1School of Earth Sciences, University of Bristol
In just over 10 years, reservoir microseismicity has gone from a niche topic to a mainstay monitoring tool in the hydrocarbon industry. Much of this activity has focussed on shale gas exploitation, where microseismic data is crucial for understanding hydraulic fracture propagation, and thereby optimising production of this resource. In addition, microseismic monitoring is a vital tool for the mitigation of induced seismicity, the consequences of which have the potential to make or break the development of this unconventional hydrocarbon resource in Europe, as well as other nascent industries like enhanced geothermal, natural gas storage and CO2sequestration.
Operators use microseismicity to evaluate the efficacy of hydraulic fracture stimulation and fluid migration. Regulators use microseismicity to assess hazards associated with oil and gas production, often employing ‘traffic light systems’. Hence, there is a need for the accurate evaluation of earthquakes magnitudes and locations. This can be a challenge with such small magnitude events, where the amplitude of the source spectra can be comparable to noise levels and corner frequencies are often above sampling frequencies. Furthermore, local magnitude scales need recalibration due to near-surface attenuation. Finally, the shale formations in these settings are often highly anisotropic, which can lead to significant event mislocations and errors in moment tensor solutions. Aligned fractures further complicate the analysis of anisotropy in these settings, but also offer insights into fluid flow.
Detecting small microseismic events (with magnitudes from -3 to -1) can be challenging, requiring either dedicated boreholes or dense surface arrays. Each type of acquisition has different costs and benefits. Beam forming and migration methods have shown great potential in improving detection levels and remove the need for picking discrete P- and S-wave arrivals. Furthermore, the use of fibre optic cables as distributed acoustic sensors (DAS) is a rapidly-developing technology. Such cables are relatively cheap and easy to deploy in boreholes and do not require a dedicated borehole. A current disadvantage of DAS is the lack of vector fidelity and the ambiguity in source location using single-well linear-arrays.