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Quantifying the Impact of Natural Fractures and Pore Structure on Borehole NMR Measurements in Multiple-Porosity Systems

Lu Chi and Zoya Heidari
Texas A&M University

This research quantitatively evaluates the hierarchical structure of multi-porosity systems (including natural fractures, inter-granular, and intra-granular pores) using nuclear magnetic resonance (NMR) measurements. Previous studies evaluated large planar fractures with apertures greater than 0.2 mm using NMR logging, assuming that the fractures are parallel to each other and mostly perpendicular to wellbore. In this research, we (a) randomly distribute and orient small natural fractures in various porous matrices, (b) demonstrate that NMR T2 (spin-spin relaxation time) distribution is sensitive to concentration, shape and size of natural fractures, and (c) investigate the possibility of quantitative detection of natural fractures and inter-/intragranular pores in different formations, in both synthetic cases and core samples from carbonate formations. Applications of this research include reliable reservoir characterization in challenging multi-porosity systems such as carbonate formations and organic-rich source rocks.

We numerically simulate NMR response using a random walk algorithm. We then show the sensitivity of NMR response to the presence of natural fractures with different shapes and to pore structure within (a) synthetic pore/fracture matrices, and then (b) three-dimensional micro- Computed Tomography (micro-CT) images of carbonate and sandstone rock samples containing synthetic fractures. Results from both synthetic and actual rock samples show that NMR T2 distribution can be significantly affected by concentration, shape (e.g., needle-like or planar shape), and width/aperture of natural fractures, as well as size and connectivity of inter-granular pores. The volume fraction of randomly oriented natural fractures could be estimated separately from the inter-/intra-granular pores under specific conditions, such as a joint interpretation of NMR measurements and other conventional well logs such as borehole electromagnetic measurements.

The natural fracture content and pore-size distribution in a porous media have significant influences on fluid transport and on the success of hydraulic fracturing. Conventional well-log interpretation techniques cannot distinguish between different pore structures. This research proposes the application of NMR in challenging formations with structural hierarchy to improve reservoir characterization and provides real-time assessment of pore structure and fracture detection in multiple-porosity formations such as carbonates.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013