An experimental investigation of improving Wolfcamp Shale-Oil recovery using Liquid-N2-assisted N2 and/or CO2 Huff-n-Puff injection technique

Abstract

Despite drilling numerous horizontal-hydraulically-fractured wells in unconventional reservoirs, the oil recovery factor (RF) of such reservoirs is still very low (<10%), leaving behind massive unrecovered oil in the formation. Improving the oil RF of unconventional reservoirs requires applying and developing enhanced oil recovery techniques. In this study, the feasibility of improving Wolfcamp shale oil RF using liquid nitrogen (LN2) combined with three cycles of N2 and/or CO2 huff-n-puff injections at reservoir temperature (77 °C) and different injection pressures was investigated. The results revealed that LN2-assisted N2 and/or CO2 huff-n-puff injections improved the Wolfcamp oil RF, outperforming the current-experimental practice of N2, CO2, or natural-gas huff-n-puff injections. Higher oil RFs with less soaking time and fewer cycles were achieved in this study. Combining LN2 with cyclic N2 and/or CO2 injections can result in flow-conductivity enhancement, induced-cracks growth, and oil–gas interaction development throughout the injection to production stages, which enhance the stimulated volumes leading to oil RF improvement. Compared to LN2-assisted N2 huff-n-puff, better performance of injecting CO2 after LN2 was observed on samples of low permeability, particularly when utilizing injection pressure near CO2-MMP, 20.68 MPa, resulted in RF of 53%, 26% higher than injecting N2 at the end of cycle#3. Injecting LN2 with N2 into samples of high permeability (greater than 0.1 mD) has the potential to recover oil up to 57% due to the presence of the moveable oil that can flow easily from deeper areas to surface, unlike the samples of low permeability (<0.1 mD), where oil RF at the end of cycle#3 was 27%. LN2-assisted N2-CO2 huff-n-puff increases the incremental oil RF on low permeable samples, resolving the issue related to the fact that the incremental RF would start declining sharply in the second cycle and as the number of cycles increases when using only N2 injection. From the results observed in this study, a combination of cryogenic treatment and huff-n-puff techniques would be potentially implemented in shale oil reservoirs, opening the opportunity to improve shale oil RF by enhancing the flow-conductivity and increasing the hydrocarbon mobility.