Study Of The Film-forming And Plugging Properties And Wellbore Stabilization Mechanism Of Lithium Silicate-based Drilling Fluids

Wellbore instability is one of the pivotal problems in drilling engineering that needs to be addressed.Especially,numerous micropores and fractures develop in the deep,hard and brittle shale.In this case,drilling fluids can be more likely to penetrate into the formation,and cause large-scale cuttings disintegration,wellbore collapse,and pipe sticking.To address the aforementioned challenges,the key to stabilize wellbore for drilling fluids is to form a stable and tough protective film on the shale surface,plug the micro-nano fractures in the near-wellbore zones,improve the cohesive strength of the wellbore rock and enhance rock cementation,and regulate the wettability of the shale surfaces to prevent filtrate intrusion into the pores of the formation.In this study,we propose a new strategy that uses the downhole temperature as the reaction parameter to allow the hybrid silicate or nanoemulsion to polymerize and seal the pores while simultaneously forming a hydrophobic film on the shale surface,thereby addressing problems associated with wellbore instability in deep well drilling.The paper elaborates on the design concepts,performance evaluation and mechanism analysis of the novel shale stabilizer,water-based film-forming drilling fluid formulation with high temperature resistance,and the oil-in-water nanoemulsion system with low surface energy.The main researches and conclusions are as follows:（1）We developed a film-forming agent composed of lithium silicate-based hybrid silicate.Firstly,we compared the inhibition performance and film-forming properties at high temperatures of three inorganic silicates（potassium silicate,sodium silicate,and lithium silicate）and three hybrid silicates created by compounding the above three inorganic silicates with potassium methylsilicate（PMS）,and the lithium silicate-based hybrid silicate with outstanding comprehensive performance was preferred.Furthermore,we systematically investigated the influence of reaction parameters,such as component,concentration,modulus,reaction temperature,reaction time and pH,on the high-temperature film-forming properties of lithium silicate-based hybrid silicates.The results showed that the hybrid silicate consisting of 0.2 mol/L PMS and 0.5-1.0 wt%lithium silicate（modulus 4.8）could form a quartz film grafed a small amount of methyl with a thickness of 130μm on the shale surface at pH>9 and temperature≥150℃after reacting for more than 8 h.The micromorphology of the film was characterized by a three-dimensional network structure formed by tightly cross-linked nanofibers with diameters<20 nm.The wellbore stabilization mechanism of the hybrid silicate can be attributed to the synergism of the potassium ion exchange and the hydrophobic effect of methyl which can inhibit the hydration swelling,especially,the hybrid silicate can generate a dense quartz film in situ on the shale surface via dehydration polycondensation reaction at elevated temperatures which can serve as an effective physical barrier to prevent the filtrate from intruding into the shale formation and thus maintain wellbore stability.（2）Based on the study of the aforementioned hybrid silicates,a water-based film-forming drilling fluid formulation with heat resistance up to 240℃was researched and developed.The formulation not only has strong inhibition with shale recovery higher than 100%.In addition,it can maintain superior rheological,excellent thermal stability and building capacity of mud cakes with low filtrate loss after aging at 150～240℃.After aging at 240℃for 16 h,the API（25℃,0.69 MPa）filtration loss was only 5.6 mL,and the HTHP（205℃,3.5 MPa）filtration loss was14.4 mL,as well it can form thin and dense mud cakes facilitating filtration reduction.（3）In order to enhance the film-forming water barrier effect and reversal of wetting on shale surfaces,the paper investigated and developed a oil-in-water hybrid silicate nanoemulsion.The desirable nanoemulsion system was screened and identified from six surfactants,four silane coupling agents,and four long-chain alkoxysilanes via comparing the stability and film-forming hydrophobicity of a series of emulsions,which was composed of lithium silicate complex silicate（0.2 mol/L PMS-0.5 wt%LS,aqueous phase）,sodium dodecylbenzene sulfonate（SDBS,surfactant）,γ-methyl acryloxypropyltrimethoxysilane（KH570,co-surfactant）,and n-octyltriethoxysilane（n-OTOS,oil phase）.The nanoemulsions have excellent storage stability,stable storage for more than one year,and resistance to high temperatures of 180℃.Moreover,it was also systematically investigated the effects of reaction parameters(e.g.SDBS concentration,M_n-OTOS/M_KH570 ratio,reaction temperature,reaction time,etc.)on film-forming and plugging effects,film composition,film structure and wettability.The results elucidated that a hydrophobic film could be formed in situ on shale surfaces in nanoemulsions with SDBS concentration of 0.3wt%,the concentration of KH570 greater than 1.0 wt%,and the M_n-OTOS/M_KH570 ratio ranged from0.5 to 1.0.Particularly,the nanoemulsion with the M_n-OTOS/M_KH570 ratio of 0.75 can not only plug pores in the near-well zone,giving excellent plugging performance,but also improve the cohesive strength of the rocks,and thus strengthen the wellbore.Besides,it can also construct a dense superhydrophobic quartz film with an average thickness of 81.2μm,the average water contact angle value of 154°,and the surface free energy of 3.17 mJ·m^-2 on the shale surface in situ.This will prevent water penetration into shale fractures or pores.The micromorphological characteristics of the superhydrophobic film are similar to the"papillary structure"of lotus leaves’surface,which was identified as a three-dimensional hierarchical superstructure composed of tightly stacked reticulate nanofilaments with a diameter of 5～20 nm and several micrometers in length,and overlapped well-distributed nanospheres with a diameter of 25～53 nm.In addition,the superhydrophobic film displayed good thermal stability and durability against dynamic water erosion.As a result,this paper presents theoretical support for borehole strengthening methods that are capable of preventing wellbore destabilization at deep intervals in HTHP conditions,through the use of a synergistic method that combines physical blocking with chemical wall consolidation.In order to plug micro-nano pores and to maintain wellbore stability,we utilize the high temperature film-forming properties of lithium silicate-based hybrid silicate and the low surface energy properties of organosilicon molecules.Sol-gel and emulsion polymerization methods are employed to build up dense film structures with a micro-nano structure on shale surfaces in situ to maintain wellbore stability.In addition,the lithium silicate-based hybrid silicate,nanoemulsion and drilling fluid formulation proposed in this paper is expected to confer practical applications in deep or ultra-deep well drilling engineering owing to its convenient fabrication,eco-friendly and cost-effective characteristics,contributing to deep oil and gas development in China.