Geomechanical Lithology-Based Analysis of Microseismicity in Organic Shale Sequences: A comparison from multiple hydraulic fracturing field sites

Abstract

With the increased application of microseismic monitoring during hydraulic fracturing, it has become increasingly important to accurately categorize “fracability” in unconventional reservoir rocks. Typically this understanding of shale fracability is based upon Poisson's ratio (PR) and Young's modulus (YM) calculated from well logs. The manifestation of fracability is assumed to be simply represented by the number and density of microseismic events generated during hydraulic fracturing stimulation. To investigate hydraulic fracturing behavior in greater detail we have analyzed five highquality microseismic datasets along with associated high-quality well logs. We investigated four field sites from the Marcellus Shale sequence and one field site from the Wolfcamp sequence in Texas. From the well log data we calculated all relevant geomechanical moduli and petrophysical parameters for a direct comparison to the vertical extent within which a high density of microseismic events were observed at each field site. A comparison of well log based (dynamic) elastic moduli to the vertical distribution of microseismic parameters, such as moment magnitude and event count, reveals both a positive and negative correlation between elasticity-based brittleness estimates and moment magnitude within shale rocks from the different field sites.

The seismological b-value has been shown to relate to the state of in-situ stress at the time of failure in microseismic datasets, enabling differentiation between fault interaction and more diffuse tensile fracturing. Preliminary analysis of b-values suggests a relationship between stress/failure style and how geomechanical properties influence the expression of microseismicity. In accordance with seismological principles regarding the statistical analysis of seismic catalogs, we have taken great care to eliminate bias in the sampling. By excluding all microseismic events with moment magnitudes smaller than the magnitude of catalog completeness, and also focusing our analysis on only the shale rocks containing the highest number of microseismic events, we are confident that the observed relationships can provide insight into the geomechanical control over how microseismicity is expressed during hydraulic fracturing in unconventional shale.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016