Earthquakes and Fracking in Oklahoma
Starting around 2009, the yearly number of small earthquakes in Oklahoma began to increase. The increase corresponded with a concurrent increase in fracking of horizontal oil and gas wells. The conclusion was often reached that well fracking caused the increased number of earthquakes.
While fracking by itself was almost never to blame, more horizontal well drilling and its relationship to earthquakes were related to the problem. However they were related in an offhand way…in a way not generally suspected by the public.
When oil and gas is produced, salt water is almost always produced with it. This salt water is similar to sea water (it once was sea water), and is not dangerous. However, it must be disposed of, and it cannot be allowed to flow down fresh-water streams or rivers. A well sometimes may produce more salt water than oil…for instance, ten barrels of water may be produced for each barrel of oil.
The water disposal is usually handled by injecting the salt water into former oil or gas wells that have been converted to salt water disposal wells (SWD), or even wells drilled just for disposal purposes. Sometimes the water is injected back into the very same zone that the oil, gas, and water came from in the first place. When this is done, the injected water may sweep oil and gas into adjacent wells in the field. This process is called “waterflooding.”
The zones chosen for salt water injection are always very deep; thousands of feet below the surface. We want to ensure that (A) the zone is capable of taking all the water that is injected (that the zone has high porosity and permeability), and (B), that the drinking water wells in the vicinity are not affected.
Oklahoma had produced relatively small quantities of salt water from oil and gas wells for a hundred years or so, and fracking is not new – it had been extensively employed for over sixty years. However, the newer horizontal well types required larger fracks and later produced increased quantities of oil and gas. This resulted in increased volumes of salt water produced along with the hydrocarbons — and the salt water had to be disposed of. The existing SWD (salt-water disposal) wells began to take more and more of this waste product in the 2000’s.
With improved well fracking, and the increased prevalence of horizontal wells, oil and gas production in the United States went up substantially during the 2010’s, reversing a downward trend that began in the 1970’s. While some people were not happy with the activity that came with increased energy production, most felt that the former heavy dependence on foreign oil was a serious security security and economic threat to the country. In addition, many good-paying jobs in Oklahoma, Texas, Colorado, Louisiana, California, Appalachia, many of the western states, and elsewhere are tied to the energy industry.
Increased production led to to the United States having the capability to become a net energy exporter. By 2016, the United States was approximately 90% self-sufficient in terms of energy, up from 53% in 1989. This recovery of self-sufficiency was almost miraculous, as numerous doomsayers had been predicting the end of oil and gas for decades.
It had been suspected for years that salt water injection had a relationship to earthquakes in Oklahoma. However, it was found that earthquakes were more likely to result when the volume of fluid injected into SWD wells was raised significantly over a short time frame. With slower rates of injection, the possibility of earthquakes was minimized, as the injected fluids had time to migrate through the permeable rocks and zones without increasing the pressure in the disposal zone too fast.
Horizontal well fracking by itself turned out to have almost no relationship to earthquakes. It was estimated by the USGS that only one to two percent of fractured wells were the cause of subsequent seismic events – mostly not in Oklahoma. While the increased volumes of waste injection fluid after horizontal fracking still had to be disposed of, these amounts were insignificant compared to the increase in produced salt water from the increased oil and gas production.
Following are several maps of Oklahoma that illustrate the relationship between disposal well injection and earthquakes. You will note that the SWD wells are noted on the first map, and the colored circles indicate the volume (indicated in barrels per month) of salt water that was disposed of, with purple wells having the most injection. The second map shows the incidence of earthquakes (black crosses, sized by strength). It is fairly obvious that many of the earthquakes have a relationship to injection volume at nearby SWD wells. While the relationship may not be 1:1 due to the migration of injected water through faults or permeable zones, the connection seemed to be clear.
The problem was eventually addressed by the State of Oklahoma. The state energy authority (The Oklahoma Corporation Commission) introduced several regulations to stop or slow the injection process in various wells, and to plug more than forty problematic disposal wells. New rules to regulate injection volume amounts for individual wells were enacted. Operators were required to report injection volumes on a weekly basis. These actions had the effect of decreasing injection volume at sites likely to cause problems, and enabled increased monitoring by the state.
The state action began having an effect on the number of small eathquakes by 2017. Eathquakes dropped in number by more than half over 2016.