基于T2和毛管压力分类转换的渗透率计算新方法

New method of permeability calculation based on classification conversion of T2 spectrum and capillary pressure curves

对于低孔-特低孔、低渗-特低渗且非均质性强的储层,其往往处于自然产能临界点附近,单靠孔隙度已无法对其物性进行准确评价,因此需要在对渗透率进行准确计算的基础上,才能进一步展开产能评价与预测等工作.然而,由于储层非均质性强、常规测井曲线影响因素多且复杂、海上钻井数少导致样本点缺乏等因素,运用常规测井曲线进行渗透率计算往往不能取得很好的效果.基于T2和毛管压力转换的方法,利用了核磁共振可以全井段连续测量和毛管压力曲线可以反映孔喉结构的优势,在渗透率的计算方面取得了突破,该方法经过多年的发展,已经成为了一种较为成熟的渗透率计算方法,但对于该方法的研究,有一个问题却往往被忽视了:即T2谱和毛管压力曲线所反映的孔喉半径r是否是同一个物理量?两者之间的相互转换是否是有先决条件的?对于这一问题,本文在对两者实验原理进行分析的基础上,使用了将恒速压汞孔喉分布和核磁共振T2谱进行对比的方法,总结得到了两者各自反映的孔喉半径r的内涵,证实了两者之间相互转换存在先决条件--稳定的孔喉半径比,并在对沉积环境、成岩作用精细分析的前提下,选取了孔隙度这一宏观参数作为分类标准,将储层划分为多个具有相对稳定孔喉半径比的区间后,再进行T2-毛管压力的相互转换和渗透率的求取.经证实,运用这种方法计算出来的渗透率,可以更好地体现出储层的非均质性,并且与产能特征具有更好的一致性.

For the reservoirs of low to super-low porosity and low to super-low permeability with significant heterogeneity, it is always located near the critical point of natural productivity. The physical properties can’t be judged only by porosity, so the work of evaluation and prediction of productivity should be established on the base of accurate calculation of permeability. However, because of significant heterogeneity, multiple and complex influence factors of conventional log curves and a lack of samples caused by offshore drilling, satisfactory effect of permeability calculation by conventional log curves can’t always be achieved. The method based on conversion of T2 spectrum and capillary pressure curve, which takes the advantage of NMR which can realizes continuous measurement of whole length and capillary pressure which can reflects pore structure, realizes breakthrough in the aspect of permeability calculation. After several years’ development, this method has become a comparatively ripe method, but a questions is usually ignored in the research:that is if the pore-throat radius obtained by T2 and capillary pressure curve are the same physical parameter? And if there is any precondition existed ahead of the conversion? According to this question, on the basis of analyzing experiment principle of NMR and capillary pressure, a contrast method is used by rate-controlled mercury penetration’s pore-throat distribution and T2 spectrum of NMR, and respective pore-throat radius’ connotation is summarized, which proves that there is precondition existed ahead of the conversion, and the precondition is stable pore-throat radius ratio. On the premise of precise analysis of sedimentary environment and diagenesis, a macro parameter, porosity, is chosen as classification standard, and as a result reservoirs is divided into several sections, each one of which has relatively stable pore-throat radius ratio, and then the work of conversion and permeability calculation is realized. It is proved that permeability calculated by the method could reflect the heterogeneity more accurately, and it also has better consistency with productivity characteristics.