Page 137 - Introduction to Petroleum Engineering

124 RESERVOIR GEOPHYSICS
TAbLE 7.1 Classification of Earthquakes
Class Magnitude Comment

>8.0 Great Can destroy communities near the epicenter
7.0–7.9 Major Serious damage
6.0–6.9 Strong Significant damage in populated areas
5.0–5.9 Moderate Slight damage to structures
4.0–4.9 Light Obvious vibrations
3.0–3.9 Minor Weak vibrations
2.0–2.9 Very minor Hardly felt

Table 7.1 presents a classification of earthquakes by magnitude. Moment
magnitudes can be negative. For example, Warpinski et al. (2012) pointed out that
microseismic events associated with fracturing operations in shale have negative
moment magnitudes on the order of −2 to −3. The fracturing operations are known
as hydraulic fracturing and are used to fracture very low permeability shale. The
fractures are kept open with proppants to provide higher permeability fluid flow
paths to the wellbore.

7.2 ACOuSTIC IMPEDANCE AND REFLECTION COEFFICIENTS

Seismic waves are detected at receivers after some of the vibrational energy of the
incident seismic wave is reflected by a geologic feature. Some of the vibrational
energy from the incident seismic wave is transmitted and some is reflected when the
incident seismic wave encounters a reflecting surface. Seismic reflection occurs
at the interface between two regions with different acoustic impedances. Acoustic
impedance Z is the product of bulk density and seismic wave velocity in a medium.
If the velocity is compressional (P‐wave) velocity, acoustic impedance of P‐wave
velocity is compressional impedance:
V
Z = ρ B P (7.7)
P
The bulk density of a rock–fluid system depends on rock matrix grain density ρ ,
m
fluid density ρ , and porosity ϕ:
f
ρ = (1 − φ ρ ) m + φρ f (7.8)
B
Fluid density for an oil–water–gas system is

ρ = ρ S o + ρ S w + ρ S g (7.9)
o
f
g
w
where ρ is fluid density of phase ℓ and S is saturation of phase ℓ. Subscripts o, w, and
ℓ
ℓ
g stand for oil phase, water phase, and gas phase, respectively. Shear impedance is
acoustic impedance calculated using shear velocity:
V
Z = ρ B S (7.10)
S