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ELASTIC ANISOTROPY OF VTI MEDIA REVISITED


Date: Tuesday, October 31st, 2017
Lunch: 11:30  Talk: 12:00-1:00
Reservation: Register Online by Monday at noon before meeting
Cost: $15-industry professionals/$10-students
Location: 5200 North Sam Houston Parkway West Suite 500 Houston Texas 77086 (Weatherford Lab)
Parking: One story building with surface parking. Visitors are requested to reverse park, note their license plate number and sign in at the main reception.

Abstract:

Shale formations, which usually show vertically transverse isotropy (VTI), comprise about 75% of the sedimentary basins. Therefore, VTI is the most common anisotropic model in geosciences, which must be taken into account for more accurate geomechanical modeling and seismic imaging.
 
C13, which is required for computing anisotropic Poisson’s ratios and Young’s moduli, is the most challenging parameter to estimate for VTI media. Monte Carlo simulation shows that while computing C13 for VTI media in a conventional way, an error of 1% in velocities can produce completely different results (e.g.; up to about 10 GPa difference in C13 and 0.4 difference in Thomsen δ, and 0.4 in Poisson’s ratios). Simulation results show that this sensitivity is mainly due to uncertainties in 45o velocities. Theoretical bounds and relationships between three directions dependent Poisson’s ratios in VTI media provide a method for determining physical bounds on C13, without requiring 45o measurements.
 
Analysis of a comprehensive collection of data shows that the range between the upper and lower bounds is relatively narrow and C13 can be estimated by averaging the bounds, with an accuracy of ± 2 GPa for 90% of the data. Monte Carlo simulation results show that the proposed method of computing C13 (and the consequent parameters such as δ and anisotropic Poisson’s ratios), by averaging bounds, is more robust to uncertainties in velocity measurements compared to the conventional way of computing C13. This is because of the high sensitivity of C13 and other parameters to 45o measurements, which is not required for computations in the proposed method. If is within the newly defined bounds, Thomsen δ, which was previously thought to have no relationship to other Thomsen parameters ε and γ, appears to be positively correlated to ε and negatively correlated to γ. Regression analysis shows that δ can also be roughly estimated from. Therefore, a second method (the first method being the averaging of bounds) for estimating Thomsen δ, and other related properties from vertical and horizontal velocities only, can be considered.

Anisotropic dynamic to static correction of Young’s moduli and Poisson’s ratios for VTI media will be discussed as well.

BIO

Mehdi E. Far is a principal scientist, working at Halliburton Technology in Houston. He holds a PhD degree in geophysics from the University of Houston, followed by a post-doctoral appointment at The University of Texas at Austin, Bureau of Economic Geology. His research interests include geophysics, rock physics, geomechanics, fracture modeling and elastic anisotropy. Mehdi E. Far is a member of SEG, SPWLA, SPE and he is a member of the SEG research committee.



 
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