Catastrophic Instability Mechanism To System Consisted Of Plugging Zone And Rock In Shale Formation

With the increasing demand of oil and gas and the shortage of conventional oil and gas resource, shale gas has become popular in the field of exploration and development of unconventional oil and gas resource. Shale gas is very rich in China, and the exploration prospect is great. However, severe lost circulation during the drilling and completion impedes the exploration and development of shale reservoirs in China, and brings great negative effects on reservoir protection, environment protection, drilling cost control and so on. Therefore, the study on bearing stability mechanism and control method of lost circulation is very significant.This paper focuses on lost circulation in shale formation, considering rock mass, rock-plugging zone and plugging zone as a plugging system, analyze influence of three factors on structural stability of the system, and reveals the destabilizing mechanism of plugging system. Firstly, based on the engineering and geological characteristics of shale formation, these inducing factors of lost circulation are analyzed. Secondly, from the view of rock mass structural failure, influencing mechanism of shale erosion by high pH drilling and completion fluid on plugging system instability was discussed. And then, rock-plugging zone interaction face fraction coefficient test was performed, basic parameters for plug-ging system model was drawn. Finally, based on inducing systematic instability, catastrophe theory was induced in, and rock mass fracture width predicting model, frictional sliding model for rock-plugging zone and instability catastrophe model for plugging system were developed. On the basis of test and typical cases analysis, reliability of the models was checked. Through above study, the following conclusions are achieved:Potential inducing factors of lost circulation are revealed. Based on engineering and geological characteristics of shale, oil-based drilling and completion fluid loss features and lost circulation materials (LCMs) are described. The inducing factors of lost circulation cover physical factors and chemical factors, the former including fracture development, working fluid lubrication, LCMs sedimentation and low bearing strength of plugging zone. This paper mainly works on fracture face traction coefficient. The latter mainly including alkali erosion caused by high pH drilling and completion fluid. Combined with in-situ data, influencing mechanism of the factors on shale formation plugging system instability was discussed comprehensively.Plugging system instability mechanism induced by shale erosion by high pH drilling and comple-tion fluid was revealed. Based on chemical kinetics principles, reaction kinetic parameters of alkali erosion at condition of pH with 9?14 and temperature with 40?90 ?, and corresponding kinetic model was drawn. Research indicates that power function could be applied to describe the relationship between the reaction rate and pH, and the exponential function reflects the relationship between the reaction rate and temperature. In addition, the reaction rate constant increases at an exponential function with the temperature increasing, while the reaction order increases at linear function. Frequency factor increases at a power function with increasing pH value, while the active energy increases at logarithmic function. The minerals and organic matter in shale are all involved in the reaction of alkali erosion, and the reaction products mostly consist of silicates, some dissolved organic matters, and a few carbonates. The alkali erosion of shale can increase fracture width, strengthen stress sensitivity, and weaken rock strength. These adverse impacts lead to lost circulation and wellbore instability in shale formation and causes strong negative impact on shale gas well productivity.Changing characteristics and influence factors of plugging system friction coefficient are described. Friction coefficients between LCMs and shale and that between LCMs under different lubrication conditions are tested by using self-made friction coefficient instrument. Results show that friction coefficient between LCMs and shale is the greatest under dry condition, and fluid lubrication can reduce friction coefficient.3%KC1 and pH=11 alkali solutions can reduce friction coefficient mildly at similar rate, while white oil can make a biggest reduction on friction coefficient. Under same lubrication condition, the changes of friction coefficient between LCMs are similar with that between LCMs and shale. In conclusion, surface roughness, interface wettability, fluid property, normal pressure and soaking time can affect friction coefficient. Introduction of fraction theory provides critical parameters for representing plugging zone fraction coefficient quantificationally, and lays theoretical basis for solving plugging system instability by system science. The friction coefficient is the basis parameter for plugging pressure prediction model.Bearing instability catastrophe model for shale formation plugging system was drawn. Rock mass, rock-plugging zone interaction face, and plugging zone were considered as one system, and instability inducing physical and chemical factors were taken into account, and then plugging zone instability mode and evolutionary stage were discussed on the basis of system failure evolutionary characteristics. Combined with catastrophe theory of system science, predicting model for fracture width increment induced by drilling operation and alkali corrosion, and constitutive model for plugging zone deformation were drawn respectively. Furthermore, cusp catastrophic model for plugging system instability based on system energy conservation principle. Combined with necessary and sufficient condition of plugging system instability, plugging zone bearing capacity predicting model was derived by solving catastrophe model. Based on plugging system catastrophe instability model, influencing mechanisms of rock strength, alkali erosion, plugging system fraction coefficient, plugging zone strength, and other parameters on plugging system bearing stability were revealed.Check results indicate that plugging pressure error of plugging system bearing instability catastrophe model is smaller than 5%. Besides, in-situ cases prove the plugging pressure prediction model is highly reliable. The research results not only can provide theoretical foundation for loss control of shale formation, but also be applied to the lost circulation control for tight sand and carbonate formations. The research results enriches the theory system of lost circulation conrol, and provide basic parameters for drilling design and theory guides for LCMs optimization and drilling design, and improve the prediction accuracy of plugging pressure.