| The problem of lost circulation in Shunbei work area is prominent,which seriously restricts the production capacity construction in the work area.At present,the lost circulation mechanism of tight formation in the work area is not clear,and the adaptability of conventional plugging technology in this kind of formation is poor.Based on the statistical analysis of field data,theoretical calculation and indoor experimental test,this paper reveals the lost circulation mechanism of dense formation in the work area;According to the relevant theories of rock fracture mechanics,the main controlling factors affecting the bearing capacity of formation are defined.Considering the instability mechanism of sealing layer and the strengthening method of pressure bearing capacity,a large-scale real triaxial pressure plugging indoor simulation experiment and experimental device are designed to form a construction scheme of cross-linked filmforming+chemical consolidation pressure plugging.The main research contents of this paper are as follows:(1)By analyzing drilling and logging data in Shunbei oil and gas field,from the perspective of formation lithology and structural characteristics,it is considered that many types of lost circulation channels,wide distribution and easy to be induced by external forces are the main plugging difficulties of tight formations in the work area;The evolution mechanism and corresponding engineering response characteristics of lost circulation channel are clarified,which provides a theoretical basis for monitoring and prevention of different lost circulation types while drilling;The large physical model experiment of fracture propagation around the well is carried out,and the effects of drilling fluid type and borehole inclination on formation stability are studied.It is revealed that the essential reason for the induced lost circulation of tight formation in the work area is the opening of weak cemented bedding and the slip dislocation of shear fractures,forming a multi fracture interleaved fracture network around the well.(2)Through the stress analysis of fracture surface,the variation law of fracture tip intensity factor with fracture length in Permian and Silurian strata is calculated,the differences of different plugging positions on regulating the pressure in the fracture are compared and analyzed,and the plugging theory of avoiding inducing the malignant expansion of the fracture to form serious lost circulation by regulating the pressure in the fracture is put forward;Based on the prediction model of shear instability,friction instability and pressure bearing capacity of plugging layer,the effects of fracture closure pressure,friction angle between plugging layer and fracture surface,internal friction angle between particles and porosity of plugging layer on the pressure bearing capacity of plugging layer are analyzed.(3)In order to objectively reflect the nature of the lost circulation layer,the true triaxial pressure plugging experimental device is optimized and improved.Based on the data of theoretical analysis,the plugging scheme evaluation experimental scheme is designed,and the effects of engineering measures such as plugging position,initial injection pressure,injection speed,pump shutdown time,plugging while drilling and re plugging after pressure failure on the pressure bearing capacity of the plugging layer are explored,The pressure bearing plugging construction technology of "high injection and high extrusion" crosslinking film formation + chemical consolidation has been formed;Based on the distribution of plugging particles on the fracture surface,the flow law of plugging slurry and the internal structure of plugging layer are analyzed.The research results of this paper provide strong technical support for the lost circulation control treatment of Shunbei tight formation,and provide an effective reference for the pressure plugging technology and construction scheme optimization of fracture propagation formation. |
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