Developing pore size distribution models in heterogeneous carbonates using especially nuclear magnetic resonance

Abstract

Petrographical examination of thin sections, scanning electron microscope (SEM) analysis, helium porosimetry, gas permeametry, low field Nuclear Magnetic Resonance (NMR) spectrometry, and Mercury Injection Capillary Pressure (MICP) measurements of cored samples were investigated in order to understand pore size distribution models in the heterogeneous carbonate formations of the Euphrates and Jeribe formations, Kurdistan. These techniques provide petrophysical parameters, which include porosity, permeability, pore size distributions and pore-throat size distributions. Relaxation time distributions (T2) of the NMR data were combined with pore size measurements derived from the thin section and SEM analysis to construct pore size distribution models in high and low reservoir quality carbonates with their different diagenetic histories. Three pore size categories were identified, each with unique NMR behaviour and different pore types based on the petrophysical and petrographical data; these were: (1) pore diameters smaller than 1 μm which had a short relaxation time equal to or less than 200 ms; (2) pore diameter ranging from 1 μm to 4 μm which had a relaxation time ranging from 200 to 600 ms; and (3) pore diameters ranging from 4 μm to greater than 10 μm with very high relaxation time greater than 900 ms, in which unimodal intercrystalline and moldic pores were observed. The pore size distribution in the low reservoir quality samples derived from the NMR (T2) distribution curves had unimodal intercrystalline pores that have T2 close to 200 ms, isolated moldic pores having T2 greater than 900 ms, and bimodal intercrystalline and moldic pores, while high reservoir quality samples had unimodal intercrystalline pores and bimodal intercrystalline pores, and moldic pores were inferred from the pore size distributions. A positive correlation has been established for samples exhibiting intercrystalline micropores, intercrystalline mesopores, moldic and vuggy mesopores using both the helium porosimeter gas expansion and NMR techniques. The petrophysical behaviour of the carbonate rock units of the Euphrates and Jeribe formations in this study can be considered to be an important tool for identifying and characterising the wide variations of carbonate reservoir heterogeneity at different scales. Pore size categories (2) and (3) show moderate to good reservoir quality, while pore size category (1) represents non-reservoir rock.