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. Monitoring microseismic activity provides a window through which to observereservoir deformation during hydrocarbon and geothermal energy production,or CO2 injection and storage. Specifically, microseismic monitoring mayhelp constrain geomechanical models through an improved understanding of thelocation and geometry of faults, and the stress conditions local to them.Such techniques can be assessed in the laboratory, where fault geometriesand stress conditions are well constrained. We carried out a triaxial teston a sample of Red Wildmoor sandstone, an analogue to a weak North Seareservoir sandstone. The sample was coupled with an array ofpiezo-transducers, to measure ultrasonic wave velocities and monitoracoustic emissions (AE) – sample-scale microseismic activity associated withmicro-cracking. We calculated the rate of AE, localised the AE events, andinferred their moment tensor from P-wave first motion polarities andamplitudes. We applied a biaxial decomposition to the resulting momenttensors of the high signal-to-noise ratio events, to provide nodal planes,slip vectors, and displacement vectors for each event. These attributes werethen used to infer local stress directions and their relative magnitudes.Both the AE fracture mechanisms and the inferred stress conditionscorrespond to the sample-scale fracturing and applied stresses. Thisworkflow, which considers fracture models relevant to the subsurface, can beapplied to large-scale geoengineering applications to obtain fracturemechanisms and in-situ stresses from recorded microseismic data.

Inferring microseismic source mechanisms and in situ stresses during triaxial deformation of a North-Sea-analogue sandstone