Micropore structure and movable fluid distribution characteristics of tight sandstone reservoirs: Taking the He 8 reservoir in the Shenmu area of the eastern Ordos Basin as an example

Abstract

Objective The complex pore throat structure of tight sandstone leads to variable distribution of movable fluid, and the micropore structure and distribution characteristics of movable fluid are the focus of the study of tight sandstone reservoirs. Methods Based on the principle of the nuclear magnetic resonance (NMR) movable fluid test, the classification standard of pore structure of the He 8 reservoir in the Shenmu area was established using centrifugal test, high-pressure mercury injection, scanning electron microscope, X-ray diffraction and casting thin section. The pore structure parameters and pore throat types of the three types of rocks are defined, and a new method for measuring the conversion coefficient suitable for tight sandstone reservoirs was proposed. The distribution characteristics of movable fluid of three types of rocks were also quantitatively evaluated. Results The results reveal that the type Ⅰ and Ⅱ rock pores in the target reservoir are mainly residual intergranular pores with pore diameters greater than 10 μm and dissolution pores with pore diameters greater than 1 μm. The throats are mainly reduced and curved flaky throats, with good pore structure parameters, a high development degree of large pore space, good connectivity between pore throats, and a large amount of movable fluid.Most of the movable fluid occurs in the macropores corresponding to the right peak of the T2 spectrum, while the content of the movable fluid in the small pores corresponding to the left peak is low. The pore structure parameters of type Ⅲ rocks are poor, the percentage of movable fluid is low, and the pore throats are mainly intergranular pores and tube bundle throats. The average conversion coefficient of the target reservoir is 0.029 μm/ms, but the conversion coefficients of type Ⅰ and Ⅱ rocks are less than that of type Ⅲ rocks. The right peak of the T2 spectrum of type Ⅰ and Ⅱ rocks after conversion corresponds to the main peak of the mercury porosimetry pore radius distribution, while the left peak of the T2 spectrum of type Ⅲ rocks corresponds to the main peak of the distribution of mercury porosimetry pore radius. The percentage of movable fluid in the pores of type Ⅰ and type Ⅱ rocks with pore diameters greater than 1 μm is high, which is the main direction of exploration and development in the future. Conclusion The results provide a reference for improving the recovery of tight reservoirs.

Keywords

• tight sandstone reservoir
• high pressure mercury injection
• nuclear magnetic resonance
• movable fluid
• micropore structure