Devastating earthquakes in Sichuan related to fracking operations

Two moderate-sized earthquakes that struck China's Sichuan Province in December and January were probably caused by nearby fracturing operations, according to a new study published in the US Seismological research letters.

The magnitude 5.7 earthquakes of December 2018 and magnitude 5.3 of January 2019 in the southern Sichuan basin caused considerable damage to farms and other structures in the region. The December earthquake was particularly deadly, leaving 17 injured and resulting in a direct economic loss of about 50 million Chinese Yuan (about US $ 7.5 million).

The Changning shale gas block in southern Sichuan has been the site of fracking since 2010, with horizontal fracking injection wells becoming more common since 2014. The tremor land in the Changning Block has increased dramatically at the beginning of systematic fracking.

In the United States, the discharge of wastewater from oil and gas activities, where the water produced during the extraction of hydrocarbons is reinjected into the rock layers, would be the main cause of earthquakes caused, especially in Oklahoma. However, it is becoming increasingly clear that hydraulic fracturing, which uses injected water to break up rocky strata during hydrocarbon extraction, could have caused moderate sized earthquakes on some sites in Ohio, Oklahoma and Western Canada.

According to Xinglin Lei of the Geological Survey of Japan and his colleagues, wastewater disposal and hydraulic fracturing have both caused earthquakes in the southern Sichuan basin. In their new study on SRL, researchers present "a complete chain of evidence" to show that the December and January earthquakes were induced by fracturing operations.

They located the location of the earthquakes, finding that they were relatively shallow (between two and ten kilometers below the surface), as could be expected for the induced earthquakes. The December and January earthquakes also coincided in time and space with an injection on nearby fracturing well plates. They did not have the exact injection volumes on these wells to better understand the relationship between injection activities and the evolution of seismicity.

The modeling of seismic activity by Lei and colleagues shows that the bulk of the activity came from initial main shocks, with low post-shock activity, which is also consistent with the pattern seen for earthquakes induced. Finally, their calculations show that the overpressure on the pores of the rock, produced by the fracturing injections, was strong enough to activate pre-existing faults in the region. The researchers noted that these faults were generally unmapped and did not have a favorable orientation for sliding under normal tectonic activity.

"For most well platforms, the associated seismicity fades quickly after the end or the end of the hydraulic fracture," said Lei, although their analysis suggests the possibility of seeing signs of reactivation of the fault resulting from an earlier seismicity.

"In my opinion, repeated mild earthquakes can be caused as long as the injection continues, since a moderate earthquake releases a very limited stress," he added. "The national regulation in China should be updated with the obligation for operators to take action if certain signs of reactivation of failures were observed."

The researchers say that more information is needed about the faults and their stress patterns in the Sichuan basin areas surrounding the fracturing wells, to guide drilling in order to avoid moderate seismic activity. "Moderate earthquakes have been observed in a limited number of sites," said Lei. "If these sites could be eliminated, the risk of moderate earthquakes would be significantly reduced."

Lei and his colleagues would like to see researchers, regulators and oil and gas operators working together to better understand the causes of injection-induced seismicity in the South Sichuan basin, to enable efficient and safe fracturing operations.