Numerical Calculation of Multi-field Damage Coupling Fracture Initiation Pressure for Ultra-deep and Extra-deep Carbonate Reservoirs
Ultra-deep carbonate reservoirs are characterized with high initiation pressure,leading to difficulties in fractures initiating.Acid can react with the reservoir matrix,enhancing porosity,increasing permeability,and deteriorating the mechanical properties of the rock,thereby reducing the initiation pressure.However,there lacks accurate calculation methods of fracture initiation pressure for acid-damaged carbonate reservoirs,making it challenging to design initiation pressure reduction measures.This study tested the dynamic Young's modulus of cores after drilling fluid immersion and acid displacement,establishing a damage evolution equation for carbonate rocks under different fluid disturbance states.In addition,a numerical calculation model was established to couple flow,chemical,and stress-damage fields during acid fracturing to estimate the fracture initiation pressure.The results indicate that when both drilling fluid and acid fluid are affected,for reservoirs with a porosity below 4.32％and an acidizing time less than 4.08 minutes,the damage factor is below 0,which means that it is unable to mitigate the increase in Young's modulus caused by drilling fluid.During the operation of"acid displacement of wellbore+acid immersion damage+acid fracturing",the 8 833 m section of Well Pl reached the fracture initiation condition at the 73rd minute with the damage factor of 0.301 and fracture initiation pressure decrease of 29 MPa;as a result,this well was opened successfully.The calculation deviation of the model ranged from 1％to 5％,resulting in an improved accuracy of 3 to 10 percentage points when compared to traditional analytical models,this indicates that the model is particularly valuable for the calculation of fracture initiation pressure and designing acid damage measures in the Dengying Formation,as well as in similar carbonate reservoirs.
carbonate rocksultra-deep and extra-deep reservoirsacid damagefracture initiation pressuretechnology of reducing fracture initiation pressure
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