MATRIX-FRACTURE TRANSFER IN NON-ISOTHERMAL CONDITIONS

本研究通过实验和数值模拟方法,对裂缝性多孔介质中的基质 - 裂缝传递现象进行了深入探究。在实验方面,于四个不同的人工裂缝岩心柱塞内安装薄膜型热通量传感器,在冷水注入过程中对裂缝内的温度和热通量进行测量,并利用实验所得的热通量值计算对流传热系数,同时借助裂缝温度校准由 CMG - STARS 模拟器构建的数值模型,以此评估基质的热特性。研究结果显示,当岩石基质的热特性较高时,裂缝内的温度降幅相对较小;由于基质 - 裂缝界面处的热通量和温差会随时间发生变化,因此在精确分析基质 - 裂缝传热时,需采用可变的对流传热系数。此外,在以 1 cc/min 的流速注入罗丹明 B 溶液且外部温度为 70°C 的示踪剂实验中,运用上述数值模型确定了弥散系数。数值模型的敏感性分析表明,基质的热特性在基质 - 裂缝质量传递过程中起到重要作用,其影响效果与注入速率类似。例如,在基质热特性较大的岩心柱塞中,溶质穿透程度更高,这是因为在基质 - 裂缝界面上会产生更大的温度梯度,此现象可通过非等温条件下的热质耦合传递效应(即索雷特效应)加以解释。
CMG 软件应用情况
本研究使用 CMG 公司的 STARS 模拟器进行单裂缝 - 基质系统的非等温过程热传输和示踪剂溶液传输的隐式有限差分数值模拟。在模拟过程中,考虑到岩心柱塞的对称形状,选取其四分之一进行模拟,其数值模型在 x、y 和 z 方向分别设置 15×120×26 个网格块,且网格尺寸在空间和方向上呈可变状态,在基质 - 裂缝界面附近设置更精细的网格,以实现对基质 - 裂缝传递的精确分析。同时,利用该软件对溶质运移模型进行历史拟合,将裂缝孔隙度、裂缝渗透率、基质体积热容和基质热导率等参数作为变量进行迭代修改,直至模拟结果与实验测量的示踪剂出口浓度相匹配,从而确定模型参数,用于预测不同场景下基质 - 裂缝溶质运移的流动和 / 或传输行为。
作者:亚沙尔・塔瓦科利・奥斯古埃(Yashar Tavakkoli Osgouei) 中东技术大学自然与应用科学研究生院

Abstract

A numerical and experimental study was carried out to investigate matrix-fracture transfer in fractured porous media. Film type heat flux sensors were installed in four different synthetically fractured core plugs to measure the temperature and heat flux in fracture during cold water injection. Experimental values of heat flux were used to calculate convective heat transfer coefficient. Fracture temperatures were used to calibrate numerical model developed using CMG-STARS simulator to evaluate contributing matrix thermal properties. The results show that the temperature decrease in fracture is lower when rock matrix has higher thermal properties. The variations in heat flux and temperature difference along matrix-fracture interface with respect to time necessitates the use of variable convective heat transfer coefficients for accurate analysis of matrix-fracture heat transfer.

Moreover, results of tracer experiments where Rhodamine B solution was injected at a flow rate of 1 cc/min and outer temperature of 70 °C was used to determine
dispersion coefficients using aforementioned numerical model. Sensitivity analysis of the numerical model indicated that thermal properties of matrix are effective in matrix-fracture mass transfer similar to injection rate. To illustrate, the solute penetration is higher in core plugs with larger matrix thermal properties that provide larger temperature gradient over matrix-fracture interface. This can be explained by the Soret effect that is kind of coupled heat and mass transfer at non-isothermal conditions.

Keywords: Fractured porous media, Matrix-fracture transfers, Convective heat transfer coefficient, Numerical simulation of tracer testing, Coupled heat and mass transfers

 

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