Comparison of different analytical methods for tight sandstone pore structure (porosity distribution)
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. It is very important to understand reservoir pore structure correctly for calculating recoverable reserves and formulating reasonable development plans.
The low magnetic field NMR T2 spectrum distribution is directly related to the pore structure and can reflect the pore distribution of the sample to some extent.
The following is a brief introduction to the differences in nuclear magnetic pore throat distribution and mercury intrusion.
Conversion of nuclear magnetic pore throat distribution curve:
The pore size of porous media varies. When mercury is pressed at high pressure, mercury preferentially enters the channel with the largest size, and gradually enters the smaller channel as the pressure increases. The mercury intrusion pore throat distribution does not reflect the information of the pores connected to the throat corresponding to the maximum mercury intrusion pressure, while the T2 spectrum of the core saturated water can reflect the distribution of all pore throats in the core.
In the calculation, only the partial T2 spectrum corresponding to the mercury melt pore throat radius distribution is compared with the mercury intrusion pore throat radius distribution. As shown in the figure below, plot the cumulative distribution curve of NMR T2 relaxation time and high pressure mercury pore throat radius. In the left end of the red line (accumulated distribution frequency < SHgmax, SHgmax is the maximum mercury saturation), the radius of any throat is rt. (i) The cumulative distribution frequency is S(i), and S = S(i) is used to interpolate the cumulative distribution curve of nuclear magnetic resonance T2 to obtain the relaxation time T2(i) when the cumulative distribution frequency is S(i).
The figure below shows the comparison of the pore-throat distribution and the pressure-hole-throat distribution of the core NMR T2 spectrum with different permeability. The two curves have good consistency in morphology and amplitude. The pore throat distribution of the sample was monomodal, and the internal pore throat was evenly distributed. At the peak, the frequency of the nuclear throat of the same throat radius is greater than the frequency of the mercury pore throat distribution, and the lower the core permeability, the more obvious this difference. According to the analysis, the mercury intrusion test is limited by the pressure of the mercury intrusion, and the pores connected to the throat corresponding to the maximum mercury intrusion pressure cannot be analyzed. The pore-throat distribution of the saturated magnetic resonance T2 spectrum reflects all the pores in the rock sample. Therefore, the above difference occurs, and the permeability of the throat control pores, the lower the core permeability, the finer the throat, and the more pores that cannot be analyzed by mercury intrusion, the more obvious this difference.
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
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