Sandia Lab explores geothermal fracture growth through controlled explosions
The Sandia National Laboratories is conducting a study on the use of explosions or propellants to predictably generate fracture networks in EGS sites.
A study being conducted at the Sandia National Laboratories in New Mexico is looking at the feasibility of using explosives or propellants to create fracture networks in subsurface formations in a manner that is controlled and predictable. Insights from this study can then be used to establish connectivity in geothermal wells in Enhanced Geothermal Systems (EGS). This study is being supported and funded by the Department of Energy’s Geothermal Technologies Office (GTO).
“Our goal was to come up with a new way of creating a geothermal fracture network that you have a clear idea where it is going to go — it’s steerable and manageable — and you are utilizing fewer resources and being more environmentally friendly,” said Sandia mechanical engineer and team leader Eric Robey.
“This is where explosives and propellants come in. The idea is that they’ll allow us to get away from pumping a lot of fluid down the wells. We’re collaborating with Lawrence Livermore National Laboratory (LLNL) to model the explosions and improve the predictability of forming fracture networks.”
Simulations with plexiglass
Explosions were done inside plexiglass cubes, a material that had mechanical properties similar to granite at about 750 degrees Fahrenheit, according LLNL computer modeling expert Oleg Vorobiev. Dozens of tons of pressure were also applies to the plexiglass vubes to see how stresses will impact fracture formation.
Small amounts of explosives or propellants were ignited in the center of the plexiglass cubes to create small-scale explosions. The outward propagation of the fractures were monitored using ultra-high-speed cameras. while tiny fractures were monitored using other sensors.
Another technique that the team employed was schlieren imaging, which allowed them to monitor the explosive shockwave ripples through “seeing” the differences in density caused by compression. Explosions were recorded at 1 million frames per second.
Listening to the weaker waves
As shockwaves reached the outside of the plexiglass cubes, the weaker waves can no longer be monitored visually. The team came up with a novel solution through the use of acoustic emission sensors. Through an array of microphones, the team was able to hear the weaker waves and identify where they were coming from by triangulation.
Similar experiments were done in plexiglass cubes that have been joined together to mimic faults. The results showed that explosive-formed fractures tend to not cross pre-exiting fault lines, but conditions such as stress and orientation of the fault also play roles in determining the results.
The significance of this technique is that it can help transition the study from the plexiglass cubes to actual granite formation. Information from the acoustic sensors is also instrumental in computer modeling efforts.
The team is currently conducting the same experiments in 1-foot cubes of granite and aim to scale up to 3-foot cubes of granite. If the results of the lab-scale experiments turn out promising, there is potential for field testing at the Utah FORGE site. This can happen in as little as three to five years.
Source: Sandia National Laboratories