James Verdon, Michael Kendall University of Bristol, U.K . james.verdon@bris.ac.uk

1. Detection of multiple fracture sets using observations of shear-wave splitting in microseismic data • James Verdon, Michael Kendall • University of Bristol, U.K. • james.verdon@bris.ac.uk • EAGE 3rd Passive Seismic Workshop • Athens • 28.03.2011

2. Acknowledgements NRCan (Weyburn): Don White. Pinnacle (hydrofrac data): Shawn Maxwell (now at SLB), Ulrich Zimmer BP, Chevron, ExxonMobil, Pinnacle, Microseismic Inc., Maersk, Schlumberger. :

3. Background • Aligned fracture sets create seismic anisotropy. • Seismic anisotropy can manifest itself in a range of observable features: VSP, AVOA, Shear-wave splitting. • To reveal more complicated anisotropy systems, the system must be illuminated from many angles. • SWS is a path averaged effect – when measured using surface seismics, the overburden must be back-stripped to reveal reservoir properties. • S-waves generated by microseismic events and recorded downhole should provide a decent range of illumination angles, and travel only through rocks of interest.

4. Shear-Wave Splitting • The Christoffel equation for body wave propagation: • has three solutions: P wave, fast S-wave and slow S-wave. All are oriented quasi-perpendicular. • We detect the fast and slow S-waves, and we are able to measure the time-lag between them, dt, and their respective polarisations (ψ is used to denote the fast polarisation). • Time lag is normalised to give percentage velocity difference between fast and slow, dVS, often referred to as splitting magnitude.

5. Inverting for Rock-Physics Properties • Rock physics theory can be used to model the effects of subsurface structures such as aligned fractures and sedimentary VTI fabrics. • The resulting systems are complicated, and non-intuitive? • Nevertheless, if we can forward model, we can invert. • By forward modelling SWS as a function of rock physics parameters, we can invert SWS measurements made on microseismic events for fracture geometries and Thomsen VTI parameters.

6. Inverting for Rock-Physics Properties • We use the additional compliance approach (Schoenberg and Sayers 1995).

7. Inverting for Rock-Physics Properties • Fracture compliance tensor:

8. Inverting for Rock-Physics Properties • Fracture compliance is computed using the low frequency endmember of Hudson’s (1996) model:

9. Inversion Workflow Loop over xi, ai, g, d Compute Cijkl For each SWS measurement: Model dVS and y for given arrival angle using Christoffel eqn. Sum RMS differences for each measurement of dVS and y Normalise the dVS and y misfits by respective minima Combine normaliseddVS and y misfits Select values ofxi, ai, g, & dthat minimise misfit Use F-test to identify confidence limits/uniqueness of the inversion

10. Case Examples: • Weyburn CCS-EOR site, Canada: Multiple fractures? • Hydraulic Fracture (Pinnacle): single fracture set? • Synthetic: Potential pitfalls caused by insufficient illumination.

11. Weyburn CCS-EOR Saskatchewan, Canada: Microseismicity recorded during CO2 injection for CCS & EOR. Array of 8 3-C geophones in a borehole just above the reservoir, installed 08/2003. 50m from vertical injection well. Events located around horizontal producers to the NW and SE.

12. Inversion for 1 fracture set x a Fracture set striking SE-NW. Weak VTI fabric. Results quality is poor.

13. Inversion for 2 fracture sets Fracture sets imaged striking NW-SE and NE-SW. Trade-off between fracture densities means we cannot know absolute values, but we can say that FNW-SE > FNE-SW.

14. Inversion for 2 fracture sets x a

15. Independent fracture calibration Fracture identification by Bunge 2000, Brown 2002, M.Sci theses from Colorado School of Mines. Borehole image logs and core samples. VSP survey (reported in Bellefleur 2003, SEG). VSP finds a weak N-S striking fracture set, and stronger VTI anisotropy. Our SWS inversion finds, in agreement with core samples and borehole image logs, a weak VTI fabric and fractures striking NW-SE and NE-SW.

16. Independent fracture calibration • VSP survey (reported in Bellefleur 2003, SEG). • Offset by ~2km from microseismic survey. • Receivers at ~900m and then ~1300m (150m above the reservoir). Direct shear arrivals from surface sources. • Image anisotropy dominated by VTI, with N-S oriented fracturing. • This system is established by 900m depth. • We interpret this as: the VSP survey is imaging anisotropy in the overburden. The anisotropy imaged through microseismics shows fractures in the reservoir and caprock.

17. Hydraulic Fracture (Pinnacle) Hydraulic fracture in a North American oilfield. Water injection in a vertical well: events occur over an 80 minute injection period. Vertical recording well containing an array of 12 3-C geophones, straddling the injection depth. Events image a fracture set developing at ~120°.

18. Inversion for 1 fracture set SWS images a VTI fabric and fracture striking at 120°. Fracture density cannot be constrained.

19. Inversion for 2 fracture sets The additional degrees of freedom provided by a second fracture set are not used to ‘invent’ spurious fracture sets. The inversion finds two fracture sets both striking at 120°: i.e., one fracture set.

20. Summary of inversion results

21. Synthetic example: a pitfall when illumination is insufficient • Often only subvertical arrivals are available (e.g., reflection seismic, global seismic), and we assume one fracture set is present. • With insufficient illumination angles (even subvertical), multiple fracture sets can masquerade as one.

22. Synthetic example: a pitfall when illumination is insufficient • Input model: two vertical fracture sets. • Strikes of 20° and 90° • Illuminated with subvertical (0 – 20°) arrivals:

23. Synthetic example: a pitfall when illumination is insufficient Inversion finds a good fit for one fracture set at an intermediate angle. With insufficient arrivals, (i.e. only subvertical), multiple fracture sets can be mistaken for only one, and a decent, yet spurious, splitting result inverted.

24. Synthetic example: a wider range of illumination (including subhorizonal) is needed to identify multiple fractures To resolve the multiple fracture sets, a wider range of arrivals, including subhorizontal angles, are needed. Because they often provide such arrivals, where controlled source surveys often cannot, microseismic events represent good shear wave sources on which to measure splitting in order to image multiple fracture sets.

25. Conclusions • We have developed a method to invert SWS measurements for fracture properties. • This method provides a good match with independent estimates of the two fracture sets at Weyburn. Discrepancies between both and the VSP survey results highlights how useful microseismic waves travelling only in reservoir/caprock rocks can be. • Inversion of the Pinnacle frac-job dataset shows that it is possible to distinguish the presence of single and multiple fracture sets. • Without sufficient illumination, SWS inversions are often non-unique. Microseismic events often cover a wide range of incidence angles in comparison with surface-based methods. • In less than 10 words: Microseismic events make excellent S-wave sources for anisotropy studies.

26. Future work • We (O. Al-Harrasi, A. Wüstefeld, G. Jones, D. Minifie) have used these methods successfully on: Cotton Valley hydrofrac, an oilfield in Oman, Ekofisk & Valhall, North Sea, block collapse mining data, and at the Ontaki volcano. • Improved inversion methods: • Nearest neighbour, misfit annealing, genetic algorithm…. • Incorporating errors in SWS measurement…. • Automation in selection of number of fracture sets…. • Application, calibration, utilisation….

27. http://www1.gly.bris.ac.uk/BUMPS/ J.P. Verdon, J-M., Kendall, A. Wüstefeld, 2009. Imaging fractures and sedimentary fabrics using shear wave splitting measurements made on passive seismic data. Geophysical Journal International,179, 1245 – 1254. J.P. Verdon and J-M. Kendall, 2011. Detection of multiple fracture sets using observations of shear-wave splitting in microseismic data: Geophysical Prospecting, Early View, DOI: 10.1111/j.1365-2478.2010.0943.x