Research On Reopening And Propagation Of The Fracture And Leakage Law In Carbonate Formation

Carbonate reservoir is an important oil and gas reservoir,the natural closure fractures in the formation are widely distributed and complex.And the reopening and propagation of the closed fractures is an important reason for leakage in the fractured formation.It is directly related to safe and efficient drilling,and even affect the process of exploration and development.In this paper,the problem of closed fracture leakage of carbonate rock is studied.Firstly,the fracture description and identification methods of carbonate rocks are analyzed,and the fracture characteristics of carbonate rocks in Middle East HF oilfield is displayed.Considering dip angle and dip direction of the fracture,the stress state on closed fracture surface near borehole is analyzed.Meanwhile,combining the critical mechanics condition of fracture reopening and propagation,a new model of the critical pressure for reopening and propagation of natural closure fracture is established based on rock mechanics theory and Linear Elastic Fracture Mechanics(LEFM),and the corresponding solution algorithm is proposed.Furthermore,the new model is utilized in investigating reopening and propagation of natural closure fracture,which intersects with the horizontal well along direction of the minimum horizontal principal stressσ_h,for R formation of HF oilfield in the Middle East.The result indicates that the normal stress and shear stress on the fracture surface near the wellbore are of central symmetry.Moreover,critical pressure distribution of reopening and propagation of closed fracture is symmetrical alongσ_h direction.The critical pressure of fracture mouth reopening and tip propagation is minimal when dip direction of the fracture is alongσ_h direction,it reaches its maximum when dip direction of the fracture is perpendicular toσ_h direction.The critical reopening pressure of natural closure fracture of R formation is consistent with the actual mud loss pressure,therefore the new model is reliable.Based on the fluid flow model,combining with the normal stress of the fracture surface,the mathematical model of unsteady crack leakage is established with the consideration of the fracture width variation with the internal pressure.The display difference on time and the central difference on space were adopted,and the computer program was compiled to simulate the leakage rate and leakage rate of non-Newtonian fluid at different time.With the parameters of crack formation and HF oilfield,quantitative analysis of the effects wellbore and formation pressure,drilling fluid viscosity,rheological index,fracture width,crack stiffness coefficient,fracture dip angle on leakage rate and the cumulative leakage loss.The wellbore and formation pressure,drilling fluid viscosity,rheological index,fracture width,fracture stiffness coefficient loss have a great influence on the crack and fracture angle of crack leakage,leakage can be neglected.In this paper,through experiments,theoretical analysis and model building,this work can effectively avoid drilling and completion leakage,which eventually prevents drilling problems and dwindles drilling cost,as well as offer a theoretical support for fast,safe and efficient drilling.