Fifth EAGE Passive Seismic Workshop

The public perception for human-induced and triggered seismicity is often very high and public safety is a major issue for approving authorities. Even weak non-damaging events felt by population have led to major debates and in some cases fostered the closing down of geotechnical projects. Often, we cannot convincingly answer the question of the likelihood that damaging earthquakes may occur or not before the start or during the course of the engineering operations. Passive seismic techniques and advances in theoretical seismicity models are needed to improve the current situation.

Seismological methods towards the characterisation of seismic sources are well advanced in mining applications (e.g. Grigoli et al., 2013 , Sen et al., 2014 , Maghsoudi et al., 2014 ). Micro-seismicity is also recognized as a valuable tool for completion, optimization, characterization and modelling of reservoirs or storage facilities (e.g. Cesca et al., 2014a ). However, although the probabilistic description of seismicity is established in seismology since decades, a probabilistic approach is still not common practice for induced seismicity and reservoir studies in industry. A possible reason may be that seismicity models in seismology are often based on steady state or quasi static loading rate conditions, a situation rarely valid for engineering activities. Therefore, seismicity models considering the stress and pressure loading conditions of engineering activities need to be further developed andvalidated.

The discrimination between natural and human related earthquakes is important for both issues. For instance, the nature of induced seismicity implies nearby geotechnical operations and engineering activity. However, the occurrence of a close-by earthquake does not always mean that the earthquake is human related, especially in regions with high tectonic activity. We need to establish community accepted methods for the discrimination of events. These should consider (geo-)physical and structural parameter from the natural background processes and the human related activities (e.g. Dahm et al., 2010b ). Beside the discrimination by means of source parameter estimations (e.g. Cesca et al., 2013b , 2014b ), probabilistic methods based on seismicity parameters are important (e.g. Dahm et al., 2012 ). A database of case studies, including both seismicity and production parameters, is important to validate such models and to establish common accepted procedures. Unfortunately, such databases are still not available or not accessible to a wider scientific community.

The presentation reviews and summarizes the basics of seismicity models and their current role in natural earthquake and triggered / induced earthquake studies. Specific problems and questions for the different cases of induced / triggered seismicity and applications will be clarified. The behaviour and different aspects of a rate and state seismicity model are demonstrated. We discuss implementations of the rate and state model ( Dietrich, 1994 ) for the characterization of triggered and induced seismicity, comprising the characterization of seismicity clouds related to hydro-fracture formation and the evaluation and discrimination of isolated significant earthquakes. Knowledge gaps and unsolved questions will be discussed.

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Enhanced oil recovery is routinely performed by water flooding in the Tazlau field in Eastern Romania. By the end of 2013, a borehole microseismic feasibility study was launched to gain better understanding of the producing reservoir behavior. Two wells were equipped with 8 geophones in approximately 1000 meters depths in two consecutive periods coinciding with the initiation of the injection phase. In the first well, a significant amount of microseismic events (2566) was recorded during 14 monitoring days, in contrast to a very small number of events (54) observed in the second well in the following 14-day period. This significant difference is related to the varying fracture density in the area and, to a lesser degree, to the amount of injected water.

In general, event hypocenters can be located within distances of 600 meters from the monitoring well with sufficient accuracy, and their moment magnitudes range from -1.5 to -3.5. Fracture network characterization based on the microseismic pattern is supported by the analysis of fullbore formation imaging. The study provides new insight on potential water paths. We conclude that borehole microseismic monitoring appears as a promising and useful technique for steering water injection.

In the scope of cavity surveillance and instability forecasting, the underground salt mine cavity of Cerville-Buissoncourt, NE France, was monitored during 5 years and represents a large collection of microseismic data, encompassing the cavity’s stability phase until its provoked collapse. The growing instability of the cavity is witnessed by several microseismic crises recorded at 1D and 3D geophone stations deployed at the surface and in boreholes. Event localizations indicate that they lie near the cavity roof and are interpreted as block falls and growing of the cavity.

Our objectives are (1) to follow the cavity’s growth over time until its collapse, in order to improve prognostics of underground cavity collapses in a general sense, and (2) to make accessible our catalog data and methods via open source web platforms, e.g. e.cenaris (INERIS) and EPOS.

In order to achieve this, a comparison is made between event localizations and characteristics along the different evolution stages of the cavity: stable, unstable and collapse. Once data processing is automated and a calibration magnitude law is found for each time period, we apply them to a much larger dataset within each time period and obtain a complete catalog describing the evolution of the salt mine cavity.

The relations between seismic moment M0 and seismic energy Es for various scale events were considered.

Averaging over the entire scale range gives relations close to the self-similar assumption. A more detailed examination reveals several hierarchical levels, in which changing the event parameters with the scale occurs by different laws, often strongly deviating from the self-similar scaling.

The first class of events is presented by small events with Mw <1 ÷ 2, located at shallow depths of about 1–2km. In this interval, the value of scaled energy Es/M0 regularly increases with the scale, which is related mainly to the dependence of rock shear modulus on scale. The second interval should include small earthquakes located at seismogenic depths. If hypocenters are confined to small cracks located aside from the central part of major faults, their parameters correspond to the self-similar laws and Es is proportional to M0. Earthquakes with moment magnitude Mw ~ 3.5–4 make the transition band, which is the boundary between the two ranges in which the scaling relations differ significantly.

We propose a method to identify repeating events and apply it to microseismic data recorded during and after the stimulation of the geothermal reservoir at Basel-1, Switzerland. Our method is based on a combination of the measured waveform similarity between event pairs and an estimate of the source radii. In our case study, at least 15% of the microseismic events can be grouped into repeating earthquake sequences. We then analyse the spatial and temporal characteristics of the repeating events and find that they differ significantly from the behaviour of the entire microseismic dataset. Repeating events are a common feature during the stimulation phase and at small distances from the injection source (<250–300m), but they are clearly under-represented after the shut-in and at larger distances. The distance interval, where the behaviour of seismicity changes abruptly, correlates with the occurrence of the largest induced events (here Mw > 2.0). The observations suggest that the repeating earthquakes define a subset of seismicity which is particularly sensitive to pore pressure changes and is less affected by earthquake interaction. Therefore, the analysis of injection-induced events may provide a tool to identify the dominant triggering mechanism of seismicity and to detect possible changes of the triggering mechanism (e.g. pore pressure versus stress transfer).

Either geomechanical modeling or reservoir engineering require a consistent microseismic cloud obtained through accurate location of very low magnitude events. In this abstract, we compare the different processing methodologies we have recently developed to improve sensitivity and accuracy of microseismic events located with a surface array.

First we developed the so-called multi-local-template approach, which strongly improves location accuracy. Relative locations using such a template remain very efficient even with an inaccurate velocity model.

We introduced two high sensitivity methods: the first is based on a joint mechanism and location inversion and the second is based on the use of a signed template, somewhat similar to a multiplet relative location scheme. There’s a twofold increase in number of detected events when using a joint inversion, as it takes into account the focal mechanism amplitude effects prior to forming the beam. The so-called signed template approach is even more sensitive.

Lastly we adapted these algorithms to 3C sensors-dedicated P+S, in order to reduce the vertical uncertainty.

The combination of these methods allows for new interpretation of the microseismic cloud. We can now observe the separated fractures growing from the perforation clusters of a stage, assessing which one is active.

Human-induced earthquakes are more and more brought into the focus of public attention. E. g. Ellsworth (2013) attracts notice to the dramatic increase of the number of earthquakes in the central and eastern United States over the past few years. Environments prone to induce or trigger seismicity are numerous, e. g. oil and gas exploration sites, large-scale surface quarries and mines, Enhanced Geothermal Systems (EGS), dam sites and injections of e.g. CO2 or waste water. Since the nature of induced and triggered earthquakes implies their occurrence near engineering activity, even earthquakes of small magnitude are a cause for concern. We present several numerical methods to enhance the understanding of the spatial and temporal occurrence of seismicity, which has been triggered or induced by human operations: a) poroelastic modelling employing elementary Green’s functions, b) analytical fracture model combined with a rate- and state- dependent constitutive model and c) a 3D FEM able to handle both heterogeneous rocks and branched fractures.

3-C seismic receivers in a single vertical well are used to monitor micro-seismic activity in unconventional reservoirs. In such cases, locations of micro-seismic events depend on a velocity model and a particle polarization of a body-wave arrival. In this abstract, we show evidence for the lack of vector fidelity on downhole receivers. We are presenting a workflow to identify receivers with the lack of vector fidelity and a methodology for correcting the sensitivity of the components of the receiver to preserve the vector fidelity. This methodology enabled us to correct a 13° error in event backazimuth in the presented case study.