天然气扩散系数研究现状及存在问题

Current situation and existing problems in research of natural gas diffusion coefficient

天然气扩散系数表征天然气在地层中的扩散能力,是油气藏盖层封堵性能、油气保存条件与油气资源潜力评价的重要参数,确定其符合地层条件的数值并提高精度,对油气勘探具有重要理论意义和实用价值。目前获取方法主要是实验模拟和数值模拟2种。前人研究表明,温压条件、岩石物性条件和孔隙流体性质是影响天然气扩散系数的主要因素。在实验室扩散系数测定过程中,温压条件的设置普遍低于地层实际情况,多会在饱和介质与温度条件方面进行系数校正,这影响了逼近实际地层条件的真实性。在数值模拟中,一些参数的设定仍然依赖于实验所得,不同模式的数学扩散模拟在实际中难以应用。通过文献资料分析认为,梳理实验室测定的扩散系数计算方法,总结现阶段天然气扩散系数研究的主要校正措施及天然气扩散的数值模拟现状,对存在的问题进行系统分析并强化天然气扩散系数测定装置与系统性能,可以有效提高天然气扩散系数测定的准确性;加强目标地层孔隙度与扩散模式研究并基于扩散模式采用相应的扩散系数计算方法,可以提高数值模拟结果精度。实验模拟与分子动力学融合将是今后实现天然气扩散系数准确获取的主要研究方向。

Gas diffusion coefficient can characterize the diffusion ability of the natural gas through formations,and can be applied as a parameter in evaluating the reservoir cap-rock sealing,the preservation conditions of oil and gas,as well as the oil and gas resource potentials.When the value accordant to the certain formation is determined and its precision is improved,the gas diffusion coefficient will have theoretical significance and practical value in oil and gas exploration.Currently,the gas diffusion coefficient is obtained mainly from two ways including the experimental simulation and the numerical simulation.Previous studies have shown that temperature,pressure,rock physical property and pore fluid property are the main factors affecting gas diffusion coefficient.During the experimental simulation of gas diffusion coefficient,the experimental temperature and pressure values are commonly lower than those in an actual formation,even though the measurement values are generally corrected in terms of the actual water-saturated situation and the formation temperature,the effectiveness of the gas diffusion coefficient in reflecting the real conditions of the actual formation is still affected.In case of the numerical simulation of gas diffusion coefficient,some parameters still rely on the values determined by experiment,leading to difficulties in applying the various mathematical diffusion simulation models in practice.By carding the diffusion coefficient calculation methods used in the experimental simulation,summarizing the current correction measures used in gas diffusion coefficient research,and the advances in the numerical simulation of gas diffusion,based on the public literatures,the authors analyze systematically the main problems existing in the current research of gas diffusion coefficient,and propose that improving the measurement device and measurement system performance can effectively improve the accuracy of gas diffusion coefficient determination;and strengthening the research of the porosity and diffusion model of the target formation and applying the gas coefficient calculation method in accordant to the diffusion model,can improve the accuracy of calculation value obtained from the numerical simulation.The authors also point out that integration of experiment simulation and molecular dynamics simulation will be the main research direction to accurately acquire the gas diffusion coefficient in the future.