Study Of High Temperature Solid Free Drilling And Completion Fluid System Of Carbonate Reservoir In Shunbei Block

The Shunbei oil and gas field is rich in reserves,but the rocks are highly inhomogeneous,fractures are developed,and solid-phase particles in the incoming fluids cause serious damage to the reservoir permeability,while the current solid-free drilling fluids have the disadvantage of insufficient temperature resistance,which affects the exploration and development process of the reservoir.Therefore,in order to reduce the damage to the reservoir caused by solid-phase particles in the incoming fluid,this paper has developed a high-temperature polymer viscosifier for solid-free drilling fluid based on the study of potential damage factors and mechanisms of the reservoir in this block,explored its mechanism of action,and used it as a viscosifier to build a high-temperature solid-free drilling fluid system suitable for the Shunbei block.The results of X-ray analysis showed that the core minerals of the Yijianfang formation in the Shunbei block were mainly calcite（>95%）,with 1%clay minerals.SEM analysis showed that the reservoir core had a dense structure and fracture development.High-pressure mercury pressure experiments showed that the pore space of the core was extremely small and non-homogeneous,with pore size distribution of 10-100μm and low porosity of the reservoir.The contact angle measurement results showed that the core wetting angle is 18.70°,with strong hydrophilicity.The sensitivity evaluation experiments showed that there were strong alkali-sensitive,salt-sensitive and stress-sensitive damages（damage rate>80%）,moderately strong velocity-sensitive and water-sensitive damages,and no acid-sensitive damages in the reservoir.Indoor evaluation experiments showed that the basic properties of on-site drilling fluid such as rheology,filtration loss and settlement stability were excellent,but the temperature resistance,pollution resistance,sealing and water locking properties needed further improvement.The solid phase content of drilling fluid is high（>18%）,and the particle size didn’t correspond with the reservoir pore size.The filtrate was well coordinated with the formation,and the damage rate to core permeability was greater than 54%on average.The damage was serious and was mainly caused by solid phase intrusion.Optimized two functional monomers,AMPS and DEAM,MBA was used as the crosslinker.A micro-crosslinked polymer viscosifier SDTV was developed by homogeneous solution polymerization method and optimization of synthesis conditions.The optimal synthesis conditions:total monomer concentration was 22 wt%,AMPS was 40 mol%,DEAM was 60 mol%,MBA was 0.9 mol%,initiator concentration was 0.25 wt%,reaction temperature was 65℃,pH was 7-8,and reaction time was 5-6 h.The structural characterization of SDTV was carried out by elemental analysis,NMR hydrogen spectroscopy,infrared spectroscopy,thermogravimetric analysis,etc.The results showed that SDTV was a polymerization product of three monomers,with good thermal stability and viscosity-average molecular weight of1.37×10⁶ g·mol^-1.The experimental results of performance evaluation showed that SDTV had excellent solubility,viscosity enhancement properties,shear resistance and temperature resistance up to 180℃,and its comprehensive performance was better than similar products80A-51 and XC.The relationship between polymer molecular structure and its viscosity enhancement ability was analyzed by cryo-scanning electron microscopy.Moreover,the viscosity enhancement mechanism and temperature resistance mechanism of SDTV were studied.By selecting the preferred treatment agent and optimizing the additive-density-cost relationship of soluble salt,the high temperature solid-free drilling and completion fluid system SDWG was constructed.The performance evaluation results showed that the SDWG system had good rheological filtration,lubricity,settlement stability and temperature resistance up to175℃.Resistant to 5%NaCl,resistant to 8%poor soil,and good permeability sealing ability.Formate was used to weight SDWG system,density could be up to 1.4 g·cm^-3.Experiments results showed that SDWG system had excellent reservoir protection performance.