Fault “Corrosion” by Fluid Injection: A Potential Cause of the November 2017 5.5 Korean Earthquake

Figure 10

The same hydrochemical data as in Figure 9, for water produced from well PX-1 during August-September 2017, compared with five analyses of the surface water used as the source and six analyses indicative of the composition of preexisting groundwater in this well, but plotted as a function of the volume of produced water rather than as a function of time. Patterns, as flowback proceeds, include the tenfold decrease in the SO₄^2-/Cl^- ratio, in a manner explicable by mixing of a progressively greater proportion of groundwater with a progressively smaller proportion of surface water. Conversely, the Na⁺/Cl^- ratio is similar in the surface water and groundwater, thus insensitive to mixing, but “spikes” by a factor of ~2 during the initial ~500 m³ or ~30 hours of flowback, this variation being explicable as a result of reaction between the injected water and the granite. Note that the first 74 m³ of produced water remained within the casing of well PX-1 after injection and so did not interact with the Pohang granite.