Study On The Performance Of Alginate As Multifunctional Drilling Fluid Treatment Agent And Its Composite Gel Plugging Properties

Drilling engineering is a direct means of exploring and exploiting natural resources,and the development of drilling technology is an important guarantee for the development and utilization of oil and gas resources.However,the wellbore instability during the process of drilling through fractures,pores,and stratified formations is considered to be one of the major technical problem in drilling engineering.Therefore,maintaining wellbore stability is of great significance for the smooth progress of drilling engineering.As the"lifeblood"of drilling engineering,the rheological properties,lubricity,shale inhibition,and fracture plugging properties of drilling fluids are key factors affecting wellbore stability and are crucial for safe and efficient drilling operations.Currently,water-based drilling fluids have become the industry’s first choice due to their low cost and environmental friendliness,while the salt invasion stability,hydration inhibition,and fracture plugging properties of drilling fluid additives still need to be improved.Therefore,the development of high-performance rheology modifiers,filtrate reducers,shale inhibitors,and lost circulation materials has become a pressing issue in drilling engineering.This study focuses on the biodegradable natural seaweed extract-alginates as the research object,systematically investigates the changes in the rheological and filtration properties of alginates drilling fluid under different electrolyte intrusions,and develops a composite shale inhibitor with potassium alginate/potassium silicate,revealing the synergistic inhibition characteristics of encapsulation/ion exchange effect,and elucidating the salt-resistant and inhibition mechanisms of alginate-based drilling fluid.Based on the chelation mechanism of alginates,a delayed composite gel formulation with cementing plugging characteristics is established,and proposed two environmentally friendly decongestants.The main research contents and conclusions of this paper are as follows:（1）Investigated the effect of Sodium Alginate（SA）on the performance of drilling fluids and its properties variation under salt intrusion.Firstly,explored the influence of SA concentration on the rheological properties and filtrate loss of the base slurry,compared with two commonly used additives,and optimized the rheological model;examined the influence of different electrolytes on the performance of SA drilling fluids,revealed the interaction between SA and clay particles and the salt resistance mechanism through a series of characterization methods.The results showed that the Bingham model had higher predictive accuracy.0.3 wt%SA significantly increased the apparent viscosity（AV）,plastic viscosity（PV）,and yield point（YP）of the base slurry from 6.5 mPa·s,3.4 mPa·s,and 3.1 Pa to 45.5 mPa·s,31.1 mPa·s,and 14.7 Pa,respectively,and reduced the dynamic plastic ratio（DPR）to 0.47Pa/mPa·s.The SA drilling fluid still exhibited excellent flow control and filtrate reduction performance under salt intrusion conditions.Under 20 wt%NaCl intrusion,the AV,PV,and YP of the SA drilling fluid only decreased by 16.4%,19.6%,and 6.4%,respectively,and maintained the DPR value within a reasonable range,which was better than common environmental-friendly additives.SA adsorbed on the surface of clay particles through hydrogen bonding and coordination bonding,playing a bridging role to form a grid structure;the relatively concentrated particle size distribution and larger absolute value of ZETA potential in the SA drilling fluid indicated that the electrostatic repulsion between molecules and within molecules resulted in highly dispersed clay particles,avoiding clay particle agglomeration or flocculation caused by electrolyte intercalation,forming thin and dense mud cakes,significantly reducing filtration loss.（2）Developed an organic/inorganic composite shale inhibitor based on Potassium Alginate（PA）.The influence of the composite inhibitor on the performance parameters of drilling fluids was systematically evaluated;the synergistic mechanism of the composite inhibitor was revealed,and a strong inhibitory drilling fluid formulation based on PA was established.The results showed that the composite inhibitor was significantly superior to the single inhibitor,with the 24-hour linear expansion rate reduced from 50.7%in single K₂SiO₃ to 22.4%,and the shale recovery rate after 16hours of hot rolling at 120℃increased from 70.1%to 92.4%,with the shale recovered from hot rolling exhibiting complete morphology and clear edges.In the inorganic component of the composite inhibitor,K⁺entered the interlayer through ion exchange,compressing the double electric layer and thinning the hydration film,inhibiting the hydration of shale;the polar groups of PA polysaccharide chains adsorbed,bridged,and encapsulated multiple clay particles through hydrogen bonding,forming a continuous polymer membrane structure,inhibiting the hydration of mud shale surface;the strongly inhibitory drilling fluid formulation formulation was 4.0 wt%Na-Bt+0.1 wt%Na OH+0.2 wt%XC+0.5 wt%PA+1.5 wt%K₂SiO₃,exhibiting good inhibitory,rheological,and filtrate reduction properties after hot rolling at 120°C.（3）Based on the ion chelation of polymer alginate,a delayed composite gel system with controllable gelation time and cementing plugging properties was constructed by introducing hydroxypropyl guar gum（Hpg）as a complex polymer,rigid particle calcium carbonate（CaCO₃）as a crosslinking agent,and environmentally friendly weak acid D-sodium gluconate（GDL）as an initiator,elucidating the crosslinking mechanism of SA-Hpg composite hydrogel.The effects of the types and ratios of polymers,crosslinking agents,and initiators on the gelation time and gel strength were studied.The optimal composition and dosage of the composite gel obtained were 0.6 wt%SA+0.6 wt%Hpg+3.0 wt%CaCO₃（800 mesh）+0.03 mol/L GDL;the mechanical properties and cementing plugging characteristics of the composite gel under different gelation times were explored.The polymer-interlaced network formed by SA and Hpg constituted the first-layer network structure,while CaCO₃ slowly released Ca²⁺under the hydrolysis of GDL to crosslink with SA,forming a uniform sodium alginate gel as the second network.The unreacted CaCO₃ served as skeleton particles filled into the three-dimensional network structure of the gel,significantly enhancing the gel cementing strength;the mechanical properties of the composite gel gradually improved with increasing time,with the tensile toughness and compression toughness at 180 min being 5.7 and 8.6 times higher than those at 120 min,respectively.Visualization plugging experiments showed that the composite gel had good plugging and cementing ability.After being pressurized at 3.0 MPa for 180 minutes,the leakage rates of fractured shale,gravel,and basalt rock layers with porosity rates of 35.7%,46.3%,and57.1%were only 55.1,44.0,and 22.4 mL,respectively.（4）Based on the ion competition chelation mechanism of chelating agents,two low-cost and environmentally friendly unblocking solutions were studied.The influence of pH and unblocking agent dosage on the unblocking time was investigated,revealing that alkaline conditions change the form of the unblocking agent and shorten or extend the gel disaggregation time.The results showed that in solutions with a pH of 11 and EDTA-2Na concentration of 7.0 mmol/L,and in solutions with a pH of 9 and SCT concentration of 25.0 mmol/L,the gel completely disaggregated within 5 hours,with a linear shrinkage rate greater than 70.0%,and the unblocking rate（permeability recovery coefficient）exceeding 95.0%.