随钻核磁共振测井横向弛豫时间截止值计算方法与应用

Calculation method and application of transverse relaxation time cutoff for NMR logging while drilling

横向弛豫时间(T2)截止值(T2cutoff)是核磁共振测井评价束缚水饱和度、渗透率的关键参数,由于随钻核磁共振与电缆、岩心核磁共振的测量在仪器参数、测量环境等方面存在较大差异,导致岩心核磁实验确定的T2cutoff评价模型在随钻核磁共振测井资料中难以取得良好的应用效果。依据含泥砂岩T2cutoff的影响因素,建立基于岩石孔隙结构特征参数的变T2cutoff评价模型;同时以岩石孔隙半径为中间参数,建立随钻核磁横向弛豫时间校正模型,消除岩心与随钻的刻度差异。研究结果表明:含泥砂岩T2cutoff主要受岩石微小孔隙及大孔隙发育程度的影响,微小孔隙越发育,T2cutoff越小;相同孔隙半径下,岩心核磁与随钻核磁横向弛豫时间呈乘幂变换关系,校正系数与随钻核磁小孔隙分量、孔隙分量中值、随钻核磁孔隙度具有高相关性。将建立的横向弛豫时间校正模型、变T2cutoff评价模型应用到渤海A油田的束缚水饱和度评价中,结果显示,束缚水饱和度计算的平均绝对误差由校正前的11.51%降低为4.62%,表明该方法有效提高了随钻核磁共振测井评价储层束缚水饱和度的精度,也为随钻核磁共振测井T2cutoff的确定提供了一个新的思路。

The transverse relaxation time cutoff(T2cutoff)is a key parameter used in nuclear magnetic resonance(NMR) logging to evaluate the saturation and permeability of bound water. In terms of instrument parameters, measurement environment, etc., there are large differences between NMR logging while drilling(LWD) and cable as well as core NMR, which lead to the difficulty of good application of the T2cutoffevaluation model determined by core NMR experiments in NMR-LWD data. In this paper, after analyzing the factors influencing the T2cutoffof mud-bearing sandstone, a variable T2cutoffevaluation model was established using the pore structure characteristic parameters of the rocks. A model for the correction of transverse relaxation time of NMR-LWD was established with rock pore radius as an intermediate parameter to eliminate the scale difference between core and LWD. Research results shows that the T2cutoffof mud-bearing sandstone is mainly influenced by the development of micro-pore space and large pore space, and the T2cutoffis smaller when the development of micro-pore space is higher;Under the same pore space radius, the transverse relaxation time of core NMR and NMR-LWD shows a multiplicative power transformation relationship, and the correction coefficient is highly correlated with the small pore space component, the median pore space component and the NMR-LWD porosity. Applying the established transverse relaxation time correction model and the variable T2cutoffevaluation model to the evaluation of bound water saturation in the Bohai A oil field, the results show that the average absolute error of bound water saturation calculation decreases from 11.51% before correction to 4.62%, indicating that the method can effectively improve the accuracy of evaluation of reservoir bound water saturation by NMR logging while drilling, and also provide a new method for the determination of NMR-LWD T2cutoff.