Study On The Effects And Mechanism Of Nano-CaCO₃ On Hydrate Formation

High-efficient and environmentally friendly non-conventional energy-natural gas hydrates havebeen attracting worldwide attentions from researchers,governments and companies for theirdistinct advantages over traditional energies,and will provide a new energy strategy in the future.In the last decade,with the rapid growth of China's economy,the consumption of fossil fuels has increased significantly,resulting in increasing air pollution,which will seriously damage the human health and the ecological environment.Therefore,there is an urgent need to adjust the consumptionstructureofenergy relying mainly on coal,and develop a clean,low carbon,high-efficient and diversified modern energy system.According to the National Plan for Medium and Long Term Energy Development,natural gas will play a more important role in the energy structure,with its consumption proportion in the primary energy structure increasing from 5.6% in 2014 to 14% in 2030.To achieve this target,besides taking energy-saving measures,using costly foreign exchange imports and making full use of the existing conventional natural gas reserves,the extraction of non-conventional energy-natural gas hydrates provides an important potential choice.The extraction of natural gas hydrates cannot realize without well drilling and drilling fluids.In order to guarantee the stability and safety of the well drilling,the overbalance drilling is the most widely used drilling method during the extraction of natural gas hydrate,however,this method leads the drilling fluid invading the natural gas hydrate reservoir.Research and practice results show that,the invasion of the drilling fluid will seriously damage the stability of the hydrate reservoir and affect the logging identification and evaluation of the drilling well.Marine hydrate reservoir usually has low permeability and obvious fracture development.Therefore,adding suitable nano-particles to drilling fluids to improve itsplugging capacity,can serve as an efficient method to weaken the damage caused by the invasion effect of the drilling fluid to the hydratereservoir,especially for the argillaceous sediment layer,which in fact is the largest and most widespread hydrate reservoir.However,adding nano-particles to drilling fluids may bring a negative effect that the nano-particles may prompt shallow gas or the hydrate decomposes gas to react with drilling fluid water and form hydrate in wellhole,which may block theannular space and lead to safety accidents.In order to solve the technical problems in practical production,the influence of type,partical size and dosage of nano Ca CO₃ on the formation of CH4 hydrate is investigated.Various experimental methods in combine with the molecular dynamics simulation are introduced to analyses the influence mechanism of nano Ca CO₃ on the formation of CH4 hydrate.This study can provide a new viewpoint and valuable reference for the construction of new hydrate drilling fluid system,and also share light to the formation and evolution mechanism of the micropore hydrates.Main conclusions can be drawn as:?1?The hydrophilic nano Ca CO₃ can inhibit the formation of CH4 hydrate.Nano Ca CO₃ with the partical size of 20 nm and the dosage at 3.0 wt% has the strongest inhibitioneffect.Whenthe CH4 hydrate is formed under this adding condition,the induction time is delayed about 38%,the formation amount and ratearerespectively reduced by about 13% and 18% when compared with the formation of CH4 hydrate in ultrapure water.Hydrophobic nano Ca CO₃ can promotethe formation of CH4 hydrate under different partical sizes and dosages,and the promoteing effect increases with the increasing dosage and decreasing partical size.?2?Owing to the hydrophilic property,the hydrophilic nano Ca CO₃ has binding effect on the water molecule.When CH4 gas dissolves into the drilling fluids,the CH4 molecules will gather to form nano gas bubbles due to the influence of hydrophilic nano Ca CO₃.The hydrophilic nano Ca CO₃ caninhibit the formation of CH4 hydrate under certain adding conditions.This is because hydrophilic nano Ca CO₃ can prevent the water molecule to generate the cage structure encapsulating water molecule,and reduce contact area of the gas-water.The smaller partical size of the hydrophilic nano Ca CO₃ is,the smaller ‘aggregate' formed in the fluid is,and the larger specific surface area of the nano particles and the ‘aggregate' is.Hence the more water molecule is adsorbed,resulting in a betterinhibition effect.There exists a threshold dosage of nano Ca CO₃,within this threshold dosage,the inhibitioneffect onthe CH4 hydrate increases with the increasing dosage.Whereas,when the nano Ca CO₃ dosage exceeds a certain value,further increase of dosage may weaken or completely eliminatethe inhibitioneffect of the nano Ca CO₃ attributed to the increasing pratical size of the ‘nanoaggregate'?3?Owing to the hydrophobic property,the hydrophilic nano Ca CO₃ promote the CH4 dissolved in the drilling fluids to spread rapidly around the Ca CO₃ particles.A part of CH4 molecules were attracted and adsorbed on the particlessurface,leading to the increase of the contact area of the gas-water molecules,which will accelerate hydrate nucleation and growth.The promotion effect of the hydrophilic nano Ca CO₃ on the generation of the CH4 hydrate mainly embodies that the induction time is reduced and the formation rete is accelerated.