Experimental Study On Sealing Of Medium Width Cracks To Improve Pressure-bearing Capacity

Fractured reservoir is one of the stratums which is frequently encountered during drilling.In the process of drilling in this kind of stratum,different degrees of drilling fluid leakage often occur,which resulting in a large amount of drilling fluid loss,increasing the drilling cost and prolonging the well construction cycle.According to the size of the leakage channel,cracks with a width of 1?10mm are of medium width.When the width of the fracture reaches millimeter level,the well leakage presents rapid leakage rate,large amount of leakage,difficult deposition of sealing material at the entrance of the fracture,and the structure of sealing layer instability and other characteristics,it is not conducive to the plugging work.At present,the most widely used sealing method is bridge sealing.However,due to the fact that bridge sealing construction depends on on-site experience,it brings a certain degree of uncertainty,such as low sealing success rate,weak pressure-bearing capacity of middle-width crack sealing layer,and poor structural stability of the sealing layer.Therefore,it is of great theoretical significance and application value to carry out experimental research on improving the pressure-bearing capacity of cracks with medium width.The paper is guided by the theories of material mechanics,rock mechanics,reservoir protection and etc.In view of the fractured reservoir of medium width of cracks,by using the combination of indoor experiment,data contrast,theoretical analysis and other research methods,put forward the plugging materials optimization index,on the basis of optimizing sealing materials,carried out by means of orthogonal test and modified compound with sealing experiment on a medium width crack,analyzed the plugging mechanism of different types of materials combination,it has been clear about the different types of sealing materials in the sealing layer formed in the process of the role of further perfecting the seal materials selection principle,formed the crack closure with medium width square optimization strategy.The multi-scale structure of fracture sealing layer is described.A single block of material particles form a microscopic scale of the containment layer,and a chain of material particles are formed by the conduction of external loads and the chain of forces that have a certain tensile strength which is a measure of the level of the containment layer,and the seal layer is the macro measure.The failure modes of fracture sealing layer are friction instability and shear instability.The mechanical and geometric parameters of the sealing materials are important factors affecting the structural stability of the fracture sealing layer.The laboratory sealing experiment of medium width crack was designed and carried out.The particle size range of the plugging materials selected in the experiment is 0.8 W?d?W,and the W is the width of the fracture section.When the fracture is within a small width range of 1-5mm,the synergistic effect of rigid granular materials,flake materials and fiber materials can be fully exerted to achieve effective plugging.The optimal plugging formula for 2mm width crack is:base slurry+5%rigid granular materials+2%flake materials+1.5%fiber materials.The pressure-bearing capacity of the plugging layer formed by the plugging slurry reached 13MPa,and the cumulative leakage was 90mL.When the crack is in a wide range of 5-10mm,the pressure-bearing capacity of the sealing layer formed by the materials combination is obviously decreased,and the sealing effect is weakened.However,adding the appropriate amount of elastic materials can effectively improve the pressure-bearing capacity of the sealing layer and the sealing effect.For cracks with a width of 5mm,the optimal combination of the three plugging materials is:base slurry+8%rigid granular materials+7%sheet materials+1.5%fiber materials,which can make the pressure-bearing capacity of the plugging layer reached 7MPa,the sealing time was 15s,and the cumulative leakage was 550mL.And the sealing layer formed by base slurry+8%rigid grainy+7%flake materials+1.5%fiber materials+5%superfine calcium carbonate+2%sqd-98+1%cottonseed shell+2%rubber particles,the pressure-bearing capacity of the sealing layer can reach 8MPa,the sealing time is 7s,and the cumulative leakage is 300mL.Compared with the combination of three sealing materials,the pressure-bearing capacity of the sealing layer is improved,and the sealing effect is greatly improved.The sealing mechanism of different material combinations was analyzed,and the roles of various types of sealing materials in the formation of the sealing layer were clarified.The rigid sheet materials mainly play the role of overturning the bridge and chocking the pressure,and the rigid granular materials mainly play the role of blocking the layer pressure skeleton.Fiber materials mainly play the role of capturing particles and improving the integrity and compactness of the plugging layer.Elastic materials mainly play a role in increasing the elastic deformation rate of the sealing layer,reducing the degradation rate of rigid particles D90,and improving the compactness and the pressure-bearing capacity of the sealing layer.The selection principle of sealing materials for medium width cracks increasing the pressure-bearing capacity is summarized.Taking the pressure-bearing capacity,accumulated leakage and sealing time as the optimization indexes,by optimizing the geometric and mechanical parameters of the plugging materials and optimizing the concentration matching of the plugging materials,the optimization roadmap of the plugging formula for medium-width cracks was constructed,and the optimization strategy of the plugging formula for medium-width cracks was formed.