Shale Fracturing and its link to earthquakes

Calgary Herald reported on Feb.13, 2017:” University of Alberta researchers find links between fracking and earthquakes”. It states: The seismic event 260 kilometres northwest of Edmonton in January 2016, which registered magnitude 4.8 on the Richter scale, was Alberta’s largest earthquake in a decade and was followed by hundreds of smaller aftershocks. The Alberta Energy Regulator said the earthquake was triggered by fracking operations that changed stresses in faults in the deep Duvernay formation.

It reminds me another earthquake, magnitude 5.8 in central Oklahoma on Sept. 3, 2016, the strongest quake ever recorded in the state. It was believed to be triggered by fracking operations too, specifically the subsurface injection of fracking wastewater.

Why fracturing/fracking?

Shale oil is unconventional oil produced from oil shale rock fragments. The oil substances in oil shale are trapped in formations that are not very porous, meaning that the oil and gas cannot easily flow out into the pipe as with traditional wells. Instead, the oil is accessed through drilling horizontally across the deposit and then fracking to open up the rock and allow the oil to flow. The following image tells us the difference between shale and conventional oil and gas.

Hydraulic fracturing is done by pumping liquids down a well into subsurface rock units under pressures that are high enough to fracture the rock. The goal is to create a network of interconnected fractures that will serve as pore spaces for the movement of oil and natural gas to the well bore.

Hydraulic fracturing combined with horizontal drilling has turned previously unproductive organic-rich shales into the largest oil and gas fields in the world.

But, earthquakes in some fracking areas used to be seemingly rare. A rapid recent increase in their occurrence has led scientists to investigate the cause. While there are a variety of potential explanations, both natural and human-induced, scientists are suggesting it is the later that is likely the cause.

How fracturing induces earthquakes?

Scientific studies have linked increased earthquake frequencies to fracking activities. Hydraulic fracturing has been linked to smaller earthquakes but not larger ones. The wastewater injection sites are suggested to increase the risk of larger earthquakes and this risk seems to be associated with the volume that is injected due to higher pressures on nearby fault lines.

In order to hydraulically fracture shale and extract the hydrocarbons, large quantities of water and chemicals must be injected underground. With the gas and oil, 18 to 80 percent of the fluid injected into the well will return to the surface. As much as it’s an oil well, it’s also a water well. For each barrel of oil produced, there are anywhere from 10 to 20 barrels of water. This water is heavily contaminated by the chemical mixtures that comprise the frack fluid, as well as dissolved salts and heavy metals from deep within the earth. It can be several orders of magnitude more saline than seawater and is often laced with naturally occurring radioactive material.

The wastewater is pumped into separate underground disposal wells, which go far deeper than the fracked well. And it’s this injection of wastewater deep underground that seismologists say is inducing earthquakes.

 

Under sufficient pressure, a cushioning layer of fluid can force its way between two sides of a deep strike-slip fault, lubricating both sides and releasing the shear stress. That’s an earthquake. When fracking waste fluids are pumped back down below the shale for what we hope will be safe disposal outside the groundwater system, we are pumping those fluids into porous rock, which requires high pressure. At the required pressures, the waste fluids can fill up the porous bedrock and open up otherwise stable faults.

Evidence suggests that the risk of induced earthquakes can persist for years after the actual fluid injection stops. The risk of induced earthquakes cannot be ignored.