Experimental study on feasibility and mechanisms of N2/CO2 huff-n-puff in the fractured-cavity reservoir

Abstract

The purpose of this experimental study is to evaluate the efficiency and performance of gas huff-n-puff in the single well on recovery of light oil from a multi-well fractured-cavity reservoir and to reveal the feasibility of gas mediums huff-n-puff. In order to reveal the production performance characteristics and mechanisms of gas huff-n-puff on activating the remaining oil, a visualized and pressure resistant two-dimensional (2D) fractured-cavity model and a three-dimensional (3D) multi-wells model were designed and fabricated respectively based on the similarity theory. With the simulated oil and brine reservoir samples in Tahe oilfield, five groups of tests in 3D model and five groups in 2D model were performed, each of which included bottom water energy depletion driving, gas injection stage, soaking stage, and gas puff stage. The production performances of N2, CO2, and N2/CO2 mixture huff-n-puff in the 3D model were firstly analyzed. Then the remaining oil recovery principles of each gas were compared and analyzed in the 2D model. Finally, the influence of gas injection position was studied both in the 2D and 3D models. The experiment results from 3D model demonstrated that higher oil recovery factor could be achieved through N2 huff-n-puff which had shown enormous advantages than CO2 and N2/CO2 mixture. Furthermore, the results from 2D model indicated that there were four types of the remaining oil, all of which could be recovered through N2 huff-n-puff if it was implemented in the relative low-position well. The results from experiments on gas injection position conducted in the 3D physical model demonstrated the better oil recovery effect if N2 was injected in low-position well which was consistent with the results concluded in the 2D model. This paper confirmed the effectiveness and advantages of N2 huff-n-puff compared with CO2 and gas mixture in the light oil-saturated fractured-cavity reservoir.