--> The Role of Pressure Diffusion in U.S. Midcontinent Seismicity

AAPG ACE 2018

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The Role of Pressure Diffusion in U.S. Midcontinent Seismicity

Abstract

The unprecedented increase in seismicity across the U.S. midcontinent since 2009 has prompted researchers to study its cause and offer solutions. It is understood that the seismicity is triggered by the recent increase in wastewater injection into the Arbuckle Group saline aquifer. The injection rate increases the pore pressure, subsequently decreasing the effective normal stress, unclamping basement faults and triggering slip. A recent study (Langenbruch and Zoback, 2016) has related the injection rates to earthquake rates and has offered an injection rate threshold for reducing seismic risks using seismic hazard models. Our study focuses on injection-induced pressure diffusion in the Arbuckle and its role in triggering fault slip in the crystalline basement. Pressure diffusion in the Arbuckle is useful for detecting areas or regions of high concern in the context of Mohr-Coulomb failure and for proposing mandated threshold on the injection rates. A 2D line source and a 3D point source pressure diffusion model is used for calculating the pressure change from UIC (Underground Injection Control) Class I and II injection data for Kansas and Oklahoma. The model incorporates well injection rates and locations, reservoir thickness, porosity, and permeability. The final pressure at a point in time is calculated by superimposing the pressure change caused by different rates of individual wells. Obtained results quantify the rate at which the pressure change diffuses through the Arbuckle. The results of analytical models are compared with a more complicated numerical model. Our results suggest that pressure diffusion is inducing mid-continent seismicity. Future studies can use the results for probabilistic assessment of potential slip for the mapped Kansas and Oklahoma faults, and for identifying regions that may be of higher risk for injection-induced earthquakes.

Langenbruch, C., and M. D. Zoback, 2016, How will induced seismicity in Oklahoma respond to decreased saltwater injection rates? Science Advances, v. 2, no. e1601542, p. 1–9, doi:10.1126/sciadv.1601542.