AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Future of Microseismic Analysis - Integration of Monitoring and Reservoir Simulation
(1) MicroSeismic, Inc., Houston, TX.
(2) Shell Exploration & Production, Houston, TX.
Monitoring of microseismic events induced by reservoir stimulation has become a key aspect in evaluation of hydraulic fractures and their optimization. Future developments of this technology are dependent on improvements in multiple discipline areas, two of which are discussed in this study: better quantification of event locations along with the velocity model, and improved understanding and calibration of the type of rock failure responsible for the seismic events.
Currently, locations of microseismic events are used to infer the geometries of hydraulic fractures. These locations are inverted from seismic signals recorded by sensors either distributed at the surface or in dedicated monitoring borehole(s). The accuracy and precision of the inverted locations depends on both the signal-to-noise ratios of seismic data and the spatial distribution of the receivers. While surface monitoring usually suffers from low signal-to-noise ratio, the ability to place receivers in multiple azimuths and offsets allows for precise event location. On the other hand, downhole monitoring provides robust detection due to a higher signal-to-noise ratio if an event is sufficiently close to the monitoring borehole; however, precise location of events might be difficult, especially in the case of a single monitoring well. Thus, integration of downhole and surface monitoring may be beneficial to both methodologies.
Observed seismic waves carry information about the reservoir properties and the mechanisms of microseismic sources, allowing determination of the type of rock failure in addition to using microseismic event to infer hydraulic fracture geometry. Fracture stimulation models are often based on generating tensile fractures parallel to the maximum stress direction in the reservoir but analyses of the observed microseismic events are dominated by shear failure mechanisms. An assessment of whether the shear failure represents creation of new fractures or reactivation of the existing ones is often based on conceptual models with little data for validation. Analysis of data obtained from a microseismic monitoring project where an image log was acquired in the treatment well allows validation of the model interpreted from the event locations and the inverted source mechanisms. Integration of source-mechanism analysis with information obtained from image logs leads to a better constrained reservoir model populated with fractures away from the wellbore.