Blowouts are bad, right? Scientists have
worked hard to prevent and minimize the damage to life, property
and the environment caused by these out of control wells.
But wouldn't it be nice to have some method to anticipate
a blowout and take measures to prevent it before it happens?
That's just what one research project could potentially
bring to the industry.
A recent three-year study conducted at Reading University
in the United Kingdom tested the use of lasers to detect and quantify
the presence of gas bubbles in the drilling slurry during drilling
This new method measures simultaneously the velocity,
size and refractive index of large, optically transparent bubbles
and droplets - based on the time displacement of refracted and reflected
beams scattered from the moving particles, according to David R.
Waterman, a physicist with the J.J. Thompson Physical Laboratory
at Reading University.
"Phase Doppler anemometry techniques used today
have an upper size limitation and do not address the velocity of
gas bubbles coming into the well," Waterman said. "The
pulse displacement technique developed in this project has the ability
to measure simultaneously the size, velocity and refractive index
of particles and bubbles up to several millimeters in diameters.
"Detection of the bubbles gives the operator
important information," he continued, "so the pressure
of the slurry can be increased to push against the imminent increase
in pressure from the gas pockets, potentially preventing a dangerous
The project was initiated when Schlumberger Cambridge
came to the university with an interest in looking at bubbles in
the drilling slurry, hoping to develop a method of using a fiber
"We started off looking at something that we
could actually put down the borehole on fiber optics to detect gas
bubbles and therefore gas pockets," Waterman said.
Researchers "realized early on" that the
limited amount of funding meant they would have to develop a system
that worked in the laboratory, and then "see where that would
"But the idea was borne specifically to address
the needs of the oil industry," Waterman added.
Basically the system used a laboratory bench laser
and diffraction grating to get three sheets of laser light.
"Sheets of light rather than a beam was necessary,"
he explained, "because, as you can imagine, these bubbles coming
into a wellbore are not moving in a nice regular column - they are
all over the place."
Detecting all the bubbles demanded a broader coverage
with the laser.
"That's one of the limitations of the Doppler
systems used today," he said, "because they only use a
beam and miss a great deal of the bubbles."
A diffraction grating is used to obtain multiple
beams from a laser light source. A cylindrical lens placed in front
of the grating transforms the three circular beams into three planar
sheets. The two outer sheets define the probe volume.
"As the bubbles move through the sheets of laser
light they light up like dust particles through a cinema projector
beam, and we look at how the light is reflected and also refracted,"
he said. "We have two detectors that pick up these different
This analytical method relates the time interval
between pulses from the refracted and reflected beams to the velocity,
diameter and refractive index of the gas bubbles.
- The velocity is obtained from the time-of-flight measurement
of refracted pulses, from either detector.
- The diameter of the particle is determined from the time of
flight of reflected pulses between the two detectors.
- The refractive index is obtained from the ratio of the time
of flight of the refracted pulse and the reflected pulse between
the two detectors.
The minimum measurable particle diameter with this
configuration is estimated at about 0.2 times the sheet thickness.
Eventually the pulses become so broad that they merge,
resulting in an inability to measure the velocity accurately.
This method potentially gives oil companies another
benefit: The refraction index allows operators to distinguish between
actual gas bubbles and other debris floating around in the drilling
"Little bits of rock and such will give you
pulses on the detectors and you need to be able to detect the difference,"
Waterman said. "By knowing the refractive index of gas you
can tell exactly when you have a gas bubble."
Show Me the Money
Initially the Reading University researchers used
the laser pulse displacement method to measure gas bubbles in a
large glass column in the laboratory. After fine-tuning the system
they moved on to water droplets.
That laboratory experiment used a laser with a beam
diameter of 1.5 millimeters, and the beam was split by a 20 lines
millimeter diffraction grating, followed by a 40-millimeter focal
length, cylindrical lens to create three laser sheets approximately
one millimeter in thickness and 10 millimeters wide.
The average spacing of the three laser sheets was
five millimeters along the direction in which the droplets fell.
Two photodiode detectors without receiving optics
were located at 80 millimeters from the point of intersection of
the falling line and the central laser sheet.
The nominal diameter of the water droplets was estimated
by determining the volume of 200 droplets and the average diameter
of water droplets were 5.78 millimeters. A typical recording signal
was captured on a waveform recorder. The experiment confirmed the
validity of the theory behind the new pulse displacement method.
Unfortunately, the project has not advanced past
the laboratory stage due to funding limitations.
"It would be nice to develop the system further
- to commercially develop the method - but until additional industry
funding becomes available we are limited," Waterman said. "While
we know the science is viable, converting the method for downhole
application would take a good deal of work."
The most challenging aspect of converting the system
for field use, he said, would be developing the best way to withstand
the downhole environment.
"Fiber optics is certainly the best option to
deliver the tool downhole," he continued, "and the one
advantage this system would have over other downhole sensors is
the fiber optic cable would simply be delivering the laser light
downhole - the sensors and other equipment would still be on the
He believes it would take another three years to
finalize a commercial instrument, but added that "this technology
could potentially save the oil industry millions of pounds a year
in blowout prevention.
"The earliest possible detection of potentially
dangerous blowouts is the key to this method."