The relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks

Abstract

Several hundred microearthquakes with local magnitudes ranging between −4 and −2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site, Fenton Hill, New Mexico. These events are probably caused by shear failure induced by high pore fluid pressures. Since the event locations seem to cluster in a narrow band near the hydraulic fracture, we were able to use micro‐seismic techniques to locate the hydraulic fracture and monitor its growth. We calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures. By comparing a pore pressure distribution, calculated using a one‐dimensional diffusion model with the distribution of event foci, we demonstrate that high pore pressures are a probable cause for these microearthquakes. Stress drops for these microearthquakes, calculated from spectral parameters, increased by a factor of 3 during the course of the experiment. The stress drop increase correlates with a migration of seismic activity away from the injection well bore, which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations.

EOS Modeling and Reservoir Simulation Study of Bakken Gas Injection Improved Oil Recovery in the Elm Coulee Field, Montana

“…Microseismic events result from rock failure due to geomechanical changes (Pearson, 1981). These microseismic events can be recorded from both surface and downhole geophones.…”

A Comparison of Hydraulic Fracture Modeling With Downhole and Surface Microseismic Data in a Stacked Fluvial Pay System

“…Such slip is induced by elevated pore pressure that reduces normal stress along pre-existing fractures (Pearson, 1981). According to Green and Sneddon (1950), slip can also occur near the tips of created fractures due to large shear stresses generated by tensile opening.…”

Numerical Investigation on the Effect of Cementing Properties on the Thermal and Mechanical Stability of Geothermal Wells

“…In order to successfully complete and operate the geothermal power plant constructed by the EGS, extensive studies have been performed with respect to the stabilization of the transmission fluid (mostly water) (Xu et al, 2005) [3], improving thermodynamic cycles (Chen et al, 2010) [4], use of carbon dioxide (CO2) instead of water for subsidiary benefits (Brown, 2000;Pruess, 2006) [5,6] and hydraulic stimulation techniques for HDR (Sasaki, 1998;Pearson, 1981) [7,8]. Among these studies, the completion of geothermal well cementing is of considerable importance for the following reasons: (1) the hydrated cement slurry must have sufficient strength to support the steel casing that tends to elongate due to the temperature changes; (2) the cement slurry should pre-flash and displace the water-based drilling fluids; and (3) complete cementing is needed to protect the steel casing from corrosive fluids (Edward et al, 1982) [9].…”