Cyclic Miscible Injection in Unconventional Reservoirs: Upscaling, Soaking, Containment, and Asphaltene Deposition

Abstract

Despite numerous cyclic miscible injection studies and field pilot tests being carried out in unconventional shale reservoirs over the last 15 years, governing mechanisms are yet to be fully understood, as there is a discrepancy between experimental results and field observations. Generally, the pilot field tests demonstrate much lower recoveries than what was originally predicted by laboratory and simulation studies. Hence, this research aims to develop a systematic approach to upscale the lab results across spatial scales, improve injectant performance, and optimize the process to achieve better recoveries. Disparate topics impacting the performance of cyclic miscible injection, including upscaling, soaking, containment, and asphaltene deposition, are addressed.

A new mathematical model is developed using classical equations to mimic the cyclic huff-and-puff miscible process. Traditionally, constant pressure or constant flow rate boundary conditions are used to describe flow between the fracture network and matrix during the injection and soaking phases, which is not realistic. The complex fracture network in this study is characterized as an ensemble of rock pillars separated by fracture discontinuities to represent field conditions better. Two scaling factors are developed to upscale the laboratory results to the field scale, and an upscaling equation is derived to better predict the recovery factors across spatial scales.

Next, the significance of the soaking phase in the cyclic process is evaluated within the complex fracture network and the mixing of the various injectants with the remaining oil. By better understanding various injectants penetration depths and mixing/swelling recovery mechanisms, recommendations are made to tailor the huff-and-puff process to improve oil recovery.

Finally, a novel liquid solvent is presented that shows potential transformative benefits compared to typical gas injectants. Compositional modeling and simulation of a volatile oil pilot in the Eagle Ford show promising results with the liquid solvent due to a more efficient recovery mechanism and improved accessibility to the remaining oil, resulting in a much better recovery improvement than gas injectants. The advantages of the liquid solvent application are complemented by significantly lower implementation costs compared to CO2 and typical hydrocarbon solvents.

A better understanding of the cyclic miscible injection processes and further optimization can extend the life of unconventional shale wells by unlocking a significant amount of hydrocarbon resources currently left behind undrained.