A Chevron official said it, in 1996:
"The time is coming when we will not drill without looking
ahead of the bit anymore than we would drive at night without headlights
— occasionally shining a lamp to see what we hit."
That time is getting even closer.
A new downhole "seismic measurement while drilling" technique
has been designed to provide information in real-time, allowing
better correlation of surface seismic and well placement.
Downhole seismic techniques have been around for years, beginning
with drill bit seismic that uses the drill bit as a source and surface
receivers to record borehole seismic information in primarily a
time versus depth measurement.
"However, over the years we have found that drill bit seismic
works very well in certain applications and not at all in others,"
said Jim Thompson, drilling and measurement acoustic product champion
for Schlumberger Oilfield Services. "The industry has been
working for years on a seismic while drilling tool that can look
ahead of the drill bit."
The alternative to seismic while drilling is intermediate wireline
check-shot or vertical seismic profile surveys, typically run to
reduce the uncertainty in putting the drill bit on seismic images
— and sometimes to provide a velocity model for seismic reprocessing.
VSP surveys, however, can be costly, since drilling must be suspended
and additional rig time taken to acquire the surveys. This is particularly
problematic for deepwater wells where rig rates are extremely high.
Also, the VSP data may come too late to have an impact on well
construction, Thompson added, since the tool in typically 100 feet
back from the drill bit.
Such limitations have pushed research on a seismic while drilling
tool that can provide critical data for well placement in real time
at a practical cost, he noted. The result came about a year ago,
when a seismic measurement while drilling tool was commercialized
that uses:
- A logging-while-drilling tool with sensitive seismic receivers,
a processor and memory.
- A seismic source at the surface.
- A measurement-while drilling system for real-time telemetry.
- A conventional seismic source activated during pipe connections.
The downhole tool calculates check-shot information and the MWD
system transmits the data to the surface after mud circulation resumes.
The entire process adds no rig time and does not disrupt the drilling
process.
Step by Step
The technological hurdle for a seismic measurement while drilling
tool was a means to synchronize the surface source with the downhole
receivers, according to Thompson.
"It was much like the problem that navigators faced in determining
longitude in the early days of exploration," he said. "They
needed a clock onboard the ship that was synchronized to Greenwich
meantime in London to account for the earth‘s rotation, but
the challenge was ruggedizing a clock for the rigors of seafaring.
"Our problem was much the same," he continued. "We
had to develop a downhole clock of sorts that could withstand the
drilling environments of 25,000 psi and 100 degrees Celsius.
"It took seven years to perfect a rugged clock to the accuracy
we needed."
The lack of efficient communication for downhole data management
was another challenge, according to Schlumberger officials; that
was solved with an automated signal recognition for stacking, time
picking and data storage.
Officials say the downhole tool calculates several quality control
indicators that are transmitted up hole in real time.
Basically, the seismic measurement while drilling tool uses a conventional
surface source such as an airgun. Offshore the source can be mounted
either on a rig for a vertical or slightly deviated well, or on
a boat for a highly deviated well.
It works like this:
-
The seismic energy is received by the specially designed downhole
LWD tool with three geophones and one hydrophone, tied to an
MWD mud-pulsing system that provides data transmission to the
surface.
- The geophones respond to the tool vibration while the hydrophone
responds to pressure waves in the borehole fluid. The technique
provides check-shot data in real time via the MWD transmission,
and waveforms are recorded in the tool memory for later VSP
processing after a bit trip.
-
A downhole algorithm continuously analyzes sensor data to determine
if suitable acoustic conditions exist and if suitable signals
are present.
-
When the source activation is complete, the tool begins to
process data for time-pick information and quality indicators.
The most important data are transmitted up hole when circulation
and MWD telemetry resume.
-
Proper depths are assigned to the real-time data at the surface,
and the time-depth pairs are used to locate the bit on the surface
seismic section.
(It isn’t possible today to send the waveforms up hole in
real time because of bandwidth and transmission speed limitations
imposed by current MWD mud-pulsing systems. However, scientists
are working to overcome this limitation.)
"This system functions much like a GPS system, placing the
well on the surface seismic map," Thompson said. "The
time image acquired with traditional seismic has depth uncertainty
associated with it, and this new technique can correct in real time
the surface seismic map."
The Holy Grail
The second level of look ahead data is the waveform information,
which allows the drilling engineer to look ahead and see the reflectors
ahead of the drill bit.
"We can do this in memory today, but by next year this data
will be available in real-time as well," Thompson said. "The
waveform data is capable of over 8,000 feet of look ahead, although
1,000 is a practical application.
"The ultimate holy grail of real-time look ahead data, which
we don’t claim we will ever be able to provide, is inversion
of these waveforms," he added. "This is why we don’t
think wireline will ever be replaced, because the waveform quality
is much better with wireline data. Waveform inversion provides the
velocity ahead of the bit, which closely follows pore pressure,
so we also can constrain the pore pressure model.
"Look ahead pore pressure data is the holy grail of downhole
information," he said.
The primary applications for the new seismic measurement while
drilling tool is in wells where it is uneconomic or impossible to
run wireline, according to Thompson.
Another important use is in wells where the driller expects to
encounter a number of problems in real time, such as the number
of liners and casing strings, the location of the salt or seismic
steering in the reservoir.
Several case studies exist, including:
-
One operation in Azerbaijan, on a well in the South Caspian Sea. The
well penetrated a steeply dipping structure in the Pliocene Fasila
formation.
The A-1 well was a deviated wildcat well designed to intersect
the reservoir section at about 4,500 meters true vertical depth
while avoiding faulting, high pore pressures and areas of poor seismic
data quality in the overburden at the crest of the structure.
Prior to drilling, the operator was not at all confident about
the depth estimates based on surface seismic; this uncertainty resulted
in a 700-meter range in depth estimates for the top of the reservoir
section.
Accurate real-time positioning on the seismic was needed to avoid
radical corrections to the well trajectory. High pore pressures
also were identified as a key risk and the interval velocities were
also used to assess this problem.
Drillbit seismic was ruled out due to the rock bit limitation and
the soft nature of the sediments.
The surface source was positioned on a boat vertically above the
downhole receiver. Data were acquired at pipe connections during
drilling and tripping in and out of the hole. The real-time check-shot
data were transmitted via mud pulse telemetry, checked by the SWD
engineer and transmitted via telephone or e-mail to the operations
geophysicist and processor onshore.
The seismic waveforms and time-depth pairs were downloaded from
the tool when it was brought to surface on bit trips. Waveform data
were transferred to a processor onshore and used to validate the
real-time results.
All the data was acquired with no impact on drilling time, and
the check-shot values that were transmitted in real time and those
derived from the memory data compared extremely well. The SWD data
accurately confirmed the location of the well picks on the seismic
down to 3,500 meters and, below this depth, resolved where the estimates
started to diverge. Access to the data in real time reduced the
depth uncertainty and allowed drilling to proceed.
Confidence in the quality of the SWD results was enhanced when
a conventional wireline survey was acquired over the same interval
— differences in the two surveys were less than three
milliseconds, indicating that using the new, less expensive tool
would lead to minimal errors of only a few milliseconds, or equivalent
to depth errors of less than 10 meters at four kilometers in depth.
-
An
exploration well offshore Brazil, where it was used to mitigate
operational risk identified in the setting of both 13 and 5/8-inch
and 11- inch casing shoes, and to avoid potential drilling problems
arising from entering the primary well objectives with significant
sections of open hole.
The B-2 well was drilled in an area where the nearest offset well
was some distance away and the velocity field was expected to be
different. Consequently, the resulting error bars on the depth picks
prior to drilling were plus or minus 10 percent, which would have
resulted in a sizable depth uncertainty while drilling, even after
the planned intermediate wireline VSP was acquired at about 1,600
meters below sea level.
Another complication was that, to be able to drill a vertical well
to test the prospect, a location had to be selected where the shallowest
primary target would be penetrated just below a fault.
To minimize drilling risk it was considered essential to set the
13 and 5/8-inch casing immediately above the target interval but
below the overlying fault to prevent up fault leakage from the open
hole section in case the target interval contained gas. However,
pre-drill planning showed that even after a VSP had been taken at
the shallower 20-inch casing shoe, the error bars for the fault
and the primary target sand would still overlap, making it impossible
to select a depth where the well had definitely passed through the
fault and was still above the primary target.
SWD allowed the progress of the well to be mapped continuously
on the surface seismic via real-time time-depth pairs. The technique
made clear when the fault had been penetrated, providing the opportunity
to set the casing above the target.
The real-time check shot information obtained from the SWD reduced
the depth uncertainty to plus or minus 1 percent compared to the
surface seismic map. Also, the uncertainty of the shallowest location
of the top of the target was reduced considerably. As a result,
the 13 and 5/8-inch casing shoe was set optimally between the deepest
fault and the shallowest target formation.
The pre-drill time-depth relationship was placing targets approximately
80 meters too shallow. If the operation had relied strictly on this
estimate, the 13 and 5/8 inch-casing shoe may be been placed above
the fault.
SWD also saved at least two intermediate wireline VSPs, which substantially
reduced overall costs.
According to Thompson, these are just the start.
"Applications for this technology will expand over time as
the technique evolves," he said. "Today it is still serving
a niche market for expensive, high risk deepwater wells, but eventually
— with further advancements — the uses will expand to
a wide variety of scenarios."