Application of Nuclear Magnetic Resonance Pore and Throat Distribution Curve in Analysis of Pore Structure of Tight Sandstone

Nuclear magnetic resonance analysis of pore structure of tight sandstone (hole throat radius)

The fluid seepage characteristics in tight reservoirs are different from those in conventional reservoirs, and have a great relationship with the microscopic pore structure of reservoirs. A correct understanding of reservoir pore structure is very important for calculating recoverable reserves and formulating reasonable development plans.

The low magnetic field NMR T2 spectral distribution is directly related to the pore structure, which can reflect the pore distribution of the sample to some extent. The following is a brief introduction to the application of the nuclear magnetic pore throat distribution curve.

Pore ​​structure analysis:
The figure below shows the pore throat radius distribution of NMR T2 spectrum conversion under arbitrary saturated water cores with different permeability. From the perspective of the pore throat size, the converted nuclear magnetic pore distribution can be analyzed to increase the pore throat size range, and the data points are significantly increased, showing the unique advantages of the T2 spectrum in reflecting minute pores and complex pore distribution. Comparing the nuclear magnetic pore throat distribution of five cores with different permeability, it is found that the pore throat of the sample is concentrated in the range of 0.01 ~ 1 micron, and the curve shows a double peak. As the permeability decreases, the right peak of the curve decreases. Increase, that is, the proportion of large pore throat decreases, and the proportion of small pore throat increases

The following figure shows the frequency distribution of nuclear magnetic pore throat distribution in different permeability cores.

Movable fluid distribution:
The fluid in the reservoir is divided into a bound fluid and a movable fluid according to its occurrence state in the porous medium. Tight reservoir rocks are important for the development of tight reservoirs due to their fine pores, large pore specific surface area and high bound fluid content. The evaluation of the binding fluid (or movable fluid) distribution is important for the development of tight reservoirs. The figure below shows the distribution of the water phase in the core pores under different displacement conditions. Among them, the curve of the bound water state represents the distribution of water in the pores after the oil flooding, the two curves of the saturated water and the bound water state, the two curves of the residual oil and the bound water state, and the saturated water and residual oil. The relationship between the two curves of the state is the distribution of the movable fluid, the movable oil and the residual oil in the pores.

References "Nuclear magnetic resonance research on the method and application of tight sandstone pore structure", Journal of China University of Petroleum (Natural Science Edition), Vol.39, No.6, 2015