致密砂岩储层可动流体分布特征及影响因素——以鄂尔多斯盆地姬塬油田延长组长8油层组为例

Distribution characteristics and its influence factors of movable fluid in tight sandstone reservoir:a case study from Chang-8 oil layer of Yanchang Formation in Jiyuan oilfield,Ordos Basin

可动流体参数是评价致密砂岩储层渗流特征的重要指标。基于核磁共振可动流体测试原理,对姬塬油田延长组长8油层组不同小层下的6块典型致密岩心样品进行了可动流体特征研究,并以毛细管压力曲线为基础,将核磁共振T_2谱分布换算为孔喉半径分布,确定了可动流体的最小孔喉半径。利用铸体薄片、黏土矿物X射线衍射、场发射扫描电镜以及纳米CT扫描等技术,分析了可动流体的影响因素。分析表明,研究区长8油层组致密储层的核磁共振T_2谱形态主要表现为4种类型:右峰发育型、单峰型、右峰微发育型、左右峰相当型;可动流体主要赋存于大孔隙和中孔隙内,部分微—小孔隙中也赋存有一定量的可动流体;可动流体百分数为6.89%～70.09%,可动流体孔隙度为0.39%～5.62%,可动流体所占的最小孔喉半径为0.024～0.555μm,其分布范围广,反映了姬塬油田长8油层组具有较强的非均质性。可动流体参数与储层孔隙度的相关性较差。造成长8油层组中可动流体参数差异较大的主要影响因素包括渗透率、孔喉结构特征、黏土矿物的含量及赋存方式、次生孔隙的发育程度及孔喉连通性、微裂缝发育程度等。

The movable fluid parameters are important indicators for evaluating the seepage characteristics of tight sandstone reservoir fluids.Based on the principle of movable fluid testing with nuclear magnetic resonance(NMR),the movable fluid characteristics of six typical tight core samples collected under different small sub-layers of Chang-8 oil layer of Yanchang Formation in Jiyuan oilfield were studied.Based on the capillary pressure curve,a conversion was performed between NMR T2 spectrum distribution and pore-throat radius distribution,thus determining the minimum pore throat radius of movable fluid.The influencing factors of movable fluid were analyzed using casting thin sections,clay mineral X-ray diffraction,field emission scanning electron microscopy and nano-CT scanning.The analysis shows that the NMR T2 spectrum of tight reservoirs of Chang-8 oil layer in the study area is mainly represented by four types:right peak developed type,unimodal type,right peak slightly developed type,and left-right peak equally developed type;the movable fluid mainly occurs in the large and medium pores,and certain movable fluid is also present in the micropores and small pores of some reservoirs;the movable fluid percent is from 6.89%to 70.09%,its porosity is from 0.39%to 5.62%,and its minimum pore throat radius is from 0.024μm to 0.555μm with a wide distribution range.Those reflect the strong heterogeneity of Chang-8 oil layer in Jiyuan oilfield.There is a poor correlation between the movable fluid parameters and the reservoir porosity.The main influencing factors causing the difference of the movable fluid parameters in Chang-8 oil layer include permeability,pore throat structure characteristics,clay mineral content and mode of occurrence,secondary pore development degree and pore throat connectivity,micro crack development degree and so on.