Theoretical Study On Plugging Leakage Zone & Water Zone In Drilling And Preparation Of New Plugging Agent

Lost circulation and formation water out are typical complex circumstance in oil & gas well engineering fields, which have detrimental effect on the high-quality design and implement of drilling and completion. The solution for the problems is that new plugging agents should be prepared while the new method is being explored theoretically in depth, and the new technology should be formed finally.In view of lost circulation which is an extreme manifestation for drilling fluid invading formation, pursuant to Darcy's equation, referring to momentum conservation law of hydromechanics theory, and based on the analysis of inhibiting drilling fluid from invading formation, one dimensional model and two dimensional model for drilling fluid invading formation were built. The model study shows that, the key for successful plugging is to decrease formation permeability and porosity and to form tight cake on the wellface, it is unfavorable for plugging that the density and volume of grain-typied plugging materials are high enough to shock the formation, grain-typied materials corresponding with grain composition theory can plug the lost circulation better than others, forming barrier bed with far low permeability in microcrack is the key to keep fracture from expanding and to plug leakage zone of this type successfully, thixotropic cement slurry can provide an effective way for the plugging of vicious leakage zone with cavities and fractures.Referring to the theory of fluid kinetics, the model for formation water out was built on the basis of the reason analysis on formation water out. Studies on the physical model show that, the more farer the intake distance of water shutoff agent in water zone is, the more higher the final viscosity of blocking agent is, the more stronger the resistance between formation and blocking agent is, it is benefit for the blocking of water zone. Gel materials with low initial viscosity and high viscosity can be selected as effective blocking agents.Based on analysis of the mechanism and deficiency of conventional polymer gel , and referring to the fundamentals of crosslinking reactions, a new gel-typed plugging agent with controllable gelation time and high strength was prepared, and its mechanism was testified by infrared analysis. A new evaluation device for gel's mechanics performance was designed and established in view of the deficiency of existing evaluation device. Abundant experiments results show that, the new plugging agent is thermostable, it has property of pH resistance and it can resist to hypersalinity. Comparing to the conventional ones, the new plugging agent has better property of right angle thickening, its thickness can exceed 100Bc, its static fluid column pressure can remain stationary during gelation process, and it also has shorter and controlled gelation time. The shear force between the gel and the pipe wall is so low that the gel can be expressed from the pipe even if the gel is formed, which can guard against downhole accidence.The pressure resisting coefficient of the gel increases with the increasing of temperature which is favor of its sealing effect in formation. The plugging agent can bear stronger pressure in fractures, which can be used to plugging vicious lost circulation in coordination with thixotropic cement slurry. The new plugging agent can be applied for plugging in reservoir, because it's property of water shutoff is better than its property of oil layer plugging. The plugging agent has good rejection capability and it keep the shale from expanding effectively. Comprehensive researches show that, the new plugging agent has good properties of plugging agent and blocking agent.