Performance And Application Of Efficient Kinetic Hydrate Inhibitors

As the oil and gas field shifting to deep water or ultra-cold areas,the thermodynamic hydrate inhibitors(THIs)gradually become absolete due to its huge amount and cost.Here,it has been clear tendency that low dosage hydrate inhibitors(LDHI)are taking place of THIs.However,kinetic hydrate inhibitors(KHI),a kind of LDHI,cannot meet the demands of industry in trems of inhibition performance,biodegradability and economy.Moreover,the current commerical KHIs had no inhibitory effect when the subcooling temperature exceeds 15.0 ?C.Three methods are used to improve the overall performance of KHI,which are adding synergists to KHIs,developing new KHIs,or combining the KHI and THIs.In this paper,we developed new synergists of polyvinylcaprolactam(PVCap)which is a kind of typical commercial KHI.Subsequently,natural pectin was developed as KHI for the first time,and variety of gas/liquid systems were used to study its inhibition performance.The synergistic mechanism of amino acid for PVCap was investigated as well as the key inhibition mechanism of pectin.On the basis of these studies,two kinds of hydrate inhibitors was applied in gas field domestically,and the recovery of MEG combined with KHI was also studied.Finally,the biodegradability and economy of hydrate inhiitors were analyzed.The results showed that four amino acids could improve the inhibition performance of PVCap during hydrate nucleation.The tolerant subcooling temperature would increase by 2.5oC,1.4 oC,1.1 oC,and 1.0 oC when the synergists were glycine,alanine,threonine,and serine corresponding.In the hydrate growth stage,glycine and alanine could aviod the failure of PVCap in high subcooling temperature.The hydrate growth rate in the presence of PVCap plus glycine(or alanine)reduced by 80.0% compared with the system adding PVCap,the complete inhibition region(CIR)of PVCap shifted to lower temperature for 2.7 oC after introducing amino acids.While the serine or threonine could not improve the inhibitory performance of PVCap when the subcooling temperature exceeded 7.0 oC.The results proved pectin possessed extraordinary performance.In the system of CH4 /H2 O as well as the CO2/CH4 and H2 O system,the tolerant subcooling of pectin was more than 12.0oC and the induction was also longer than 48 h.Even the dosage of pectin was 66.6% of PVCap,the induction time of pectin was 7.4 times more than PVCap.Moreover,in terms of typical natural inhibitors(chitosan or starch),the inhibition activity of pectin increased 19.2-fold in induction time.Besides,in multicomponent gas water system,the subcooling of pectin was as high as 18.8 oC.The induction time of it was longer than 48.0 h at 19.0 oC of subcooling.What’s more,pectin began to retard hydrate formation when the concentration exceeded 0.3wt%.The inhibition mechanism was also studied by raman spectroscopy.During the nucleation stage,amino acid such as glycine disrupted the liquid water structure around PVCap polymers so that enhanced the inhibition strength of PVCap.At the growth stage,glycine as a small molecule might easily penetrate into spaces that polymer chains could not cover,leading to increase the mass transferring resistance of water so that improve the inhibition performance of PVCap.Pectin inhibited hydrate formation by two steps.On the first step,the carbonyl and hydroxyl groups of pectin could form hydrogen bonds with water molecules which would disrupt the water structure,resulting in restarding hydrate nucleation.On the second step,pectin increased the transferring resistance between bulk phase and the hydrate phase by threedimensional network and then hindered hydrate growth.The application results showed the composite inhibitors(such as pectin plus MEG,HI-121 plus MEG)could meet the need of oil and gas field distributed in the South China Sea.In order to reduce the cost,the recovery of MEG and HI-121 together was studied.The MEG recovery rate was influenced by temperature but not HI-121.The presence of the HI-121 had a negative impact on the thermodynamic inhibition efficiency of the MEG.The HI-121 was recovered along with the MEG,the KHI performance of the recovered solution obtained at the polymerization temperature of HI-121 could reach the level of the fresh inhibitors.The synergists and new KHI developed in this work reduced the cost as well as improved the biodegradability.Glycine could reduce the cost of PVCap by 63.0% and also improve its biodegradability by 17.1%.Biodegradable natural pectin made the cost decreased by 33.3% compared with HI-121 based on PVCap.