Improving oil recovery from shale oil reservoirs using cyclic cold nitrogen injection - An experimental study

In this experimental study, effects of injecting temperature and pressure on oil recovery factor (RF) of shale oil reservoir was investigated by implementing cyclic cold nitrogen injection on Eagle Ford core samples. Four outcrop core samples from Eagle Ford were used in this study. Nitrogen was injected at various temperatures (-26 degrees C (-15 degrees F), -18 degrees C (0 degrees F), 0 degrees C (32 degrees F), and 23 degrees C (74 degrees F)) and pressures (6.9 MPa (1000 psi), 10.3 MPa (1500 psi), 13.8 MPa (2000 psi), and 20.7 MPa (3000 psi)) into the saturated core samples. Oil recovery factor for each experiment was calculated during three days of production period. Using average Young's modulus and Poisson's ratio of the samples, applied thermal stress due to the thermal shock was calculated. Furthermore, Computed Tomography (CT) scanner was deployed to scan the core samples prior to and after performing the experiment. The results demonstrated that injecting nitrogen at low temperatures increases oil RF than does injecting nitrogen at ambient temperature. Injecting cold gas resulted in increase in the cumulative oil recovery factor by 10% and the highest recovery factor was observed at the operating pressure and temperature of 20.7 MPa (3000 psi) and -26 degrees C (-15 degrees F), respectively. The results demonstrated that thermal stress applied to the core samples owing to injecting cold nitrogen resulted in creating new cracks and/or extending the existing ones. Additionally, the results demonstrate that injecting cold nitrogen resulted in widening the existing crack in the core samples. Hence, implementing cyclic cold nitrogen injection could potentially improve the efficacy of the current industry practice of cyclic gas injection technique in shale oil reservoirs.