Fracture characterization using frequency-dependent shear wave anisotropy analysis of microseismic data

Abstract

S U M M A R YThe presence of fractures in hydrocarbon reservoirs can enhance porosity and permeability, and consequently increase production. The use of seismic anisotropy to characterize fracture systems has gained much interest in the last two decades. However, estimating fracture sizes from observations of seismic anisotropy has not been possible. Recent work has shown that frequency-dependent anisotropy (FDA) is very sensitive to the length-scale of the causative mechanism for the anisotropy. In this study, we observe FDA in a microseismic data set acquired from a carbonate gas field in Oman. The frequency-dependent shear wave anisotropy observations are modelled using a poroelastic model, which considers fluid communication between grain size pore spaces and larger scale fractures. A grid search is performed over fracture parameters (radius, density and strike) to find the model that best fits the real data. The results show that fracture size varies from the microscale within the shale cap rocks, to the metre-scale within the gas reservoir, to the centimetre-scale within the non-producing part of the carbonate formation. The lateral variation in fracture density agrees with previous conclusions from ordinary shear wave splitting (SWS) analysis. Cumulatively, the results show the potential for characterizing fracture systems using observations of FDA.

Microseismic Monitoring of Fracture Networks During Hydraulic Stimulation: Beyond Event Locations

“…The splitting parameters are then estimated for each frequency-band. The results presented in Al-Harrasi et al (2011) reveal a lithology dependent variability in the nature of frequency-dependent splitting. …”

“…In the past, we have successfully applied such an inversion to a dataset from a carbonate reservoir (Al-Anboori and Al-Harrasi et al, 2011). Our results show large meter-scale fractures in the gasproducing reservoir and micrometer scale cracks in the sealing shale.…”

“…The frequency content in datasets is somewhat variable with depth and lithology, but is generally between 10-400Hz. The analysis of frequency-dependent shear-wave splitting has been described in Al-Harrasi et al (2011). The data are filtered with a one-octave passband (i.e., a constant ratio of high to low frequencies of 2).…”

“…Microseismic data are generally rich in frequency content so there is also much potential to look at frequency-dependent wave phenomena. It has been shown that frequency-dependent shear-wave splitting is sensitive to crack size, aspect ratio, and fluid properties (Chapman, 2003;Al-Harrasi et al, 2011). Cumulatively, shear-wave splitting is sensitive to a range of crack or fracture parameters and is a potentially valuable tool for reservoir management and exploitation.…”

Experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles

“…This fluid dependent SWS is indeed frequency dependent as shown by theoretical studies and field studies (Marson-Pidgeon and Savage, 1997, Al-Harrasi et al, 2011, Chapman, 2003. Theoretical models are needed to interpret seismic data in terms of fracture properties and fluid properties.…”