Oklahoma drilling, earthquake and Nigeria!

In a new study published in the June 19, 2015 issue of the journal; Science Advances, Professor Mark Zoback, and a PhD student, Rali Walsh showed that the state’s rising number of earthquakes coincided with dramatic increases in the disposal of salty waste water into the Arbuckle formation, a 7,000-foot deep, sedimentary formation under Oklahoma.

In addition, they showed that the primary source of the quake-triggering waste water is “produced water” that naturally co-exists with oil and gas within the earth. “What we’ve learned in this study is that the fluid injection responsible for most of the recent quakes in Oklahoma is due to production and subsequent injection of massive amounts of waste water, and is unrelated to hydraulic fracturing,” said Zoback, the Benjamin M. Page Professor in the School of Earth, Energy and Environmental Sciences.

Before 2008, Oklahoma experienced one or two magnitude four earthquakes per decade, but in 2014 alone, the state experienced 24 of such. In the study, Zoback and Welsh looked at three areas-centered around the towns of Cherokee, Perry and Jones in Oklahoma that have experienced the greatest number of earthquakes in recent years. All three areas showed clear increases in quakes following increases in waste water disposal. Three nearby control areas that did not have much waste water disposal did not experience increases in the number of quakes.

Since the team was able to review data about the total amount of waste water injected at wells as well as the total amount of hydraulic fracturing happening in each study area, they were able to conclude that the bulk of the injected water was produced by water generated using conventional oil extraction techniques, not during hydraulic fracturing. “We know that some of the produced water came from wells that were hydraulically fractured, but in the three areas of most seismicity, over 95% of the waste water disposal is produced water, not hydraulic fracturing flowback water,” said Zoback.

The three study areas in Oklahoma that Zoback and Welsh looked at all showed a time delay between peak injection rate and onset seismicity as well as spatial separations between the epicenter of the quakes and the injection well sites. Some of the quakes occurred months or even years after injection rates peaked and in locations that were sometimes located miles away from any well. These discrepancies had previously puzzled scientists, and had even been used by some to argue against a link between waste water disposal and triggered earthquakes. But Zoback said they are easily explained by a simple conceptual model for Oklahoma’s seismicity that his team has developed.

According to this model, waste water disposal is increasing the pore pressure in the Arbuckle formation, the disposal zone that sits directly above the crystalline basement, the rock layer where earthquake faults lie. Pore pressure is the pressure of the fluids within the fractures and pore spaces of rocks at a depth interval.

The earth’s crust contains many pre-existing faults, some of which are geologically active today. Sheer stress builds up slowly on these faults over the course of geologic time, until it finally overcomes the frictional strength that keeps the two sides of a fault clamped together. When this happens, the fault slips and energy is released as an earthquake. By increasing the fluid pressure through disposal of waste water into the Arbuckle formation in the three areas of concentrated seismicity-from about 20 million barrels per year in 1997 to about 400 million barrels per year in 2013, humans have sped up this process dramatically.

“The earthquakes in Oklahoma would have happened eventually”, Welsh said. “But by injecting water into the faults and pressurizing them, we’ve advanced the clock and made them occur today.” Moreover, because pressure from the waste water injection is spreading throughout the Arbuckle formation, it can affect faults located directly beneath a well, but several miles away, it would take time for the fluid pressure to propagate”, Welsh said.

With the source of the recent quakes in Oklahoma known, scientists and regulators can work on ways to stop them. One possible solution, Zoback said, would be to cease injection of produced water into the Arbuckle formation entirely, and instead inject it back into producing formations such as the Mississippian Lime, an oil-rich limestone layer where much of the produced water in Oklahoma comes from in the first place. Some companies already injected water back into reservoirs in order to displace remaining oil and make it easier to recover. The Stanford study found that this technique, called enhanced oil recovery, does not result in increased earthquakes. Even if companies opt to use producing formations to store waste water, however, the quakes won’t cease immediately.

“They’ve already injected so much water that the pressure is still spreading throughout the Arbuckle formation”, Zoback said. “The earthquakes won’t stop overnight, but they should subside over time.”

Although, Nigeria is not sitting on any major tectonic disequilibrium, it will be much better if a think tank is put together to come up with ways to mitigate against tremors and earthquake triggered drilling with a view to incorporating and following through recommendations arising therefrom into the petroleum roadmap, even if as an addendum, for the benefit of Nigerians and their environment. This, is the message from Oklahoma!
Kayode Adeoye is an energy expert in Lagos