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New viscous and dissolution fingering instabilities in porous media with dead-end pores in miscible displacements
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  • Qingwang Yuan,
  • Zhiwei Ma,
  • Jinjie Wang,
  • Xiang Zhou
Qingwang Yuan
Texas Tech University

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Zhiwei Ma
Stanford University
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Jinjie Wang
China University of Geosciences
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Xiang Zhou
Southwest Petroleum University
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Improving the understanding of mechanisms involved in low miscible displacement efficiency is significant for a wide spectrum of applications in subsurface, from environment such as groundwater remediation and CO2 sequestration to energy extraction such as enhanced oil recovery and geothermal recovery. Two key limiting factors to the efficiency are viscous fingering (VF) instability and dead-end pores in porous media. Previous research on VF simply assumes all pores are well connected and fluids can be mobilized by convection. However, fluids trapped in dead-end pores, such as non-aqueous phase liquids (NAPLs) in groundwater remediation, are inaccessible to convection, resulting in even less efficient displacements. Instead of the classic convection-diffusion/dispersion equation, in this work, we use a fundamentally different capacitance model to incorporate the mass transfer between two pore types in miscible displacements. The hybrid pseudo-spectral and high-order finite difference methods are employed to solve the governing equations in a fixed reference frame for simulating the flow dynamics. A new dissolution fingering (DF) mechanism is identified for the first time in miscible displacements. It is induced by VF and caused by slow dissolution of trapped NAPLs from dead-end pores to their adjacent well-connected pores. It is found the two fingering mechanisms interact and together determine the remaining NAPLs in the full ‘life cycle’ displacements. A simple model is also developed to accurately predict the NAPL concentration behind the finger trailing front which has not been examined previously. Six flow regimes, four of which are new, are then identified.