致密砂岩全息三维数字岩心建模和电阻率数值模拟研究

Tight gas/oil sandstone features more complicated rock properties than conventional sandstone in many aspects such as higher volume fraction of clay minerals and co-existence of multiple scales of porous space. These features highly affect the accuracy of formation evaluation if we try to use classical Archie's law to estimate the oil/gas saturation within a certain tight formation. New techniques such as numerical simulation of electrical properties on digital image of rock samples have been applied effectively to conventional sandstone but the limit of these simulation methods on tight gas sandstone is inevitable. Due to the multiple scale property of tight gas sandstone, simulated electrical properties of tight gas sandstone on digital images with only one single scale of visible pore space always show a tendency of either overestimate or underestimate, if compared with experimental measurements. In this project, we will tackle the challenge of multiple scale porous space of tight gas sandstone. First, by combining digital images of tight gas sandstone acquired from X-ray Micro-CT and Scanning Electron Microscopy Energy Dispersive Spectrum (SEM-EDS), different types of clay minerals embedded between solid grains are identified. These clay minerals are believed to be the main cause to the blockage of electrical currents within the macroscopic pore space. In addition, large field-of-view MAPS will focus onto various types clay minerals to distinguish their unique characterization of porous structure, e.g., the intragranular dissolved pore in feldspar, micro-pores in kaolinite, illite and chlorite. Next, on quantitatively analyzing porous structures of various clay minerals and calculating their effective resistivity respectively, the electrical properties of corresponding clay type will be registered back into larger scale digital image of tight gas sandstone to construct a comprehensive 3D digital image containing not only electrical property of large pores and grains but also that of clay minerals at microscopic scale. Finite element methods will be employed to simulate how electrical currents are conducted within the constructed 3D digital model of tight gas sandstone. The advantage of the model proposed in this project will be verified by comparing the simulated results with and without considering the effect of clay mineral conductivity. The coupling style of large pore and micro pore in tight sandstones is investigated by H-K labelling algorithm. The effective resistivity of percolated pore space alone is calculated by the finite element method. Then the resistivity of the digital rock containing all pore space is simulated by the same method. The coupling factor between large and micro pore system are related to the large porosity and micro porosity, which will improve the accuracy of hydrocarbon saturation evaluation.

致密砂岩物性差，发育多尺度孔隙，黏土含量高，导电路径复杂，制约含油气饱和度评价精度，传统三维数字岩心建模和电阻率模拟方法不适用。结合X射线CT和扫描电镜散射能谱SEM-EDS测试，识别致密砂岩粒间孔隙，区分主要矿物组分，建立多矿物组分三维数字岩心。选择合适分辨率，采用大视域高精度扫描电镜MAPS依次识别粒内溶蚀孔和高岭石、伊利石、绿泥石填隙物中的微孔。分析主要矿物组分的微孔隙率和孔隙尺寸分布，计算各矿物组分的等效电阻率，将上述等效物性参数赋给三维数字岩心的各矿物组分，建立体现多尺度孔隙和矿物组分的全息三维数字岩心。采用有限元方法准确计算全息三维数字岩心电阻率，模拟完整饱和度区间电阻率，拓展三维数字岩心技术应用范围。基于数字岩心分析不同储层参数条件下，大孔隙和微孔隙导电路径的耦合关系。采用有限元方法计算不同导电路径单独存在和共存时的电阻率，归纳导电路径耦合函数，有助于改进该类储层饱和度评价。

致密砂岩油气是全球非常规油气发展的重点之一，我国的致密油气资源丰富，主要分布在鄂尔多斯、准噶尔、松辽和四川等多个盆地。致密砂岩物性差，发育多尺度孔隙，黏土含量高，导电路径复杂，制约含油气饱和度评价精度。数字岩石物理已经发展为岩石物理研究的重要方法，但传统三维数字岩心建模和电阻率模拟方法无法直接应用至致密砂岩储层。针对致密砂岩储层，本项目建立了全息三维数字岩心建模方法，构建了升尺度电阻率数值模拟方法，明确了不同导电路径的耦合关系，提出了双孔隙导电模型，改进了致密砂岩储层饱和度评价精度。首先，采用X射线CT扫描获取标准柱塞致密砂岩样品的三维灰度图像，基于Qemscan测试结果进行图像配准和分割，准确识别粒间孔和主要矿物组分，采用MAPS测试识别孔径小于CT扫描分辨率的微孔隙，建立了致密砂岩全息数字岩心，其孔隙度和矿物组分含量与真实岩心吻合。然后，基于矿物组分的性质和微孔隙率，计算其等效电阻率，采用有限元方法升尺度模拟致密砂岩全息三维数字岩心的电阻率，同时考虑多尺度孔隙中流体和粘土附加导电作用，电阻率数值模拟结果与实验结果吻合，并分析了粘土、样品尺寸和流体分布对模拟结果的影响。最后，结合流体注入法和X射线CT扫描，明确了致密砂岩储层大尺寸粒间孔隙和小尺寸填隙物微孔隙的串联关系，建立了双孔隙导电模型，准确描述了致密砂岩储层胶结指数m的变化规律，应用该模型评价鄂尔多斯盆地长7段致密砂岩储层含油气饱和度，提高了饱和度评价精度。项目研究期间，发表论文8篇，其中SCI收录7篇，EI收录1篇。申请国家发明专利3项，其中授权1项，在审2项。授权软件著作权2项。受邀参加2020年海上油田测井技术研讨会1次，做40分钟专题报告，参加第11届测井新技术国际学术研讨会1次，参加第12和13届国际多孔介质协会(Interpore)年度会议3人次。项目研究期间累积培养硕士研究生4名。

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