MODELING THE CONTRIBUTION OF WASTEWATER INJECTION TO SURFACE DEFORMATION IN THE RATON BASIN, CO/NM

Land surface deformation has been observed near some Class II wastewater injection sites, where injection can change the pore pressure regime within an aquifer. Rocks can expand and contract in response to changes in pore pressure, a concept known in poroelasticity. Cumulative elastic deformation can be linked to observations on the land surface using geodetic techniques. The Raton Basin, in southern central Colorado and northern central New Mexico, has experienced wastewater injection related to coalbed methane and gas production starting in the early 1990s. The depth of injection is roughly 2 km targeting the permeable Dakota formation. Groundwater flow models indicate changes in pore pressure sufficient to trigger seismicity on critically stressed faults, but how the surface deforms in response to pore pressure changes from wastewater injection remains unknown.

Here, we estimate the extent and magnitude of total vertical deformation in the Raton Basin since injection began in 1994 to 2020, and incremental deformation between the years 2017 to 2020 by using a multifaceted modeling approach. Years 2017 to 2020 were chosen to match dates of available differential interferometric synthetic aperture radar (DInSAR) data which can later be compared to the results of the study. First, we obtain pressure change from a groundwater flow model. We then apply a relation between pressure change and the change in vertical rock thickness. Calculated cumulative deformation over time is used to create regional-scale surface deformation maps and time series at select locations.

The results indicate uplift of as much as 3.5 cm between 1994 and 2020, with most occurring near high-rate injectors in New Mexico. Between 2017 and 2020, around 0.4 cm of uplift occurred in some areas, largely near the northwestern wells. Most total uplift between 1994 and 2020 occurred near the wells in the southern part of the Raton Basin, where the greatest cumulative volume of wastewater was injected. However, subsidence occurred around the same wells between 2017 and 2020, after the rate of injection decreased. This suggests that while the magnitude of surface deformation corresponds to cumulative injection volume and maximum rate in the long-term, short-term incremental deformation is controlled by changes in the rate of injection.