A New Unfree Fluid Index in Sandstones Through NMR Studies

Summary
Nuclear magnetic resonance (NMR) of 1H nuclei of liquids in saturated porous rocks furnishes an increasingly important tool in core analysis as well as in well-logging. In particular, there are several basic questions regarding NMR downhole logging in reservoir sandstones. For this reason, it is of interest to investigate the laboratory NMR properties to identify some important basic mechanisms. This paper presents new correlations among porosity, ϕ, irreducible water saturation, Swi, formation resistivity factor, F, and the geometric mean longitudinal relaxation time, T1g. Equilibrium magnetization and longitudinal relaxation, T1, measurements were performed on 57 clean sandstone samples from 12 oil and gas reservoirs. NMR measurements ϕNMR of ϕ agreed very well with conventional porosity measurements. The product of porosity and irreducible water saturation is approximately equal to an unfree fluid index, UFFI=(ϕNMR/T1g)2/3, giving an error-of-estimate factor of 1.24. The best estimate of Swi based on a simple cut-off relaxation time (35 ms) gave an error factor of 1.6. The present correlation of Swi with ϕNMR and Tig is nearly as good as the correlation using T1g and F, and it has the advantage of requiring only NMR measurements on fully saturated samples. No further improvement in correlation with Swi was obtained by using T1g with both ϕ and F over that with T1g and F only. The correlation with F was slightly better than that with ϕ. The effectiveness of using both T1 and either F or ϕNMR is plausible, since T1, being sensitive to surface-to-volume ratio, gives some information on pore or pore channel dimensions, while ϕNMR, being correlated with F, gives some information on connectivity within a sandstone. Large pores connected only by very small channels contribute to long T1 but contribute also to Swi. The above use of ϕNMR and T1 should be useful for laboratory measurements on cores; furthermore, since T2 is closely related to T1 at the low measurement frequencies used in nuclear magnetism logging, it is very likely that a similar relationship (probably with different parameters) may relate Swi to ϕNMR and T2 obtained in downhole measurements.