基于核磁共振新参数的致密油砂岩储层孔隙结构特征——以鄂尔多斯盆地延长组7段为例

以核磁共振技术为基础，结合高压压汞、扫描电镜等实验资料，对鄂尔多斯盆地合水区块延长组7段致密油砂岩储层的66个岩心样品进行了分析，提出了一种新型孔隙结构参数--可动流体有效孔隙度。该参数不仅克服了核磁共振实验所测孔隙度偏小的影响，还将孔隙内的可动流体同时受到孔隙表面亲水性颗粒的黏滞及临近细小喉道的束缚这2个因素的影响考虑在内，对可动流体占据的有效孔隙空间的大小定量化标定出来，更加准确地反映了致密油储层的孔隙结构特征，弥补了可动流体孔隙度等现有参数对储层孔喉有效性评价的不足。研究孔隙结构特征与新参数影响关系发现，孔喉的连通性越好，可动流体有效孔隙度越大；孔喉分选系数介于1.4~1.9时，可动流体有效孔隙度达到最高；不同级别的孔喉配置关系是致密油砂岩储层孔喉有效性的关键。以可动流体有效孔隙度为主要标准，将研究区孔隙结构类型划分为3类：I类孔隙结构的可动流体有效孔隙度大于2 % ，II类孔隙结构的可动流体有效孔隙度介于1 % ~2 % ，III类孔隙结构的可动流体有效孔隙度小于1 % 。据此建立了适合致密油砂岩储层孔隙结构的分类评价标准，以指导油田的勘探开发。

Pore structure characteristics of tight-oil sandstone reservoir based on a new parameter measured by NMR experiment: a case study of seventh Member in Yanchang Formation,Ordos Basin

Based on NMR (nuclear magnetic resonance) technology in combination with high-pressure Hg injection, scanning electron microscope (SEM) and other experimental data, 66 core samples of tight-oil sandstone reservoir in seventh Member of Yanchang Formation, Heshui block, Ordos Basin were analyzed in this study. Then a new pore structure parameter was proposed, i.e., effective porosity of movable fluid. This parameter not only can overcome the impact of small porosity measured by NMR experiment, but also takes into account the movable fluid in the pores influenced by two factors, i.e., the viscosity of hydrophilic particulate on pore surface and the constraint of tiny throats nearby. The size of effective pore space occupied by movable fluid can be quantitatively calibrated to more accurately reflect the pore structure characteristics of tight-oil reservoir and make up the defects in effectiveness evaluation of pore throat using the existing parameters such as the porosity of movable fluid. Through analyzing the relation between pore structure characteristics and the influence of new parameter, it has been found that the better the connectivity of pore throat is, the larger the effective porosity of movable fluid will be; when pore-throat sorting coefficient is within the range of 1.4-1.9, the effective porosity of movable fluid can reach the maximum; different levels of pore-throat configurations are the key for pore-throat effectiveness in tight-oil sandstone reservoir. Then, based on the effective porosity of movable fluid as major criteria, pore structures in the study area can be divided into three types, i.e., Type I - the effective porosity of the movable fluid is large than 2 % ; Type II - the effective porosity is ranged between 1 % and 2 % ; Type III - the effective porosity is smaller than 1 % . Accordingly, the classified evaluation standard should be established for tight-oil sandstone reservoir to guide the exploration and development of oil field.