Induced seismicity once again primarily focused on disposal wells

Published: October 13, 2014

The article "The 2001–Present Induced Earthquake Sequence in the Raton Basin of Northern New Mexico and Southern Colorado" (Weblink) published in October 2014 is not directly related to shale gas production but, instead, deals with seismicity created in connection with the disposal of reservoir water co-produced during coalbed methane production. However, it is of some interest here, because the origin of the disposed water does not really matter when considering seismicity in connection with disposal wells.

Disposal wells have been getting more and more attention when considering seismic risks ever since events in the Rocky Mountain Arsenal, in Denver, Colorado were reported in 1966 and experiments in the Rangley oil field in Colorado showed a clear correlation with the disposal rates. Other examples are: Prague, Oklahoma, 2011, M5.7; Youngstone, Colorado, 2011, M4.0; Paradox Valley, Colorado, M4.4; Guy-Greenbeer, Arkansas, M4.7. Research conducted in the Raton Basin adds to these already published reports.

Coal seam was produced in the Raton Basin on the border between Colorado and New Mexico, USA between 1862 and 2002. Between 2001 and today, coalbed methane (CBM) was produced, mostly from coal seams in the Raton, Vermejo and Trinidat formations at a depth of 200 to 800 meters. The production of coalbed methane always goes along with the production of a large amount of water (the article does not include the relation of the amount of gas and water), that needs to be disposed of.

In the Raton Basin, only small amounts of water can be disposed of without previous treatment (into discharge systems). Most of the water must be disposed of underground due to its chemical composition. In the Raton Basin, this is realized by means of a number of disposal wells (>20) into the Dakota formation, a conglomerate sandstone at a depth of 1,250 to 2,100 meters that is suitable for this purpose. Due to the underpressure in this formation disposal could mostly be carried out taking advantage of gravitation and without the need for additional pressure at the wellhead.

The Raton Basin does have a natural seismicity; however, only one earthquake is known to have occurred with a magnitude of >M3.8 before 2011. During the time water was disposed (2011 to today), on the other hand, there were 16 events >M3.8, the largest event on August 23, 2008, with a magnitude of M5.3. A similar increase applies for events of other orders of magnitude. The authors conduct a thorough analysis of the connection between the disposal of water and seismicity. They come to the conclusion that such a connection is hard to prove for an individual event. This would require more exact knowledge of the conditions within the seismic focus: the stress field, friction resistances, injection pressures, injection rates and injection volume as well as an exact localization of the events.

This is why the article concentrates on showing that the increase of the events as such is induced. Based on an estimated stress change of 4 kPa determined by means of a model calculation, the authors eliminate the possibility that events could already be triggered by the removal of water, because it is generally assumed that, to trigger such an event, at least 20 kPa would be necessary. It can therefore clearly be assumed that the disposal of water in the Dakota formation triggered most of the events that occurred between 2001 and today.

According to the data the authors present, the water that was disposed of and the disposal rates amount to 2 million barrels/ month (1 m³ = 6.29 barrels). The data shows that the event rates of the earthquakes are temporally related to the disposal rates. A correlation between the magnitudes and the disposal rates or the cumulative volume of the disposed water cannot be found.

The localization of the events clearly allocates the events to the disposal wells, but also to the indicated fault systems. Here, it must be taken into account that the accuracy of the localization was initially very small (+- 15km) and only improved after a local network was put into place. For most events, a depth of 4-6 km was determined (i.e. in the bedrock). Where the depth could not be determined, a depth of 3.5 km was assumed, the disposal wells being less than 2 km deep. The authors state that there is no hydraulic connection between the Dakota formation and the bedrock, because there are several hydraulically restrictive layers in between. The authors do not see any connecting faults either, but it is obvious that the fault inventory is little known and that no information on the drilling activities of the companies was available. An explanation for if or how the events in the much deeper bedrock could be triggered by the injection of water into the sediment at the end of the slope without a hydraulic connection is not offered.

Conclusions

Similar to shale gas production, in coalbed methane production the disposal of co-produced water is the critical factor with regard to seismicity.
The event rate of the earthquakes is temporally related to the disposal rate of the water.
Event rates and magnitudes do not depend on the cumulative volume of the disposed water.
Disposal wells can also trigger events if no additional injection pressure was created.
With regard to the focal mechanism, it remains unclear how events in the bedrock kilometers below the formation into which water was injected can be triggered without an existing hydraulic connection.
In the US alone there are several thousand disposal wells in which induced seismicity did not occur. Therefore, the question why earthquakes could be triggered here over a duration of 13 years with a maximum magnitude of 5.3 without the water needing to be disposed of in a different way or a different location, remains unanswered.