Subsurface carbon sequestration, which stores carbon in underground rocks, is a partial solution to removing carbon from the atmosphere. Geologic carbon sequestration, when used in conjunction with emissions reductions could help to mitigate anthropogenic climate changes. It comes with risks, however, just like any underground operation.Geophysicists continue to investigate what causes earthquakes that are human-induced. This has been known since the 1960s. The new study published in Geology Thursday explores the reasons why earthquakes are occurring in part of an oilfield that is heavily produced in the U.S. The authors show that past oil drilling has caused changes in faults that make it less likely that fluids are used to cause earthquakes.The Delaware Basin is an oil-and-gas-producing field that stretches the border between West Texas, New Mexico and West Texas. The region has been drilled since the 1970s. There are over 10,000 individual wells that are active. No'am Dvory, a Stanford geophysicist, and Mark Zoback observed a pattern in seismic activity. Recent shallow earthquakes occurred mainly in the southern part of the basin. However, seismic activity is low in the northern half despite the presence of shallow wastewater injection across the basin."The compelling question is, therefore, why are all the shallow earthquakes restricted to a single area and not more widely distributed?" Zoback agrees.Injecting fluids such as wastewater underground can cause earthquakes. By injecting wastewater into rocks, the pressures in the rock increase, placing the rocks and faults under greater stress. An earthquake can occur if these stresses and pressures are too high.The earthquakes that result from injection in southern Delaware Basin are usually shallow and low-magnitude. They can rattle dishes but not cause serious damage. Higher-magnitude earthquakes, however, can be caused by deeper faults. In Mentone, Texas, for example, a magnitude 4.6 earthquake occurred in March 2020. This was likely to be due to deep injection which interacted with faults within the crystalline basement rocks around five miles belowground.AdvertisementDvory explained that the size of an earthquake depends on the fault slippage. Quake magnitudes tend not to increase in areas where faults are small and shallow (less than a few kilometers wide). It's still possible to feel it, but it's much less dangerous.Subsurface operations, regardless of whether they are oil and gas production or carbon storage, must minimize the chance of earthquakes. Dvory and Zoback were drawn to the Delaware Basin because of its unusual pattern of earthquakes. It was a natural experiment of geomechanics and the "why" behind inducible earthquakes.Dvory and Zoback first calculated the underground pressures that would cause the basin to slip, and then connected these values to the estimated stress values. After establishing a baseline, they calculated the Delaware Basin's pore pressures. The results revealed a clear pattern. The pore pressures of the geologic formations within the northern basin were lower than those in "unperturbed rock" and there was no earthquake. The southern basin had much lower 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 hydrocarbon production was substantial, the pressure was reduced and formations became more stable."When fluids are injected into previously drilled rocks that have been'stable', the starting pressure will be lower than it was the first time.Zoback explained that oil production was not possible before current injection. This makes it less likely that earthquakes will occur. It's possible that an earthquake could be triggered by enough oil injection. We are able to show that the past strongly influences the current operational processes and the probability of an earthquake triggering.These sites, which have lower earthquake risks, might be an option for carbon sequestration.Zoback states that "we have a global challenge in storing enormous quantities of carbon dioxide in subsurface over the next ten- to twenty years." We need safe places to store large quantities of carbon dioxide over hundreds of years. This includes not allowing earthquakes to be triggered by pressure increases. Geoscience is crucial in meeting this challenge. This is a huge problem, but geoscience can be a crucial place to begin.
