Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 6.
GENERATING ANISOTROPIC SEISMIC MODELS BASED ON GEOMECHANICAL SIMULATION
4000
σ33
3500
Velocity (m/s)
3000
2500
σ11
2000
1500
0 5 10 15 20 25 30 35
Hydrostatic Pressure (σ 11
= σ 22
= σ 33
) (MPa)
40
σ33 > σ11= σ22 = 0
(a)
(b)
150
4000
Shear Stress (σ 33
− σ 11
) (MPa)
100
50
0
5 10 15 20 25 30 35
Confining Stress (σ 11
) (MPa)
40 45
(c)
Velocity (m/s)
3800
3600
3400
3200
3000
2800
2600
2400
2200
2000
0 5 10 15 20 25 30
Confining Stress (σ 11
) (MPa)
35 40
(d)
Figure 6.9: Panel (a) shows observed and back-calculated P- and S-wave velocities for hydrostatic
compression of Berea sandstone. The best fit parameters (ξ 0 and a 0 ) for the hydrostatic case are
used to model the uniaxial case (d). The details of the uniaxial experiment are shown in (b) and
(c). Experimental data (symbols) from Scott and Abousleiman (2004). Red - V P z green - V P y ,
black - V P 45 , cyan - V Sxy , yellow - V Syz .
due to the fact that cores generally sample more competent zones of a reservoir, and that they may
miss larger scale fractures which could increase stress sensitivity, it is of interest to consider how to
account for the damage due to coring (or other mechanisms) within the framework of my model.
MacBeth and Schuett (2007) demonstrate the effect that damage can have on a sample, though in
this case the damage is caused not by coring but by thermal expansion of grains during heating. Figure
6.10 shows measurements of ultrasonic P- and S-wave velocities from samples before and after they
have been damaged by heating. Assuming the isotropic background compliance given by MacBeth
and Schuett (2007), and an isotropic α, we use (6.37) and (6.38) to find the optimum values of ξ 0 and
a 0 that minimise misfit between observed and modelled velocities.
Table 6.4 shows the values of ξ 0 and a 0 used to calculate the modelled velocities in Figure 6.10.
It is clear that the differences between damaged and undamaged samples can be accounted for solely
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