Preparation Of Self-generated Proppant And Study Of Phase Transition And Growth Process

Driven by the continuous increase in energy demand and the depletion of conventional oil and gas wells,the development of fracturing technology to exploit unconventional oil and gas resources is of specific strategic significance to China’s energy security.Self-generated proppants are the best materials for the effects of this technology.However,self-generated proppants have been reported to be difficult to accurately regulate the phase transition time,slow growth rate,and difficult to monitor the growth and phase transition process,which fail to achieve the ideal effect of self-generated proppants used in fracturing stimulation.Combined with the actual wellbore and fracture environment,this paper proposed a self-generated proppant with high strength(56.2 N),controllable phase transition time(9-66 min)and rapid growth based on construction of a molecular structure of “functional monomer-styrene(St)-divinyl benzene(DVB)” copolymer,regulating initiation system or doping inorganic filler.In view of the lack of reliable research methods on the process of proppant phase transition and growth,the monitoring of the process of proppant phase transformation and growth was carried out.1.According to different well exploitation environments,the optimum technological conditions of self-generated proppant was determined from the perspective of rigid structure and reaction rate by adjusting the water-oil ratio of the system,introducing surfactants and optimizing rotation speed.When the rotational speed was 120 rpm,phase change fracturing fluid was formed by mixing of phase change liquid(St,DVB,benzoyl peroxide)and non-phase change liquid(tap water,PVA-1788,MC-400,sodium dodecyl benzene sulfonate)at a volume ratio of 1:12.The phase change fluid requires no organic solvent and is environmentally friendly.It consists of phase change liquid and non-phase change components.The phase change liquid can begin phase change in 33 to 60 min at the simulated fracture temperature of 60 to 110℃ and nonphase change liquid improves the problems of easy aggregation,low production rate and low pellet formation rate of self-generated proppant.The average single particle strength of the self-generated proppant generated from this phase change fracturing fluid is 32 N,and the yield is up to 78%.60% of the microsphere sizes are distributed in 20 to 40 mesh.2.Based on the chain initiation rate equation,from the initiation rate of the initiator and the migration law of the proppant in the bottom well,a phase transition fluid system with controllable phase transition timing in a wide time range was constructed.Simulation of self-generated proppant in wellbore and fracture temperature field(65~85℃),based on the relationship between the half-life of the initiator and the time required for the liquid-solid phase transition of the self-generated proppant,azobisvaleronitrile(ABVN),azobisisovaleronitrile(AIBN),azodicarboxylate(AMBN) and benzoyl peroxide(BPO)were selected as the initiator to realize the controllable phase transition of the self-generated proppant in a wide time range.By changing the type of initiator,the phase transition can be started at 9~66 min.The average single particle strength of the self-generated proppant produced by the phase change fracturing fluid containing azobisvaleronitrile is 42 N,and the yield can reach about 70%,and 70%of the self-generated proppant is distributed in 20 to 40 mesh.3.Based on the mechanical isolation and induced nucleation of inorganic nano-filler,nano-graphite was selected as inorganic dispersing agent to improve the microsphere formation rate and accelerate the strength growth process of self-generated proppant microspheres.Simulation of self-generated proppant in fracture temperature fields(65~85℃),by doping different kinds of inorganic fillers(nano graphite,hydroxyapatite,calcium carbonate)into the phase change liquid,establish the relationship between inorganic fillers and the growth rate and strength of self-generated proppant,and prepare a high strength,rapid growth of self-generated proppant.Among them,the enhancement effect of nano-graphite is outstanding.The average single particle strength of the self-generated proppant generated by the phase change fracturing fluid containing graphite can reach 56.2 N,the growth rate is 20% higher than that before doping inorganic filler,the yield can reach about 80%,and the size is almost distributed in 20~40 mesh.4.In order to solve problem that the absence of reliable research method on the phase changing process and growing process of self-generated proppants in the lab,the direct and indirect research methods are put forward in this paper.The phase changing process can be studied by the combination of subjective observation(direct)and the measurement of time-varying viscosity(indirect).the difference between the beginning and end points of the phase changing are determined by the two methods is only 0-2min.The growth process of microsphere can be studied directly by the change of microsphere size,compressive strength and yield,as well as the change of glass transition temperature and monomer conversion(indirect),The evolution rules of selfgenerated proppant microspheres over time revealed by the above methods are basically consistent.The research in this paper makes up for the shortage of self-generated proppant at present,and can promote the practical application of self-generated proppant in fracturing technology,so as to improve oil and gas reservoir quality and production,especially to improve the recovery efficiency of tight reservoirs.