Assessment of Micro-Fracture Density using Combined Interpretation of NMR Relaxometry and Electromagnetic Logs
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Copyright 2014, Unconventional Resources Technology Conference (URTeC). Assessment of micro-fracture density in hydrocarbon-bearing reservoirs is of special interest for designing production plans and selecting zones for fracture treatment. NMR (nuclear magnetic resonance) T2 (spin-spin relaxation time) distribution has been traditionally considered insensitive to the presence of fractures. However, in a recent publication, we documented a measureable NMR sensitivity to the existence of micro-fractures and proposed a new concept of fracture-pore diffusional coupling. The quantification of micro-fracture density in multipleporosity systems is a challenging issue, and distinguishing fractures from pore space is not possible from NMR T2 measurement alone. However, the inclusion of additional borehole measurements, such as induction logs, enables evaluation of micro-fracture density. In this paper, we introduce a new method to evaluate the porosity associated to micro-fractures and intra-/inter-granular pores in complex formations using combined interpretation of NMR and EM (electromagnetic) measurements. We used a previously-introduced NMR analytical model for fracture-pore coupling to account for micro-fractures in the rock. This model was verified through NMR numerical simulations (using a random walk algorithm) in our previous work. Next, we simulated EM responses in the fluid-bearing fractured media using our previously developed software, Seatem. This program solves the diffusive Maxwell equations, and describes spatial heterogeneity through representation of anomalous diffusion of quasi-free charges generating electromagnetic eddy currents in the fracture network. Finally, these simulations were jointly interpreted to solve for micro-fracture density and intra-/inter-granular porosity. We applied the described technique on synthetic cases devised from pore-scale images of carbonate and organicshale formations. The estimated micro-fracture density was in satisfactory agreement with the actual value. The results showed that assessment of fracture content is possible by combining the NMR analytical model of fracturepore diffusional coupling, and EM simulations. Our introduced method for quantifying micro-fracture density can improve reservoir characterization, and contribute to operational decisions regarding number and location of fracture treatments for enhanced production from tight carbonate and organic-shale formations.
author list (cited authors)
Chi, L. u., Elliot, M., Heidari, Z., & Everett, M.