Quantitative Evaluation on Natural Gas Huff-n-Puff in Fracture-Matrix Tight Cores Based Experimental Method

The yield of tight reservoirs declines rapidly in depletion development, gas injection after volume fracturing is an effective means to develop tight reservoirs. As the associated gas in oilfields, natural gas is a good injection gas with abundant resources and non-corrosive. In addition, in the gas injection process, gas channeling after volume fracturing is also a difficult problem that seriously affects the development effect, but huff-n-puff can effectively alleviate it. For these reasons, tight cores and tight oil were selected from the actual tight reservoir, and fracture-matrix tight cores were designed and made. Then, natural gas huff-n-puff experiments with different injection pressure and injection gas volume were conducted. Every experiment included three cycles, and the development effects and characteristics of different cycles were analyzed. The gas to oil replacement ratios (GORR) under different conditions were compared to evaluate the injection schemes. The development effects of different gas injection schemes were analyzed using NMR technology. The results demonstrated that natural gas could effectively develop tight oil by huff-n-puff. High injection pressure contributes to improving the oil recovery, and oil is mainly recovered in the first two cycles. At high injection pressure, the difference in oil recovery between two tight cores with different permeability is obvious. The higher permeability core has a higher oil recovery. Under the promise that the total gas injection is the same, the more gas injection in cycle 1, the greater the final oil recovery. Increasing the injection volume in a single cycle can improve the oil recovery in this cycle. The higher pressure and higher gas injection of the first cycle can cause a higher GORR. The research is conducive to the application of natural gas in tight oilfields.