WO2013112719A1

WO2013112719A1 - Method and system for displaying microseismic event locations

Info

Publication number: WO2013112719A1
Application number: PCT/US2013/022950
Authority: WO
Inventors: Sean COFFIN; Jonathan S. Abel
Original assignee: Octave Reservoir Technologies, Inc.
Priority date: 2012-01-24
Filing date: 2013-01-24
Publication date: 2013-08-01

Abstract

A problem with current approaches to displaying microseismic events is that the greater the event energy, the more accurately the event can be localized— it has greater amplitude, and can better overcome the background noise, As a result, it is misleading to plot energetic events with markers that span larger volumes, Aspects of the invention account for the statistics of the event location estimates, so that poorly localized events occupy a large area (say plotted with a light shade), and accurately localized events appear only in a small area about the estimate (say, plotted in a dark shade).

Description

Docket No: 071850-0420277

OTI-007 PCT

METHOD AND SYSTEM FOR DISPLAYING MICROSEISMIC EVENT LOCATIONS

[0001] Passive localization of microseismic events is commonly used to monitor resource extraction processes such as hydraulic fracture stimulation, or 'Tracking." In a typical scenario, geophones in a monitor will record seismic energy released impulsively from failure events as the fracture forms. The relative arrival times and amplitudes of P-waves and S-waves among other features are used to estimate the location of each detected event. Inaccuracies in event location estimates result from seismic background noise and uncertainties in seismic wave propagation, and are related to the array and fracture geometry.

[0002] In the current art, event location estimates are displayed as spheres, centered on the estimated locations. Conventionally, the spheres are sized to indicate event energy. In this way, more energetic events are plotted as large spheres, whereas weak events are plotted as small spheres,

[0003] The problem with the current approach is that the greater the event energy, the more accurately the event can be localized— it has greater amplitude, and can better overcome the background noise. As a result, it is misleading to plot energetic events with markers that span larger volumes. In the present invention, we account for the statistics of the event location estimates, so that poorly localized events occupy a large area (say plotted with a light shade), and accurately localized events appear only in a small area about the estimate (say, plotted in a dark shade). Docket No: 071850-0420277

OTI-007 PCT

[0004] In one embodiment, all position estimates are plotted as points, with the shade or color indicating localization accuracy and/or energy. For instance, on a white background, more accurately localized events could be plotted using darker colors.

[0005] In another embodiment, the volume is divided into voxels, preferably no larger than the greatest possible localization accuracy. For each event, each voxel is adjusted (e.g., darkened) according to the probability that it contained the event. This can be done, for instance, by assuming that the estimate errors are Gaussian and unbiased, and by using the information inequality or other means to infer the covariance of the estimate. In this way, an estimate with high variance will slightly darken a region in the space, whereas an accurately localized event will noticeably darken just a few voxels. The benefit is that the fine structure of the fracture is not obscured.

[0006] This could be done for instance by, for each event, drawing a semi-transparent ellipsoid, centered on the estimated location and sized according to the estimate standard deviation. The transparency of the ellipsoid would be inversely related to its volume: the smaller the ellipse the less transparent it would be. The transparencies could be nonlinearly combined such that a number of nearby inaccurately localized events could produce a relatively small opaque region. Furthermore, erosion and dilation operations could be applied to bridge small gaps in the estimated fracture structure resulting from undetected events, and the like,

Claims

Docket No: 071850-0420277 OTI-007 PCT WHAT IS CLAIMED IS:

1. A method for displaying microseismic events, comprising:

obtaining location estimates of a plurality of microseismic events;

obtaining statistics of the event location estimates; and

based on the statistics, displaying the events such that poorly localized events occupy a large area of the display, and accurately localized events appear only in a relatively small area around the estimate.

2. A method according to claim 1, wherein all location estimates are plotted as points, with a shade or color indicating localization accuracy and/or energy.

3. A method according to claim 2, wherein on a white background, more accurately localized events are plotted using darker colors.

4. A method according to claim 1 , further comprising:

dividing a display volume into voxels, each preferably no larger than the greatest possible localization accuracy; and

for each event location estimate, adjusting each voxel according to the probability that it contained the event.

5. A method according to claim 4, wherein adjusting includes assuming that the estimate errors are Gaussian and unbiased, and using the information inequality or other means to infer the covariance of the estimate, Docket No: 071850-0420277

OTI-007 PCT

6. A method according to claim 5, wherein an estimate with high variance will slightly darken a region in the space, whereas an accurately localized event will noticeably darken just a few voxels.

7. A method according to claim 6, wherein the fine structure of the fracture is not obscured.

8. A method according to claim 6, wherein adjusting includes, for each event, drawing a semi- transparent ellipsoid, centered on the estimated location and sized according to the estimate standard deviation.

9. A method according to claim 8, wherein a transparency of the ellipsoid is inversely related to its volume, such that the smaller the ellipse the less transparent it would be.

10. A method according to claim 9, wherein the transparencies are nonlinearly combined such that a number of nearby inaccurately localized events produce a relatively small opaque region,

1 1. A method according to claim 9, further comprising performing erosion and dilation operations to bridge small gaps in an estimated fracture structure resulting from undetected events.