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Summary Equinor in collaboration with Shell and Total is working on maturing the carbon storage project to store industrially produced CO2 into geological subsurface offshore Norway. However, the selected candidates for CO2 storage are part of a region with moderate natural seismicity. In order to assure a safe storage, the background seismicity should be monitored to understand both the nature of natural seismicity and to detect possible events induced by the imposed pressure changes due to CO2 injection. Current existing seismic network onshore is rather sparse and due to the limitation of recording mostly from the Norwegian side, event location uncertainty is rather high which complicates associating individual earthquakes to specific faults. Integration of onshore network with selected offshore PRM stations improved detectability. However, a more local monitoring system is required to improve the detectability and reduce the location uncertainty. We here present a proposal for a seismic monitoring array design for the offshore setting around the Smeaheia and Aurora sites that includes an upgrade of the onshore monitoring network as well as some ocean bottom nodes. We argue that such a network will provide an adequate baseline dataset, which is crucial for understanding the site prior to injection.

Fractures and faults within a reservoir can provide important pathways for the movement of reservoir fluids. Understanding the character and properties of these features on a range of length scales can be vital for the efficient exploitation of natural resources, whether it be enhanced oil and gas recovery, the safe storage of CO2 ,o r better exploitation of geothermal heat. The monitoring of microseismicity within a reservoir illuminates active faults, but these events can be also used to characterize fracture networks through measurements of seismic anisotropy. In this study we use microseismic data acquired over an 18-day period in April 1997 at the Ekofisk oil field in the North Sea. Using the analysis of seismic multiplets we delineate a number of sub-vertical tectonic faults, which are consistent with previous core data analysis and seismic reflection work. We use shear wave splitting measurements, which are indicative of fracture-induced seismic anisotropy, to infer the orientation of aligned aseismic fracture sets within the reservoir. The estimated fracture dip and strike from the shear wave splitting analysis are consistent with the active tectonic fractures characterized by the multiplets, but this analysis also illuminates spatial variations in fracture properties. Such monitoring on a longer term and with multiple wells is a promising tool for better understanding fracture and fault-controlled flow within reservoirs.

SUMMARY Hydraulic overpressure can induce fractures and increase permeability in a range of geological settings, including volcanological, glacial and petroleum reservoirs. Here we consider an example of induced hydraulic fracture stimulation in a tight-gas sandstone. Successful exploitation of tight-gas reservoirs requires fracture networks, either naturally occurring, or generated through hydraulic stimulation. The study of seismic anisotropy provides a means to infer properties of fracture networks, such as the dominant orientation of fracture sets and fracture compliances. Shear wave splitting from microseismic data acquired during hydraulic fracture stimulation allows us to not only estimate anisotropy and fracture properties, but also to monitor their evolution through time. Here, we analyse shear wave splitting using microseismic events recorded during a multistage hydraulic fracture stimulation in a tight-gas sandstone reservoir. A substantial rotation in the dominant fast polarization direction (ψ )i s observed between the events of stage 1 and those from later stages. Although large changes in ψ have often been linked to stress-induced changes in crack orientation, here we argue that it can better be explained by a smaller fracture rotation coupled with an increase in the ratio of normal to tangential compliance (ZN/ZT) from 0.3 to 0.6. ZN/ZT is sensitive to elements of the internal architecture of the fracture, as well as fracture connectivity and permeability. Thus, monitoring ZN/ZT with shear wave splitting can potentially allow us to remotely detect changes in permeability caused by hydraulic stimulation in a range of geological settings.

Terra Nova, 25, 57–64, 2013 Abstract Characterizing the interaction of a fault with its surroundings is vital to fully understand the tectonic processes involved and predict future behaviour. Regional and local stress orientations affect different fracture length scales, manifested by numerous associated fault, fracture and crack structures. We use seismic anisotropy to constrain the dominant orientation of aligned rupture planes of various length scales. In particular, we study shear-wave splitting of regional seismic events in Trans-Alboran Shear Zone (TASZ), south-east Spain. The TASZ consists of three major left-lateral strike-slip faults and numerous secondary strike-slip and thrust faults. The observed orientations for S-waves vary from roughly N–S in the northern segment of TASZ, to E–W in the centre, to NNW–SSE and NNE–SSW in the south. We show that the strikes of fast polarizations reflect both structural and lithological differences, indicating complex interactions of principal and secondary faults within the crust to accommodating tectonic stresses.

Monitoring of induced seismicity is gaining importance in a broad range of industrial operations from hydrocarbon reservoirs to mining to geothermal fields. Such passive seismic monitoring mainly aims at identifying fractures, which is of special interest for safety and productivity reasons. By analysing shear-wave splitting it is possible to determine the anisotropy of the rock, which may be caused by sedimentary layering and/or aligned fractures, which in turn offers insight into the state of stress in the reservoir. We present a workflow strategy for automatic and effective processing of passive microseismic data sets, which are ever increasing in size. The automation provides an objective quality control of the shear-wave splitting measurements and is based on characteristic differences between the two independent eigenvalue and cross-correlation splitting techniques. These differences are summarized in a quality index for each measurement, allowing identification of an appropriate quality threshold. Measurements above this threshold are considered to be of good quality and are used in further interpretation. We suggest an automated inversion scheme using rock physics theory to test for best correlation of the data with various combinations of fracture density, its strike and the background anisotropy. This fully automatic workflow is then tested on a synthetic and a real microseismic data set.

Summary Fracture networks stimulated by hydraulic fracturing follow pre-existing planes of weakness in the rock. We analyze a stimulation program in a tight gas formation in Texas. Microseismic Frequency-Magnitude analysis is used to infer fracture and stress characteristics. Data from a microseismic Shear-Wave Splitting analysis gives dominant fracture orientation. The static stress interaction of the microseismic events on the fracture network are then calculated. The resulting changes in Coulomb Failure Stresses (ΔCFS) thus constrain the zone of stress induced cracks and fissures, which are activated below microseismic detectability.

A tight gas case history from the North America Rocky Mountains shows the capability of borehole microseismic and 3D seismic anisotropy to delineate fracture systems induced in a clastic reservoir by hydraulic fracturing of a horizontal well. Dual monitoring of the horizontal well completion indicates improvements in fracture mapping by compressing hypocenters of the microseisms to more linear features that are corroborated by production logging flowmeter. Fracture half-lengths and vertical heights of fractures are indicated for reservoir modelling. Advanced processing of the microseismic 3-component geophone first motions indicates fracture azimuth and dips from a moment tensor inversion and suggests a conjugate set of orthogonal fractures in the reservoir. 3D seismic anisotropy results, using a fast and slow velocity direction calculated from the data, suggest inherent microfractures present in the rock prior to completion. Surface fault mapping in the field supports fractures mapped by both the microseismic and the 3D seismic anisotropy. Multiple regression analysis of drilling, completion and 3D seismic anisotropy parameters versus initial production for 30, 90 and 180 days results show independent contributions to production. A predictive production model based on the multiple regression analysis allows for future well planning and completions to optimise well performance.

SUMMARY Changing stress conditions are well known to cause rupturing of rock. This is well constrained on a small scale from laboratory experiments and inferred on a much larger scale from tectonic earthquakes. Here, we present a study of rock fracturing induced by changes in stress state during block-caving operations in an Australian mine. This intermediate-scale study provides further evidence of the scalability of processes involved in rock fracturing and thus helps to link laboratory and seismological observations. We analyse the temporal evolution of rock fracturing during a production cycle using the analysis of fracture-induced anisotropy. Fracturingoftherockmassismonitoredusingevidenceofseismicanisotropyfromestimates of shear wave splitting and their subsequent inversion for fracture parameters. The data set consists of more than 40 000 three-component seismograms recorded by an array of sensors, which provides excellent ray coverage. We applied a novel automatic quality assessment technique to handle this large data set and find that anisotropy, and thus fracturing, correlates strongly with the excavation process. We then perform a grid search over a series of synthetic models based on rock physics to invert the splitting parameters for fracture orientation and density. Finally, by applying a sliding window on our results, we are able to identify production related fracture evolution. During production the fracture density increases, with horizontal fracture density being stronger than the vertical fracture density. This can be explained by the removal of the supporting rock during caving. During short intervals of reduced production, the horizontal fracture density decreases, whereas vertical fracture density increases. We relate this to change in stress regime with reducing overburden mass during cavity collapse. This scenario is similar to a collapsing caldera or the inverse of an inflating magma chamber.

We summarize observations of shear-wave splitting from passive seismic monitoring in a range of petroleum settings. Such measurements are relatively rare, but the emerging picture is one of fracture-induced anisotropy where a number of factors control both spatial and temporal variations. These observations offer insights into the state of stress in a reservoir and can serve as a ground-truth for geomechanical models.

The successful exploitation of tight-gas reservoirs requires fracture networks, sometimes naturally occurring, often hydraulically stimulated. Borehole microseismic data acquired in such environments hold great promise for characterising such fractures or sweet spots. The loci of seismic events delineate active faults and reveal fracture development in response to stimulation. However, a great deal more can be extracted from these microseismic data. For example, inversions of shear-wave splitting data provide a robust means of mapping fracture densities and preferred orientations, useful information for drilling programs. They can also be used to track temporal variations in fracture compliances, which are indicative of fluid flow and enhanced permeability in response to stimulation. Furthermore, the frequency-dependent nature of shear-wave splitting is very sensitive to size of fractures and their fluidfill composition. Here we demonstrate the feasibility of using such analysis of shear-wave splitting measurements on data acquired during hydraulic stimulation of a tight-gas sandstone in the Cotton Valley field in Carthage, West Texas.