In an oil field far, far away ... the future is about to arrive.
Laser drilling is poised to be the first fundamental change to the rotary drilling concept since its inception nearly 100 years ago.
Some might say, it's about time.
The earliest studies of laser drilling possibilities date to the 1960s and 1970s, but these were primarily theoretical -- physical tests were limited by the laser technology and low power available at that time.
But the then-Gas Research Institute (GRI; now the Gas Technology Institute) resurrected the idea of using lasers to drill oil and gas wells in 1997, when the institute initiated a two-year study to determine the feasibility of using the high power lasers developed by the U.S. military as part of the Reagan-era Star Wars Defense Initiative.
Those first steps investigated the interaction of lasers with different rock lithologies as the first step toward determining the energy required to remove rock with laser beams.
"The study began in earnest when the Star Wars effort was winding down and some in the industry realized these big, high-powered military lasers could provide sufficient power to blast through rock," said Claude B. Reed, with the Argonne National Laboratory. "So GRI, along with the Colorado School of Mines, got access to two military lasers and ran some initial tests."
Reed presented a paper updating the status of laser drilling at the recent AAPG Mid-Continent Section meeting in Tulsa.
GRI, he said, was keen to continue its work with lasers, but the large military lasers were not readily available or reliable enough to mount a systematic test program. Plus it was realized that industrial lasers had enough power to perform additional tests.
"GRI contacted Argonne, because we have two of the largest industrial lasers in the country," Reed said, "and we have quantified the power levels, the shape of the beam and other parameters necessary to calculate just what you have done with the laser."
< h5>Spalling
In 1999 Argonne, GRI and the Colorado School of Mines again joined forces under a U.S. Department of Energy National Energy Technology Laboratory agreement in an effort to more quantitatively determine the energy required for lasers to remove rock.
Halliburton Energy Services and Petroleos de Venezuela-Intevep, SA, were industry partners on the project.
This investigation identified the specific energy requirements to remove rock from test samples of sandstone, shale and limestone using a 1.6 kilowatt pulsed neodymium aluminum garnet (Nd:YAG) laser, according to Reed.
Those test results indicated:
- In general, shales required the least energy to remove a unit volume of rock by an order of magnitude -- good news, since 70 percent of the rock drilled in most oil and gas wells is shale, according to the study.
- The sandstone samples exhibited higher specific energy values than the shales, although still quite low compared to mechanical drills.
- The energy requirement differences between spalling or cutting the rock and melting the rock became more defined -- particularly in the shale samples, where clear and sharp increases in the specific energy values of melted samples were observed.
- Sandstone samples showed more of an increasing trend in specific energy values at the onset of melting, not the sharp definition seen in the shales.
- The limestones seemed to spall through a thermal dissociation mechanism rather than breaking or melting, so the onset of melting was not observed.
"Determining the different regimes for vaporizing rock, melting rock or spallation was important," Reed said.
Spallation occurs when the laser beam heats up moisture, which is present in virtually every kind of rock. The moisture is heated to the point it causes a steam explosion that acts as a subsurface explosion to fracture rock and spall off the surface layer.
"We found that you can get the lowest specific energy in this spallation regime," Reed said. "That means you would be drilling the largest hole with the least amount of energy if you could set your parameters to spall but not melt the rock."
< h5>Testing Continues
Last year the consortium followed that study with a series of experiments lasing multiple holes into rock samples to determine effects of specific energy.
Initial results indicated:
- As continuous beam exposure time on a single point increased, the probability of melting and observing higher specific energy values increased.
- As the depth of a single point hole increased relative to its diameter, deleterious secondary effects contributed to a higher specific energy value.
- An eight-inch diameter beam that can provide the required power density to spall rock the full length of the well is not technically feasible with current technology.
- Rastering the beam in a pattern necessary to create the hole may prove too complex in powering and maintaining the rasterizing mechanism downhole, and an alternative method would be preferable.
This research focused on a laser-based drilling system making use of multiple beams of near-infrared energy placed adjacent to one another, collectively creating a hole. The size of the hole would then depend on the number, arrangement and burst frequency of the beams.
Reed said the set up was not unlike a mechanical drill bit that uses individual teeth or cutters to chip small pieces of rock as the bit turns under the weight of the drill string. In both cases, a specific amount of controlled energy is repeatedly delivered from the system to a point on the rock, causing the rock to fail in a predetermined path.
Researchers used both the Nd:YAG pulsed laser and a six kilowatt carbon dioxide gas-type laser capable of both continuous wave and super-pulsed beams. Both lasers are located at the Argonne laboratory.
The first series of tests repeated application of laser energy on one spot with varying amounts of time between laser exposures. The second series created two spots, varying the spacing between the spots and the time between repeats. The third series created three spots arranged in an equilateral triangle, and four spots in a parallelogram.
Test results on sandstones were encouraging, in that both the two and three spot tests indicate that the weight loss levels off as the number of bursts per shot increases. The two-shot tests indicated a precipitous increase in specific energy before the leveling occurs, but the three-shot tests indicate a very small increase in specific energy, which seems to be related to the relaxation time.
The shale samples cut more easily than the limestone or sandstone, but specific energy values were significantly higher than the optimized single shot, single burst tests done in the earlier study. The specific energy behavior of the shale samples was similar to those of the sandstone.
While the recent tests are encouraging, a great number of questions must be answered before a laser drilling system can even be field tested.
"For example, we are starting to look at what happens when you have moisture in the wells and then with high pressure liquids, because that is what you must deal with in the real world," Reed said. "Our early work was dedicated to dry rocks, but we know that if you drill a hole in the ground you are going to have water in it. We have to be able to deal with that.
"Those are future hurdles to clear, but first we have to understand what is the best we can accomplish in an ideal environment," he said. "So far, we have seen nothing to tell us that we shouldn't move forward."