slippage location and orientation consistent with the data is achieved.
8. The method according to claim 1 , which includes a step (i) wherein the most likely location and orientation of slippage planes found from steps (a) to (h) are input into a complex fracture simulator.
9. The method according to claim 1 , wherein in step (f), the one of the combination with and the alternative to amplitudes of the primary and secondary sound waves for each microseismic event comprises a polarity of the primary and secondary sound waves.
10. The method according to claim 1 , which includes a step (j) for repeating steps (b) to (h) to analyze a number of microseismic events to generate the most likely slippage plane for each microseismic event.
11. The method according to claim 10 , wherein in step (j), steps (b) to (h) are carried out for all or nearly all of the microseismic events.
12. A method of monitoring hydraulic fracturing of a subterranean formation, the method comprising:
using a plurality of receivers to detect primary and secondary sound waves from microseismic events produced by the hydraulic fracturing of the subterranean formation; and
using the detected primary and secondary sound waves with non-transient instructions on a processor to:
(a) process a geomechanical model for a region of the subterranean formation bounding locations where the detected microseismic events occurred, wherein the geomechanical model includes at least one of a vertical stress, a reservoir pore pressure, a minimum horizontal stress or an orthogonal horizontal stress;
(b) for one of the detected microseismic event locations, postulate an associated slippage plane and an orientation of the associated slippage plane;
(c) apply the geomechanical model to the orientation to determine a resulting shear stress and normal stress applied to the orientation;
(d) repeat steps (b) and (c) for different postulations of associated slippage planes to produce a probability distribution of possible combinations of shear stress and normal stress attributable to a set of slippage planes associated with the one of the detected microseismic event locations;
(e) using the processor to compare the set of slippage planes of the microseismic event location established in steps (a) to (d) with slippage planes that are consistent with one of a combination with and an alternative to: amplitudes of primary and secondary sound waves associated with the one of the detected microseismic events to obtain a fit;
(f) comparing the fit obtained in step (e) to a statistical baseline where the plane orientation information in the detected microseismic events is destroyed, wherein the statistical baseline is generated by repeating step (e) using randomly shuffled the one of the combination with and an alternative to: amplitudes of the primary and/or secondary sound waves and is a distribution of random fit probability for the set of slippage planes associated with the one of the microseismic event locations; and
(g) selecting one or more geomechanical-model-consistent slippage planes associated with the detected microseismic events from the set of slippage planes, wherein the one or more geomechanical-model-consistent slippage planes have highest ratios of shear stress to normal stress, and wherein the probability that each of the one or more geomechanical-model-consistent slippage planes is representative of a real slippage plane is found using the distribution of random fit probability.
13. The method according to claim 12 , wherein steps (a) to (e) are performed many times using different geomechanical models, wherein each of the different geomechanical models comprises an arbitrarily assigned value for a parameter in the geomechanical model and the arbitrarily assigned value is different for each of the geomechanical models.
14. The method according to claim 13 , wherein steps (a) to (e) are performed many times and the parameters in the geomechanical models are varied or evolved until a consistent geomechanical model that produces the most likely plane slippage location and orientation consistent with the microseismic data is determined.
15. The method according to claim 12 wherein the geomechanical-model-consistent slippage planes are used to process an image or description of a fracture produced by the hydraulic fracturing.
16. The method according to claim 15 , wherein the image or description of the fracture is used to control the hydraulic fracturing.
17. The method according to claim 15 , wherein the image or description of the fracture is applied to a reservoir model.
18. The method according to claim 12 , wherein in step (e), the one of the combination with and the alternative to amplitudes of the primary and secondary sound waves for each microseismic event comprises a polarity of the primary and secondary sound waves.
19. The method according to claim 12 , which includes a step (h) for repeating steps (b) to (g) to analyze a number of microseismic events and generating the geomechanical model consistent slippage plane for each microseismic event.