Microseismic Monitoring and Geomechanical Modelling of CO2 - bris

CHAPTER 3.
INVERTING SHEAR-WAVE SPLITTING MEASUREMENTS FOR FRACTURE PROPERTIES
Parameter Value
Frequency (ω) 100kHz
Fluid viscosity (η) 1×10 −3 Pa.s
Fracture aperture (c) 2×10 −5 m
Fracture aspect ratio (a) 0.0036
Fracture density (ξ) 0.1
Porosity (ϕ) 0.3460
Fluid bulk modulus (K fl ) 2.16GPa
Rock shear modulus (µ r ) 12.7GPa
Rock bulk modulus (K r ) 16.6GPa
P i 50
P ep
3×10 −4
Table 3.1: Physical properties of the synthetic fractured rock created by Rathore et al. (1994),
used to test the influence of fractures on ultrasonic velocities.
ometry and orientation. These were subsequently dissolved from the matrix using a chemical leachate,
leaving the resultant voids to represent a network of fractures whose geometries were known a priori.
The seismic velocities of these samples were then measured in the lab at ultrasonic frequencies when
both dry and water saturated. Their results provide a benchmark against which the above models
can be tested.
Table 3.1 lists the relevant parameters for the brine saturated synthetic sandstone constructed by
Rathore et al. (1994). Rathore et al. did not compute κ, so I use the estimate given by Hudson et al.
(2001) as κ = 300mD. Using these values, P ep ≈ 3 × 10 −4 and P i ≈ 50. Therefore I expect fluid
flow to occur and pressure gradients to be fully equalised. As a result, Hudson’s (1981) model, which
assumes that fractures are hydraulically isolated, will not be appropriate. Figure 3.3 shows Rathore
et al. (1994)’s experimental results along with the modelled values for P- and S-wave velocity assuming
isolated fractures in (a) and using frequency dependence in (b). I note in Figure 3.3(a) the P-wave
velocity from the experimental results does not match the predictions from Hudson’s 1981 isolated
fracture model, of a cos(4ζ) periodicity, where ζ is the incidence angle between the aligned fracture
faces and the ultrasonic waves. In contrast, Figure 3.3(b) compares experimental velocities with those
predicted from Hudson’s 1996 fluid exchange model, which does account for frequency-dependent
flow. It is clear that the fit is far superior, with a cos(2ζ) periodicity. This testing demonstrates
the sensitivity of rock physics modelling to the extent of fluid flow, and highlights the need when
interpreting data for awareness of the likely extent of fluid flow through estimation of P i and P ep and
use of an appropriate model.
For seismic waves passing through reservoir rocks, ω is small (10 < ω < 250) and κ is likely to be
high. Table 3.2 shows values for the relevant parameters for a generic reservoir example with CO 2
and brine filled fractures, and the resultant values of P ep . It can be seen that P ep for both cases
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