low

Background
As it was introduced on opening part of this
section, monitoring of IOR/EOR projects
is being considered necessary to guide and
guarantee the success of these projects. In lack of
a proper monitoring program, the fate of injected
material would be unknown with high risk of
injected material (e.g., CO 2
, Methane, Polymer,
Modified water) to be migrated to another layer.
There have recently been introduced variety of
monitoring programs, however, 4D seismic, due
to its full and 3 dimensional coverage of reservoir,
is being considered as the main monitoring
program. Utilising 3D and 4D seismic data,
international oil and gas companies have improved
the reservoir recovery factor in North Sea above
30% and reaching at around 60% [1] . By measuring
the water and gas fronts and pressure variation
in the reservoir as well as by identifying bypassed
oil and gas, it has been successfully used on most
of IOR/EOR projects on the North Sea reservoirs
to optimise IOR/EOR projects, well placement
and production/injection plans. On the other
hand, most of failed injection projects have been
typically suffering from lack of comprehensive
monitoring programs.
What Does Seismic Data Offer to
Us?
Seismic data contains quantitative and valuable
information that has widely been used
during reservoir exploration, development,
production and monitoring steps. Quantitative
seismic interpretation such as rock physics,
AVO modelling, inversion and geomechanics is
normally ended up with extracting reservoir static
parameters such as porosity, shale content and
oil and gas saturation from the seismic data. 4D
seismic, which is a series of repeated 3D seismic
surveys over time, has been extensively used
by oil and gas companies to monitor reservoir
production and injection in time and space. The
elastic and acoustic parameters of fluid and rock
changes by production activities, and this affects
the reflection coefficients at the top as well as at
the base of reservoir. These changes are detected
by amplitude changes in different angles,
timeshift or even frequency-derived attributes
(Figure 1). 4D seismic has the potential to provide
information regarding fluid movements, pressure
changes, reservoir compaction, barriers and
compartments, fault transmissibility and general
connectivity.
These information assist to optimise IOR/
EOR projects in the filed scale, improve well
performance and possibly increase a field’s
economic life. Time lapse seismic applicability has
been proven for monitoring of gas injection for the
storage purposes, water injection and managing
the gas coming out of solution. It has also been
used in monitoring of heavy oil reservoirs, gas
injection and gas reservoir production [3] . Figure
1 shows repeated saturation logs on 1989, 1992,
1993, 1994, 1995 and 1997 on Gullfaks field
(North Sea). It also shows seismic data on 1985
(before production) and 1999 after production
and injection. Utilising Rock Physics analysis, top
of oil bearing sandstone is represented by yellow
colour on the seismic sections. As it can be seen
from Figure 1, yellow colour signal disappears
once it reaches at oil-water contact. OWC
movement can be observed on both well logs
and seismic data that matches with the animated
figures on the right hand side.
The Selected Case Studies
In this brief article, there are presented a few
successful examples from the literature. The
first example (Figure 2) is from North Sea. Halfdan
reservoir is a Carbonate (Chalk) oil reservoir that is
under FAST (Fracture Aligned Sweep Technology)
production method [1] . Horizontal wells produces
for 6 months until it is converted to the water
injection well. Water front is monitored by 4D
seismic data. Water replaced by oil presents
hardening signal (increase in acoustic impedance)
on the 4D seismic maps that is shown by blue
colour on Figure 2-a and b. A highlighted area
is shown on d and e for better visualisations. As
an example, the water front around the injector
well (dashed blue line) is presented on 2005-1992
maps (a and d). Water front movement towards
production wells (green lines) can be detected on
2012 on b and e maps. White colour represents unswept
oil in these figures. Figure 2-c and f shows
2012-2005 to understand the water movement
over time in one map. Red colour signals on these
maps represent the softening signal (decrease in
Figure 1: Repeated saturation logs on 1989, 1992, 1993, 1994, 1995 and 1997 with seismic data before production
(1985) and after production and injection (1999). Oil-Water contact movement can be observed by comparing
two seismic sections that matches with the saturation logs over the time [2] .
Figure 2: 4D seismic maps on 2005-1992, 2012-1992 and 2012-2005 on Halfdan oil field. The bottom figure shows
the selected area in detail. Blue colour represents the hardening 4D seismic signal that is due to oil replacing by
water. On the other hand, red colour signal represents the oil replacing by gas due to gas coming out of solution
mainly on the northern parts in which pressure goes below bubble point pressure. Production and injection wells
re shown by green and dashed blue lines [1] .
44 OIL INNOVATORS International Journal MAR. 2018 45