Experimental Research On Oil Shale In-Situ Conversion By Acid Fracturing-Heat Injection

With the rapid development of economy in twenty-first century, energy will undoubtedly become the foundation of national economic development, and it is also an important guarantee of social stability. Petroleum, natural gas and other traditional energy as non renewable resources, plays an important role in the energy structure in the early time, But in China, the traditional resources are scarce, and the energy structure of our country is not reasonable, which leads to the increase of the dependence on foreign crude oil year by year. The development of unconventional oil and gas resources attract more and more attention in China and even in the world. Oil shale is a kind of sedimentary rock containing organic matter, it has a broad prospect for development and utilization depending on its huge reserves. The reserves of oil shale resources in China ranks the second in the world, and 47.644 billion tons of shale oil can be extracted. Compared with the traditional way of high pollution ground retorting, the in-situ conversion technology will become the main utilization mode of oil shale to the lower cost and lower pollution. But the in-situ conversion technology is complex and still in the experimental stage. This paper proposed acid fracturing-high temperature nitrogen injection technology for oil shale in-situ conversion. Lots of laboratory test and computer simulation were done in order to achieve higher energy utilization ratio in the conversion process and it has very important practical significance.Crosslinked acid fracturing fluid was prepared according with oil shale, the rheological properties, temperature resistance, shear resistance, sand carrying performance and corrosion inhibition performance were tested. The properties of the crosslinked acid fracturing fluid could meet the need of practical engineering. Than a variety of experiments were done with this kind of fracturing fluid and oil shale sample, and analyze the difference between the acidified oil shale sample and original sample. In the aspect of the composition and structure of oil shale, XRD and FTIR test showed carbonate minerals were corroded and the organic matter remained unaffected in the acidified oil shale sample. Scanning electron microscope and mercury injection experiment data showed that significant changes occurred in the pore structure of oil shale samples after acidification. Its porosity increased significantly. In the aspect of pyrolysis behavior, the weight loss rate of the samples after acidification was significantly increased, and Fischer test indicated that oil shale samples after acidification could get higher oil and gas yield and more content of low molecular hydrocarbon. The activation energy of pyrolysis was calculated, the pyrolysis activation energy decreased compared with the original sample, which means the pyrolysis reaction is easier to carry out.The above experiments showed that in the oil shale in-situ conversion process, acid fracturing to the oil shale layer before the heat injection is meaningful. Acid fracturing could not only form macro fractures to provide a channel for the heat carrier but also could change the microstructure of the oil shale, thus, it improves the oil yield and energy utilization in the later pyrolysis process.In the fracturing-nitrogen injection technology for oil shale in-situ conversion, high temperature nitrogen should be injected to heat the oil shale layer after fracturing. In this paper, the process was simulated by a self-made pyrolysis test device and the composition and experimental principle of the pyrolysis unit were introduced in detail. Set the experimental conditions for the total nitrogen injection volume is 12 m3,the injection temperature is about 550℃,several group experiments were carried out with different nitrogen injection flow rate, and the results showed that: With the increase of gas flow rate, the oil and gas yield increased firstly and then decreased. The largest amount of shale oil products were collected when the nitrogen injection flow rate was 5m3/h. The mass of shale oil was 202 g and the oil production rate was 8.63%. The energy utilization rate reached to the highest value at this nitrogen injection flow rate. After the experiments, the collected shale oil was analyzed and tested, its composition and properties were similar to that of crude oil.Finally, combined with the practical engineering background of Nong’an oil shale in-situ conversion project organized by Jilin University, computer simulation was done to the process and optimized the parameters of the process. The technology and principle were introduced, established the mathematical and physical model according to the actual oil shale formation. Using FLUENT software to simulate the flow of nitrogen and the heat transfer process to the oil shale. The temperature distribution nephograms of oil shale when heated a certain time with different nitrogen injection flow rate were got and also got the accurate time when the average temperature of oil shale layer reach to 400℃.The simulation results showed the same trend with the laboratory pyrolysis experiment mentioned above, with the increasing of the nitrogen flow rate, the time to heat the oil shale to 400℃ decreased initially and then increased. The reason and mechanism were analyzed and discussed. In the end, we concluded that when the nitrogen flow rate was 30m3/h, the total injection energy reached to the minimum value and got the highest energy utilization ratio. The energy utilization ratio was 61.1% and the heat injection time was 125 days.In this paper, the acid fracturing-high temperature nitrogen injection technology for oil shale in-situ conversion was put forward. Starting from the improvement of oil shale layer structure, which laid the foundation for higher oil production rate in the later heating process. Through the experiment and numerical simulation of the actual engineering, the qualitative relation between the nitrogen injection flow rate and the heating energy utilization was obtained and optimized the flow rate. Thus reduced the energy consumption of the in-situ conversion process and get the higher energy utilization ration on the other hand.