Microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight Bakken reservoir

Abstract

To improve the accuracy of microseismic event locations, we developed a new inversion method with double-difference constraints for determining the hypocenters and the anisotropic velocity model for unconventional reservoirs. We applied this method to a microseismic data set monitoring a Middle Bakken completion in the Beaver Lodge area of North Dakota. Geophone arrays in four observation wells improved the ray coverage for the velocity inversion. Using an accurate anisotropic velocity model is important to correctly assess the height growth of the hydraulically induced fractures in the Middle Bakken. Our results showed that (1) moderate-to-strong anisotropy exists in all studied sedimentary layers, especially in the Upper and Lower Bakken shale formations, where the Thomsen parameters (ϵ and γ) can be greater than 0.4, (2) all the events selected for high signal-to-noise ratio and used for the joint velocity inversion are located in the Bakken and overlying Lodgepole formations, i.e., no events are detected in the Three Forks formation below the Bakken, and (3) more than half of the strong events are in two clusters at approximately 100 and 150 m above the Middle Bakken. Reoccurrence of strong, closely clustered events suggested activation of natural fractures or faults in the Lodgepole formation. The sensitivity analysis for the inversion results showed that the relative uncertainty in parameter δ is larger than other anisotropy parameters. The microseismic event locations and the anisotropic velocity model are validated by comparing synthetic and observed seismic waveforms and by S-wave splitting.

On the evolution of the elastic properties of organic-rich shale upon pyrolysis-induced thermal maturation

“…By understanding the relationship between the thermal maturity of the organic matter (i.e., the degree of hydrocarbon generation) and the elastic properties of the composite organic-rich shale, we may provide important inputs to rock-physical models that better improve the characterization and development of unconventional reservoirs. For instance, the effect of velocity anisotropy on the determination of microseismic locations and mechanisms during hydraulic fracturing has been well documented lately (e.g., Grechka et al, 2011;Li et al, 2014). By establishing deeper understandingwell-derived anisotropy measurements to thermal maturity through laboratory-derived relationships may, in combination with organic richness estimates (e.g., Passey et al, 1990), provide more robust estimates of horizon quality from well logs.…”

Relative location of microseismicity

“…Because of the importance of the subject, numerous techniques for locating events pioneered by the classical Geiger's method (Geiger, 1912) have been proposed in seismology and similar developments and refinements, accounting for salient features of unconventional reservoirs absent in the earth on a larger scale, are currently taking place in the microseismic arena. Among the features necessitating special treatment are the strong seismic anisotropy of kerogen-rich shales (Vernik and Nur, 1992;Vernik and Liu, 1997;Miller et al, 2012) and the high velocity contrasts observed on sonic and microseismic data in certain hydrocarbonbearing formations (e.g., Li et al, 2014;Yang and Zoback, 2014).…”

“…This recognition has led to three groups of approaches that differ from each other in how the velocities are handled: first, a velocity model might be fixed a priori and the hypocenters can be computed in that model (e.g., Geiger, 1912;Asch et al, 1996;Gambino et al, 2004;Chambers et al, 2010;Ito et al, 2012); second, initially estimated velocities and possibly anisotropy coefficients might be iteratively updated based on the data supplied by events themselves and simultaneously with obtaining their hypocenters (e.g., Thurber, 1986;Iyer and Hirahara, 1993;Thurber and Rabinowitz, 2000;Zhang et al, 2009;Jansky et al, 2010;Zhou et al, 2010;Grechka and Yaskevich, 2014;Li et al, 2014); and, third, the influence of velocities on the hypocenters of events within a selected cluster might be reduced by relying on the previously located events in the same cluster and finding the hypocenters of other events relatively to them (Waldhauser and Ellsworth, 2000;Waldhauser, 2001;Waldhauser and Schaff, 2008;Li et al, 2013).…”

Microseismic Event Location Using Multiple Arrivals: Demonstration of Uncertainty Reduction

“…As an inverse problem, the microseismic event location in downhole monitoring can be carried out in various ways. Commonly used methods include leastsquare travel time inversion (Douglas, 1967;Li et al, 2014), doubledifference (Waldhauser and Ellsworth, 2000), coherence scanning (Drew et al, 2005;Duncan and Eisner, 2010), fullwaveform inversion, and et al Though effective to a certain extend, these methods don't follow a rigorous statistical framework. Thus, they may either have difficulty in predicting uncertainty with their location estimation or can only give a rough value of estimation uncertainty.…”

“…An additional constraint on the event location usually comes from direct Pwave polarization (Dreger et al, 1998;Li et al, 2014). Thus, three component data will be necessary.…”