Study On Degradation Characteristics Of High Water Absorbent Polymer Temporary Plugging Cement

Geothermal energy is a kind of green renewable energy.The main way to develop geothermal energy is to drill geothermal wells.In the process of geothermal well drilling and EGS fracturing,in order to prevent the leakage of drilling fluid and fracturing fluid,plugging is needed.Conventional cement plugging will damage the reservoir and is irreversible.Therefore,it is necessary to use geothermal temporary plugging cement for plugging.Geothermal temporary plugging cement is a kind of degradable cement which can be used here.It can maintain stable strength during drilling fluid circulation(about 85 ?)and prevent drilling fluid leakage;After the completion of drilling,the temperature in the well rises(about 250 ?),and the degradation can be achieved by adding normal temperature water stimulation,and the degradation products will not damage the reservoir.Sodium carboxymethyl cellulose(CMC)is the key to the degradation of geothermal temporary plugging cement,which can effectively promote the self degradation of temporary plugging cement.However,the significant tackifying effect of CMC leads to the limitation of CMC dosage.Therefore,in this paper,the super absorbent polymers such as polyethylene glycol(PEG),sodium polyacrylate(PAAS),sodium starch phosphate(PES),soluble starch(SS)and polyvinyl alcohol(PVA)were selected as the degradation agents in the geothermal temporary plugging cement to replace CMC.In this paper,the role of different additives in geothermal temporary plugging cement is explored.Based on the experiments of fluidity,setting time,compressive strength,apparent viscosity and water loss,the potential of each polymer as degradation agent was discussed,and the influence of each polymer on cement was clarified.Combined with the water absorption characteristics and compressive strength loss of the polymer,PVA has the best degradation potential as a degradation agent.The basic formula of geothermal temporary plugging cement is slag / fly ash 80 / 20 + 6% sodium silicate + 3% PVA,and the water cement ratio is 0.6.Then,the degradation effect of the basic formula was optimized by adding sodium dodecyl sulfate(K12)as a degradation aid and SS as a composite material.The final formula of geothermal temporary plugging cement is slag / fly ash 80 / 20 + 6% sodium silicate + 3% PVA + 0.02%K12 + 0.2% SS,and the water cement ratio is 0.6.At this time,the compressive strength of the cement at 85 ? is 9.375 MPa,and the compressive strength after meeting water is less than 1.5MPa.Combined with thermogravimetric analysis experiment and infrared analysis experiment,the mechanism of the whole reaction process of geothermal temporary plugging cement is studied,which is divided into two parts: hydration and degradation.The hydration process mainly includes hydrolysis reaction,nucleophilic reaction,coordination reaction,dehydration condensation reaction,crosslinking reaction and so on,which belongs to alkali activated gel reaction.The main hydration products are gelatin,calcium hydroxide,calcium carbonate and so on.The degradation process is divided into two stages(high temperature stage and water contact stage)and three parts(polymer degradation,silicate structure destruction and bonding point shedding).The main degradation products are water,small molecular substances and silicate residue.Combined with the previous reaction model,the reaction mechanism involved is analyzed and supplemented in the form of chemical reaction formula,experimental photos and schematic diagram,so as to improve the reaction mechanism of geothermal temporary plugging cement.