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Can Lasers Help Stop Blowouts?

UK Research Bobs for Bubbles

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 operations.

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 explosion."

Tiny Bubbles

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 optic sensor.

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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 operations.

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 explosion."

Tiny Bubbles

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 optic sensor.

"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 take us.

"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 pulses."

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 slurry.

"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 surface."

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."

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