Recovery Mechanisms for Nano-Confined Oil in Source Rocks using Lean Gas Injection
- Seunghwan Baek (Texas A&M University, College Station) | I. Yucel Akkutlu (Texas A&M University, College Station)
- Document ID
- Society of Petroleum Engineers
- SPE Western Regional Meeting, 23-26 April, San Jose, California, USA
- Publication Date
- Document Type
- Conference Paper
- 2019. Society of Petroleum Engineers
- Ethane injection, Shale oil, Enhanced oil recovery, Lean gas injection, Molecular simulation
- 1 in the last 30 days
- 135 since 2007
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Organic matter in source rocks stores oil in significantly larger volume than that based on its pore volume due to so-called nanopore confinement effects. However, during production and depletion, recovery of that oil is low. In this paper, we introduce the nano-confinement effects and explain their impact on the release of oil molecules. We propose to control these effects and increase the oil recovery using lean gas injection, such as ethane or carbon-dioxide. We identify and discuss microscopic-level oil recovery mechanisms that appear during the soaking and production stages.
Due to the nature of the problem, molecular Monte Carlo simulation method is used for the investigation. A multi-component hydrocarbon mixture is considered in model organic pores under reservoir conditions. The fluids stored in pores are in thermodynamic equilibrium with a nearby bulk fluid in a fracture. The fluid composition in pores varies with the size of the pore and becomes progressively heavier during the production as the bulk fluid pressure is reduced. The lean gas molecules are introduced to the nanopores by adjusting the bulk fluid composition and pressure to the desired values. Simulations are used to predict fate of in-situ and the injected molecules when the system is reached to equilibrium.
Results show that oil in smaller nanopores is richer in heavy components compared to the bulk oil outside in the micro-crack. Compared to gas reservoirs, the impacts of the nano-confinement on in-place fluid volume is not significant. Recovery of the confined oil is typically below 15 % indicating that pressure depletion and fluid expansion is no longer an effective recovery mechanism. Ethane injection shows higher recovery performance than CO2 injection; it improves recovery up to 90 %, depending on its composition in the fracture. Ethane recovers 5-20 % higher than carbon dioxide in both large pores and nanopores, because ethane molecules are more effective in vaporizing the heavier molecules in the pore. In addition, ethane reduces viscosity of the confined oil, and its diffusion is faster than CO2. In summary, lean gas injection is effective in recovering the oil but its delivery to the matrix using fractures and micro-cracks under closure stress makes injection operations challenging in the field.
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