# Experimental Study on Fluid Dissipation Effects in Core Samples by NMR Measurement
## 1 Introduction  
In the domain of oil exploration and development, oil and gas reserves serve as a crucial indicator of the outcomes of oilfield exploration. Parameters such as porosity and oil saturation are essential for evaluating the physical properties of formations and their associated reserves [1]. Numerous methods exist for calculating these essential parameters. The Archie equation is the most widely utilized method for calculating oil saturation [2]. The Archie equation determines rock saturation and evaluates reservoir oil potential by measuring core porosity, resistivity, and fluid resistivity. However, its reliance on specific assumptions and sensitivity to factors like rock uniformity and pore structure limit its applicability. To address these limitations, alternative methods have been explored [3, 4], including NMR (nuclear magnetic resonance) technology. Unlike traditional approaches, NMR directly measures the hydrogen nuclear signal in formations and remains unaffected by rock skeleton and lithology [5]. This enables the precise determination of critical reservoir parameters, such as porosity [6], saturation, and permeability [7], offering a robust and accurate method for evaluating oil saturation.  
With the increasing complexity of exploration targets, especially in unconventional reservoirs, traditional NMR technologies struggle to meet the demands for rapid and precise analysis [8]. NMR logging technology is constrained by the wellbore environmentand exhibits limited capability in distinguishing thin interlayers [9]. While laboratory NMR analysis achieves high measurement accuracy [10], it faces challenges such as small sample size, limited reservoir representativeness, lengthy measurement cycles, and inadequate timeliness. To address these, advancements such as mobile full-diameter core NMR scanning technology have been introduced [11], enabling immediate on-site measurements to improve timeliness [12; 13]. However, even with this technology, cores inevitably experience exposure to air during sample handling and pretreatment, resulting in fluid dissipation within the pores. This time delay, often spanning minutes to hours, remains a challenge for accurately capturing the fluid distribution during NMR analysis. The impact of pore fluid dissipation on the results of nuclear magnetic resonance experiments warrants in-depth study. To investigate the specific impact of core dissipation time on NMR measurement results, this article analyzes the dissipation process of pore fluid in sandstone cores through physical simulation experiments. Furthermore, it obtains the dissipation rates of different fluid components through one-dimensional and two-dimensional NMR experimental monitoring of cores at various dissipation stages. The dissipation rate of bounding water is markedly higher than that of crude oil, leading to a significant increase in oil saturation. After the core is exposed to air for a period of time, it is observed that the T\({}_{2}\) values of crude oil components decrease, while the T\({}_{1}\)/T\({}_{2}\) ratio markedly increases. These conclusions can serve as a reference for the accurate calculation of reservoir parameters.