Abstract

Fluid identification is one of the greatest challenges in Sichuan Basin, China. The tight carbonate reservoirs have very strong heterogeneity in mineralogy and pore structure without a unified free water level, resulting in the frequent failure of water saturation calculation by Archie’s equation. The scope of this paper is to show how a novel methodology based on non-electric open-hole logs reveals the fluid type of complex tight carbonate reservoirs.

The workflow introduced in this paper incorporates neutron-induced spectroscopy, nuclear magnetic resonance (NMR) and borehole image. Spectroscopy data not only describes the mineralogy, but also provides key measurements for fluid identification, which are thermal neutron capture cross-section (sigma) and chlorine concentration. The value of sigma and the amount of chlorine ion in the formation are both proportional to the water volume, thus water saturation. Furthermore, Two-dimensional NMR data enable to identify different types of fluid by T1-T2 maps, especially beneficial for gas reservoirs owing to the high T1/T2 ratio of gas. The integrated measurements provide water saturation estimation without resistivities and mutually complementary with each other.

By detailed analysis of the borehole image and well-test data, we found that the resistivity logs are significantly affected by the rock framework, apart from the fluid type. The resistivity of rocks with interbedded frameworks is apparently lower than that of rocks with massive frameworks. However, sigma and chlorine concentration from spectroscopy measurement are less affected by rock framework, they are mainly dominated by the volume of formation water with high salinity. Case studies presented in this paper prove the fluid identification workflow utilizing sigma and chlorine concentration from spectroscopy, integrated with the movable water and gas volume estimation from 2D NMR measurements works successfully in both water and oil-based mud environments. The coincidence rate of the fluid identification reaches above 95% by means of the gas saturation calculation and the discrimination charts of sigma, chlorine concentration, and 2D NMR T1-T2 data.

This manuscript illustrates a successful and novel integrated workflow that combines multiple wireline measurements for fluid identification in tight carbonate reservoirs when the uncertainty of the resistivity method is high due to the complex mineralogy and pore structure. The non-electrical methodology significantly lowers the uncertainty for water saturation estimation, it also ensures the correct mapping of fluid typing in various reservoirs with different petrophysical properties, such as sandstone and igneous reservoirs.

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