Inferring Bulk Reservoir Properties From Microseismcity

Abstract

Microseismic monitoring is becoming an increasingly valued technology in the interpretation of dynamic processes, for example, such as hydraulic fracturing, cyclic steam injection, waterfloods, and gas storage to name a few. Locations and parameterization of the individual sources of the microseismicity are often used to gain insight into reservoir processes and can be critical for use in the calibration of geomechanical models of the reservoir. However, the microseismic signal are not only sensitive to the processes that occur in the source region, but also to the rock between the source regions and the recording system. Structural variations within lithological units, as well as the effect of injection processes themselves, contribute to the observed effects on the microseismic waveforms. The need to isolate the areas of the reservoir where velocity variations are observed imposes a strong constraint on the acquisition geometries: in order to observe the crossing raypath geometries necessary to localize velocity anomalies the waveforms need to be observed from a number of azimuths around the reservoir. We present a study from a zipper-frac complete of ten horizontal wells drilled at different formation depths in the Horn River Basin. Over 70000 microseismic events were observed from three observation tool strings deployed near reservoir depth in nearby wellbores. To invert for the three-dimensional variations in the structure of the reservoir, we considered the highest-quality 3000 events distributed over the extent of the study area. Also, because the reservoir is undergoing strong deformation during this process, due to hydraulic injection, we consider that the reservoir velocities are changing through the process. When we invert for the velocities, we observe signals in rough agreement with the rock property ratios observed from 3D elastic inversions of seismic reflection data. However, when we look for velocity changes we observe an overall reduction of velocities, especially in the shear wave, potentially associated with the injection process. The one aberration in the overall pattern of velocity reduction is an increase of the reservoir velocities towards the heels of the wells, suggesting that the zipper-frac may be concentrating stresses as it progresses up the horizontals. By utilizing such an approach, we have been able to identify the dynamic nature of the underlying bulk rock properties which result from the injection program.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016