Geothermal wells are drilled deep into the ground in order to tap into the heat radiating from the Earth's core and transform it into electricity. However, one remaining challenge associated with drilling deep geothermal wells is the presence of hard rocks, such as granite, that slow down the process and wear down drill bits. In turn, this causes drilling time and expenses to increase.
To combat this, a team of researchers from the J. Mike Walker '66 Department of Mechanical Engineering at Texas A&M University, led by Dr David Staack, is developing Shockwave and Plasma Accelerated Rock Cracking (SPARC) drilling technology. By making the creation of wells more efficient, accessible and cost-effective, their US Department of Energy-funded project aims to make geothermal energy a more viable alternative to fossil fuels.
The idea behind SPARC technology is to equip traditional drill bits with high voltage electrodes on the tip that emit a microscopic plasma discharge to shock the rock and crack it like a tiny explosion. Creating fractures and weakening the rock will allow the drill head, affixed with conventional diamond cutters, to have an easier time breaking through the material.
Research into the use of SPARC technology began because the team believes that the process of acquiring the required power downhole has been a failure mode of similar technology in the past and, in addition, some of the business cases it examined call for large pressures and temperatures which provides a challenging environment for electronics.
Expanding upon these themes, Dr Staack explains: "Sonic drilling seems like a soil penetration technique, which is apparently not aiming for hard rock like granite which is encountered in most industrial geothermal wells. Sonic technique judging by its description is for domestic geothermal usage like heating or cooling, but not for power generation. Other techniques for industrial hard rock application are like Foro (laser technique) and Tetra (Large Power Pulsed Spark Discharge) but their constraints are 'how to get power downhole', 'how to get long cables downhole'. Large energy requirement (Power per pulse) and cable requirement (either electric cable or fibre optical cable) are keeping these techniques from thriving in the drilling market."
The current research is inspired by the shock wave generated in cavitation study
To overcome these issues, Dr Staack and the team were inspired by nature. And, in particular, by the snapping pistol shrimp which uses mechanical means to produce shock waves underwater where they use cavitation technique for tunnelling activities in nature. "After studying this animal we arrived at an idea of accomplishing similar underwater shock waves using plasma, says Staack.
He then goes on to explain: "The current research is inspired by the shock wave generated in cavitation study. When cavitation collapses, it reaches high pressure (~GPa) and high temperature (several thousand degrees) inside the collapsed cavitation, this high energy density states can generate shock waves. When the shock waves impact the hard rock surface, it will initiate micro cracks on hard rock. With small pulsed electric power, we are able to generate cavitation bubbles in a controlled manner so that the pre-cracks are induced on the rock surface, making it much easier for mechanical drill bits to cut or chip hard rocks."
Among the problems that SPARC overcomes is the issue of running cables long distances down holes because the new technology uses low power electric plasma bursts, only consuming about ~80J per pulse compared to ~kJ per pulse that other large power pulsed electric discharge consumes. "Since this is low power, we can get power down the hole much easier by adding power conversion subassembly module in the mud motor, which requires no long-distance cables from surface to downhole to power the circuit," explains Staack. He also notes that the energy requirement for the plasma discharge is lower than creating the same impact mechanically.
Although SPARC technology initiates micro cracks in hard rock, Staack is keen to point out that the technique is very different from fracking. "While we are both looking to crack the rock, our approach is much more localised than fracking. We are looking to use the plasma burst to impact the rock directly below the drill bit - something that happens naturally in today's current drilling processes. Our cracks extend ~1cm into the rock directly preceding the drill. Fracking impacts rock formations as significantly large much deeper and broadly than what we are looking to accomplish."
While it may seem fanciful that shrimps can influence modern drilling techniques the project's ideas could very easily be incorporated into current practices. "There are two key elements to implement our technology," says Staack. "The first is the drill head. We expect our product to be a plug and play adaptation from current drill heads. That is to say, our product will represent a normal drill head with additional plasma producing features. Those additions will obviously require power input in a subassembly. We expect to use a mud motor (mud turbine), which is commonly used in the drilling industry today, and batteries with some specialisations."
Fortunately, the potential impact of changing over to SPARC technology for drill contractors should be minimal as it will require only the modified drill bit as well as the mud motor/generator item. All other aspects of the drilling platform should remain the same.
"SPARC drilling is a hybrid technology, a combination of traditional PDC drill bit and low power spark discharges and as the current hardware is a major part of the SPARC technology, therefore it is completely compatible with current hardware," adds Staack.
This hybrid model means that the expense of introducing the technology should not be too excessive. The only cost difference will be the initial increase in cost for the SPARC equipment. Staack expects that the modified bit and the specialised subassembly will be associated with an increased one-time cost. He also anticipates an improved rate of penetration in addition to reduced tool wear. "These two aspects are key the reason that our product is superior to present methods," states Staack.
At this stage, the technology is two to four years away from being commercially available but Staack is confident that the timeline could be accelerated. "We currently have funding from the DOE [United States Department of Energy] to pursue this idea. As we move forward and approach commercialisation, we intend to look for commercial partners and a company to prove our technology in a commercial setting." Such potential partners would be service companies and drilling companies.
Dr David Staack, Associate Professor, Sallie and Don Davis '61 Career Development Professor, and College of Engineering Director of laboratory instruction, is leading the SPARC project. His team includes Dr Dion Antao, Assistant Professor, Dr Alan Palazzolo, James J. Cain Professor I and Dr Bruce Tai, Assistant Professor.