High density oilfield wastewater disposal causes deeper, stronger, and more persistent earthquakes

Abstract

Oilfield wastewater disposal causes fluid pressure transients that induce earthquakes. Here we show that, in addition to pressure transients related to pumping, there are pressure transients caused by density differences between the wastewater and host rock fluids. In northern Oklahoma, this effect caused earthquakes to migrate downward at ~0.5 km per year during a period of high-rate injections. Following substantial injection rate reductions, the downward earthquake migration rate slowed to ~0.1 km per year. Our model of this scenario shows that the density-driven pressure front migrates downward at comparable rates. This effect may locally increase fluid pressure below injection wells for 10+ years after substantial injection rate reductions. We also show that in north-central Oklahoma the relative proportion of high-magnitude earthquakes increases at 8+ km depth. Thus, our study implies that, following injection rate reductions, the frequency of high-magnitude earthquakes may decay more slowly than the overall earthquake rate.

Fig. 1

Summary earthquake and wastewater data within study areas. Annual M2.5+ earthquake count (gray bars), salt water disposal volume (light blue line), and mean earthquake depth (circles) for a Alfalfa, b Oklahoma, and c Lincoln Counties, which are located in Oklahoma, USA. Panel (d) presents the same data for the Raton Basin of Northern New Mexico and Southern Colorado. Mean annual earthquake depth from 2013 to 2018 (solid circles) are weighted by inverse square depth error. Before 2013, mean annual earthquake depths (open circles) are calculated as an arithmetic mean because depth errors were infrequently reported. Error bars correspond with two standard errors. Note that open circles lacking error bars arise because shallow earthquakes were commonly reported to occur at 5 km depth before 2013

Fig. 2

Models of variable- and constant-density salt water disposal. Results for a single salt water disposal well operating for 10 years at 2,080 m³ day⁻¹ (13,000 bbl day⁻¹). Left column shows variable density results after a 1 year, b 5 years, and c 10 years of injection. Black contour lines denote fluid pressure change above initial conditions in 10 kPa increments. Background shading for the variable density models is fluid density, which decreases with depth due to increasing temperature. Right column shows constant-density results after d 1 year, e 5 years, and f 10 years of injection. White contour lines denote fluid pressure change above initial conditions in 10 kPa increments, and background shading is the mass fraction of injection water

Fig. 3

Time series of fluid pressure above initial conditions (ΔP_f). Simulation data are recorded at monitoring points located within the injection well (black) and directly below the well at 4 km (blue), 5 km (green), and 6 km (red) depth. Solid and dashed lines are ΔP_f curves for the variable density and constant-density models, respectively. For a given depth, the difference between solid and dashed lines are due to the advective transport of high-density wastewater

Fig. 4

Post-injection fluid pressure recovery. Simulation results showing post-injection fluid pressure patterns for the variable density model following ten years of wastewater disposal at 2080 m³ day⁻¹ (13,000 bbl day⁻¹). Remaining fluid pressure above background (ΔP_f) is shown after a 1 year, b 5 years, and c 10 years of post-injection fluid pressure recovery. Contour lines denote ΔP_f in 10 kPa increments. The corresponding constant-density model recovers to <10 kPa within 1 year post-injection, and the results are presented in Supplementary Materials

Fig. 5

Earthquake depth-magnitude analysis between 1 Jan 2013 and 31 Dec 2018. a Areal extent of the earthquake catalog within northern Oklahoma and southern Kansas. b Earthquake distribution in 1 km depth intervals (gray bars) with the corresponding Gutenberg-Richter b-value shown as red circles. The b-value systematically decreases beyond 8 km depth indicating that the relative proportion of high-magnitude earthquakes is larger in the 8–10 km depth interval. Error bars correspond with two standard errors of the regression slope for each depth interval. c Annual distribution of M2.5+ (gray), M3.0+ (blue), M3.5+ (yellow), and M4.0+ (red) earthquakes, as well as the percent change from the prior year (circles) beginning when the overall earthquake started to decline