Preparation And Application Of Gas-sensitive Hydrogels With High Strength

Hydrogel is a kind of three-dimensional crosslinking network formed by physical or chemical interaction with water as dispersion medium.Its unique controlled release and stimuli-responsive characteristics make it have great application and development potential in the fields of agriculture,medicine and petrochemical industry.In the field of oilfield chemistry,gel material is an important functional additive,which is widely used in hydraulic fracturing,lost circulation prevention and plugging and other processes.In the oil and gas exploitation process,the associated petroleum gases such as H₂S,SO and CO₂ often damage the cementing safety,reduce oil and gas recovery,and even cause major safety accidents.In this paper,acrylamide（AM）,acrylonitrile（AN）and N-vinylformamide（NVF）were used as functional monomers to prepare high-strength,salt-tolerant,gas-sensitive hydrogels.And using its own special gas sensitivity,salt resistance and controlled release characteristics to prepare gas-triggered cementing cement ring micro-fracture repair agent and CO₂-sensitive tracer controlled release material.1.Sulfur-containing toxic gases such as H₂S are extremely common associated gases in underground oil and gas reservoirs.During cementing construction and after well completion,toxic gases leak from micro-cracks of cement ring or cementing interface,seriously damaging wellbore integrity and shortening the life of oil and gas wells.In this study,in order to prepare a repair agent that can be applied to the micro-cracks of cement sheath in gas-triggered cementing,AM,AN and NVF were used as monomers,and laponite was added to prepare a nanocomposite hydrogel（L-PMAN）with excellent mechanical strength.Considering the difference of chemical environment in the process of wellbore cement slurry sealing and subsequent micro-fracture plugging,different structural units were selected to design amphoteric polyelectrolyte gel structure.During the curing process of cement slurry,the amide group of AM structural unit is hydrolyzed into carboxylic acid under alkaline conditions.After curing,the AN and NVF structural units of the hydrogel material in the microcrack generate an amine-containing vinylamine segment and amidine functional group in the H₂S environment,thereby constructing an amphoteric polyelectrolyte structure that exhibits obvious swelling characteristics in salt solution while maintaining 80%of the original mechanical strength.Finally,H₂S triggered self-healing cement slurry was prepared by using G-grade cementing cement as the main raw material and adding L-PMAN.After curing,cement stone（C₁）with 50μm artificial cracks was cured in H₂S environment.The experimental results show that C₁ can achieve good repair of microcracks under the trigger of H₂S,and the maximum gas breakthrough pressure can reach 2.5 MPa,which provides strong evidence for L-PMAN as a self-healing material for plugging microcracks in cement sheath.2.Multistage hydraulic fracturing is an important means of oil and gas exploitation at present,but the fractures formed by hydraulic fracturing have an extremely high degree of complexity,and it is difficult to effectively evaluate the fracturing effect in general.In this study,in order to prepare a hydrogel material suitable for controlled release of CO₂-sensitive tracers,a hydrogel（PMAN）was prepared by free radical polymerization using AM,AN and NVF as monomers and N,N’-methylene-bis-acrylamide（MBA）as crosslinking agent.Hydrolyzed hydrogel（H-PMAN）was obtained by hydrolysis of PMAN in a high temperature and strong acid environment.In the process of strong acid hydrolysis,the network structure and mechanical properties can be adjusted through imidization reaction,physical interaction between hydrolytic groups and chemical crosslinking.At the same time,the hydrolysis process resulted in the formation of vinylamine segments and cyclic amidine with primary amine groups in the gel network.It was found that the above functional groups could regulate the swelling ratio through protonation in the acidic environment of CO₂,so as to realize the controllable tracer release of gel materials under CO₂ stimulation.By further analyzing the release kinetics,it can be determined that CO₂ can effectively change the release mechanism of tracer in the hydrogel.The above results prove that H-PMAN can be used as a CO₂-sensitive tracer controlled release material to achieve the purpose of transmitting information on the changes of underground oil,gas and CO₂ concentrations with formation storage conditions.