Abstract
Because of the ultra-low permeability of tight formations, gas injection is believed to be an effective method to enhance oil recovery (EOR). However, it has two modes: huff-n-puff injection (HNP) and continuous flooding. Which mode is more efficient? This study aims to answer this question through an experimental and simulation study. In the experimental study, tight core samples of permeabilities of 0.16 mD, 0.045 mD, 0.001 mD, and 0.00029 mD were used to compare the oil recovery from huff-n-puff gas injection with flooding. The gas used was nitrogen. Both flooding and huff-n-puff experiments were conducted on the same core in each core sample. Experimental results show that gas flooding had higher recovery in the cores than HNP with the permeability above 0.001 mD, but lower when the permeability was lower 0.00029 mD. To further analyze the gas injection performance, a numerical simulation was performed. The conducted experiments were used to validate the numerical models by historically matching the experimental data. Based on the validated simulation models, HNP is optimized. Simulation results show that optimized HNP is superior to gas flooding when the permeability is lower than 0.001 mD core. Finally, based on published data, a filed-scale model consisting of two well-propped fractures was established to compare the performance of the two gas injection modes. Filed-scale simulation results show that gas flooding can achieve a higher recovery when the matrix permeability is above 0.01 mD, while HNP injection can yield more oil when the matrix permeability is lower than 0.001 mD. However, from an economic point of view, the HNP performance is more justified than the corresponding flooding when the permeability is higher than 0.01 mD, and it is difficult to be economic for both HNP and flooding when the permeability is lower than 0.001 mD.
NSTL