基于分形理论的致密砂岩储层孔隙结构研究——以鄂尔多斯盆地庆城地区延长组长7段为例

Study of pore structure of tight sandstone reservoir based on fractal theory:A case study from Chang 7 tight sandstone of Yanchang formation in Qingcheng area,Ordos Basin

以鄂尔多斯盆地庆城油田庄183井区延长组长7段致密砂岩储层为例,通过铸体薄片、扫描电镜和压汞实验对储层的孔隙结构进行研究;结合高压压汞和恒速压汞开展基于分形理论的致密砂岩储层孔隙结构特征研究.通过分析全孔径分形维数(宽喉道分形维数D_(2),窄喉道分形维数D_(4))与矿物含量和孔隙结构参数的关系,明确了D_(2)和D_(4)的控制因素,认为长7段致密砂岩储层孔隙和喉道类型多样,主要发育残余粒间孔、长石溶孔和微孔隙群,喉道以片状喉道和管束状喉道为主,高压压汞和恒速压汞分形维数拟合曲线相结合可用来表征全孔径分形特征.研究结果表明:研究区储层孔喉分布差异显著,非均质性强.全孔径分形维数D_(2)和D_(4)能更有效地表征孔隙结构非均质性;D_(2)越大,宽喉道越发育,使孔隙常具有较好的连通性,致密砂岩储层具有较好的渗流能力;D_(4)越大,高度不规则的微纳米级喉道越发育,导致孔隙结构复杂,使致密砂岩储层的物性变差,同时不利于油气渗流;D_(2)和D_(4)主要受黏土矿物含量、钾长石含量、平均喉道半径、排驱压力和迂曲度的综合控制.通过对比常规0.12μm处拼接表征全孔径孔喉分布与基于分形维数拼接点表征全孔径孔喉分布两种方法,基于分形维数拼接点表征全孔径孔喉分布更合理.根据孔喉组合类型、孔喉大小和分布以及分形维数特征,将研究区致密砂岩储层孔隙结构划分为4类,分别为大粒间孔-宽片状喉道、小—中粒间孔-窄片状喉道、粒内孔-窄片状喉道和微孔-极窄片状喉道组合.

The tight sandstone reservoir of Chang 7 member of Yanchang formation in well block 183 of Qingcheng oil field in Ordos Basin were taken as a research object.The pore structure of the reservoir was studied through cast thin section,scanning electron microscope and mercury injection experiment.Focusing on the pore structure characteristics of tight sandstone reservoir based on Fractal Theory combined with high-pressure mercury injection and constant velocity mercury injection.It is considered that there are various types of pores and throats in the tight sandstone reservoir of Chang 7 member,mainly developing residual intergranular pores,feldspar dissolved pores and micropores,and the throats are mainly flaky throats and tube bundle throats.The fractal dimension fitting curves of high-pressure mercury injection and constant velocity mercury injection were combined to characterize the fractal characteristics of full-range fractal dimension.The controlling factors of D_(2)and D_(4)were determined by analyzing the relationship between the full-range fractal dimension(wide throat fractal dimension D_(2),narrow throat fractal dimension D_(4))and mineral content and pore structure parameters.The results show that the distribution of pore throats is significantly different and the heterogeneity is strong in the study area.The pore structure heterogeneity can be more effectively characterized by the full-range fractal dimensions D_(2)and D_(4).The larger D_(2)is,the wide throats are more developed,so that the pores often have better connectivity,and the tight sandstone reservoir has better seepage capacity.The larger D_(4)is,the more irregular micro and nano throats are developed,resulting in complex pore structure and poor physical properties of tight sandstone reservoir,which is not conducive to oil and gas seepage.D_(2)and D_(4)are mainly controlled by clay mineral content,potassium feldspar content,average throat radius,displacement pressure and tortuosity.By comparing the conventional method that 0.12μm radius was used as the splicing point to represent the full pore size distribution with Characterization of full pore size distribution based on fractal dimension stitching points.The former is more reasonable.According to the pore throat combination type,pore throat size and distribution and fractal dimension characteristics,the pore structure of tight sandstone reservoir in the study area can be divided into four categories.They are composed of large intergranular pore-wide sheet throat,small medium intergranular pore-narrow sheet throat,intragranular pore-narrow sheet throat and micropore-very narrow sheet throat.