超低渗透储层全孔径分布及其分形特征研究

Study of the full pore size distribution and fractal characteristics of ultra-low permeability reservoir

为深入研究超低渗透储层孔隙结构的全孔径分布特征及不同孔喉尺度下的分形特征,本文选取鄂尔多斯盆地镇北地区延长组长8段10块超低渗透岩心样品,开展铸体薄片、扫描电镜、X衍射、高压压汞和恒速压汞等实验,运用联合压汞法对储层全孔径大小及分布进行分类并定量表征,采用Brooks-Corey模型计算孔喉的分形维数,研究不同孔隙空间和整体孔隙结构的分形特征及其影响因素.认为超低渗透储层在不同孔喉尺度下其孔隙结构存在较强的非均质性,对应的分形特征差异明显。研究结果表明:该类储层孔隙类型主要为残余粒间孔、溶蚀孔和晶间孔.全孔径分布范围从3.6 nm到400μm之间,曲线呈多峰形态,主要发育纳米孔(孔径小于0.1μm)和巨孔(孔径大于10μm)。研究区储层孔隙结构可划分为Ⅰ类和Ⅱ类,对应的平均总分形维数DT分别为2.586 2和2.605 5,Ⅱ类储层物性和连通性较差,孔隙结构非均质性较强.超低渗透储层全孔径孔隙结构具有多重分形特征,对应孔喉段的分形维数具有D1>D2>D3的关系.孔隙半径大于50μm的巨孔具有分形特征且结构复杂程度较高;喉道分布介于0.1~10.0μm之间的微孔和中孔,其分形特征与储层溶蚀孔发育程度有关;纳米孔相对于巨孔、中孔和微孔而言,具有相对均匀和规则的喉道分布,对应分形维数最小.超低渗透储层不同孔喉类型及孔径大小的发育造成了孔隙结构非均质性各异,其分形维数大小与储层物性和孔隙结构参数之间有较好的相关性,且储层矿物组成及其含量的差异加剧了储层孔隙结构的复杂程度,影响着储层质量评价和油田后期开发.

This study explores the full pore size distribution and fractal characteristics with different pore throat scales of the ultra-low permeability reservoir. Casting thin section, scanning electron microscope(SEM), X-ray diffraction(XRD), high pressure mercury injection and constant rate mercury injection tests were conducted on ten ultra-low permeability core samples from Chang 8 member of Yanchang formation in Zhenbei area of Ordos Basin. The full pore size and distribution of the reservoir were quantitively characterized by the combined mercury injection method. The Brooks-Corey model was used to calculate the fractal dimension of pore throat. The fractal characteristics of different pore space and overall pore structure, as well as their influencing factors, were studied. It is considered that the pore structure of the ultra-low permeability reservoir has high heterogeneity with different pore throat scales, and the corresponding fractal characteristics are significantly different.The results suggested that this reservoir mainly contains three pore types i.e. residual intergranular pores, dissolution pores and intercrystalline pores. The full pore size distribution is in the range of 3.6 nm and 400 μm, showing a multi-peak curve peaking, and mainly developing nanopore(radius less than 0.1 μm) and macropore(radius greater than 10 μm).The pore structure of the reservoir in the study area can be classified into type I and type II, with an average total fractal dimension DT of 2.586 2 and 2.605 5, respectively. Type II reservoir has worse physical properties and connectivity, and higher heterogeneity of pore structure.The pore structure with full pore size distribution of the ultra-low permeability reservoir is multifractal, and the corresponding fractal dimension of pore throat sectionhas a relationship of D1>D2>D3.The macropores with pore radius greater than 50 μm have fractal characteristics and high structure complexity. The fractal characteristics of mesopores and micropores with throat radius between 0.1 and 10.0 μm are related to the development degree of reservoir dissolution pores. Compared with macropores, mesopores and micropores, nanopores are relatively uniform and regular in throat distribution, and they have smallest fractal dimension. The development of different pore throat types and pore sizes in ultra-low permeability reservoirs resulted in the heterogeneity of pore structure and the strong correlation of the fractal dimension with the reservoir physical properties and pore structure parameters. The difference in mineral composition and content of reservoirs aggravates the complexity of pore structure and affects the quality evaluation of reservoirs and the subsequent oilfield development.