Research On The Water-based Clay Free Drilling Fluid With Temperatue And Salt Resistance

With the development of oil and gas resources,drilling operations are gradually developing to deep Wells.Water-based soilless drilling fluid,as a kind of drilling fluid with low cost and excellent performance,has been widely used in drilling operations in reservoir sections.In order to ensure the good performance of carrying cuttings,stabilizing borehole wall and controlling the filtration loss of the system,rheology modifier and fluid loss reducer are usually added to adjust the rheology and filtration property of the water-based drilling fluid,so as to realize the role of cleaning the borehole,balancing formation pressure and stabilizing the borehole wall.With the development of material technology,more and more synthetic polymers are used as the core additives of water-based drilling fluids,which play an important role in adjusting system rheology and controlling system filtration.However,in the high temperature and high salt environment,the synthetic polymer is easy to degrade and break the chain,which leads to the instability of the drilling fluid system and cannot meet the safety requirements of the operation.Therefore,more and more attentions to the research of temperature and salt resistant polymers for water-based caly free drilling fluid,especially the development of synthetic polymers with temperatures over 200℃.In this paper,the temperature and salt resistant polymer used in water-based clay free drilling fluid is studied.Based on the design of molecular structure,two kinds of thermal and salt resistant polymers with special structures are prepared.The synthesis and preparation parameters of two kinds of polymers are optimized,their temperature and salt resistance properties are investigated,the mechanism of temperature and salt resistance is analyzed,and the thermal degradation kinetic characteristics of the two polymers are studied.The main contents and results are as follows:1.A"mesh and lock"temperature and salt rheology modifier named PRA which take AM/NVP/AMPS/C₁₈DMAAC as the main monomer,PTE as the cross-linking agent is designed and synthesized by free radical polymerization in aqueous solution.Fourier transform infrared spectroscopy（FTIR）and nuclear magnetic resonance carbon spectroscopy（NMR）are used to confirm the structure of the polymer which is in accordance with the theoretical design,and the temperature-salt resistance of the synthesized product is investigated.The results showed that the optimal synthesis parameters of PRA are:The total monomer concentration（AM+AMPS+NVP）is25%（w/w）,the other components accounted for the total monomer mass ratio is,C₁₈DMAAC 0.2%,PTE 0.05%,initiator 0.05%,reaction at 40℃for 8h.In the total concentration of monomers,the mass ratio of each monomer is AM:AMPS:NVP:C18=5:12.5:7.5:0.05.the PRA salt resistance at the end of 1.3 g/cm³（potassium formate）,resistance to temperature of 220℃.2.A kind of copolymer function as filter Loss reducer with"core-shell"structure,named PDL which take AM/NVP/AMPS as main monomer,N-AM/MAAas temperature sensitive monomer and p H sensitive monomer,PTE and MBA as cross agent is designed and synthesized by free radical polymerization in aqueous solution.Fourier transform infrared spectroscopy（FTIR）and nuclear magnetic resonance carbon spectroscopy（NMR）are used to confirm that the structure of the polymer which is in accordance with the theoretical design,and the temperature-salt resistance of the synthesized product is investigated.The results showed that the optimal synthesis parameters of PDL are as follows:the total monomer concentration（AM+AMPS+NVP）is 23%（w/w）,the other components accounted for the total monomer mass ratio is,cross-linking agent dosage 0.008%(m_{pentaerythritol triallyl ether}:m _{N,N methylene diacrylamide}=2:1),initiator concentration 0.03%,reaction time 10h at 35℃.In addition,the mass ratio of each monomer is AM:AMPS:NVP:N-AM:MAA=4.46:11.15:6.69:0.46:0.23.The salt resistance of PDL reached（potassium formate）1.3g/cm³,and the temperature resistance reached 200℃.3.Based on the method of thermal analysis kinetics,the thermal degradation kinetic behavior of temperature and salt resistant rheology modifier and temperature and salt resistant fluid loss reducer is explored as well as the thermal degradation kinetic equation.At the same time,the effects of salt solution density and deoxidizer on thermal degradation kinetics of PRA and PDL are also studied.The results show that the thermal degradation kinetics equation of PRA is:dΔη/dt=2.5×10^-1（1-Δη）^1.098,E_a=64.6 k J×mol^-1while PDL is:dΔAPI_FL/dt=2.4×10^-1(1-ΔAPI_FL)^1.179,E_a=24.4 k J×mol^-1.as the increase of salt solution density,the order of thermal degradation kinetics of the two polymers did not change,but the reaction rate constant decreased.In the three deaerating agents investigated,the decrease of the reaction rate constant is greatest after the addition of sodium ascorbate,which had the strongest inhibition of thermal degradation.The thermal degradation kinetics equation of PRA at 220℃after adding sodium ascorbate is as follows:dΔη/dt=3.5×10^-2（1-Δη）^1.411.The thermal degradation kinetics equation of PDL at 200℃after adding sodium ascorbate can be expressed:dΔAPI_FL/dt=9.04×10^-2(1-ΔAPI_FL)^1.277.4.The mechanism of PRA,an anti-temperature and anti-salt rheology modifier,is that it forms a special"net-lock"molecular structure.Through the crosslinking,polymer molecular chains are wound together to form a certain degree of spatial"network".At the same time,the cross-linking reduces the distance between polymer molecules and strengthens the non-covalent bond between hydrophobic monomers on the molecular chains.Thus the network structure of the polymer molecules is further tightened,such as"locking",thus improving the overall stability of the polymer chain of the rheology modifier.5.The mechanism of the anti-temperature and anti-salt filtration reducer PDL is as follows:sensitive groups（thermosensitive monomer,Ph-sensitive monomer）in the molecule can make the molecular chain continuously shrink into clusters under high temperature and high salt environment,forming a certain degree of"nuclear"structure;Using pentaerythritol triallyl ether and N,N-diacrylamide as double crosslinking agent,a certain degree of cross-linked shell structure can be formed outside the copolymer.The above two effects enable the polymer to maintain a certain"core-shell"structure at high temperature,so as to control the filtration channel of the system and achieve filtration loss reduction.6.A Temperature and salt water resistant drilling liquid named Drinflow is constructed by using PRA and PDL and its overall performance is investigated.The results showed that the salt resistance of Drinflow is 1.3 g/cm³（potassium formate）,resistance to temperature of 210℃.it has good calcium resistance and anti-contaminated soil ability,while maintaining strong inhibition and low corrosion（the average corrosion rate of N80 steel sheet is 0.1302mm/a;The average corrosion rate of 13Cr steel sheet is 0.0903mm/a）.In addition,the Drinflow system has excellent flowback capacity and reservoir protection performance.After inflow,the sand disc return breakthrough pressure is 0.0124MPa,and the core permeability recovery value is higher than 85%.