THE DISTANCE OF CONTINUED INCREASING PORE PRESSURE AFTER INJECTION STOPS

Occurrence of injection induced earthquakes following the cessation of injection is not uncommon. Instances of this have been observed at both wastewater disposal and geothermal induced seismicity sites. Understanding the mechanics of why earthquakes occur after injection stops is important in deciding mitigation strategies. While multiple mechanisms are likely involved in triggering induced seismicity, elevated pore pressure reducing the effective normal stress along critically stressed faults is widely accepted as the main cause. After injection stops, pore pressure diffusion continues, as the pressure decreases closer to the well, the pressure continues to increase further out.

At what distance from the injection well does the pore pressure continue to increase after injection ceases and what controls that distance? What are the controlling factors that dictate the distance? We use the Theis solution for pore pressure change caused by injection and the Principle of Superposition for pore pressure change after injections stop to derive an equation for the radial distance at which the pore pressure is increasing through time. We find that this distance can be described as a power function of the time since injection ceased. The equation is also a function of the injection aquifer’s diffusivity and the total length of injection. As pore pressure continues to increase at distances farther form the well, the likelihood of encountering a critically stressed fault increases. We do find limits on this phenomenon where the pore pressure does not continue to increase after injection ceases when the diffusivity of the aquifer is very high (e.g. on the order of thousands of square meters per second) and the injection rate is not excessively high (e.g. less than several thousand cubic meters per day).