Fracture Characterization in Unconventional Reservoirs Using Active and Passive Seismic Data With Uncertainty Analysis Through Geostatistical Simulation

This study discusses a new workflow for fracture characterization and modeling using geophysical (microseismic and 3D surface seismic) data along with independent reservoir information (such as well logs). The framework is ideally suited for unconventional environments such as shale and tight reservoirs where modern technologies such as the use of hydraulic fracturing and passive seismic monitoring allow application of the proposed workflow. The workflow involves generating geomechanical property estimates (including stress and weakness estimates) as derived from passive seismic data analysis and relevant seismic attributes derived from 3D seismic data combined using ANN based reservoir property modeling framework. The training information for the networks is generated based on a-priori information through image logs. Resolution of passive seismic derived velocity models is improved by using sequential Gaussian co-simulation by combining low resolution velocity maps high resolution seismic impedance data for phase velocity estimation. Uncertainty estimates are quantified by adequate number of realizations and associated probability density functions for fracture properties within study volume. In this paper, different properties estimated through ANN modeling have been shared. New fracture identifier (FZI) properties have been defined and the models have been used to characterize fracture zones and major discontinuities for a representative unconventional reservoir (geothermal setting) used in our study. We also share uncertainty estimates for the identified fracture zones for improved characterization. Finally fracture property estimates for the study area (derived using FZI and other properties) have been generated for future reservoir simulation studies. The proposed method allows for improved understanding of shale and other unconventional reservoirs through fracture mapping and provides a workflow for improved volumetrics of the reservoir by making use of identified properties for fracture modeling. This work validates the potential for using relatively low resolution passive seismic data for improved reservoir characterization using Geostatistical tools. It also provides a valuable framework for pseudo 4D characterization where a single 3D seismic survey can be used as the basis to characterize the reservoir in a time lapse fashion using new information collected in time through passive seismic arrays as well as new well logs being obtained within the area of interest. Introduction Use of geophysical tools for reservoir characterization and monitoring is fairly well understood and has been used extensively in recent years. Passive seismic monitoring has found applications in development of unconventional reservoirs such as geothermal systems involving injection and production of hot water or steam from the reservoir, tight gas and oil systems which require hydraulic fracturing and finally monitoring of injection wells (waste water, CO2, etc.) among others. In the field of conventional seismic, techniques such as multi-attribute analysis and integrated analysis techniques are being used extensively for reservoir characterization. While conventional seismic data is rarely available for small unconventional reservoir developments, the use of microseismic data is limited to better understanding of the fracturing process including diagnosis and volumetrics but is seldom used