4D Seismic Monitoring Of Gas Production And CO Sequestration

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					seismic technology: carbon capture geophysics




4D Seismic Monitoring Of Gas
Production And CO2 Sequestration,
North West Australia
David Lumley, Winthrop Professor, Woodside-Chevron Chair, Director, Centre for Petroleum Geoscience,
University of Western Australia



                                                                           Mesozaic Sub-basin
                                                                           Mesozaic Platform-Shelf
                                                                           Palaeozaic Sub-basin
                                                                           Precambrian Basement
                                                                          Oil field       Oil show
                                                                          Gas Field       Gas show




                            Fault        Gas accumulation
                            Articulate   Gas show
                            Syncline     Gas indication
                                         Oil accumulation
                            Bathymetry
                            (metros)     Oil indication

                                                            (a)                                                                                                        (b)

Figs. 1(a) and (b). Maps showing the major gas fields offshore northwest Australia in: (a) the Carnarvon Basin, and (b) the Browse and Bonaparte basins (courtesy of
Geoscience Australia).



                                                                  The major undeveloped gas fields in the             120 MMbbl, 12% CO2), and Crux (Shell; 2 Tcf
Major northwest Australia gas
                                                                  Carnarvon Basin (Figure 1a) are: Greater            + 71 MMbbl). It is currently undecided where
development projects
                                                                  Gorgon (Chevron; 40 Tcf gas, 14% CO2), Io/Jansz     the LNG facilities for Browse gas production



C
        urrently, there are estimated to                          (ExxonMobil; 21 Tcf, 1% CO2), Pluto/Xena            will be built; development of an onshore
        be at least $250 B of investments                         (Woodside; 5 Tcf, 2% CO2), Scarborough              plant at James Price Point near Broome is
        planned for developing major gas                          (ExxonMobil; 8 Tcf, 1% CO2), and Wheatstone/        a leading candidate, and some operators
fields offshore northwest Australia in the                        Iago (Chevron; 6 Tcf; 2% CO2). A new LNG            (such as Shell) are considering building FLNG
coming years. These fields make up over                           plant is planned for development on Barrow          plants offshore at their reservoir sites. Since
130 Tcf of undeveloped gas discovered to                          Island to process the Gorgon and Io/Jansz gas       Browse gas is relatively high in CO2 content,
date, located in the Carnarvon, Browse and                        production, and a major CO2 sequestration           it is likely operators will have to develop
Bonaparte basins (Department of Mines and                         project is planned to inject and store the excess   geosequestration projects. If the LNG facility
Petroleum, Geological Survey of Western                           CO2 in the Dupuy Formation. The low-CO2 gas         is based onshore, the CO2 can be separated
Australia). Most of the gas fields are far                        from Pluto/Xena will likely be processed at         at the plant and injected into suitable storage
from shore and their ultimate market,                             a new or expanded LNG plant onshore near            reservoirs, onshore or offshore. For FLNG
making them ideal candidates for LNG                              Dampier-Karratha.                                   facilities, the CO2 will have to be separated
processing and shipping. Since many of the                                                                            offshore and injected in nearby saline/depleted
gas fields have a high natural CO2 content,                       The major undeveloped gas fields in the             reservoirs, or back into the producing gas
and the LNG process requires removal of                           Browse Basin (Figure 1b) are: Scott Reef/Torosa     reservoir itself.
CO2 to liquefy the gas, there is a major                          (Woodside; 11.5 Tcf + 120 MMbbl condensate,
interest in mitigating CO2 emissions via                          8% CO2), Ichthys (Inpex; 12.8 Tcf + 527 MMbbl,      The major undeveloped gas fields in the
geosequestration (injection and long-term                         9% CO2), Brecknock (Woodside; 5.3 Tcf +             Bonaparte Basin (Figure 1b) are: Sunrise
storage of CO2 in deep rock formations).                          107 MMbbl), Calliance (Woodside; 5.3 Tcf +          (Woodside; 9 Tcf gas + 300 MMbbl
                                                                  87 MMbbl, 12% CO2), Prelude (Shell; 3 Tcf +         condensate, 4% CO 2), Barossa/Caldita


14   | PESA News | February/March 2010
                                                                                                seismic technology: carbon capture geophysics


                                                                                                                                              Due to the geologic complexity of the
                                              Monitoring gas movement                                                                         reservoirs involved, these gas production
                                                                                                                                              and CO2 sequestration projects may benefit
                                          Before                                        After                                                 greatly from 4D seismic monitoring to
                                                                                                                                              help optimise gas recovery and to monitor
                                                                                                                                              injection and long term storage of CO 2 in the
                                                                                                                                              subsurface.


                                                                                                                                              4D seismic fundamentals
                                                                                                                                              4D seismic involves repeating 3D seismic
                                                                                                                                              surveys in time-lapse mode to image changes
                                                                                                                                              in the subsurface over time, whether due
                                                                                                                                              to injection or depletion in a hydrocarbon
                                                                                                                                              reservoir, injection and storage of CO2 for
                                                                                                                                              sequestration projects, or other time-variant
                                                                                                                                              subsurface processes (Lumley, 2001). An
                                                                                                                                              example of 4D seismic monitoring images
                                                                                                                                              showing the movement of gas is shown
                                                                                                                                              in Figure 2. To first order, seismic waves
                                                                                                                                              measure the compressibility of the subsurface
                                                                                                                                              porous rock-fluid system. Thus, in order to
                                                                                                                                              create a 4D seismic signal, the fluid-saturated
                                                                                                                                              rock compressibility must change over
                                                                                                                                              time. Since gas and CO2 are much more
                                                                                                                                              compressible than water, the compressibility
Fig. 2. Example of 4D seismic images showing gas movement in a reservoir. The left panel shows the image at                                   of the reservoir rock depends strongly on
the start of gas injection (southern well), the right panel shows the image several months later, showing an                                  the gas or CO2 saturation levels (Figure 3).
unexpected path of gas flow across a fault that was originally thought to be sealing (courtesy of 4th Wave
                                                                                                                                              In general, the more compressible the rock
Imaging and Statoil).
                                                                                                                                              matrix (e.g. unconsolidated sand), and the
                                                                                                                                              larger the compressibility contrast between
(ConocoPhillips; 3.4 Tcf + 17 MMbbl, 12%                               Darwin, or other possible onshore/offshore                             the fluids of interest (e.g. water versus gas
CO2), and Evans Shoal (Santos; 6.6 Tcf +                               locations. Because of moderate-to-high CO 2                            or CO2), the larger the resulting 4D seismic
31 MMbbl, 26% CO 2). New gas production                                content, especially for Evans Shoal with                               signal. Conversely, stiff rocks and/or similarly
from the Bonaparte Basin may be processed                              26% CO 2, operators will need to develop                               compressible fluids produce a weak 4D signal,
via the existing/expanded LNG facility at                              geosequestration project plans.                                        as shown in Figure 4.


                                                                                                                                         4D Sensitivity to rocks & fluids
                                  Vp – CO2 Saturation (Critical saturation : 0.6)
                                    2.8                                    Uniform
                                    2.7                                    Patchy
                                    2.6
         P-wave Velocity (km/s)




                                                                           Voigt
                                    2.5                                                                                                                             strong
                                                                           Brie (e=3)
                                    2.4
                                                                           Brie (e=2)                                                                 4D
                                    2.3
                                    2.2
                                                                           log(18–26th)                                                            sweet spot
                                                                                                                rock compressibility




                                    2.1
                                    2.0
                                    1.9                                                                                                             4D
                                    1.8                                                                                                                  noi
                                                                                                                                                             se
                                    1.7
                                    1.6                                                                                                   weak
                                    1.5
                                          0   0.1    0.2   0.3       0.4   0.5       0.6
                                                                                                                                       fluid compressibility contrast
                                                    CO2 Saturation

Fig. 3. Seismic P-wave velocity versus gas or CO2 saturation. The dark blue nonlinear curve           Fig. 4. 4D seismic sensitivity matrix with respect to dry rock frame
is the low frequency Gassmann bound, the green quasi-linear curve is the high-frequency               compressibility (vertical) and fluid compressibility contrast (horizontal),
patchy saturation bound. The blue dots are field data measurements of Vp and CO2                      overlain with the 4D seismic non-repeatable noise envelope (pink). Stiff
saturation from time-lapse well logs during CO2 injection at the Nagaoka test site (from              rocks and fluids of similar compressibility produce weak 4D signals (lower
Lumley, 2010; courtesy of C. Konishi et al, OYO Corp. Japan, with support from RITE and               left quadrant); soft rocks and large fluid compressibility contrasts produce
METI).                                                                                                strong complex nonlinear 4D seismic responses (top right quadrant), and
                                                                                                      moderate combinations of rock and fluid compressibility produce a 'sweet
                                                                                                      spot' of both good 4D seismic detectability and interpretability in the
                                                                                                      centre of the matrix (Lumley, 2010).


                                                                                                                                                           February/March 2010 | PESA News |   15
seismic technology: carbon capture geophysics


As with the measurement of any physical
phenomenon, the ability to detect a 4D signal
                                                                                     1994                          1999                                2001
in field data depends both on the magnitude
of the signal and on the noise level in the data.
With 4D seismic, the largest source of noise
tends to be what we call 'non-repeatable'
noise, which results from the fact that we
cannot perfectly repeat the seismic imaging
experiment or its environmental conditions                                           2002                          2004                                2006
from one survey to the next. In the offshore
environment, the largest sources of
non-repeatability are due to variations in:
source-receiver positioning and geometry,
water column properties (tide heights, sea
conditions, water temperature and salinity,
etc), and source waveforms. On land, the                                   3 km             S                          N
largest sources of non-repeatability are due to
variations in: near surface conditions (water       Fig. 5. 4D seismic cross-section images of injected CO2 at the Sleipner CO2 sequestration project, offshore
                                                    Norway. The 1994 image is before CO2 injection, the subsequent images are after injection of about 1 MMt/y
table levels, soil moisture content, ground
                                                    CO2. The effects of the injected CO2 are very strong in the seismic images (courtesy Statoil).
coupling, etc), source-receiver equipment and
waveforms, and source-receiver positioning
and geometry. A major challenge going
forward will be to further reduce 4D noise                                        1999               Parallel Diff                Simult Diff
levels by improving repeatability in 4D seismic
                                                                     650                                                                              650
acquisition and processing techniques to detect
                                                                     700                                                                              700
even weaker 4D signals.
                                                                     750                                                                              750
                                                                     800                                                                              800

Monitoring gas production                                            850                                                                              850
                                                                     900                                                                              900
Geologic complexity in gas reservoirs can                            950                                                                              950
lead to significant loss of recovery efficiency.                 1000                                                                                 1000
                                                         Time (ms)




                                                                                                                                                             Time (ms)
Since gas is highly mobile compared to                           1050                                                                                 1050

water, it can be quickly produced from high                      1100                                                                                 1100

permeability zones, leaving large amounts of                     1150                                                                                 1150
                                                                 1200                                                                                 1200
unproduced gas in lower permeability areas.
                                                                 1250                                                                                 1250
Compartmentalisation of the reservoir by
                                                                 1300                                                                                 1300
stratigraphic or structural/fault mechanisms
                                                                 1400                                                                                 1400
can lead to decreased recovery. Uncertainty
                                                                 1450                                                                                 1450
in the strength of the aquifer water flow,                       1500                                                                                 1500
from beneath the gas-water contact or                            1550                                                                                 1550
from the reservoir flanks, can also impact
ultimate gas recovery. 4D seismic can be
useful to monitor gas production over time
                                                    Fig. 6. 4D seismic difference images using current best practice approach (centre) compared to 4D
to help identify permeability pathways and
                                                    simultaneous image processing method (right).The left panel is the 3D baseline survey image. Note the
barriers, areas of unproduced gas, reservoir
                                                    improvement in the imaging of the weak injection anomalies in the right panel compared to the conventional
compartmentalisation, and flow of gas and           result in the centre panel (Lumley et al, 2004).
water associated with aquifer support or
injection (e.g. Figure 2).
                                                    For this reason, with conventional 4D seismic
                                                                                                              Monitoring CO2 sequestration
As Figure 3 shows, the compressibility (and         technology we may only be able to image
thus velocity) of a rock varies non-linearly        and monitor large changes in gas saturation.              4D seismic can be extremely useful for
as a function of gas-water saturation. A rock       Increasing the ability of 4D seismic to detect            monitoring CO2 injection and storage in
will be fairly stiff when it is water full and      smaller gas saturation changes is an active               subsurface geologic reservoirs (Lumley,
becomes more compressible (softer) as gas           area of research and will likely require new              2010). The main objectives are to image the
saturation increases. However, it does not          techniques to understand heterogeneous                    location of the CO2 plume as it is injected
take much gas to get the maximum effect,            'patchy' gas saturation effects, to improve               into the reservoir, estimate its spatial extent
so that a rock with 10–30% gas saturation           seismic repeatability (i.e. decrease 4D noise),           and volume as it grows over time, and ensure
may be as compressible as a rock with 80%           and will involve the use of complementary                 that the CO2 remains safely stored in the
gas saturation. This leads to a flat portion of     geophysical techniques like electromagnetics              containment reservoir for the long term and
the velocity/compressibility-saturation curve       and gravity to increase the sensitivity of the            does not leak through confining seals or faults.
within which the 4D seismic signal, due to a        gas saturation signal.                                    Most CO2 sequestration projects will inject the
change in gas saturation, may be fairly weak.                                                                 CO2 at depths greater than 1 km, which is at



16   | PESA News | February/March 2010
                                                                                seismic technology: carbon capture geophysics


pressures and temperatures that place the CO2       to avoid releasing it into the atmosphere. 4D         WAERA, CSIRO and the CO2CRC among
in the 'supercritical' part of its phase diagram,   seismic will be useful to monitor the location        others.
implying the CO2 will have complex physical         and extent of the CO2 plume over time, and
properties of both a gas and liquid. These          to ensure that the CO2 is safely stored for
                                                                                                          References
complex physical properties, and the variable       the long term and does not leak through
properties of the reservoir rocks saturated         the geologic seals or faults that are intended        Lumley, D.E., 2010, 4D seismic monitoring of
with CO2-fluid mixtures after injection, will       to contain it. There are many active areas of         CO2 sequestration: The Leading Edge, February
determine the strength of the 4D seismic signal.    research underway to improve the 4D seismic           2010 issue, in press.
The quality of the seismic data and images,         techniques for monitoring gas and CO2,
especially the level of non-repeatable noise and    including detecting 4D signals in hard rocks,         Lumley, D.E., 2004, Business and technology
complexity of wavefields, determines whether        and for smaller gas saturation changes. These         challenges for 4D seismic reservoir monitoring:
the CO2 can be accurately detected, imaged          topics of research are currently being investigated   The Leading Edge, 1166–1168.
and quantified.                                     in Australia via the industry-sponsored UWA:RM
                                                    research consortium (University of Western            Lumley., D.E., 2001, Time-lapse seismic reservoir
Reservoir rocks favourable for 4D monitoring        Australia Reservoir Management), and also             monitoring: Geophysics, 66, 1, 50–53. ■
of CO2 include soft, unconsolidated sands
and turbidites, common in the Carnarvon
Basin, for example. At the Sleipner CO2
sequestration project offshore Norway, CO 2
is injected into a highly unconsolidated
sand containing salt water, creating a huge
change in seismic velocity/compressibility
of up to 60% (Figure 5). The effect of the
CO2 shows up dramatically in the Sleipner
4D seismic images, showing the location
of injected CO 2 as well as strong complex
imaging artifacts. Rocks that are less
favourable for 4D monitoring of CO 2 are
well-cemented sandstones, tight sands,
and stiff carbonates, which are found both
offshore and notably onshore in North West
Australia. At the Weyburn CO 2 sequestration
project in Canada, the injection of CO 2 into
stiff carbonate rocks containing residual oil
creates a small velocity/compressibility change
of only a few percent, which is at or near the
seismic noise level for detection (Figure 6). In
North West Australia, CO2 injection projects
are likely to be better 4D seismic monitoring
candidates in offshore soft-rock reservoirs, and
more difficult in onshore/offshore hard-rock
reservoirs, or in gas reservoirs that will be
re-injected with CO2 (since the residual gas
and CO2 have similar compressibility values
and thus will be difficult to discriminate from
each other).                                                The more extreme the environment, the more you need Geokinetics. Because our
                                                            highly capable crews have earned their world-class reputation by adapting to the
                                                            toughest challenges and fielding an impressive array of data-acquisition tools.
Conclusions
                                                            They have planted geophones, transported drills and deployed telemetry units in
In the coming years, there will be a                        some of the planet’s most hostile environments: from Indonesia’s steamy jungles
tremendous increase in gas project activity                 to Colombia’s treacherous mountains to Bangladesh’s precarious transition
(130 Tcf ) offshore North West Australia. 4D                zones. The tougher the challenge, the greater their satisfaction.
seismic will be useful to monitor and improve               Which is why more and more results-oriented energy companies depend on
the recovery efficiency of the gas production               Geokinetics. We deliver the decision-critical intelligence it takes to cut the cost
in reservoirs with complex geology and aquifer              of every barrel of oil you discover.
support. Since many of the gas fields have
moderate-to-high amounts of natural CO2
content, and since most of the gas will be
processed as LNG, which requires removing
the CO2 to liquefy the gas, these projects will
require injection and long-term storage of CO2
in deep rock formations (geosequestration)




                                                                                                                       February/March 2010 | PESA News |    17

				
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