A study of earthquakes triggered by underground liquid injection has shown surprising patterns, suggesting that current recommendations for hydraulic fracturing, sanitation and geothermal drilling may need to be revised.
UC Santa Cruz researchers have collected and analyzed data from around the world for earthquakes that are clearly related to injection wells. They found that a single injection well can cause earthquakes at distances of more than 6 miles (10 kilometers) from the well. They also found that injecting fluids into sedimentary rocks can generally cause larger, more distant earthquakes than injecting into the underlying bedrock.
"This is problematic because the current advice is to inject preferably into the sedimentation sequence theoretically safer alternative to bedrock," said Emily Brodsky, Professor of Earth and Planetary Sciences at UC Santa Cruz.
Postdoctoral researcher Thomas Goebel said the key issue was the spatial footprint of induced seismicity around the injection well. "It's not that the basement is safe because there is still the possibility of finding a fault in the basement rock that can cause a major earthquake, but the likelihood is lower because the spatial footprint is smaller," he said ,
In a work published on August 31 in Science Goebel and Brodsky described two distinct patterns of induced seismicity, which linked them to various physical mechanisms in basement and sedimentary rocks. In the first pattern associated with bedrock injection, earthquakes occur in a compact cluster around the borehole, with a strong fall in earthquakes farther from the borehole. In the other pattern associated with sedimentary rock, induced earthquakes gradually decrease with distance from the source and occur at much greater distances.
It has been hypothesized that the physical mechanism by which injection wells induce earthquakes is a direct consequence of increased fluid pressure in the pores of the rock, making it easier for slips to slip. This mechanism can explain the spatial pattern of seismicity observed in the basement injection, said Goebel. However, the pattern observed when injected into sedimentary rocks suggests another mechanism that results from an efficient "poroelastic coupling" that controls the ability of the rock to transfer fluid stresses into the solid rock matrix.
it presses on the surrounding rock and creates an elastic tension in the rock that can exert pressure on faults at a distance without putting water into these faults. Thus, if poroelasticity is dominant, you have a bigger footprint because of it Neighboring errors over the area with increased pore pressure, "said Brodsky.
According to Goebel, the crystalline bedrock is stiffer and has lower porosity than sedimentary rock." Therefore, the increase in pore pressure on isolated pockets around the borehole is limited and coupling with The whole field of tension is low, "said Goebel.
Goebel said that their results help explain the extent of induced seismicity in regions like Oklahoma, where there are many injection sites in oil and gas fields There is a dramatic increase in earthquakes, so there are more earthquakes in Oklahoma each year than in California However, Brodsky did not include sites in Oklahoma in their study because there were so many injection wells that they could not isolate the effects of individual wells.
"In Oklahoma they inject into the highly porous sedimentary unit above the basement, but these elastic tensions can be transmitted over a long distance, so that a large basement fault could be triggered 10 kilometers away," said Goebel. "That's maybe what we see in places like Oklahoma."
Oklahoma is a laboratory for human-induced earthquake research
T.H.W. Goebel et al., "The Spatial Footprint of Injection Drilling in a Global Compilation of Induced Seismic Sequences" Science (2018). science.sciencemag.org/cgi/doi … 1126 / science.aat5449