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

Abstract

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.

Depletion-induced seismicity in NW-Germany: lessons from comprehensive investigations

“…A more detailed insight, also with seismicity arising in the sample, requires smaller wavelengths. For instance, Griffiths et al [10] employed them with dry sandstone samples by means of 14 piezo-electric transducers at the confining membrane of a triaxial setup. Thus, they tracked down micro-seismic sources along a major shear band, including its alignment.…”

Induced-seismicity geomechanics for controlled CO2 storage in the North Sea (IGCCS)

“…In addition, multistage triaxial tests are performed on two caprock lithologies as well: one with shallow (700 m) mudstone from Nordland Group and the other with a deeply-buried (2581 m) shale from the Draupne Formation (CLIMIT, 2020;Grande et al, 2020). Furthermore, two reservoir sandstone analogues, Red Wildmoor and Grès des Vosges, are also tested (Griffiths et al, 2019(Griffiths et al, , 2021, which can be relevant for the intermediate depth Sognefjorden (Smeaheia) and the deeper Johansen Formations (Aurora), respectively. Tables 1-3 summarize, respectively, all the tests performed in IGCCS, the geomechanical parameters extracted from the tests performed on the overburden and reservoir lithologies, and the compiled in-situ stresses for different lithologies and depths.…”

“…The main test results are summarized briefly here with particular focus on: (1) the influence of pore fluid type on fracturing and AE response of a deep reservoir sandstone; and (2) geomechanical material models (covering sandstone, mudstone, and shale). More details, including the AE data processing workflow developed in IGCCS, are reported at CLIMIT, 2020, Grande et al (2020), and Griffiths et al (2019.…”

“…Our focus is particularly on the following fundamental mechanisms: (1) differences in geomechanical response between CO 2 and brine injections; and (2) thermal effects due to cold CO 2 injection. The current paper provides a brief summary of the main findings and learnings from IGCCS, and for more details, refers to the IGCCS-related publications of Bjørnarå et al (2021aBjørnarå et al ( , 2021b, CLI-MIT, 2020, Fawad and Mondol (2019), Grande et al (2020), Griffiths et al (2019Griffiths et al ( , 2021, Mondol et al (2018) and Mondol (2019).…”

Frictional Properties and Seismogenic Potential of Caprock Shales

“…Another laboratory study on the Draupne shale samples was performed by Griffiths et al [49] where they monitored acoustic emission (AE; laboratory-scale seismicity) events during the triaxial testing of rock cores. AE monitoring was performed during both isotropic and anisotropic loading, undrained shearing to failure and subsequent reactivation of the shear fracture created during the triaxial test.…”