Abstract
Tight sandstone reservoirs are affected by various factors such as pore structure, formation water salinity, and siliceous cementation, which lead to the abnormal phenomenon of high-resistivity water layers and increase the difficulty in identifying gas and water layers by conventional logging. In this study, the pore types and pore size distribution characteristics of tight sandstone reservoirs were firstly determined by NMR and high-pressure mercury injection experiments, and then the iterative least-square method was used to automatically optimize the inversion method of pseudo-capillary pressure curve and search for the optimal conversion coefficient. Finally, the apparent free water porosity was inversed and the fluid identification standard was obtained and applied. The results showed that the reservoirs mainly developed intergranular pores, cutting solution pores, and intergranular pores. The pore throats were poorly sorted, and the displacement pressure was high. The median radius ranged from 0.01 to 0.48 μm, and the main peak range was from 0.02 to 0.06 μm. Pores were of mainly small-hole fine throat type. In the inversion results of the optimal conversion coefficient, the correlation coefficient between the aperture parameters and the results of high-pressure mercury injection experiments was greater than 0.93. According to the fluid property identification standard based on nuclear magnetic apparent free water porosity, the high-resistivity water layers were effectively identified and its coincidence rate with the final field test was 10.7% higher than that of the conventional method. This identification method can be used to identify complex fluids in tight sandstone reservoirs.
Highlights
Introduction eP2h formation and P1s formation of the Upper Paleozoic Permian in Tianhuan Sag belong to typical tight sandstone reservoirs in China [1, 2]. e water layers in the area have no unified gas-water interface, and disconnected water bodies are distributed in the area [3, 4]
According to the fluid property identification standard based on nuclear magnetic apparent free water porosity, the high-resistivity water layers were effectively identified and its coincidence rate with the final field test was 10.7% higher than that of the conventional method. is identification method can be used to identify complex fluids in tight sandstone reservoirs
The pseudo-pore size distribution curve calculated with nuclear magnetic resonance logging data and the pore size distribution curve measured with core data are different in the pore size distribution position and pore shape
Summary
E CO2/N2 adsorption and high-pressure mercury injection experiment was performed for the quantitative characterization of pore structure [23]. According to the thin section analysis and statistics of reservoir samples of sections P2x8 and P1s in the study area, in the south and west of Sugri Gas Field (Figures 2 and 3), the clastic content in Section P2x8 was 83.97% and the clastic components were mainly quartz. Displacement pressure distribution range of P2x8 sandstone section was 0.03 to 60 MPa with the average of 5.73 MPa and the main peak range of 0.5 to 1 MPa (Figure 4(a)). E comparative analysis of mercury injection test and gas test results of samples indicated that with the improvement of pore structure, gas production gradually increased and water production gradually decreased The proportion of small holes decreased and the proportion of large holes increased. e proportion of bound water porosity decreased, indicating that the pore structure of cores from left to right was gradually improved in turn. e sample permeability was greatly increased from 0.16 × 10−3 μm of the No 16 sample to 30.441 × 10−3 μm of the No 6 sample, and the gap was nearly 200 times. e difference indicated that pore structures significantly affected the reservoir seepage capacity. e better the pore structure, the higher the corresponding reservoir permeability. e comparative analysis of mercury injection test and gas test results of samples indicated that with the improvement of pore structure, gas production gradually increased and water production gradually decreased
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