buzz aplenty that 4-D, or time-lapse, seismic is the next new big
thing in the geophysical arena, particularly as it applies to the
not talking the traditional 4-D method, where the operator commissions
additional towed streamer 3-D surveys at specific time intervals
to get a handle on whats happening in the reservoir. Todays
world is becoming increasingly specialized, demanding some very
specialized equipment, especially in the deepwater environs.
the anticipated need for more frequent yet cost-effective surveys,
contractors and manufacturers have engaged in heavy duty R&D
to come up with some amazingly sophisticated, high-tech tools for
this challenging environment.
continue to be made in the realm of permanently placed cable systems,
for instance, and significant advances in flexible, self-contained
nodal systems have nodal technology poised to become a system-of-choice
for many operators.
this is an industry that essentially has redefined the meaning of
slow adaptation; "wait and see" remains the mantra.
period between trial status to project status for permanently placed
cable technology is a prime example: An experimental permanent seafloor
seismic array was first installed at Foinhaven in 1995 by Shell
and BP. It wasnt until last August, however, that the first
permanently placed cable system to be financed and purchased by
a business unit was finally installed, according to Gary Owens,
chairman, president and CEO of OYO Geospace, which manufactured
is the BP-operated Valhall Field in Norway, where the presence of
a gas cloud over part of the reservoir distorts the conventional
seismic image acquired using towed streamers. Another constraint
to streamer technology there is in-place infrastructure.
survey was shot in December 2003, according to Rodney Calvert, geophysical
consultant at Shell, which is a partner at Valhall.
done a couple of repeat lines on it," Calvert said, "and
it is very repeatable, so it looks excellent."
has been one of the limiting aspects of using towed streamer data
for 4-D programs, various experts say; and, according to Dave Ridyard,
business development manager at Input/Output, towed streamer technology
has fundamental limits of resolution.
starters, how repeatable is it, or how accurately can you put it
back in the same place and repeat the noise problem?" he queried.
"And you cant record shear wave with towed streamer,
so if you think the primary fluid movement information is going
to be in the shear wave information, then you need a seafloor full-wave
more subtle limitation I'm seeing is quite a few oil companies are
starting to use 4-D not just as a reservoir monitoring tool but
as part of an active reservoir management tool," Ridyard added.
"Reservoir monitoring tells you what happened in the reservoir,
but reservoir management tells you fast enough you can do something
about it before the bad thing happens.
towed streamer 4-D, by the time I get a boat, shoot, process the
survey, take out the differences and get the right answer, Im
just about in time to explain the disaster that just occurred,"
intervention is as big a driver toward permanent monitoring systems
as the imaging repeatability issue," Ridyard noted.
has the potential to be substantial.
the life-of-field seismic reservoir management program at Valhall,
where productive life exceeds 20 years, is anticipated to lead to
improved drainage and well placement resulting in an additional
60 million barrels of production, according to Olav Barkved, lead
geophysicist for the BP Valhall subsurface team.
curve is high.
have field tests and related experience (for permanent cable),"
Barkved said, but theres no textbook or reference case.
are a number of issues and concerns," he added. "For instance,
there is a lot of equipment on the seafloor with a lot of connectors
in place, and one concern is, will this last for long enough?
element is how far to push the 4-D response what type of
sensitivity can we really get out of the system," Barkved said.
due in March from the initial survey will address this question,
among others. Plans include a minimum of three more surveys this
year alone, with mobilization already under way for the first.
millions of dollars already expended on new and improved seismic
technology for programs such as Valhall, the equipment manufacturers
have a lot riding on a success there.
though 4-D has been talked about extensively and encouraged by the
E&P companies, in terms of dollars spent 4-D has yet to be a
salvation for the geophysical industry," said Steve Mitchell,
vice president operations at Fairfield.
is in the air, and in a sense Valhall may prove to be an exception
in 4-D application rather than the rule to the advantage
of the equipment providers.
are beginning to take a serious look at how to accelerate a drilling
program at the start of a fields life as opposed to the usual
approach of applying reservoir evaluation via 4-D late in the game
to try to stem the production decline. The goal is to add oil early
on when it has more economic impact.
also falls in with when people sometimes have a discovery, and they
say OK, how big is it?" Fairfields Mitchell said. "Top
side equipment to develop a 400 million barrel field is far greater
than with a 100 million barrel discovery.
a better seismic survey after a discovery which used relatively
inexpensive towed streamer is far less expensive than a $200 million
mistake on topside equipment," Mitchell noted. "And it
gives a baseline for 4-D."
to good 4-D is to repeat the ray paths in the acquisition process,
using the same shot and receiver positions, according to Calvert.
(ocean bottom cable) is one way to do this," Calvert said,
"but to trench OBC in the seabed where its in a fixed
location is very expensive.
potentially cheaper [bottom reference survey] solution is OBS (nodes),
where you dont buy the equipment but rent it for the duration
of the survey.
cables pay out if there are many, many surveys because you only
need a shooting vessel for repeats," Calvert noted. "It
looks like six to 10 repeats would be needed before it starts to
be cheaper than other methods. If you dont know how many times
you want to survey a field, you may find the answer to your questions
after only two surveys. Then youve got expensive stuff on
the seabed thats not being used.
number of times you will shoot is an important parameter in deciding
on cable versus nodal."
In the cyclical
oil and gas industry, where operators cant afford to ever
avert their eyes from the bottom line, unnecessary expense ranks
right up there with the plague. Economics aside, however, there
are other noteworthy differences between permanently placed cable
configurations and the less expensive nodal systems that are drawing
self-contained nodal units are placed on the seabed via remote operated
vehicles (ROV), which enable the equipment to be deployed in high-productive
you move in with 4-D, you must place it where it was the first time,"
said Hal Haygood, engineering manager at Fairfield. "ROV technology
lets you do this, and you know both the tilt and the direction of
the unit within two degrees and placement accuracy within one meter.
Repeatability is high"
of any hard link between units circumvents noise issues common to
most cable systems, enhancing vector fidelity. This also enables
deployment flexibility in areas with subsea infrastructure, where
pipeline operators frown at the prospect of cables placed over their
typically are left in place to collect data, including multi-component,
for 10-15 days or so before being brought up to extract data from
each unit in a matter of seconds.
clock crystal technology advances over the last five years that
has allowed us to do this, because all the units must be synchronized
to take samples at the same time," Haygood said. "We know
the exact time each shot was fired."
uncertainties inherent with most cable and streamer systems are
circumvented using nodes because the units are azimuthly symmetrical,
meaning the response is the same in all directions. The system can
shoot any type survey design, e.g., high-fold wide azimuth for subsalt
imaging, whereas cables are incapable of reaching out to accomplish
proven in shallow water applications, nodal technology for deployment
in deep water programs is being readied for a field debut in the
coming year, according to Mitchell.
aside, the consensus is theres a target market for both cable
and nodal technology.
big picture is theyre tools, and all tools have a place,"
Mitchell said. "Sometimes you need a socket, and sometimes
you need a wrench."
for these tools has the potential to enlarge considerably
its a mindset thing.
push for 4-D seismic technology has been confined to the geophysical
community for the most part, and some folks think that must change.
reservoir engineers must take it more on board," Calvert said.
of the reservoir engineers in the world are keen on 4-D, and the
other half have resisted," he said. "It has tremendous
potential for them because they can see what happened in a field.
need a revolution in thinking to really exploit 4-D.
you start with a field you can make a model, but you should not
believe it and should know its a first approximation
fluid flow through the earth is much more complicated than expected,"
Calvert said. "If you plan from the start for 4-D, you can
proceed to develop the field with monitoring in a rational way.
a sort of myth that you make models, give predictions and all are
happy," Calvert said. "Those predictions and models are
wrong and why we are getting low recovery rates.
we used a phased production approach and are honest about our uncertainty,"
Calvert said, "then with 4-D we could do a better job."