Research On Instability Mechanism And Drilling Fluid Technology Of Coal Seam Borehole Wall In Yanchang Gas Field

With the development of oil and gas,the instability of coal seams in Yanchang gas field is becoming more and more serious.This paper mainly focuses on the mechanism of borehole instability and drilling fluid technology in Yanchang coal seam.Taking Shanxi formation coal and rock of Yanchang gas field as the research object,this paper systematically analyzes the general situation of actual drilling in Yanchang gas field,the characteristics of coal and rock fabric and the mechanical properties of coal and rock,and from the point of physics,The mechanism of coal seam instability in prolonging gas field is revealed in chemical and mechanical aspects,and the technical countermeasures of coal seam drilling fluid with reasonable density,strong plugging,effective inhibition,good lubrication and reasonable rheology are formulated.It provides experimental and theoretical basis for the study of water-based anti-sloughing drilling fluid technology in coal seam of Shanxi formation.In view of the characteristics of coal seam micro-fissure development,fragmentation,poor cementation and easy collapse in Shanxi formation of Yanchang gas field,a "simulated core" was prepared to simulate the pore fractures of coal and rock,and the evaluation method of coal-rock plugging was established.A blocking agent CRPA,was synthesized by emulsion polymerization using styrene,butyl methacrylate,Allyl trimethyl ammonium chloride as monomer.The average molecular weight of blocking agent CRPA,was 68505 and the particle size was 8.94 μ m.The best addition of CRPA was 2.0%,the permeability reduction rate Kr was 60.53%,and it had better plugging ability.The best plugging agent was 3.0%ultra-fine CaCO3(2200 mesh:1250 mesh:400 mesh=1:3:1)1.5%white asphalt 2.0%CRPA.The permeability reduction rate Kr was 78.42%.According to the drilling fluid technical requirements and technical countermeasure s,a strong plugging and anti-collapse drilling fluid system(KCl polysulfonate drilling fluid system)suitable for Shanxi Formation coal seam in Yanchang gas field was cons tructed.Its formula are respectively as follows:4.0%mud+0.15%XC+0.5%HF-1+7.0%KC1+2.0%M-SMC+2.0%CMJ-2+3.0%CaC03(2200 mesh:1250 mesh:400mesh=1:3:1)+1.5%white asphalt+2.0%CRPA+0.5%JN-303(300 mesh to 400 mesh CaCO 3 aggravated to 1.20g/cm3).4.0%mud+0.20%XC+0.5%HF-1+7.0%KCl+3.0%M-S MC+3.0%CMJ-2+3.0%CaC03(2200 mesh:1250 mesh:400mesh=1:3:1)+1.5%whit e asphalt+2.0%CRPA+0.5%JN-303(300 mesh to 400 mesh CaCO3 aggravated to 1.25g/Cm3).After aging at 120℃ for 16 hours,its properties were comprehensively eva luated.The system had good rheological properties,wall-building property,inhibition a nd plugging property,lubricity,Caking property,anti-pollution,temperature resistance and thermal stability,the performance of the system is significantly better than the pro ject evaluation index.Comparison with clear water and on-site drilling fluid,The compressive strength of coal and rock in Shanxi formation of Yanchang gas field was reduced by 53.26 MPa,and the increment of coal-rock collapse pressure was the lowest,only 0.06g/cm3,which fully showed that the system had excellent sealing and inhibiting performance.The coal and rock wall stability of Shanxi formation in Yanchang gas field can be effectively guaranteed.In view of the coal seam of Shanxi formation in Yanchang gas field,the design and preparation of drilling fluid properties,maintenance and treatment measures and application technical requirements are integrated into one,and the matching application technology is put forward.