致密砂岩微裂缝分形定量识别方法研究

Micro-fractures not only serve as the effective storage space for tight sandstone gas, but also as important conduits for the migration of tight gas. With complex generation mechanism and various controlling factors, micro-fractures possess high heterogeneity on its distribution. Thus, the quantitative identification and characterization of micro-fractures remain major problems in the geological domain. Fractal geometry has an advantage in quantitatively characterizing irregular objects in nature and is widely employed in the quantitative characterization of fractures. However, fractal characterization of fractures remains mostly at the macroscale and further researches are urged in the study of micro-fractures. In this project, the tight sandstone of Shan 1 Member of Upper Paleozoic in the Ordos Basin was selected as the research object. As to the problem of quantitative identification of micro-fractures by fractal method, observations and fractal characterization of micro-fractures will be carried out firstly with the help of SEM. On this basis, the fractal analysis on the high pressure mercury intrusion experiment, the spontaneous imbibition experiment and the low-field nuclear magnetic resonance experiment are integrated to: 1)further verify the validity of "mercury intrusion fractal anomaly" as a criterion for identifying micro-fractures, 2) establish fractal methods for the identification of micro-fractures by analyzing the data of "low field nuclear magnetic resonance" and "spontaneous imbibition", 3) innovatively establish integrated multi-technical fractal method for quantitative identification of micro-fractures. The achievements of this project have important practical significance for improving quantitative evaluation system of tight reservoirs and improving exploration efficiency.

微裂缝不仅可以作为致密砂岩气的有效储集空间，同时也是天然气运移的重要通道。由于微裂缝的成因复杂，控制因素多样，分布具有高度非均质性，其定量识别与表征仍是地质界的一大难题。分形几何能够定量表征自然界不规则物体，广泛地应用于裂缝的定量研究中。目前，裂缝的分形表征多停留在宏观尺度，在微裂缝中的研究尚需深化。本项目以鄂尔多斯盆地上古生界山1段致密砂岩为研究对象，针对微裂缝分形定量识别这一难题，在扫描电镜观察、微裂缝分形表征的基础上，综合高压压汞实验、低场核磁共振实验、自发渗吸实验的分形表征技术，进一步验证“压汞分形异常”作为微裂缝识别标准的有效性，建立“低场核磁共振分形”识别微裂缝、“自发渗吸分形”识别微裂缝的方法，并创新性地建立多技术综合微裂缝分形定量识别方法。本项目成果对于完善致密储层定量评价体系、提高勘探效率具有重要的现实意义。

微裂缝不仅可以作为致密砂岩气的有效储集空间，同时也是天然气运移的重要通道。由于微裂缝具有高度非均质性，其定量识别与表征仍是地质界的一大难题。本项目以川中地区须家河组须二段致密砂岩为研究对象，优选气层（微裂缝发育的）、干层（微裂缝不发育）样品，在铸体薄片、扫描电镜定性观察的基础上，综合孔隙结构二维分形表征（铸体薄片）、孔隙结构三维分形表征（高压压汞、低场核磁共振、自发渗吸）技术，厘清了微裂缝的存在对二维分形维数、三维分形维数的控制作用。本研究表明：（1）气层样品的二维分形维数明显大于干层样品。气层样品铸体薄片孔隙结构分形维数在1.44～1.80范围内，平均值为1.65。相比之下，干层样品铸体薄片孔隙结构分形维数在1.12～1.45范围内，平均值为1.39。（2）不能简单地以“压汞分形异常”作为微裂缝识别标准。几乎所有气层样品的D1均处于正常范围（2, 3），平均值为2.71；而几乎所有干层样品的D1均明显大于3，平均值为3.62，偏离正常范围。（3）富含微裂缝的岩心样品，其高压压汞分形维数更小。气层的高压压汞分形维数（D2）分布范围在2.09和2.13之间，平均值为2.11；与之相比，干层的高压压汞分形维数（D2）大于气层的分形维数，分布于2.15和2.26之间，平均值为2.21。（4）须二段气层样品的自发渗吸分形维数整体大于干层样品的分形维数。（5）须二段干层样品的分形维数整体大于气层样品的分形维数。