Abstract: To clarify the degree of damage caused by drilling fluid to low permeability tight reservoirs and to quantify the depth of drilling fluid contamination,a prediction model for drilling fluid contamination based on dual-size solid particles was established on the basis of long core injection displacement experiments.The effects of seepage time,wellbore pressure drop,solid particle mass concentration,and filter cake porosity on drilling fluid contamination damage were analyzed,and Well Y88-32 in the Y88 Block of the Changqing Oilfield was taken as an example for calculation and analysis.The results show that when the injection rate is 0.2 mL/min and the pressure limit is 30 MPa,distilled water can penetrate the long core of the Y88 interval,whereas the maximum displacement depth of the drilling fluid does not exceed 150 mm.The predictions of the dual-size solid particle drilling fluid contamination model are highly consistent with the experimental results,indicating that the model can simulate the damage process induced by solid particles and can satisfy the prediction requirements for the radius of drilling fluid contamination in the target block.Controlling the bottomhole pressure differential and reducing the invasion of drilling fluid solid particles by lowering their mass concentration can effectively reduce the contamination radius.Under field drilling parameters,the optimal evaluated contamination radius of drilling fluid for the reservoir in Well Y88-32 is 131.1 mm.The results provide a reasonable method and theoretical guidance for damage evaluation in tight reservoirs and are helpful for efficient reservoir development.

Key words: low permeability, tight reservoir, reservoir damage, drilling fluid, drilling contamination, prediction model