Integration of Seismic Anisotropy and Reservoir Performance Data for Characterization of Naturally Fractured Reservoirs Using Discrete Feature Network Models

This paper proposes a method for quantitative integration of seismic (elastic) anisotropy attributes with reservoir performance data as an aid in characterization of systems of natural fractures in hydrocarbon reservoirs. This method is demonstrated through application to history matching of reservoir performance using synthetic test cases. Discrete Feature Network (DFN) modeling (Dershowitz et al.) is a powerful tool for developing field-wide stochastic realizations of fracture networks in petroleum reservoirs. Such models are typically well conditioned in the vicinity of the wellbore through incorporation of core data, borehole imagery, and pressure transient data. Model uncertainty generally increases with distance from the borehole. Threedimensional seismic data provides uncalibrated information throughout the inter-well space. Some elementary seismic attributes such as horizon curvature and impedance anomalies have been used to guide estimates of fracture trend and intensity (P32) (Dershowitz and Herda) in DFN modeling through geostatistical calibration with borehole and other data. However, these attributes often provide only weak statistical correlation with fracture system characteristics. The presence of a system of natural fractures in a reservoir induces elastic anisotropy that can be observed in seismic data. Elastic attributes such as azimuthally dependent normal moveout velocity (ANMO), reflection amplitude versus azimuth (AVAZ), and shear wave bi-refringence can be inverted from 3D seismic data. Anisotropic elastic theory provides physical relationships among these attributes and fracture system properties such as trend and intensity. Effective elastic media models allow forward modeling of elastic properties for fractured media. A technique has been developed in which both reservoir performance data and seismic anisotropic attributes are used in an objective function for gradient-based optimization of selected fracture system parameters. The proposed integration method involves parallel workflows for effective elastic and effective permeability media modeling from an initial DFN estimate of the fracture system. The objective function is minimized through systematic updates of selected fracture population parameters. Synthetic data cases show that 3D seismic attributes contribute significantly to reduction of ambiguity in estimates of fracture system characteristics in the inter-well rock mass. The method will benefit enhanced oil recovery (EOR) program planning and management, optimization of horizontal well trajectory and completion design, and borehole stability studies. Introduction Anisotropy and heterogeneity in reservoir permeability present unique challenges during the development of hydrocarbon reserves in naturally fractured reservoirs. Predicting primary reservoir performance, planning development drilling or EOR programs, completion design, and facilities design all require accurate estimates of reservoir properties and the predictions of future reservoir behavior computed from such estimates. Over the history of naturally fractured reservoir development many methods have been employed for characterization of fracture systems and their effect on fluid flow in the reservoir. These include the use of geologic surface outcrop analogues, core, single and multi-well pressure transient analysis, borehole imaging logs, and surface and borehole seismic observations. To date, efforts to integrate seismic data into the workflow for characterization of naturally fractured reservoirs have been focused on the use of post-stack data. Seismic data are typically used to define main structural elements of the reservoir. Fracture density has been successfully correlated with horizon curvature determined from seismic horizons. Seismic attributes can frequently be correlated with reservoir properties such as shale fraction, which often correlates with fracture population statistics. Acoustic impedance computed from seismic data frequently exhibits dim spots in the presence of fractures. Linear elastic theory provides the framework for computation of elastic behavior of a generalized medium. Oda developed a fabric model for discontinuous geologic materials, which he later developed into effective media models for both elasticity and permeability using discrete models for discontinuities (fractures). Schoenberg introduced the linear slip model for fractured media which, further elaborated on the elastic characteristics of the discrete discontinuities. This theory was further developed by SPE 84412 Integration of Seismic Anisotropy and Reservoir Performance Data for Characterization of Naturally Fractured Reservoirs Using Discrete Feature Network Models Robert Will, SPE, Schlumbeger; Rosalind Archer, SPE; Bill Dershowitz, SPE, Golder Associates