Abstract. Monitoring microseismic activity provides a window through which to observe
reservoir deformation during hydrocarbon and geothermal energy production,
or CO2 injection and storage. Specifically, microseismic monitoring may
help constrain geomechanical models through an improved understanding of the
location and geometry of faults, and the stress conditions local to them.
Such techniques can be assessed in the laboratory, where fault geometries
and stress conditions are well constrained. We carried out a triaxial test
on a sample of Red Wildmoor sandstone, an analogue to a weak North Sea
reservoir sandstone. The sample was coupled with an array of
piezo-transducers, to measure ultrasonic wave velocities and monitor
acoustic emissions (AE) – sample-scale microseismic activity associated with
micro-cracking. We calculated the rate of AE, localised the AE events, and
inferred their moment tensor from P-wave first motion polarities and
amplitudes. We applied a biaxial decomposition to the resulting moment
tensors of the high signal-to-noise ratio events, to provide nodal planes,
slip vectors, and displacement vectors for each event. These attributes were
then used to infer local stress directions and their relative magnitudes.
Both the AE fracture mechanisms and the inferred stress conditions
correspond to the sample-scale fracturing and applied stresses. This
workflow, which considers fracture models relevant to the subsurface, can be
applied to large-scale geoengineering applications to obtain fracture
mechanisms 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