Preparation Methods And Mechanisms Of High-strength Low-grade Bauxite Based Fracturing Proppants

Hydraulic fracturing is an important engineering technology for increase production of oil and gas.Fracturing proppants is a key material to success of fracturing and the fracturing effect.High-grade bauxite based proppants composed of corundum show the high crush resistance and is widely used in hydraulic fracturing.But the high-grade bauxite causes the high density of proppants and high cost of manufacture.Low-grade bauxite is abundant and cheap in China.Exploration of low-grade bauxite based high strength proppants is beneficial to reduce the cost of fracturing operation.Low-grade based proppants mainly composes mullite and glass,and show the obviously different structure from high-grade bauxite based proppants.It is necessary to study the relevant technology of preparation of high-performance proppant from the low grade bauxite.In this thesis,high-strength bauxite based proppants was successfully prepared from two kinds of low-grade bauxite(?40 wt.%Al2O3)and low-grade bauxite(?55 wt.%Al2O3),respectively.The relationship between structure composition and performance of proppants was studied by XRD,SEM and EDX.Strength enhancement mechanism and the sintering mechanism of proppants were also discussed.The main work and conclusions are summarized as follows:Firstly,a method of preparation of high-strength bauxite(?55 wt.%Al2O3)based proppants by introducing the additives of CaO and Fe2O3 based on liquid sintering mechanism and solution strength effect was put forward.The effects of CaO and Fe2O3 on the structure,crystal growth and characterization of proppants were systematically studied.Strength enhancement mechanism was discussed and the best formula was given.CaO promoted the formation of low temperature liquid phase and improve the density and crush resistance of proppants.But CaO also can restrain the mullite crystal growth.Excessive CaO led to the decrease of the strength of proppants for losing mullite whisker reinforcement.Fe2O3 promoted the crystal growth of mullite and corundum,causing the increase of crush resistance.In the sintering process,the reaction of excessive Fe2O3 to Fe3O4 can take place to liberate O2 gas and is detrimental to the densification and strength of proppants.The proppants doped with the additives composed of CaO and Fe2O3 sintered at 1300? had the best performance with 2.981 g/cm3 of apparent density and only 5.13wt.%of breakage ratio under 69 MPa.Secondly,low-temperature sintering of proppants was explored.The effects of pre-calcination of bauxite,the particle size distribution of raw materials,bingders,and the additives of CaO-MgO or CaO-MnO2 on the sintering temperature and characterization of bauxite(?55 wt.%Al2O3)based proppants were systematically studied.The best formula of CaO-MnO2 was given.The low-temperature liquid can be formed at the sintering temperature between 1140? and 1160? by introducing the additives of CaO and MnO2.MnO2 also can restrain the mullite crystal growth.Excessive MnO2 led to the decrease of the strength of proppants for losing mullite whisker reinforcement.The additives of CaO and MnO2 was firstly used in low-temperature sintering of low-grade bauxite proppants and caused the significant decrease of sintering temperature.The lowest sintering temperature of 1180? was achieved by introducing the additives of CaO and MnO2,and obviously lower than the sintering temperature of above 1300? of common bauxite based proppants.Thirdly,phase transition of bauxite(?40 wt.0/%Al2O3)in heating process was analyzed.The effects of pre-calcination of bauxite,Fe2O3 and MnO2 on the characterization of bauxite(?40 wt.%Al2O3)based proppants were systematically studied.The temperature of pre-calcination of the bauxite was put forward and the high-strength bauxite(?40 wt.%Al2O3)based proppants was finally successfully prepared in this thesis.The additive of Fe2O3 restrained the transition from cristobalite to galss in the sintering process and reduced the glass content.The method of increasing the strength of proppants by introducing Fe2O3 was proposed for the first time.The importance of complete mullite reaction in the pre-calcination process of natural bauxite for high-strength low-grade bauxite based proppants was explained.A new technology of preparing high-strength proppants was completely explored.The pre-calcination bauxite based proppants doped Fe2O3 sintered at 1400? had the best performance with 2.782 g/cm3 of apparent density and only 2.62wt.%of breakage ratio under 52 MPa.Fourthly,the effects of 12wt.%HCl+3wt.%HF acid on crush resistance and flow capacity of proppants composed different phases were studied.The mechanism of property change of proppants composed different phases after the acid corrosion was put forward.Compared to the bauxite(?40 wt.%Al2O3)based proppants mainly composed of mullite whisker,common high-strength bauxite based proppants mainly composed of corundum showed the significant decrease of crush resistance and proppants flow capacity after acid corrosion at the same condition.Staggered mullite crystal provided the strength of erosion proppants.Breakage ratio of bauxite(?40 wt.%Al2O3)based proppants under 52 MPa only increased 3.5wt.%after 6h acid corrosion at the temperature of 66?.Fifthly,high-performance rod-shaped proppants was prepared by sintering with extrusion method.The effects of binder,lubricants and plasticizer on the losing power rate,extrusion pressure and properties of proppants were systematically studied.The difference of fracture pattern,accumulation mode and flow capacity of proppants between sphere-shaped proppants and rod-shaped proppants was researched.Bridge structure can be formed in the accumulation process of rod-shaped proppants and improve the proppants flow capacity by increasing the high porosity.The flow capacity of sphere-shaped proppants can be increased by adding proper amount rod-shaped proppants.