Oilfield operations have likely caused earthquakes in California a few miles from the San Andreas Fault

Thomas H. Goebel, University of Memphis

Posted at 2:04 PM EST, Tuesday November 10, 2020

(The Conversation is an independent, not-for-profit source for news, analysis, and comment from academic experts.)

Thomas H. Goebel, University of Memphis

(The Conversation) The way companies search for oil and gas and dispose of wastewater can trigger earthquakes, sometimes in unexpected places.

In West Texas, earthquake rates are 30 times higher than they were in 2013. Studies have also linked earthquakes to oilfield operations in Oklahoma, Kansas, Colorado and Ohio.

California was thought to be an exception, as oilfield operations and tectonic faults appear to coexist without major problems. Now, new research is showing that the state’s natural earthquake activity may mask the earthquakes caused by the industry.

As a seismologist, I have been investigating induced earthquakes in the United States, Europe and Australia. Our latest study, released on November 10, shows how oilfield operations in California are compressing tectonic faults in an area a few miles from the San Andreas fault.

Seismic rise

Earthquakes caused by industry have been a growing concern in the central and eastern United States for more than a decade.

Most of these earthquakes are too small to be felt, but not all of them. In 2016, a 5.8-magnitude earthquake damaged buildings in Pawnee, Oklahoma, and federal and state regulators shut down 32 sewage disposal wells near a newly discovered fault. Large earthquakes are rare, far from tectonic plate boundaries, and Oklahoma suffers from three earthquakes of magnitude 5 or greater in one year, as in 2016, unheard of.

Oklahoma’s earthquake rate has decreased with lower oil prices and regulators decision to require companies to reduce the volume of well injection, but there are still more earthquakes today compared to 2010.

A familiar pattern has emerged in West Texas in the past few years: earthquake rates have increased dramatically beyond the normal range. A magnitude 5 earthquake jolted west Texas in March.

How it works

At the roots of the problem of induced earthquake are two different types of fluid injection processes: hydraulic fracturing and sewage disposal.

Hydraulic fracturing involves the injection of water, sand, and chemicals at very high pressures to create flow paths for the hydrocarbons trapped in tight rock formations. Wastewater disposal involves injecting fluids into deep geological formations. Although wastewater is pumped at low pressures, this type of process can disturb both natural pressures and pressures over large areas, several miles from injection wells.

Tectonic faults beneath geothermal and oil reservoirs are often unstable. Even a slight disturbance of the natural tectonic system – due to deep fluid injection, for example – can cause faults to slip and trigger earthquakes. The consequences of fluid injection can be seen easily in Oklahoma and Texas. But what are the implications for other places, like California, where earthquake-prone oilfields and failures are so close?

Hidden Risks of California Oilfields

California provides a particularly interesting opportunity to study the effects of fluid injection.

The state has a large number of oil fields, earthquakes and many tools that detect even small events, and it was believed to be largely free of abnormal earthquakes.

My colleague Manu Sherzai of Virginia Tech and I wondered whether the induced earthquakes could be masked by nearby natural earthquakes, and thus have not been missed in previous studies. We conducted a detailed seismic study of the Salinas Basin in Central California. The study area is distinguished by its proximity to the San Andreas fault and because waste fluids are injected at high rates close to the seismically active faults.

Using satellite radar images from 2016 to 2020, Sherzai made a surprising observation: some areas in the Salinas Basin were rising by about 1.5 cm per year, that is, just over half an inch. This rise was the first indication that fluid pressures are unbalanced in parts of the San Ardu oil field. The increase in fluid pressure in the pores of the rock causes the surrounding rock matrix to expand like a sponge that is completely pumped out with water. The resulting reservoir expansion raises the forces acting on the surrounding tectonic faults.

Next, we examined the seismic data and found that fluid injection and earthquakes were significantly correlated over a period of more than 40 years. Surprisingly, this extended 15 miles from the oil field. These distances are similar to the large spatial footprint of Oklahoma’s injection wells. We analyzed the spatial pattern of 1735 seismic events within the study area and found a group of events near the injection wells.

Other areas of California may have a similar history, and more detailed studies are needed to distinguish between natural and induced events.

How to reduce the risk of earthquakes

Most sewage disposal and hydraulic fracturing wells do not produce earthquakes that can be felt, but the problem wells have three things in common:

– These are large injection wells;

It is injected into highly permeable rock formations; And the

These formations are located directly above tectonic faults in the deeper geological basement.

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Although the first problem is difficult to solve because reducing the volume of waste fluids will require reducing the amount of oil produced, sites for injection wells can be planned more carefully. The seismic integrity of oil and gas operations can be increased by selecting geological formations separate from deep faults.

This article is republished from The Conversation under a Creative Commons license. Read the original article here: https://theconversation.com/oil-field-operations-likely-triggered-earthquakes-in-california-a-few-miles-from-the-san-andreas-fault-149207.

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