Abstract
The complex pore structure of rock may influence resistivity logs and make it challenging to perform saturation estimations with better precision. For example, water saturation may go higher or lower when Archie’s formulae are used to obtain the saturation as the logging resistivity may become greater or smaller, due to the influences of complex pore structures in the reservoirs. In this paper, having considered the influences of pore structure on logging resistivity, we have developed a different algorithm to improve the equivalent rock element model (EREM, a rock conduction model) to have better water saturation. First of all, the nuclear magnetic resonance (NMR) echo data of cores are inverted by using the optimized inversion to calculate pore parameters, such as the average radius of spherical pore tubular pore, Cd path, and sorting coefficient. Secondly, according to the parameters derived from the optimization inversions, the pore structures of the core are divided into different groups. Based on the classifications of the pore structure, the conduction parameters of rock for different pore structures are determined by fitting the experimental conduction data of rock with a damping factor. Finally, the working flow of calculating fluid saturation according to the EREM model is determined for different conduction parameters. The key feature of the improved EREM saturation model is to eliminate the influence of pore structure on rock conduction, highlight the influence of pore fluid on rock conduction, and then improve the calculation accuracy of saturation in it. The applications of our research show that the saturation estimations derived from the improved EREM model appear to be more reasonable in terms of the saturation value and its numerical trend. Thus, the improved EREM model has potential in petroleum exploration and exploitation.
Highlights
The exploration and development of complex oil-bearing reservoirs are becoming increasingly important in the petroleum industry worldwide
Our research showed that the median value of the initial pore structure efficiency (C0) is a good coefficient suitable for the equivalent rock element model (EREM) model
Complex pore structure may have various effects on the rock conductivity in oil-bearing and/or gas-bearing reservoirs, including the effects of increasing or decreasing resistivity, which significantly reduces the accuracy of saturation estimation in the classical saturation model
Summary
The exploration and development of complex oil-bearing reservoirs are becoming increasingly important in the petroleum industry worldwide. Even if the porosity and saturation are identical, the resistivity of the rock may change when the pore structure changes significantly and/or the fluid shape in the pore changes significantly In this case, the classic Archie formula (Archie., 1942) cannot be applied in the complex reservoir. He argued that rock wettability seems variable in analyzing experimental data, and the non-Archie phenomena exist in different samples To understand these relationships, Man proposed a new pore network model according to the pore morphology and the shape combinations of the different pores. It can be considered that the pore structure does not constitute any influence on rock conduction in the reservoir with the same or similar pore structures, and the rock resistivity is entirely dependent on the response of pore fluids Based on these considerations, the accuracy of saturation estimations is improved
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