Based on the NMR T 2 spectra of all of the samples, the applicability of the single fractal model and the multifractal model for the quantitative investigation of pore diameter distribution heterogeneity is analyzed. Then, the relationships between pore structures, physical properties, and fractal dimensions calculated by each model are discussed. The following results have been achieved. (1) Model 1 can be used to study the heterogeneity of different fluid states (including saturation and bound water) or partial pore size distribution, and the calculated fractal dimension has the best correlation with permeability. This model is mainly suitable for type I samples with macropores developed. (2) Models 2, 3, and 4 can quantitatively analyze the heterogeneity of the overall pore diameter distribution. These calculated fractal dimensions have a good linear relationship with porosity, T 2cutoff, T 2gm, and other physical parameters, which are mainly suitable for type II, III, and IV samples with micropores developed. (3) There is a negative correlation between multifractal dimensions calculated by model 4 and single fractal parameters calculated by models 2 and 3. Micropores and macropores are the key pore sizes that affect the variation of single fractal parameters, while mesopores are the key size affecting the variation of multifractal parameters. (4) The selection of a reasonable T 2 spectrum range has become a key to determine the calculation accuracy of models 2 and 3. Above all, models 1 and 4 should be given priority in characterizing pore distribution heterogeneity of tight sandstone gas reservoirs.

Thirty-eight samples from tight sandstone reservoir in the upper Paleozoic layer of Ordos Basin, China were examined. The micropore structure of the reservoir was observed by casting thin sections, which were analyzed via scanning electron microscopy. The pore size distribution characteristics of the reservoir were studied via high-pressure mercury injection. The fit of five different models to the fractal characteristics of the tight gas sandstone reservoir were analyzed, and the fractal characteristics of pores in tight sandstone reservoirs were further revealed. Pores were divided into microscale macropores (P 1 ) (>10 μm), microscale micropores (P 2 ) (1−10 μm), submicron pores (P 3 ) (100 nm−1 μm), and nanopores (P 4 ) (2−100 nm). The results show that P 2 account for 63.24% of the total pore volume in type I reservoirs. P 3 are dominant in type II reservoirs, accounting for 51.74% of the total pore volume. Type III reservoirs are mainly composed of P 3 and P 4 , accounting for 48.96% and 41.88% of pore volume, respectively. In type IV reservoirs, P 4 is the main pore size followed by P 3 , accounting for 54.98% and 36.96%, respectively. The Brooks−Corey model can characterize the pore fractal characteristics of the tight sandstone reservoir effectively, while other models presented some limitations. The evaluation of fractal characteristics showed that the three-segment fractal fitting curve was closely related to the inflection point of the S Hg /P c −S Hg curve. The fractal dimensions of P 1 , P 2 , P 3 , and P 4 were 2.904−2.998, 2.384−2.999, 2.155−2.951, and 2.151−2.911, with average values of 2.9826, 2.951, 2.69, and 2.604, respectively. The correlation between the stage pore fractal dimensions and reservoir parameters showed that the fractal dimensions of P 3 and P 4 better reflected the complexity of the pore throats and were more suitable for pore throat heterogeneity characterization. In tight gas sandstone reservoirs, porosity shows relatively low sensitivity to fractal dimensions, while permeability is controlled by the fractal dimensions of P 1 to a great extent. Thus, reservoirs featuring regular macropores are favorable target areas for oil and gas filling.

Nuclear magnetic resonance (NMR) T2 cutoff value is an important parameter for pore structure evaluation. It is complicated and uneconomical to obtain T2 cutoff value by an experimental method; therefore, it is necessary to explore a prediction method of T2 cutoff value. In this paper, 10 samples of tight gas reservoirs in the eastern Ordos Basin were selected, and then saturation and centrifugal experiments of nuclear magnetic resonance were carried out. On this basis, multifractal theory was introduced to calculate the multifractal characteristics of the NMR T2 spectrum of each sample, and the relationship between multifractal parameters and T2 value was analyzed. The influencing factors of the T2 cutoff value were clarified, and the prediction model of the T2 cutoff value was constructed accordingly. The results show that the T2 spectra of sandstones in the study area can be divided into three types: single steeple peak, double steeple peak, and irregular double peak. The pore diameter of the three types is 1 nm ~ 3×104 nm, 1 nm ~ 104 nm and 1 nm ~ 4×103 nm, respectively. The T2 cutoff value ranges from 9.72 to 35.16 ms. The correlation analysis suggests that the symmetrical fractal dimension difference and symmetrical multifractal dimension ratio (Dmin−Dmax, Dmin/Dmax) shows a positive linear correlation with the T2 cutoff value. The value of T2 cutoff gradually decreases with the increase of the flow zone indicator (FZI). Therefore, three parameters, including symmetrical fractal dimension difference, symmetrical multifractal number ratio, and FZI are optimized, and the prediction model for the NMR T2 cutoff value of sandstone samples in the study area is proposed. The introduction of porosity‐related parameters compensates for the shortcomings of previous T2 cutoff value prediction models. At the same time, the prediction model is proven to be accurate and reliable by testing the measured data of the samples near the study area. The results of this paper can be used for further study of the NMR T2 cutoff value prediction of tight sandstone reservoirs in different areas.

No abstract

A series of studies were carried out on 11 tight sandstone samples of Upper Carboniferous in Ordos Basin. Firstly, the deposit composition and pore structure characteristics are investigated based on analysis and experiments including cast thin section scanning electron microscope high-pressure mercury intrusion and nuclear magnetic resonance Then, combined with DP-P test, the stress-dependent permeability change and pore compressibility characteristics of sandstone reservoirs were studied to reveal the influencing factors and mechanism of reservoir pore compressibility. The detrital particles of the sandstone reservoir in the study area are mainly quartz (75.8%–89%), followed by fragments (3%–16.1%), and almost no feldspar. The content of interstitial materials is 6.5%–11.2%. The type I reservoirs mainly consist of mesopores and macropores, accounting for 60.57% and 32.84% respectively. Mesopores are dominated in Type II reservoirs, accounting for 78.98% of the total pore volume. There are almost no macropores, while a similar proportion of mesopores, micro mesopores and micropores in the type Ⅲ reservoirs. The study of pore compressibility shows that the pore compressibility coefficient decreases with the increase of effective stress, and the reduction rate shows the two-stage characteristics of rapid in the early stage and slow in the later stage. The pressure turning point is between 3 and 10 MPa. The average pore compressibility coefficient increases from type I to type Ⅲ reservoirs. The compressibility coefficient is directly proportional to the changing rate of the pore volume. The higher the content of rigid detrital particles, quartz and carbonate cement in sandstone, the smaller the pore compressibility coefficient, while the higher the content of ductile components such as soft rock fragments and clay minerals, the greater the pore compression coefficient. The pore-throat structure is closely related to the pore compressibility, reservoirs with low displacement pressure, T2glm value, and large average pore-throat radius show lower compressibility coefficient. In addition, the compressibility coefficient of the reservoir is positively correlated with DL (dimension of large pores such as mesopores and macropores), and negatively correlated with DS (the fractal dimension of micropores and micro mesopores). It is considered the pore compression of sandstone including two stages, viscoplastic destructive deformation of ductile components for the first and then the small-scale non-ideal elastic deformation on rigid particles.