Induced Seismicity and Underground Injection of Drilling Fluids
At the recent midyear meeting of the Independent Petroleum Association of America (IPAA) geological experts from Kansas, Oklahoma and Texas provided updates and analyses of the historic and current seismic activity in their respective states. They offered that while some of the seismic activity can be linked to the injection of fluids into geologic formations (in this case drilling related fluids) via underground injection control wells, other recent seismic activity appears to have no causal relationship to deep well injection operations. While state and federal geologic agencies continue to investigate, the US EPA has taken up the cause and is also looking into any relationship between underground injection operations and induced seismicity.
The EPA’s Dallas Regional office is leading the effort and is preparing a report likely to be titled “Draft Report on Injection Induced Seismicity – Practical Tools for UIC Regulators.” This report will come on the heels of other recent reports including one done jointly by the Groundwater Protection Council and the Interstate Oil and Gas Compact Commission. While those interested wait for the EPA report to be released, EPA Region III in Philadelphia published its own framework for evaluating the seismic potential of UIC wells.
The Region III document, which has a May, 2014 posting date on the EPA website, looks specifically at Class II injection wells. The document notes that the three conditions must exist for seismic activity to occur. These include: (1) a fault in a near failure state of stress, (2) the injected fluid has a pathway to the fault and (3) injection pressure is high enough and long enough to cause movement along the fault line. Region III also notes that they are aware of fewer than 10 cases of injection induced seismicity out of 30,000 injection wells in operation.
EPA notes that scientists believe the rock mechanics in play during induced seismic activity is caused by increased pore pressure that overcomes the friction forces that otherwise stabilize the fault and this pore pressure increases with volume and rate of injected fluid. It goes on to state that most induced seismic occurrences are caused by injecting into formations with low permeability. The document also notes that injecting into formations that have a history of oil or gas production is unlikely to result in induced seismic events. These formations are generally those with high permeability and low pore pressure and can readily accept the injected fluid with little resistance. Basically the injected fluid replaces the oil or gas that was extracted.
This EPA document also notes that of the hundreds of thousands of injection wells in operation in the US, they are not aware of any case where seismic activity, whether natural or induced, resulted in contamination of an underground drinking water source. This is certainly good news for those concerned about groundwater impacts from deep well injection operations. These experiences parallel experiences with hydraulic fracturing of shale gas and deeper conventional wells where there is no evidence of communication between the formation being fractured and near surface groundwater zones.