Abstract
Capillary imbibition in unsaturated rocks is important for the exploitation of tight reservoirs, such as oil and gas reservoirs. However, the physical properties of natural rocks tend to be relatively uneven, mainly in the heterogeneity of material composition and pore space. Reservoir heterogeneity is an important factor affecting the exploitation of oil fields and other reservoirs, which can be evaluated by the pore structure tortuosity fractal dimension DT of rock. The greater the value of DT, the stronger the heterogeneity of sandstone. Two types of sandstone with high and low permeability were selected to study the effect of heterogeneity on the imbibition behavior by using high-resolution X-ray imaging and neutron radiography. Quantitative results of the wetting front position for each specimen were extracted from the neutron images. The wetting front advanced linearly with the power index of time t1/(2DT). Different values of DT were selected to estimate and discuss the effect of the heterogeneity on sorptivity. A modified L-W equation was employed to predict the sorptivity. Comparing with the experimental results, the heterogeneity plays a significant role in determining the sorptivity. The modified model provides a reference for the prediction of the sorptivity of the same types of sandstones studied in this paper.
Highlights
Natural rock in reservoirs, such as underground aquifers, oil, and natural gas reservoirs etc., contain a large number of microporous structures, whose physical properties tend to be relatively uneven, and are mainly manifested by the heterogeneity of the rock material composition and pore space.The imbibition of water in the pore structure has great significance for oil and gas exploitation [1,2], geothermal reservoirs [3], and the utilization of groundwater resources [4,5]
The vertical monitoring line was located at the center of the net-water image of the specimen to avoid the effects of boundary effects
Linear regression analysis found that the DT values for both specimens, W1 and S1, are larger than 1.0, i.e., specimens W1 and S1 are heterogeneous, and the heterogeneity for W1 is stronger than that for S1
Summary
Natural rock in reservoirs, such as underground aquifers, oil, and natural gas reservoirs etc., contain a large number of microporous structures, whose physical properties tend to be relatively uneven, and are mainly manifested by the heterogeneity of the rock material composition and pore space. The imbibition of water in the pore structure has great significance for oil and gas exploitation [1,2], geothermal reservoirs [3], and the utilization of groundwater resources [4,5]. Due to the diversity and complexity of the pore structure, it is difficult to visually describe the effect of heterogeneity on the spontaneous imbibition behavior within rock. The exploitation of nondestructive testing technology, such as high-resolution X-ray imaging (HRXI) and neutron radiography (NR), provides an effective means for the study of imbibition behavior within rock. NR demonstrates unique advantages [8] to quantitatively analyze the distribution and movement of water owing to the strong attenuation of neutron rays by hydrogen, and its relative insensitivity to the gas phase and mineral composition
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