| Proppant has played a decisive role in hydraulic fracturing technology which is one of the most widely used technology for oil and gas exploitation. It is demonstrated that the efficiency of hydraulic fracturing and the life time of the oil and gas wells are depended on the quality of proppant.In the experiments, the X-ray diffraction, scanning electron microscope and thermogravimetric analysis were used to analyze the thermal decomposition and high temperature sintering behavior of coal gangue, the sintering reaction process of coal gangue, bauxite respectively with the magnesite.The influence of the content of coal gangue and sintering temperature on the properties of sintered samples has been investigated. On the basis of the above results, a primary recipe was selected. Then, waste coal gangue, bauxite and magnesite, were used as main raw materials and mineralizer respectively to prepare proppant.Ceramsite proppants with low density and high strength have been fabricated with coal gangue and using the traditional method which includs steps of mixing, powder preparation and sintering. The main conclusions are as follows:Coal gangue thermal decomposition was mainly related to the thermal decomposition of kaolinite. Pure coal gangue needed a high sintering temperature upto 1450 oC. Moreover, in the sample sintered at 1450 oC, cristobalite crystal formed, causing the appearant of cracks which would do harm to mechanical strength. The addition of magnesite could reduce the sintering temperature of the coal gangue. The reaction process involved the reaction between magnesium oxide from the decomposition of magnesite with quartz and the mullite generated from the coal gangue, forming the cordietite phase. Carbon dioxide, which was from the the decomposition of magnesite, resulted in pores in the sintered samples. Furthermore, magnesium aluminum spinel was produced by the reaction of Al2O3 and MgO which were from bauxite and the decomposition of magnesite respectively. Then cordierite was also formed via the reaction between magnesium aluminate spinel and silicon dioxide.In the system of coal gangue-bauxite-magnesite,the sintered specimens would get lower apparent porosity and higher strength when the content of coal gangue declined from 85.46wt% to 55.46wt%. However, more liquid phases resulted in high acid solubility of the samples. The apparent porosity and strength of the sintered samples became lower and higher respectively. when the sintering temperature increased from 1300 °C to 1450 °C. The specimens with 55.46wt% coal gangue, 31.80wt% bauxite and 12.74wt% magnesite sintered at 1450 °C for 2h had a compact structure with the main phases of mullite and magnesium aluminate spinel. In addition, rod-like mullite grains were intertwined. The sintered samples had high bending strength and corrosion resistance. However, the ceramsite proppants based on this recipe showed a high breakage ratio due to the large porosity.Further, the coal gangue and magnesite were reduced to 45.46wt% and 3.20wt% respectively, bauxite was increased to 51.34wt%. The green bodies were sintered at 1450 oC for 2 h. According to testing standard issued by the China Petroleum and Chemical Group co.Ltd. The prepared ceramsite proppants had the main phase of mullite, bulk density of 1.49 g/cm3, apparent density of 2.75 g/cm3, breakage ratio under 69 MPa of 9.5%, acid solubility of 5.24%.The roundness, sphericity and sieving results could also meet the requirements of the standard.In summary, it is feasible to fabricate ceramsite proppants by using coal gangue. |
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