Old oil fields may be less prone to induced earthquakes

Red dots indicate earthquakes in the southern Delaware Basin. These are areas where there has not been any historical oil or water production (purple circles; the size of the circle is indicative of the amount of oil and/or water produced). Credit: Dvory and al.Subsurface carbon sequestration, which stores carbon in rocks deep underground, is a partial solution to removing carbon from the atmosphere. Geologic carbonsequestration, when used in conjunction with emissions reductions could help to mitigate anthropogenic climatechange. It comes with risks, however, just like other underground operations.Scientists still trying to figure out what causes earthquakes in humans, as documented since the 1960s, are geophysicists. On Thursday, Geology published a new study that explores why earthquakes are occurring in part of an oilfield in the United States. The authors show that past oil drilling has changed fault stresses in such a manner that injectingfluids to cause or trigger earthquakes today is less likely.The Delaware Basin is an oil-and-gas-producing field that stretches along the border of West Texas and New Mexico. The region has been drilled since the 1970s. There are over 10,000 individual wells. No'am Dvory and Mark Zoback, Stanford geophysicists, noticed a pattern in seismic activity. Recent shallow earthquakes were concentrated in the southern half, while seismic activity is quieter in the northern half."The compelling question is, therefore, why are all the shallow earthquakes limited to a single area and not more widely distributed?" Zoback agrees.Injecting fluids such as wastewater underground can cause earthquakes. This increases the pressure and puts the rocks and faults under greater stress. An earthquake can occur if these stresses and pressures are high enough.The earthquakes that result from injection in southern Delaware Basin are usually shallow and low-magnitude. They can rattle the islands but not enough damage. Higher-magnitude earthquakes, however, can occur if deeper faults become active. For example, a magnitude 4.6 earthquake struck Mentone, Texas, in March 2020. This was likely to be due to deep injection, which interacted with faults within the crystalline basement rock about five miles belowground.Dvory explains that the faults that slip are what limit the earthquake's size. Quake magnitudes are smaller in areas where faults are small and shallow (just a few kilometers wide). It's still possible to feel it but it's much less dangerous.Subsurface operations, whether they are oil and gas production or carbon storage, must minimize the chance of earthquakes. TheDelaware Basin with its unusual pattern of earthquakes was a prime target for Dvory and Zoback. It was a natural experiment of geomechanics, the reason behind induced earthquakes.Dvory and Zoback first modelled the undergroundpressures that would cause the basin to slip, and then connected these values to the estimated stress values. After establishing a baseline, they calculated the pore pressures surrounding the Delaware Basin. The results showed a clear pattern. The pore pressures of the geologic formations around the Delaware Basin, where hydrocarbons were previously produced, were lower than those in "unperturbed" rocks. There was no earthquake. The southern basin had higher initial pressures and earthquakes than the northern basin.Dvory states that there is evidence of oil-and-gas development in some areas dating back to the 1950s. "Where significant hydrocarbon production was observed, pressure was reduced and formations became more stable."When fluids are injected into'stable' rocks that were previously drilled, the starting pressure will be lower than it was the first time.Zoback explained that oil production in areas where it was previously possible to inject is now at a significantly lower pressure, making it less likely to cause earthquakes. It's possible that an earthquake could be triggered by enough oil injection. We are ableto document in the area that past events have a significant impact on how current operational processes affect the probability of an earthquake triggering.These sites, which have lower earthquakerisks, might be good targets for carbon sequestration.Zoback states that "we have a global challenge to store immense volumes of carbon dioxidein thesubsurface over the next ten- to twenty years." Zoback says that we need places to store large quantities of carbon dioxide safely for hundreds of years. This includes not allowing pressure to increase to trigger earthquakes. Geoscience is crucial in meeting this challenge. This is a huge problem, but geoscience can be the key place to begin.Continue reading Geoscientists discover that deep earthquakes are caused by shallow wastewater injection in TexasMore information: NoamZ.Dvory et.al, Prior oil-and-gas production can limit the occurrences of injection induced seismicity: A case report in the Delaware Basin, Texas, USA, Geology (2021). Information from the Journal: Geology Noam Z. Dvory et. al. Prior oil and gas production can reduce the occurrences of injection-induced seismicity. A case study in Delaware Basin, Texas, USA (2021). DOI: 10.130/G49015.1