核磁共振测井T2截止值的确定方法

在缺乏岩心核磁共振实验资料的情况下,确定合适的核磁共振测井T2截止值(即T2cutoff)对于准确评价地层非常重要。NHA井砂岩储层核磁共振束缚水饱和度对T2cutoff的敏感度分析表明,T2cutoff取值对低孔低渗层束缚水饱和度的计算影响较大,当T2cutoff从21.3ms变化至52.9ms,低孔低渗层束缚水饱和度的变化量高达22.7%。可通过对比其他高含油(气)饱和度地层常规资料含水饱和度与核磁共振束缚水饱和度差值来确定低孔低渗层合理的T2cutoff取值。通过拟合得到的NHA井储层核磁共振束缚水饱和度与储层物性指数及T2cutoff的经验关系式,可用于分析该地区储层核磁共振束缚水饱和度对T2cutoff的敏感性,以及估算某T2cutoff下的束缚水饱和度值。

A method to determine T2 cutoff value in nuclear magnetic resonance log

To determine a proper T2cutoff in nuclear magnetic resonance(NMR) log is important to accurate evaluation of formation in absence of core NMR experiment data.An analysis of sensitivity of NMR irreducible water saturation to T2cutoff in well NHA has indicated that T2cutoff has a relatively larger effect on NMR irreducible water saturation in a formation with low porosity and permeability,with its variation of irreducible water saturation up to 22.7% when T2cutoff increases from 21.3 ms to 52.9 ms.By comparing conventional water saturation with NMR irreducible water saturation for other formation with high oil(gas) saturation,a proper T2cutoff can be determined for a formation with low porosity and permeability.Some empirical equations between NMR irreducible water saturation,petrophysical parameters and T2cutoff are derived from fitting for the reservoir in well NHA,which can be used to analyze sensitivity of NMR irreducible water saturation to T2cutoff and to estimate NMR irreducible water saturation under a certain T2cutoff in this area.