Abstract
Core samples of the tight sandstone reservoir in the Bonan Oilfield were analyzed by using multiple petrophysical experimental techniques, then a multi-scale three-dimensional digital rock model was constructed. The pore structure parameters of tight sandstone and homogeneous Berea sandstone were compared. The electrical simulation method based on the digital rock model was utilized to quantitatively reveal the influence of five micro-pore structure parameters (pore size, throat size, pore-throat size, coordination number, and shape factor) on the rock’s electrical properties. In addition, the saturation of tight sandstone reservoirs was evaluated in combination with the three-component automatic mixed-connection conductivity model. The results show that the “non-Archie” phenomenon in sandstone is obvious, which is mainly caused by the small radius of the maximum connected pore throat and the complex structure of the pore throat. We noted that: with an increase in pore radius, throat radius, and coordination number, the formation factor decreases and tends to be stable; the pore-throat size increases and the formation factor decreases in the form of power function; the shape factor increases, and the formation factor increases; the larger the pore–throat ratio and shape factor, the greater the resistivity index; with an increase in coordination number, the resistivity index decreases; and the pore-throat size has no effect on the resistivity index. The calculation accuracy of oil saturation is improved by 6.54% by constructing the three-component automatic mixed-conductivity saturation model of tight sandstone.
Highlights
The tight sandstone reservoir has the characteristic of low porosity and low permeability, it contains considerable oil and gas resources, which are an important force to replace conventional oil and gas energy and support the future oil and gas revolution [1,2,3]
In order to characterize the pore structure of tight sandstone, various experimental testing techniques have been widely used, including X-ray diffraction (XRD) [6], highpressure mercury injection (HPMI) [7], small-angle neutron scattering (SANS) [8], lowtemperature gas adsorption (N2/CO2, GA) [9], nuclear magnetic resonance (NMR) [10,11,12], cast thin section (CTS) [13], scanning electron microscope (SEM) [14,15,16], focused ion beam microscope (FIB-SEM) [17], micro/nano CT scanning and the resulting digital rock technology [18,19]
We proposed a method that comprehensively utilizes the results of conventional rock physical experiments to assist CT scanning in constructing highprecision digital rocks; this compensates for the lack of accuracy of micron CT in the process of constructing digital rocks of tight sandstone
Summary
The tight sandstone reservoir has the characteristic of low porosity and low permeability (less than 1 × 10−3 μm), it contains considerable oil and gas resources, which are an important force to replace conventional oil and gas energy and support the future oil and gas revolution [1,2,3]. Poor physical properties (porosity, permeability), strong heterogeneity, and complex pore structure are typical features of tight sandstone reservoirs, and the pore size distribution can be divided into microscale pores and nanoscale pores [4,5]. A variety of experimental methods and theories have been comprehensively used by many scholars, to accurately and comprehensively characterize the full pore-throat size distribution of tight sandstone reservoirs [20,21]
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