Abstract
The production of a low permeability reservoir decreases rapidly by depletion development, and it needs to supplement formation energy to obtain stable production. Common energy supplement methods include water injection and gas injection. Nitrogen injection is an economic and effective development method for specific reservoir types. In order to study the feasibility and reasonable injection parameters of nitrogen injection development of fractured reservoir, this paper uses long cores to carry out displacement experiment. Firstly, the effects of water injection and nitrogen injection development of a fractured reservoir are compared through experiments to demonstrate the feasibility of nitrogen injection development of the fractured reservoir. Secondly, the effects of gas-water alternate displacement after water drive and gas-water alternate displacement after gas drive are compared through experiments to study the situation of water injection or gas injection development. Finally, the reasonable parameters of nitrogen gas-water alternate injection are optimized by orthogonal experimental design. Results show that nitrogen injection can effectively enhance oil production of the reservoir with natural fractures in early periods, but gas channeling easily occurs in continuous nitrogen flooding. After water flooding, gas-water alternate flooding can effectively reduce the injection pressure and improve the reservoir recovery, but the time of gas-water alternate injection cannot be too late. It is revealed that the factors influencing the nitrogen-water alternative effect are sorted from large to small as follows: cycle injected volume, nitrogen and water slug ratio, and injection rate. The optimal cycle injected volume is around 1 PV, the nitrogen and water slug ratio is between 1 and 2, and the injection rate is between 0.1 and 0.2 mL/min.
Highlights
Water flooding is the most applied traditional enhanced oil recovery technology, and it can replenish formation energy and expand oil sweeping efficiency compared to depletion development [1, 2]
It is shown that miscible-phase displacement will happen by injecting high pressure compressed carbon dioxide which is better in maintaining formation pressure and enhancing oil recovery [7]
Displacement experiments are conducted using carbonatite samples, and it showed that injecting nitrogen and carbon dioxide will obtain different effects, which is determined by gravity drainage and component exchange of gas in fractures and oil in matrix [15]
Summary
Water flooding is the most applied traditional enhanced oil recovery technology, and it can replenish formation energy and expand oil sweeping efficiency compared to depletion development [1, 2]. Displacement experiments are conducted using carbonatite samples, and it showed that injecting nitrogen and carbon dioxide will obtain different effects, which is determined by gravity drainage and component exchange of gas in fractures and oil in matrix [15]. The component simulation technique is applied to verify the feasibility of nitrogen flooding in enhanced oil recovery. Numerical simulation results show that oil recovery can be improved by nitrogen flooding, and it is highly influenced by the time of gas channeling [19]. For the carbonate reservoir with natural fractures using nitrogen flooding to enhance oil recovery, the injecting rate of nitrogen can reduce gravity differentiation of gas and water [20]. Researching results can provide some guidance for developing fractured reservoirs
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