Earth boring with laser energy
The time is right to make the change and train new crews in laser technology
The time is right to make the change and train new crews in laser technology
For the past six years, a team of scientists has been assessing the feasibility of using high-power lasers to create boreholes into the earth for oil and gas extraction. Using high-power lasers to drill wells for oil and gas extraction will allow drilling surface-to-target depth without stopping or having to pull out of the hole, create a hole lining (casing) to protect the hole during and after drilling, and complete and stimulate the well as part of the drilling process, not as a separate operation as is done now. A technology that can drill any type of rock faster and that has the potential to drill, case, and complete a well in one pass will reduce the number of days a rig is on location, which is attractive to the drilling industry.
Figure 1. Photograph of a modern drilling rig.
Using lasers to drill oil and gas wells requires a radical shift in thinking about how the process works. There is no weight applied to the bit and there are no moving parts down hole. The entire down-hole assembly can be made of composite material, which means that the surface equipment can be much lighter than the rigs currently in use. There is no mechanical wear on the down-hole assembly due to rotation of the drill string.
Rotary drilling as practiced today consists of threading together at the surface sections of heavy steel pipe to lengthen the drill string as the hole is deepened. Because the bottom hole assembly usually includes one or more sections of thicker wall pipe that help to apply downward force onto the cutting bit, the total weight suspended by the surface equipment can exceed one million pounds. Current drilling techniques require specialized bits for different rock types. Any time another rock type is encountered, the bit has to be changed. For instance, the bit used to drill soft shale is ineffective when a ledge of harder sandstone or limestone is reached. The pipe and bit have to be pulled out of the hole and a different bit attached. The entire process may have to be repeated several times.
The rotary drilling method has been greatly refined during the last hundred years. New bit designs make it possible to drill economically through all types of rock at increasing rates. Still, much of the rotary drilling method is time consuming and inefficient. Only 50 percent of the time is spent with the bit on the bottom, turning right, as shown in a 1993 study that identified the major activities that take up most of the time of creating a well. The rest of the time is spent lowering the drill string into or raising it out of the hole and other non-drilling activities such as deploying instruments into the hole.
Figure 2. Shale cutting behavior as average power is increased. Note the increase of S.E. upon onset of melting. From SPE Paper No.71466.
The drilling industry is facing shrinking profit margins, so there is limited room for technological innovation in rotary drilling development. The once revolutionary changes to, for instance, diamond impregnated bit teeth have evolved to the point where incremental improvements are all that are possible. The energy industry is in the position of having to drill more wells per year just to replace reserves that are being produced each day. Because of a slow time over the past decade, the drilling industry is facing the prospect of having to build more rigs and train more crews to operate them.
In the laser testing process, the number of laser parameters to be explored is almost as large as the variables of the rock a well has to penetrate. Work started with simple parameters: power, time, and wavelength. The first large-scale tests aimed at finding out if this hole-making process could be done were performed on a continuous wave (CW) chemical oxygen iodine laser (COIL; 1.32 μm wavelength and up to 6kW). Some of the samples were penetrated completely, and the melt created in the form of a sheath was very strong. However, the holes were too deep for their diameter, which meant that the purging did not remove particles, which then were melted and re-melted, using energy that would otherwise have cut fresh rock.
The test procedure shifted to determining the minimum amount of energy (the Specific Energy, or S.E.) necessary to cut the three main lithologies drillers have to deal with, sandstone, shale, and limestone. Two parameters were added to the three originals, power density and investigating the potential benefits of using pulsed lasers instead of CW beams.
The 2kW Nd:YAG laser available at Argonne National Laboratories (Argonne, IL) has the pulse characteristics that were needed to test the pulse laser concepts, calculated at about 1 ms. A range of laser pulse schedules were used around that value, keeping the programmed average power at 1.6 kW. The measured average powers ranged from about 250 W to 1.4 kW. One surprise was that even low average powers, just over 500W, could cut a lot of rock. Higher repetition rates allowed faster cutting, as long as the measured average power remained high.
After encouraging tests, the task became to visualize how a drilling assembly would look-how the actual creation of a typical oil or gas well would be accomplished. It was clear that a single beam from a single lens would not be able to create a one-inch diameter hole. The concept became to add beams together to create a larger hole. The advantage of this method is that the hole size can be changed by adding lenses.
Combinations of different size spots, different amounts of spot overlaps, and varying time between rehitting a spot (relaxation time) were all tested. The research team is increasing the rock sample sizes. There are definite edge effects from the laser and thermal energy reflecting off of the bottom and sides of the sample, causing secondary effects (such as melting and cracking) as the heat builds up in the sample.
One of the most encouraging results of the tests last year is the improvement in S.E. of limestone after finding the right combination of average power and power density. Early tests indicated that limestone required an order of magnitude higher S.E. than sandstone, but the most recent tests on the Argonne CO2 laser showed that the true S.E. will be much closer to that of sandstone
One challenge is how to get the energy to the working face at the bottom of the hole. Options include the current generation of fiberoptics. At the power densities needed to make holes efficiently, current industrial fibers will convey enough energy down hole for at least a kilometer and probably two. New technologies, such as hollow fibers, are emerging that may solve this problem. Development of hollow fibers would allow the use of the longer wavelength lasers, such as CO2 (10 μm), which would provide advantages in absorption efficiency.
Another option would be to put the laser in the bottom hole assembly and send power down to it. This would require creating a laser-friendly environment as well as compressing the lasing medium and the optics into a package that will fit into the desired hole size. The compact diode lasers and fiber diode lasers may have some application here.
Another challenge is how to make the laser cut rock with water or other fluids in the hole. As a hole is drilled into the earth, it can hit zones of abnormal pressure. If the pressure is not controlled, the well can blow out, destroying equipment and endangering the lives of the workers. Current technology is to fill the hole with weighted fluids that keep the formation fluids in the formation. While we know that we can make a hole under water, and even make a hole under a layer of drilling mud, these tests are mostly rudimentary. We have several ideas of how to deal with this problem, but have not had sufficient funding to attack it properly.
The research team is encouraged with the results to date and is confident that the solutions to the remaining problems are forthcoming. More than half of the wells drilled each year in the U.S. are in the range that our technology can reach in the near future. The fundamental tests have shown that the amount of laser energy needed to cut rock is the same or less than the amount needed for mechanical drilling. Laser energy has been shown to be more than competitive with current mechanical drilling. The rest is just engineering.
Dr. Richard Parker (firstname.lastname@example.org) is president of Parker Geoscience Consulting, LLC, and president of Subsurface Laser Applications Inc.
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