| Hydraulic fracturing is an effective technique applied to the petroleum and gas industry. With the development of unconventional reservoir, hydraulic fracturing has become one of the important technologies related to the energy development of our country. Fracturing proppants are small spherical particles used to improve the reservoir permeability, which prop fractures and then improve the oil and gas fracture permeability.Low-weight mullite-based fracturing proppants were prepared from Xiaoyi lightly calcined bauxite and Inner Mongolia kaolin clay with TiO2 and MgO as sintering aids by the granulation process of intensive mixer and pressureless sintering. In order to meet the requirement of hydraulic fracturing of low-permeability oil and gas reservoirs especially coalbed methane and shale gas, kaolin-based low weight fracturing proppant was prepared via granulating and sintering from kaolin clay and its calcinate with AlF3 and（NH4）6Mo7O24·4H2O as the sintering aids. The influence of granulating process, additives and sintering temperature on the properties, phase and microstructure structure of proppant was investigated systematically. The conclusions are divided into the following several parts.（1） The granulation of sugar coating machine need a long period and water spray is used in the forming process. Green ceramic proppant shaped by this method is layered greatly and shows a low strength. By comparison, green ceramic proppant shaping in intensive mixer has a shorter granulating period and a more homogeneous structure with high efficiency and more simple granulating process. This paper prepared satisfactory green ceramic proppants used the second method and explored a new granulation technology to prepare an ultralow weight proppant shaping in oil.（2） The effect of additives on mullite ceramics was studied and drew some conclusion. Sample with the lowest density was added TiO2 and MgO addition made the highest apparent density. With Mo O3 addition, sample shows the lowest bulk density, porosity and flexure strength. It indicates that mullite crystal occurs in ceramics material with MoO3 at 950 oC by XRD analysis, but mullite diffraction peaks has not yet appeared with TiO2 and Mg O.（3） The addition of MgO to proppant can decrease the apparent density, but increase the bulk density and crush resistance. The results indicate that MgO promotes the formation of mullite, especially proppant sintering at 1450 oC with 3 wt% MgO which shows a significantly increasing number of rod-like mullite crystals on the fracture surface and only a little high temperature liquid phase filling in the pores of mullite grains. This proppant sample exhibits a bulk density of 1.42 g/cm3, an apparent density of 2.63 g/cm3 and a breakage ratio of 4.06% under 28 MPa pressure.（4） The addition of（NH4）6Mo7O24·4H2O to proppant can decrease the bulk density, and increase the apparent density and the crush resistance. With this addition, the rod-like mullite crystals formed a porous interlocked structure, which provided the proppant with strengthen mechanism of fiber and whisker reinforcement. The introduction of（NH4）6Mo7O24·4H2O and AlF3 is not only more conducive to the strength of proppant, but also decreases the bulk density and apparent density.（5） The proppant sample shows a bulk density of 1.069 g/cm3, an apparent of 2.751 g/cm3 and a breakage ratio of 7.034% under pressure of 35 MPa with the amount of 5 wt%（NH4）6Mo7O24·4H2O at 1400 oC. Proppant samples sintering from 1250 oC to 1400 oC containing 1 wt% AlF3 and 3 wt%（NH4）6Mo7O24·4H2O respectively exhibits a breakage ratio below 9% under pressure of 35 MPa. Among them, low weight proppant sample sintering at 1350 oC shows a bulk density of 1.027 g/cm3, an apparent density of 2.713 g/cm3 and a breakage ratio of 5.451% under pressure of 35 MPa. |
