C. Macbeth et al., ESTIMATION OF RESERVOIR FRACTURING FROM MARINE VSP USING LOCAL SHEAR-WAVE CONVERSION, Geophysical prospecting, 46(1), 1998, pp. 29-50
A marine VSP is designed to estimate the orientation and density of fr
acturing within a gas-producing dolomite layer in the southern North S
ea. The overburden anisotropy is firstly estimated by analysing shear
waves converted at or just below the sea-bed, from airgun sources at f
our fixed offset azimuths. Full-wave modelling helps confirm that the
background has no more than 3% vertical birefringence, originating fro
m TM anisotropy with a symmetry axis orientated perpendicular to the m
aximum horizontal compressive stress of NW-SE. This finding concurs wi
th current hypotheses regarding the background rock matrix in the uppe
r crust. More detailed anisotropy estimates reveal two thin zones with
possible polarization reversals and a stronger anisotropy. The seismi
c anisotropy of the dolomite is then determined from the behaviour of
locally converted shear waves, providing a direct link with the physic
al properties of its fractures. It is possible to utilize this phenome
non due to the large seismic velocity contrast between the dolomite an
d the surrounding evaporites. Two walkaway VSPs at different azimuths,
recorded on three-component receivers placed inside the target zone,
provide the appropriate acquisition design to monitor this behaviour.
Anisotropy in the dolomite generates a transverse component energy whi
ch scales in proportion to the degree of anisotropy. The relative ampl
itudes, for this component, between the different walkaway azimuths re
late principally to the orientation of the anisotropy. Full-wave model
ling confirms that a 50% vertical birefringence from TM anisotropy wit
h a similar orientation to the overburden is required to simulate the
field observations. This amount of anisotropy is not entirely unexpect
ed for a fine-grained brittle dolomite with a potentially high fractur
e intensity, particularly if the fractures contain fluid which renders
them compliant to the shear-wave motion.