Development And Application Of Starch-based Degradable Temporary Plugging Agent In Drilling Fluid

Reservoir protection is one of the key technologies in well drilling and well completion.The temporary plugging agent directly affects the quality of the temporary shielding plugging zone and the recovery value of the reservoir permeability.At present,the temporary plugging agents have some problems,such as high requirements for the matching of the borehole throat in the formation,single function,poorly environmental protection and so on.Aiming at those technical diffculties,a starch-based biodegradable temporary blocking agent with excellent properties was developed based on the characteristics of starch-based water absorbing resin and the basic principles of molecular structure design.The adaptive temporary blocking mechanism and its application in environmentally friendly water-based drilling fluids was studied.Starch and acrylamide,a salt-resistant monomer,acrylic acid were polymerized by grafting and crosslinking reaction,while mixing nano-calcium carbonate to prepare an environmentally friendly degradable temporary blocking agent,STA-2.Infrared spectrum analysis showed that the carboxyl characteristic peak appeared at1654cm^-1,the NH₂ bending vibration peak at 1540cm^-1,and the S=O absorption peak at1202cm^-1,which proved that STA-2 was the target product.Scanning electron microscopy showed that the surface of STA-2 had a porous structure.The nano-calcium carbonate significantly improved the shear resistance of STA-2.The total weight-loss rate of STA-2 to900°C was 65.4%,which was better than that of starch,and the soil degradation rate was more than 50%at 90 days.The the water absorption properties of the starch-based temporary plugging agent and its effect on the bentonite based slurry was investigated through experiments.Compared with other commonly used temporary plugging agents,the experiments results showed that the newly developed STA-2 had excellent properties of filtration loss and plugging,temporary plugging and environmental performance.Through the microscopic analysis of mudcake and the temporary blockage and temporary plugging of cores with different permeability,the mechanism of starch-based temporary plugging agent was explored.STA-2 ffectively blocked the holes in the mud cake which made the structure of the mud cake more compact.STA-2had good plugging performance for low permeability,medium permeability and high permeability cores.The core permeability recovery value was more than 85%,which indicated that STA-2 had self-adaptability to formation of different permeabilities.When the cores with low permeability and medium permeability reached 0.4MPa and the core with high permeability reached 0.6MPa,the recovery value of permeability could reach 85%.Based on the starch-based temporary plugging agent developed in this paper as the key agent,the agents with good performance and good compatibility with STA-2 were obtained.And a set of environmental friendly water-based drilling fluid system was further optimized.The indoor evaluation experiments showed that the system had excellent rheology,filtration and sedimentation stability with the density of 1.2g/cm³.Through evaluation experiments,it was found that expansion rate of the drilling fluid was 9.7%,shale recovery rate was 92.2%,lubrication coefficient was 0.061,and anti-pollution property was good when encountering NaCl?6%?,CaCl₂?0.5%?,poor clay?5%?.The drilling fluid system has good reservoir protection and sealing performance,biodegradability.