Abstract
This study aims at investigating the applicability of the commonly used water saturation test methods such as distillation extraction method, nuclear magnetic resonance (NMR) method, constant weight method, and Coulomb method in the water saturation test of tight sandstone gas reservoirs. The above methods were studied and optimized to overcome the difficulties of low water content determination caused by low porosity, low permeability, small pore throat, high capillary force, and high bound water saturation of tight sandstone, to obtain relatively accurate water saturation data. The results showed that the water content data determined by the constant weight method were consistent with the known water content saturation, and the average error was 3.6%. By shortening the echo interval, the average error of water content saturation tested by the NMR method for tight sandstone samples was 4.8%. The two methods of the distillation method and Coulomb method had large errors, and both failed to meet the requirements of water saturation determination of tight sandstone, of which the average error of the former was 27.9% and the average error of the latter was 50.7%. Therefore, for tight sandstone, the constant weight method and NMR method were recommended to be used for water content saturation testing, and the distillation extraction method and Coulomb method were not recommended. The research results provided a new method for the accurate determination of water content saturation in tight sandstone gas reservoirs.
Highlights
The water saturation of tight sandstone gas reservoirs is a key parameter for reservoir evaluation, and its accuracy is directly related to the reliability of reservoir calculation results
The experimental results of 22 pieces of fresh tight sandstone showed that the water saturation of tight sandstone remained stable when the drying temperature reaches above 105°C–116°C as required by Recommended Practices for Core Analysis, and the drying time is 8 h for each temperature point; after that, the moisture in the rocks remained almost unchanged after the samples were placed in the desiccator for 48 h
The above results show that for tight sandstone samples, the TE value is the key to affecting the signal volume, and it is suggested that the TE value is taken to be less than 0.2 ms, and the TW value has no obvious effect on the signal volume. This is because micro- and nanopores are developed in tight sandstone, and the nuclear magnetic resonance (NMR) instrument can completely polarize the water in the pores with a short echo interval and waiting time, and the TW value can be consistent with the conventional NMR test range
Summary
The water saturation of tight sandstone gas reservoirs is a key parameter for reservoir evaluation, and its accuracy is directly related to the reliability of reservoir calculation results. The currently used methods of water saturation testing include constant weight method (an optimized method based on dry distillation), distillation extraction method, Coulomb method, and NMR method. The constant weight method is prone to clay water precipitation when the test temperature is too high [2, 23], which results in abnormally large values of water saturation and leads to great difficulty in reservoir water saturation determination. The Coulomb method is commonly used in conventional oil and gas reservoirs, and the extraction degree is relatively low for low-permeability as well as dense reservoirs, which affects the experimental accuracy. The test results of water saturation by NMR method are influenced by reservoir factors such as lithology and pore structure, and by measurement parameters such as echo interval TE and waiting time TW, the larger the echo interval TE is, the larger the error of the collected data is. It is necessary to systematically study the test method of water saturation for tight sandstone and carry out method optimization and parameter optimization to meet the demands for water saturation determination, so as to provide a new method for the accurate determination of water saturation in tight sandstone gas reservoirs and accurate data support for tight gas reservoir evaluation
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