Pore structure characterization of tight sandstones via a novel integrated method: A case study of the Sulige gas field, Ordos Basin (Northern China)

摘要

Pore structure characterization is a very crucial work to understand rock properties. In this work, a novel integrated method was proposed to characterize the pore structure of a real tight sandstone reservoir in the Sulige gas field. This integrated method requires a comprehensive analysis of experimental results based on cast thin section (CTS), nuclear magnetic resonance (NMR), constant-rate mercury intrusion (CMI) and X-ray computed tomography (XCT) techniques. Five main pore types were firstly identified from CTS images. Bimodal distribution curve of surface relaxivity rate, mercury injection curves of pore body and throat, 3D visual image of pore reconstruction were then generated from NMR, CMI and XCT measurements, respectively. Pore size distribution (PSD) was also obtained by and compared among NMR, CMI and XCT techniques. Full size PSDs of pore body and throat were ultimately determined using a combination analysis method of NMR and CMI. In addition, fractal dimension was calculated by NMR- and CMI-deduced formulas. Moreover, pores were classified into three categories and the contribution of different category pore to rock properties was qualitatively ascertained. At the end, a novel permeability prediction model in consideration of the contributions of three category pore was established. It was found that radii of pore body and throat ranged in 0.21?42.01 ?m and 0.006?3.7 ?m, respectively. Sandstone pore has multifractal characteristics and its fractal dimension ranged in 1.335?2.982. Overall, sandstone pore structure of the example reservoir was characterized by multiple pore types, nanomicrometer pore scale, irregular geometric shape, non-uniform distribution, poor connection status and bad development degree of pore space. The findings of this study can help for better understanding of pore structure characteristics of tight sandstones.