Geology Reference
In-Depth Information
a)
b)
a)
b)
Top reservoir subsidence from
reservoir geomechanical model
Baseline, T
0
=1988
Time lapse, T
1
=1999
Top reservoir subsidence from
seismic time-lapse timeshifts
630ms
∆
t
(ms)
∆
z
(m)
-6
700ms
2948ms
Top reservoir
-4
-1.0
-2
-0.5
3008ms
3032ms
0
0.0
3092ms
3092ms
3152ms
Base reservoir
0100
500m
Figure 10.32
Time-lapse time shifts related to subsidence on
Ekofisk Field North Sea (after Nickel et al.,
2003
). The figure shows
baseline sections (a) and monitor sections (b) for various two-way-
time intervals. The dashed lines are seismic picks made on the
baseline survey; comparisons of baseline and monitor sections
illustrate the timing effect related to reservoir compaction and
subsidence.
Figure 10.33
Observed and predicted reservoir compaction on
South Arne Field, offshore Denmark (after Herwanger et al.,
2010
);
(a) compaction-induced travel time changes to the top-reservoir
reflector (time-lapse time-shifts
) as a measure for top-reservoir
subsidence, (b) predicted vertical displacement
Δ
t
of the top-
reservoir surface from a geomechanical model. Note that a 6 ms
increase in travel time corresponds roughly to 1.5 m top-reservoir
subsidence. Yellow areas
Δ
z
observed subsidence larger than
predicted subsidence, red areas
¼
A commonly used generalised measure of repeat-
ability is the normalised root mean square (NRMS)
(Kragh and Christie,
2002
). This is calculated in zones
where no production changes have taken place and is
defined as:
observed subsidence is less than
predicted subsidence. Note how faults exert a large degree of
control on both observed and predicted subsidence.
¼
Beasley et al.,
1997
). For this type of acquisition the
accuracy of cable deployment as well as receiver/sea
floor coupling are important success factors. The
same is true for ocean-bottom nodes (e.g. Beaudoin
and Ross,
2007
). Further improvements in repeatabil-
ity might be gained from permanent seabed receiver
installations entrenched in the seabed (e.g. Jack et al.,
2010
). On land, repeatability of acquisition geometry
is not a problem; instead, the main difficulty lies
in changes to source and receiver coupling and scat-
tering in the weathered zone, which may vary through
the year with changes in soil saturation and position
of the water table. Pevzner et al.(
2011
) describe an
NRMS value of about 20% for a time-lapse survey on
the Otway CO2 sequestration project in Australia.
Seismic processing for time-lapse aims to minim-
ise differences in the amplitude, phase and timing for
non-reservoir reflections and thereby enhance signals
related to production differences (e.g. Ross et al.,
1996
; Rickett and Lumley,
2001
). Experience has
shown that it is more beneficial to combine the moni-
tor survey processing with re-processing of the base-
line survey than attempt to match different surveys
with different acquisition and processing (e.g. John-
ston,
2013
). Cross-equalization tools such as space
2RMS a
ð
b
Þ
NRMS
¼
,
ðÞ
ðÞ
RMS a
+RMS b
where a and b are the two surveys being compared.
Typical values for NRMS are around 10%
30%. How-
ever, what is acceptable will depend on numerous
factors including acquisition and processing as well
as the magnitude of the time-lapse signal.
A key issue in achieving acceptable levels of
repeatability is that the acquisition should reproduce
the source and receiver positioning as accurately as
possible. This is quite difficult in the case of marine
streamer data, because currents in the seawater cause
the receiver streamer to be pushed to one side rather
than being towed in a straight line directly behind the
ship, an effect usually called streamer feathering.
These currents often vary significantly with time due
to tidal or larger-scale circulation effects. Matching
feathering angles and overlapping streamer coverage
as well as shooting infill lines on the basis of repeat-
ability rather than simple fold (i.e. coverage) criteria
are typical strategies used to address this situation.
Better receiver-position repeatability is possible where
receiver cables are deployed on the ocean floor (e.g.
-
245
