Hydraulic fracturing is widely used in the petroleum industry to enhance oil and gas production, especially for the extraction of shale gas from unconventional reservoirs. A good understanding of the vertical distance which should be preserved between hydraulic stimulation and overlying aquifers (potable water) has been demonstrated as being greater than 600 m (2000 feet). However, the effective application of this technique depends on many factors; one of particular importance is the influence of the fracturing process on pre-existing fractures and faults in the reservoir, which, however, to date, has had little analysis. Specifically, the identification of the required respect distance which must be maintained between the hydraulic fracturing location and pre-existing faults is of paramount importance in minimizing the risk of felt, induced seismicity. This must be an important consideration for setting the guidelines for operational procedures by legislative authorities. We investigate the respect distance using a Monte Carlo approach, generating fifty discrete fracture networks for each of three fracture intensities, on which a hydraulic fracturing simulation is run, using FracMan Ò . The Coulomb stress change of the rock surrounding the simulated injection stage is calculated for three weighted source mechanisms combining inflation, strike-slip and reverse. The lateral respect distance is obtained using values from literature of the amount of stress required to induce movement on a pre-existing fault. We find that the lateral respect distance is dependent on fracture intensity and the failure threshold. However, the weighting of the source mechanism has limited effect on the lateral respect distance.

The shale gas industry has significant impact on economies around the world, however, it is not without risk. One of the primary concerns is felt seismicity and recent earthquakes, caused by fault reactivation related to hydraulic fracturing operations, have escalated uncertainty about hydraulic fracturing methods. Mitigating these risks is essential for restoring public confidence in this controversial industry. We investigate the effect that changing two operational parameters (flow rate and pumping time) and differential pressure has on the flow distance, fracture network area and the minimum lateral distance that hydraulic fracturing should occur from a pre-existing fault in order not to reactivate it (lateral respect distance); thus reducing the risk of felt seismicity. Sensitivity analyses are conducted using a Monte Carlo approach. The lateral respect distance is obtained from calculations of the Coulomb stress change of the rock surrounding the injection stage, for four stress threshold values obtained from the literature. Results show that the flow rate has the smallest rate of change for fracture area (3700 m 2 per 0.01 m 3 /s) and flow distance (8.3 m per 0.01 m 3 /s). We find that differential pressure has the largest impact on stimulated fracture area, when less than 2 MPa, at 31,029 m 2 /MPa. The pumping time has the most significant effect on the flow distance (48 m/hr) and the stress threshold value the most significant effect on the lateral respect distance. This study suggests that to reduce the lateral distance, a compromise is required between flow distance and fracture area. The results obtained by this research provide invaluable guidance for operational practice in determining the potential area of the induced fracture network and generated stress field under realistic hydraulic fracturing conditions, an important aspect for risk assessments.

This paper presents a case study of the ground-borne vibrations generated by small wind turbines.

Ambient seismic noise can often be seen as problematic, but with the right analysis can act as a tool to image the Earth. Wind turbines are known to generate low frequency vibrations, however, the wave types that are generated are currently unknown. Characterizing these vibrations will allow wind turbines to be used as a seismic source and be of value to geotechnical applications and seismic interferometry. This paper uses polarization analysis of the seismic wavefield around a small wind turbine to identify the type of wave being generated by the turbine and to clarify the source. The seismic data recorded 190m from the wind turbine are processed using a window length of 0.1 seconds and bandpass filtered on a selection of frequency ranges. Polarization analysis is performed for two different wind speed ranges, in order to show the variation of wave characteristics between operational and non-operational modes of the wind turbine. Polarized surface waves are identified as the predominant wave type at blade rotation harmonics, making this work particularly relevant to multichannel analysis of surface waves and seismic interferometry.

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