Enhanced oil recovery (EOR) by solvent injection offers significant potential to increase recovery from shale oil reservoirs, which are typically between 3 and 7% OOIP. The rather sparse literature on this topic typically models these tight reservoirs based on conventional reservoir processes and mechanisms, such as by convective transport using Darcy's law, even though there is little physical justification for this treatment. The literature also downplays the importance of the soaking period in huff'n'puff

In this paper we propose for the first time a more physically-realistic recovery mechanism based solely on diffusion-dominated transport. We develop a diffusion-dominated proxy model assuming first-contact miscibility (FCM) to provide rapid estimates of oil recovery for both primary production and the solvent huff'n'soak'n'puff (HSP) process in ultra-tight oil reservoirs. Simplified proxy models are developed to represent the major features of the fracture network.

The key results show that diffusion-transport only can reproduce the primary production period within the Eagle Ford shale and model the HSP process well, without the need to use Darcy's law. The mechanism for recovery is based solely on density and concentration gradients. Primary production is a self-diffusion process, while the HSP process is based on counter-diffusion. Incremental recoveries by HSP are several times greater than primary production recoveries, showing significant promise in increasing oil recoveries. We calculate ultimate recoveries for both primary production and for the HSP process, and show that methane injection is preferred over carbon dioxide injection. We also show that the proxy model, to be accurate, must match the total matrix contact area and the ratio of effective to total contact area with time. These two parameters should be maximized for best recovery.

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