The ultimate recovery from shale formations is relatively low compared to the total in-place reserves. The important role that these reservoirs have in the future of the oil and gas industry generates a significant need for cost-effective and environmentally-friendly enhanced oil recovery techniques. This study evaluates the application of enzyme EOR in shale reservoirs. Laboratory results of tests conducted on multiple cores taken from Woodford shale outcrops demonstrate an average of 20% improvement in the recovery factors.
Enzymes change the wettability of formation rocks and fluid systems by changing the interfacial tension which results in releasing hydrocarbons from the rock. In this study, enzyme solutions with concentrations of 10 and 5 wt.% are used on 6 different cores. Recovery factors of the spontaneous imbibition tests at this concentration were 11 and 22%, respectively, after a soak time of 200 hours. This compares to 2 and 12% recovery factors, respectively, in the control tests. The experiment is performed under static condition, addressing the change in the recovery of crude oil from each sample using spontaneous imbibition tests.
A suitable concentration of enzyme is identified to be 5 wt.%. Some of the shale samples used in this study were clay-rich and some were carbonate-rich. There was no significant difference in the EOR performance between the two categories of rocks.
Oil recovery in shale formations could be improved up to around 10% using cyclic gas injection. However, this study proves that adding biological enzymes has the potential to improve this recovery factor up to 25%. Moreover, the effect of enzymes is everlasting in the formation because the enzyme dissolved in formation water continues to invade pores and fractures in lower concentrations.
All oil recovery factors from shale and tight formations, in which the permeability is less than 0.1 mD and pore sizes with less than 100 nm in diameter are abundant, are reported to be less than 10% (Sheng, 2017; Salahshoor et al., 2018). The average oil recovery factor of Woodford shale is reported as 8.4% by Energy Information Administration (Advanced Resources International, 2013). Therefore, EOR techniques are massively explored to help with improving the recovery from these formations. However, the complex fluid flow and phase behavior of these formations make them more challenging in many aspects including finding the best EOR practices (Salahshoor and Fahes, 2018; Salahshoor and Fahs, 2018). Common EOR practices in shale and tight formations, including gas injection, water injection, and surfactant injection, are studied in numerous industry and academic research publications.