Determination of Irreducible Water Saturation Using Different Experimental Methods

Bound water saturation is a key parameter in reservoir evaluation and plays a significant guiding role in the formulation of oilfield development strategies. This study focuses on the G region and systematically analyzes the reservoir’s microscopic pore-throat characteristics, original oil saturation, and bound water saturation using integrated experimental methods. Specifically, high-pressure mercury intrusion (MICP) was employed to characterize the microstructure of pore-throats, while bound water saturation was determined using a combination of relative permeability tests, nuclear magnetic resonance (NMR) technology, and MICP. The results show that: 1) The main types of reservoir space in the study area are primary and secondary pores. Primary pores are predominantly residual intergranular pores, with chlorite films and authigenic clay minerals developed along pore edges. 2) When using the T2 spectral area ratio method in NMR logging to determine bound water saturation, the difficulty in accurately defining the T2 cutoff value (T2 cutoff) leads to imprecise results. Furthermore, the calculated saturation does not account for bound water present in movable water pores, resulting in systematically underestimated values. 3) The MICP method offers advantages such as fast testing speed, wide measurement range, and minimal requirements on sample size and shape. However, it also presents a discrepancy in wetting characteristics compared to actual oil-water systems. 4) Given the tight nature of the reservoir in the study area, treating the critical water saturation point as a proxy for bound water saturation may introduce certain errors. Thus, the applicability of this approach is relatively limited compared to the other two methods and should be applied with caution.