Calculation of Irreducible Water Saturation (S_(wirr)) from NMR Logs in Tight Gas Sands

摘要

It is difficult to calculate irreducible water saturation (S_(wirr)) from nuclear magnetic resonance (NMR) logs in tight gas sands due to the effect of diffusion relaxation on the NMR T_2 spectrum at present. By combining with classical Timur and Schlumberger-Doll Research (SDR) models, a novel model of calculating S wirr is derived. The advantage of this novel model is that S_(wirr) can be calculated without a T_2 cutoff, and all input information can be acquired from NMR logs accurately. With the calibration of 36 core samples, which were drilled from Xujiahe Formation in Bao-jie region of Triassic, Sichuan basin, southwest China, the values of these statistic model parameters are defined. Field examples of tight gas sands show that the proposed model is reliable. The S_(wirr) calculated with the proposed model match well with core analyzed results both in tight gas formations and water-saturated layers, the absolute error is in the range of ±4%. The calculated results by using 20.75 ms as the T_2 cutoff are accurate in water-saturated layers but are overestimated in gas-bearing intervals. Defining 33 ms as the T_2 cutoff is unusable both in gas-bearing and water layers.