Study On The Effects Of Nano-SiO₂ And Hydrate Inhibitors On Hydrate Formation In Drilling Fluid

As a potential alternative energy source,well drilling operations are integral to the exploration and exploitation of natural gas hydrates.To maintain the borehole stability and safety,drilling operations in gas hydrate-bearing sediment(GHBS)are typically performed under overpressure conditions,in which drilling fluid would penetrate into the GHBS due to differential pressure.To eliminate the adverse effects while maintaining borehole stability,the filter loss of drilling fluid should be reduced as much as possible,and the application of nanoparticles is an effective way to reach such goal.Considering most of natural gas hydrates in nature are formed in argillaceous and sandy sedimentary strata,nano-SiO2 was introduced into hydrate drilling fluid system in this paper.However,nanoparticles' presence in hydrate drilling fluid may also pose new safety risks.Therefore,the question that whether it is possible to select a optimized nanoparticle formula with appropriate type/types,grain size and amount,which would not promote,or even possess the ability to inhibit the formation and aggregation of hydrates,becomes the crux in deciding if certain nanoparticle is suitable for hydrate drilling fluid preparation.In order to avoid the massive decomposition of hydrate in reservoir,low-temperature drilling fluid is usually pumped into the borehole.As a result,after a critical depth in the wellbore,low-temperature drilling fluids will be under a strong driving force for hydrate formation.In order to prevent gas cut induced hydrate formation and aggregation in the wellbore,existing hydrate drilling fluid system often changes hydrate phase equilibrium conditions by adding a large amount of thermodynamic inhibitors to completely block hydrate formation.However,the cost of such drilling fluids is very high and these drilling fluids could cause serious pollution.Therefore in this paper,a new formula where different types of hydrate inhibitors are mixed is proposed according to the characteristics of marine hydrates drilling.However,many studies have shown that the nanomaterial added to the drilling fluids is likely to adsorb the drilling fluid treatment agent on its surface and weakens the effectiveness of treatment agent.In this prerequisite,the effect of nanoparticles on the inhibition of a single hydrate inhibitor was studied in this paper,and the effect of nanoparticles on inhibition ability of compound hydrate inhibitors was further investigated through experiments.Finally,the basic formula of drilling fluid system which is optimized for marine hydrate drilling was obtained.In view of the above content,firstly,the effects of nano-SiO2 with different types,grain sizes and amounts on the formation of methane hydrate was investigated under different temperature-pressure conditions,and the mechanisms of these effects were multi-angled,thoroughly analyzed and revealed in combination with experiments of how aggregation of nanoparticles in fluid,the effects of nanoparticles on the Zeta potential,water activity,thermal conductivity of the fluid and the equilibrium conditions of methane hydrate.Then we determined which type,grain size and amount of nano-SiO2 is more suitable for the preparation of hydrate drilling fluid.Secondly,the effects on the formation and aggregation of methane hydrate were investigated under different temperatures and pressures when hydrate kinetic inhibitors and anti-agglomerant were used alone or compounded with thermodynamic inhibitor separately.For these simple systems cannot effectively inhibit the formation and aggregation of hydrate under strong driving force,it is found that when all three types of inhibitors were compounded,the system is able to significantly reduce the formation rate of hydrate and effectively inhibit hydrate aggregation.Finally,on the basis of the above,the effects of hydrophilic nano-SiO2 on the inhibition of single hydrate inhibitor were clarified and the mechanism of these effects were preliminarily revealed,then the performance of simulating drilling fluid with a combination of hydrophilic nano-SiO2 and compound hydrate inhibitor on the formation and aggregation of hydrate was further studied under the experimental conditions with strong driving force for hydrate formation.Based on the experimental research and theoretical analysis,several conclusions can be drawn as:(1)Although hydrophilic nano-SiO2 cannot inhibit the aggregation of hydrate,it can inhibit hydrate formation when the grain size and amount are appropriate,besides,hydrophilic nano-SiO2 with a grain size of 80 nm and an amount of 4.0 wt% has the strongest inhibiting ability.And the addition ofhydrophilic nano-SiO2 can improve the performance of drilling fluid on multiple aspects,thus,hydrophilic nano-SiO2 can be used to prepare hydrate drilling fluid.(2)Regardless of its size and amount,hydrophobic nano-SiO2 has a promoting effect on the formation of hydrate,and this effect develops with the increase of grain size and amount of hydrophobic nano-SiO2,which makes it not suitable to prepare hydrate drilling fluid.(3)Although the compound of kinetic inhibitor and thermodynamic inhibitor cannot inhibit the aggregation of hydrate,it can improve the drilling fluid's inhibition ability on the formation of hydrate.The compound of anti-agglomerant and thermodynamic inhibitor,on the other hand,has no effect on hydrate formation,but it can efficiently inhibit the aggregation of hydrate.Because these two compound formulations can hardly satisfy inhibit hydrate formation and aggregation simultaneously when the driving force of hydrate formation is strong,all three types of inhibitors should be compounded.When the amounts of NaCl,VC-713 and KAP-1 in simulated drilling fluid are set as 3.5 wt%,1.25 wt% and 1.50 wt%,respectively,hydrate formation rate can be greatly slowed down and hydrate aggregation is suppressed effectively.(4)When the hydrophilic nano-SiO2 is compounded with VC-713 or KAP-1,due to the adsorption and package interaction between them,the inhibiting ability of VC-713 and KAP-1 will decrease.Also,the inhibition of hydrophilic nano-SiO2 on hydrate formation is continually weakened while more and more VC-713 and KAP-1 is added to the fluid system,until it completely fades away.When hydrophilic nano-SiO2 is compounded with NaCl,although still unable to inhibit hydrate aggregation,the nanoparticles,the Na+ and Cl-ions will have a certain binding effect on water molecules synergistically,enhancing the overall inhibition of hydrate formation.(5)Experiments show that the addition of hydrophilic nano-SiO2 will weaken the inhibition ability of compound inhibitor,but this negativity can be partially or completely neutralized by increasing the amounts of VC-713 and KAP-1 properly.Finally,the basic formula of new marine hydrate drilling fluid system is proposed as: simulated seawater + 4.0 wt% nano-SiO2(hydrophilic,80 nm)+ 1.50 wt% VC-713 + 1.75 wt% KAP-1.