Summary

We show source mechanism and stress inversion of microseismic events induced by hydraulic fracturing of shale reservoirs in two different case studies using surface and downhole monitoring. The first study used dual downhole monitoring arrays in a nearly optimal geometry and the second case study used a star-like surface monitoring array. We detected and located microseismic events, and then determined their magnitudes, source mechanisms. We then used inversion to determine stress field orientation, analyzed shearing/non-shear source mechanisms, and showed consistency of the stress field with regional stresses and hydraulic fracture results.

We show that both surface and downhole source mechanisms of microseismic events are dominated by shear failure with negligible (and not interpretable) non-shear components. This is a key observation resolving many of the recent professional debates. In both case studies representative of different areas, the source mechanisms were consistent with the locations of events, and the strikes of the nearly vertical fault planes agree with the strikes of the location trends. We showed that the source mechanisms constrained the orientation of principal stress axes well and that this orientation of stress was also consistent with the event locations and regional stress. This result makes it possible to build and develop a new reservoir simulation model based on advanced interpretation of microseismicity.

Introduction

The microseismic monitoring of the events induced during hydraulic fracture stimulations provides a great opportunity to examine the role of pressurized fluids in initiating earthquakes and opening fractures. This is a crucial tool for observing and mapping reservoirs. Locations of microseismic events determine basic information about fracture geometry such as direction of fracture propagation, and their length and height. Recently, additional fracture characterization using source mechanisms of induced microseismic events is being provided. To assess the mode of fracturing during injection, the source mechanisms have to be determined for a statistically representative number of events. This knowledge of mechanisms can clarify the question whether the induced microearthquakes occurred on pre-existing faults or they represent opening of new fractures. The double-couple parts of moment tensors can be used for determination of local tectonic stress. Locations and source mechanisms allow building of reservoir models such as discrete fracture network, stimulated rock volume, or propped volume.

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