Techbits: Understanding Naturally Fractured Reservoirs

Abstract

A recent Applied Technology Workshop (ATW) held in Houston, "Naturally Fractured Reservoirs," explored reservoir characterization, fracture-network modeling and reservoir simulation, and experience with improved-oil-recovery (IOR) and enhanced-oil-recovery (EOR) processes in naturally fractured reservoirs (NFRs). The ATW featured a technical agenda divided into four sessions and included more than 20 presentations. The four main sessions were:NFR Characterization (cochaired by El Paso Energy Petroleum Engineer Enrique Peralta and Texas A&M U. Assoc. Professor David Schechter).Constructing NFR Geologic and Flow Models (cochaired by Ecopetrol Chief Engineer and Vice President-Exploration Andreas Mantilla and Repsol Reservoir Engineer Hugo Araujo).Predicting NFR Performance (cochaired by Texas A&M U. Professor Thomas Blasingame and Colorado School of Mines Research Professor Hossein Kazemi).Improved and Enhanced Oil and Gas Recovery in NFRs (cochaired by Kazemi and Schechter). NFR Characterization Heterogeneity: Cause and Effect—The presentation by Chevron Energy Technology Center Structural Geologist Wayne Narr, titled "Guilt and Absolution in NFR Characterization: The Cause and Devilish Effect of Heterogeneity," examined the concept of representative elemental volume, illustration of fracture-network intersection with the well, and evaluation interval of the fracture density. Outcrop analogs necessary for qualifying heterogeneity and modeling of fractured-formation systems also were addressed. The presentation concluded with discussion among workshop participants on fracture allocations for simulation, the combination of primary and secondary porosity in simulation, and numerical modeling. Dissimilar Fracturing—Sandia Natl. Laboratory Geologist John Lorenz presented "Difference Between Fractures in Limestones and Sandstones," in which observations regarding lithology were made that supported the conclusion that production-induced pressure (stress) changes can destroy permeability. Regarding fracture creation, discussion centered around two schools of thought: the idea of more fractures creating less matrix permeability and whether extension fractures are conductive. Seismic for Reservoir Characterization—Prism Seismic Director of Consulting Abdel Zellou, in "Seismically Driven Fractured Reservoir Characterization," illustrated high-resolution inversion (elastic and post-stack) and other processes that can provide a basis for modeling, the use of seismic attributes (e.g., impedance) to derive geologic models (e.g., porosity) constrained by the seismic data, and fracture drivers (seismic, geological, and geomechanical) in 3D. One topic of discussion was drilling recommendations based on seismic attributes. La Concepción—Denis Marchal, Petrobras Technical Advisor in Structural Geology, delivered a presentation on "Reservoir Characterization and Tailored Technology Application in the Mature Cretaceous NFR of La Concepción Field, Venezuela." Presenting the key operational strategies used in redevelopment of the mature Cretaceous NFR in this field, Marchal emphasized the successful application of selective technologies and methodologies and the need to fully integrate the structural model with the dynamic one.