Pore structure and movable fluid characteristics of tight sandstone reservoirs in the Lower Shihezi Formation in the Hangjinqi area, Ordos Basin

Abstract

Objective In recent years, new discoveries have been made in the He 1 Member of the Hangjinqi Gas Field. And the initial gas production of some wells by fracturing can reach 10×104 m3/d, which shows that the He 1 Member has great exploration potential. However, due to the strong heterogeneity of the He 1 Member, the gas production mechanism of the He 1 Member is not clear at present, which restricts its efficient development. To accurately and quantitatively characterize the microscopic pore structure and movable fluid characteristics of tight gas sandstone reservoirs. Methods In this paper, taking the tight sandstone reservoir in the He 1 Member of the Shangshihezi Formation in the Hangjinqi Gas Field as an example, NMR and CT tests were used to study the response characteristics and fluid identification ability of different types of pores in tight sandstone. Results The research shows that the porosity of the test samples is mainly distributed in 1.7%-10%, and the gas permeability is mainly distributed in 0.1×10-3-1.4×10-3 μm2, which belongs to the typical low-porosity and low-permeability porous tight sandstone reservoir. According to the T2 relaxation time curve of the saturation component before centrifugation, the pore type of the reservoir in the He 1 Member is bimodal (mainly the left peak, the right peak is not obvious), including 3 subtypes: micropore-small pore type, small pore-mesopore type, micropore-small pore-mesopore type. The T2 relaxation time intervals corresponding to the above three subtypes of pore types are 0.1-10 ms, 1-100 ms, and 0.1-100 ms, respectively. The results of 3D CT scans show that the micropore-mesopore reservoir has the best physical properties, followed by the micropore-small pore-mesopore type, and the micropore-small pore type reservoirs have relatively poor physical properties. The T2 cut-off values of the tested samples were mainly distributed between 1 and 14 ms, with an average value of 6.11 ms. There is a certain negative correlation between the T2 cut-off value and the movable fluid percentage of the rock samples. The movable fluid porosity and permeability have a very good positive correlation, reflecting that the amount of movable fluid is significantly affected by the reservoir permeability and the number of throats. The movable water saturation in the tight sandstone reservoirs of He 1 Member in the study area is mainly distributed at 4%-9%, with an average value of 5.8%. Conclusion Overall, the original movable water saturation of the He 1 Member is low and has great development potential.

Keywords

• hangjinqi area
• nuclear magnetic resonance
• ct scanning
• lower shihezi formation
• tight sandstone
• reservoir property
• pore structure
• movable fluid