UC Riverside

The Geysers geothermal field in northern California has seen subsidence, attributed to net volume loss during power production, since at least the 1960s. Over the last three decades this has been accompanied by reductions in reservoir steam pressure and power generation. To combat these effects, wastewater has been injected in the field since 1997. In order to better understand the effects of variations in production and wastewater injection on geothermal reservoir volume and surface subsidence over time, two continuously-recording GPS stations (TG01 and TG02) in the northern Geysers in 2012 and one in the southern Geysers (TG03) in 2013 were installed in the field.

In this thesis, I will present our first analyses of the continuous GPS data and our first attempt at modeling the data. Both TG01 and TG02 show early periods of uplift and later subsidence while TG03 shows ongoing subsidence. Next, we downsample steam extraction and injection data onto a rectangular grid and calculate ‘observed’ monthly volume changes as a function of position. We then use these to drive a forward elastic dislocation model to predict surface deformation changes each month in The Geysers field. The models cannot reproduce the GPS data, specifically the uplift that occurs at TG01. We then compare the observed volume changes with inverse elastic dislocation models of the volume changes required to reproduce the GPS time series. Due to low spatial resolution, we compare the total volume change in the field, which is comparable amplitude, and shows periods where the peaks are in phase, and periods where they are out of phase. In these cases, the peaks in the inverse modeled volume changes lag those in the reported data by one month. These 'out-of-phase periods' correlate with periods of peak injection in the field, typically in the winter and early spring months, suggesting that the GPS data are detecting a delayed deformation response to injection in the field, possibly related to the finite permeability of the geothermal reservoir rocks.

We also look at a case study of The Geysers Mw 5.0 earthquake. This earthquake is a rare small shallow strike-slip earthquake capable of producing surface displacements. Our three continuous GPS stations in the field recorded coseismic displacement of up to 2 cm. We also were able to see the earthquake displacement in a 6-day descending interferogram. We ran forward and inverse models of both nodal planes provided by earthquake catalogs and found that both nodal planes produce consistent deformation patterns although the inverse model closely matched the NNW-striking nodal plane.