| With the continuous development of the oil and gas industry,the development and utilization of oil and gas wells in China is gradually changing from shallow wells to deep wells and even ultra-deep wells.Hydraulic fracturing technology is an effective means to achieve this goal.This puts higher requirements on the quality of the proppants used in the hydraulic fracturing process.At present,ceramic proppants are mainly prepared from high-grade bauxite,which is expensive and has a high sintering temperature;while the low-grade bauxite has abundant reserves and low price,but the strength of the proppants prepared are low.Therefore,the preparation of low-density and high-strength ceramic proppants at lower temperatures can greatly reduce production costs.Combined with the production requirements of Yangquan Changqing Petroleum Fracturing Proppant Co.,Ltd.,low-grade bauxite and feldspar with a certain particle size(0.980～43.96μm)were used as raw materials to prepare corundum-mullite based low density and high strength ceramic proppants,and investigated the effects of sintering temperature,holding time and heating rate on the properties of ceramic proppants.In addition,the ceramic proppants semi-finished products prepared by experiments were respectively carried out 1～8weeks stale experiments,and compared with the samples without conducting the stale experiment,discussed the influence of stale on the properties of ceramic proppants.Finally,low-density and high-strength ceramic proppants were prepared from coal gangue and calcined flint clay from Yangquan,Shanxi Province,which provided an effective way for the comprehensive utilization of solid waste coal gangue,and explored the effect of sintering temperature on the properties of ceramic proppants.The main experimental conclusions are as follows:(1)The sample was prepared by mixing low-grade bauxite(85wt.%)and feldspar(15wt.%),and the corundum-mullite based ceramic proppants were sintered at 1200℃,1220℃,1240℃,1260℃ and 1280℃.The sintering temperature range was determined to be 1200～1260℃.The proppant samples has the best performance when the sintering temperature is 1260℃.The bulk density is 1.53 g/cm~3,the apparent density is 2.79 g/cm~3,and the breakage ratio at 52MPa is 5.81%,which meet the requirements of the oil and gas industry standards.(2)By changing the sintering process parameters,the corundum-mullite based ceramic proppants were prepared at sintering temperatures of 1200℃,1220℃,1240℃ and 1260℃.When the sintering temperature was 1260℃,the holding time was 2.5 h,and the heating rate was 7℃/min,the proppant samples breakage ratio was at least 4.86%.(3)In the stale experiments on the ceramic proppant semi-finished products,the breakage ratio of the proppant samples first decreased and then increased with the extension of the stale time.The bulk density and apparent density both showed an increase first and then basically remained trend.The proppant samples perform best at two weeks of stale.(4)With the increase of sintering temperature,the corundum phase,mullite phase and kyanite phase are formed in the matrix.The mullite is mainly in the form of needles,interspersed in the corundum matrix to form a dense network structure with good mechanical properties.(5)The ceramic proppants prepared by using 40 wt.%of coal gangue and 60wt.%of calcined flint clay as raw material,the main crystal phase is mullite phase and cristobalite phase.As the sintering temperature increases,the mullite grains gradually change from a needle-like morphology to a short columnar equiaxed crystal.The granular cristobalite is filled in the skeleton structure formed by the short columnar mullite,and the pores in the proppant matrix are reduced.The structure tends to be densified.At 1400℃,the sintering reaction has been completed.The bulk density of the proppant sample after firing is 1.43 g/cm ~3,the apparent density is 2.73 g/cm ~3,and the breakage ratio at 35 MPa and 52 MP closed pressure are 3.77%and 8.87%respectively. |
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