Technology capable of "looking" ahead of the
drill bit continues to become less a theory and more a reality.
Its something major oil companies have sought for years.
As one Chevron official said 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."
Now, its about time.
"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.
"It works in shallow, vertical wells but not in soft sediments,
horizontal wells and in conjunction with polycrystalline diamond
compact bits, thus eliminating a substantial number of high risk
or very expensive wells that could most benefit from seismic while
drilling," he said.
"There has been a great deal of interest, so the industry
has been working for years on a seismic while drilling (SWD) tool
that can look ahead of the drill bit."
Alternatives to SWD are intermediate wireline check-shots or vertical
seismic profile surveys, typically run to reduce target uncertainty
or to provide a velocity model for seismic reprocessing.
However, VSP surveys can been very costly, because drilling must
be suspended and additional rig time taken to acquire the surveys
(particularly problematic for deepwater wells, where rig rates are
Also, the VSP data may come too late to have an impact on well
construction, because the tool is typically 100 feet back from the
Such limitations have pushed SWD research for a tool to provide
critical data for well placement in real time at a practical cost,
As a result, an SWD tool was recently introduced that uses:
- A logging-while-drilling tool with sensitive seismic receivers.
- A processor and memory.
- A surface seismic source.
- A measurement-while drilling system for real-time telemetry.
The entire process adds no rig time and does not disrupt the drilling
The primary applications are in wells where it is uneconomic or
impossible to run wireline.
Another important use is in wells where the uncertainty is high
and 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, he said.
The technological hurdle for a SWD tool was a means to synchronize
the surface source with the downhole receivers, Thompson said.
"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
mean time in London to account for the earths 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."
Another challenge for downhole data management was the lack of
efficient communication. An automated signal recognition for stacking,
time picking and data storage was developed to solve this issue.
Also, the downhole tool calculates several quality control indicators
that are transmitted up hole in real time to build confidence in
the calculations of the downhole tool, Thompson added.
Step By Step
Basically, the SWD 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
The seismic energy is received by the specially designed downhole
LWD tool with three geophones and one hydrophone, which is in communication
with an MWD mud-pulsing system that provides data transmission to
The geophones respond to the tool vibration while the hydrophone
responds to pressure waves in the borehole fluid, Thompson explained.
The seismic signals are acquired, stored and processed to provide
key information for transmission up hole.
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
It isnt 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.
"There are various levels of look ahead data companies would
like to have," Thompson said. They are:
Real-time check shot data, which this tool provides.
"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, eliminating that uncertainty."
ahead data -- the waveform information, which allows the drilling
engineer to see 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."
look ahead data -- the "ultimate holy grail,"
"This is why we dont think wireline will ever be replaced,"
Thompson added, "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."