Abstract
It is generally difficult to identify fluid types in low-porosity and low-permeability reservoirs, and the Chang 8 Member in the Ordos Basin is a typical example. In the Chang 8 Member of Yanchang Formation in the Zhenyuan area of Ordos Basin, affected by lithology and physical properties, the resistivity of the oil layer and water layer are close, which brings great difficulties to fluid type identification. In this paper, we first analyzed the geological and petrophysical characteristics of the study area, and found that high clay content is one of the reasons for the low-resistivity oil pay layer. Then, the formation water types and characteristics of formation water salinity were studied. The water type was mainly CaCl2, and formation water salinity had a great difference in the study area ranging from 7510 ppm to 72,590 ppm, which is the main cause of the low-resistivity oil pay layer. According to the reservoir fluid logging response characteristics, the water saturation boundary of the oil layer, oil–water layer and water layer were determined to be 30%, 65% and 80%, respectively. We modified the traditional resistivity–porosity cross plot method based on Archie’s equations, and established three basic plates with variable formation water salinity, respectively. The above method was used to identify the fluid types of the reservoirs, and the application results indicate that the modified method agrees well with the perforation test data, which can effectively improve the accuracy of fluid identification. The accuracy of the plate is 88.1%. The findings of this study can help for a better understanding of fluid identification and formation evaluation.
Highlights
Fluid identification is the basis for reservoir evaluation and research
The great difference in formation water salinity reduces the contrast between the oil layer and the water layer to some extent, which brings great difficulties to fluid identification [2]
According to the production materials, the water saturation value of 0–50% is converted to 30%, the water saturation value of 50%–80% is converted to 65% and the saturation of exploration wells with water saturation greater than 80% is converted to 80%
Summary
Fluid identification is the basis for reservoir evaluation and research. For tight sandstone, complex pore structure and minerals weaken the fluid identification ability of conventional logs [1]. With large changes in formation water salinity, figuring out the characteristics of formation water and finding the right fluid identification method are critical [3,4,5]. Variable formation water salinity often results in low-resistivity pay layers and ambiguity in the log interpretation [8]. It is necessary to construct fluid identification methods based on making clear the reasons for low-resistivity pay zones and the characteristics of formation water. Exploring a new method for fluid type identification using resistivity logging data in the background of regional formation water salinity changes is very important. Considering the effect of changes in formation salinity, the modified resistivity–porosity cross plot method is established, combined with linear interpolation, which can effectively improve the fluid identification accuracy [18,19].
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