Pore Pressure Diffusion and Onset of Induced Seismicity

Abstract

The Raton Basin of Southern Colorado and Northern New Mexico experienced a significant increase in seismicity over the last several decades, including two M5+ earthquakes. The increase in seismicity started soon after an increase in wastewater disposal associated with coal‐bed methane production. Twenty‐nine wastewater injection wells have operated in the Basin, with several high‐rate injectors located within a kilometer of the most seismically active regions. We developed a pore pressure model to investigate the relationship between pore pressure diffusion and onset of induced seismicity, with an emphasis on the Trinidad fault zones. Our model indicates that sufficient pore pressure increase had accumulated on the northern extent of the Trinidad fault zone to induce seismicity at the time of early‐stage seismic activity. The hydrogeologic architecture of the Basin played a key role in controlling pore pressure perturbation. The basal sedimentary interval with moderately low diffusivity restricted vertical pore pressure diffusion, creating a substantial lag time between wells becoming active and the expression of perturbation in the fault zones. The spatiotemporal relationship between early‐stage seismicity and wastewater injection in conjunction with modeled pore pressures suggests that the early‐stage seismicity in the Raton Basin was likely triggered by pore pressure diffusion.

Plain Language Summary

In the Raton Basin, coal‐bed methane production sparked the need for deep wastewater disposal starting in 1994. Prior to wastewater injection few earthquakes were recorded within its boundaries but a large increase in earthquakes has been recorded over the last several decades. Pore pressure increase, caused by the deep wastewater injection, is thought to be the reason for the increase in local earthquakes. We developed a model to estimate pore pressure increases on several fault zones that have recently exhibited increased earthquake frequency. We found that wastewater injection produced increases in pore pressure on the fault zones large enough to cause earthquakes. This suggests that increased seismicity is, at least in part, the result of pore pressure change caused by deep wastewater injection.

Key Points

• Pore pressure change between 0.007 and 0.03 MPa initiates the onset of induced seismicity on the Trinidad fault zone

• The basal sedimentary interval restricted pore pressure diffusion, limiting the impact of high‐rate injection wells in the short term

• The depth varying permeability of crystalline basement effects the manner and magnitude of pore pressure diffusion into basement faults