Use of microseismicity for determining the structure of the fracture network of large-scale porous media

Abstract

We show that microseismic events---earthquakes with small magnitudes---can be fruitfully used to gain insight into the properties of the fracture network of large-scale porous media, such as oil, gas, and geothermal reservoirs. As an example, we analyze extensive data for the Geysers geothermal field in northeast California. Injection of cold water into the reservoir to produce steam leads to microseismic events. It is demonstrated that the analysis can also lead to insight into whether the fractures are of tectonic type or induced by injection of cold water. To demonstrate this we estimate, using the catalogue of the microseismic events, the fractal dimension ${D}_{f}$ of the spatial distribution of hypocenters of the events in three seismic clusters associated with the injection of cold water into the field, as well as the $b$ values in the Gutenberg-Richter frequency-magnitude distribution. The fractal dimensions are all in a narrow range centered around ${D}_{f}\ensuremath{\simeq}2.57\ifmmode\pm\else\textpm\fi{}0.06$, comparable to the measured fractal dimension of fracture sets in the greywacke reservoir rock. For most cases the $b$ values are about $b\ensuremath{\simeq}1.3\ifmmode\pm\else\textpm\fi{}0.1$, consistent with the Aki relation, ${D}_{f}=2b$. Both ${D}_{f}$ and $b$ are significantly higher than those commonly observed for regional tectonic seismicity or aftershock sequences for which ${D}_{f}\ensuremath{\approx}2$ and $b\ensuremath{\approx}1$ are typical. Our results do not imply that no tectonic triggering exists in the reservoir, but rather that the overpressure allows the activation of less favorably oriented fractures that produce an increase in both $b$ and ${D}_{f}$. The estimate ${D}_{f}\ensuremath{\approx}2$ for tectonic seismicity has been interpreted as indicating that most tectonic events occur on the subset of near-vertical faults---because they have lower normal stress---or that they occur on the backbone of the fracture and fault network, the multiply connected part of the network that enables finite shear strain. Our results lend support to the latter. The results that the entire fracture network, and not just its backbone, is active at the Geysers indicate that the seismicity is not a result of the triggered release of tectonic stress, but is induced by the release of local stress concentrations, driven by thermal contraction that is not constrained by friction. The possible implication for hydraulic fracturing---so-called fracking---is also briefly discussed.