Updated Non-Electrical Solutions for Unlocking Gas Potential in Ultradeep Tight Reservoirs

Abstract

The Tarim Basin is distinguished by its substantial natural gas production, as exemplified by the West-to-East Gas Transmission Project. The gas reservoirs are located beneath a salt formation at depths of approximately 8,000 meters, primarily within the Kuqa thrust belt. These reservoirs are characterized by well-developed open fractures, yet they exhibit relatively low petrophysical properties. The occurrence of water in specific blocks underscores the importance of precise fluid identification to delineate optimal targets for continued development. Previous studies have established that resistivity measurements are affected by multiple factors, such as pore structure, sedimentary dips, remote fractures, and neighboring lithologies. To mitigate these complexities, non-electrical open-hole technologies—particularly two-dimensional NMR with T1-T2 joint analysis—have demonstrated considerable efficacy in differentiating between gas and water under such challenging conditions. It is established that diffusion-induced relaxation affects only T2, not T1. When using high-frequency magnetic field gradients typical in logging tools, the oil and gas signals can be differentiated based on their distinct T1/T2 ratio responses. Thus, with a continuous measurement of T1 and T2 spectrum, it helps enable the separation and quantification of different existing fluids in the pore systems, thereby solving the fluid typing issue of the target reservoir. The T1-T2-based NMR evaluation demonstrated significant success in differentiating between gas and water; however, the invasion of mud filtrate was detected in certain wells, despite the tight petrophysical properties. This phenomenon presents a challenge to consistently achieving high accuracy in fluid identification, primarily due to the limited depth of investigation of the T1-T2 NMR services. To address these limitations and enhance fluid identification, additional measurements with better depths of investigation were incorporated. The workflow described herein is an integrated approach that leverages advanced spectroscopy measurements. Advanced spectroscopy employs both capture and inelastic gamma ray spectroscopy to deliver precise dry weight for elements and minerals, as well as total organic carbon (TOC) content. Furthermore, it provides thermal neutron cross-section (sigma) and chlorine concentration (DWCL) measurements, both of which are sensitive to saline water and thus well-suited for fluid characterization in saline formations. The Fast Neutron Cross Section (FNXS), derived from inelastic gamma ray measurements, was also introduced. FNXS demonstrates insensitivity to variations in matrix and liquid-filled porosity, while exhibiting sensitivity to gas-filled porosity. Therefore, it serves as a reliable indicator for the presence of gases, including CO₂. Integrating Sigma, FNXS, and neutron porosity measurements reduces the uncertainties associated with individual measurements and yields a more dependable result for saturation calculations. A case study from this ultradeep reservoir was used to demonstrate the effectiveness of the updated non-electrical fluid identification method, which offered improved results in environments with better petrophysical properties and potential invasion effects. The interpretation outcomes closely matched the test data, presenting a new approach to identifying sweet spots and reducing costs associated with unnecessary development. This paper presents a successful and novel integrated workflow that utilizes multiple wireline measurements for fluid identification in Ultra-deep tight reservoirs where the uncertainty of the resistivity method is high due to multiple factors. Also, the nonelectrical methodology significantly lowers the uncertainty for water saturation estimation. This result has helped to enhance the methodology for fluid identification and earn considerable economic value by escaping the water zones in the further development of the reservoir. This workflow is also promoted to other locations in China containing high-salinity formation water environments.