| There is abundant heavy oil resource in China, with reserves of 15×108 ton, which is mainly distributed in Liaohe, Xinjiang, Shengli, Henan and other oilfields. Heavy oil occupies very important position in the world oil resources. But due to high crude oil viscosity, the waterflood development effects of heavy oil is poor and the recovery efficiency is at least 10% lower than conventional oil. Gas is easy to flow, and it has the functions of reducing viscosity, increasing energy, extracting and reducing interfacial tension. Besides, gas drive is more economical than thennal recovery, which was widely used in heavy oil development.Natural depletion is used in YD heavy oil reservoir now. But due to insufficient formation energy, many well could not be filled with enough liquid, resulting in a very low recovery percent of reserves. According to this situation, it is decided to carry out CO2/N2 huff and puff in YD heavy oil reservoir for enhancing oil recovery. In this paper, according to the full investigation of gas huff and puff development technology over the world, the interaction mechanism between crude oil and CO2/N2 was studied by high pressure physical properties experiment, and the oil incremental effect of CO2/N2 huff and puff was studied by three-dimensional physical simulation. Furthermore, mechanism model was established to analyze the sensitivity of reservoir parameter to CO2/N2 huff and puff. In addition, based on the fluid phase behavior modeling and history matching, CO2/N2 huff and puff injection parameters of target zone are optimized by numerical simulation. Finally, development efficiency of CO2/N2 huff and puff were predicted and compared. The main achievement and the understanding of this paper can be divided into the following several aspects:(1) CO2 solubility in heavy oil is much higher than N2, and oil viscosity reduction effect and expansion effect on oil is stronger than N2. At formation temperature and pressure conditions, dissolved N2 gas-oil ratio is 9.23 m3/m3 while CO2 is 87.85m3/m3, and N2 injection can increase oil volume factor by 3.7%, reducing oil viscosity by 34.84%, showing no extraction effect while CO2 can increase oil volume factor by 18.5%, reducing oil viscosity by 84.33%, showing very weak extraction effect; (2) At physical simulation experiment, when the pressure drop to 7MPa, N2 huff and puff could enhanced oil recovery by 1.78%while CO2 by 3.23%; (3)The parameter, which have most influence on CO2/N2 huff and puff effect are permeability, formation thickness, oil viscosity, followed by porosity and reservoir heterogeneity. Compared with the CO2, N2 huff and puff effect is more sensitive to oil saturation. (4) Through numerical simulation CO2/N2 injection parameters optimization results of target zone were obtained. For N2, optimal injection volume was 360000m3, injection rate was 15000m3/d, soak time was 2-4 days, and 2-3 times cycle was better. For CO2, optimal injection volume was 216000m3, injection rate was 20000m3/d, soak time was 10 days, and CO2 cycle could be higher than N2. At the optimal injection parameter, CO2 to oil exchange ratio is much greater than N2; (5) For YD oil reservoir, the effect on reducing oil viscosity plays a leading role in enhancing oil production for in CO2 huff and puff, while supplying formation energy and maintaining reservoir pressure is generally dominant in N2 huff and puff.The research results can provide some theoretical basis and technical guidance for the application of CO2/N2 huff and puff in YD heavy oil reservoir. Besides, it can also provide guidance for the development of similar oil reservoirs in a certain extent. |
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