| In the process of horizontal well drilling,due to the positive pressure,the drilling fluid invades the reservoir and causes damage,which is mainly manifested as the invasion radius and permeability changes.Therefore,the evaluation of the degree of reservoir damage cannot solely focus on the size of the invasion radius or the permeability recovery of the damaged area.The impact of the two on reservoir damage should be considered comprehensively.The method of directly using the results of laboratory experiments to evaluate the damage degree of the on-site reservoir is not from the perspective of actual formation changes and is not accurate enough;the method of decomposing the well test interpretation results to obtain the drilling fluid invasion skin factor has a large workload and ignores a part of the pseudo-skin factor.This article starts with the model of fluids in porous media,analyzes the difference between the core intrusion radius and permeability changes in laboratory conditions and single well conditions,and studies the influence of various parameters on the skin factor of horizontal well drilling fluid during the drilling process,and establishes a method for accurately calculating the skin factor of drilling fluid invasion in horizontal well.The main research content and research results of this paper are as follows:(1)Using the results of dynamic fluid loss experiments that simulate actual drilling conditions,establish a mathematical model of fluid loss rate and time per unit length,and calculate the equivalent damage radius of single-well-scale drilling fluid based on the volume method.In the process of mud cake formation,the fluid loss rate gradually decreases until it becomes constant,which leads to a gradual increase in the invasion radius until it is basically unchanged.The shape of the drilling fluid damage area in the horizontal section is elliptical platform from the toe end to the root end.If the actual length of the horizontal section is longer than the equivalent length of the horizontal section of the ellipse platform is an elliptical platform first and then an elliptical cylinder.(2)Use the permeability gradient method to measure core linear permeability,establish a mathematical model of linear permeability recovery rate and measurement location and apply it to single well radial seepage.Using the intrusion radius calculated in the previous chapter,the radial permeability at different locations in the reservoir damage area is calculated.(3)According to the Hawkins formula and the above results,the horizontal well drilling fluid invasion local skin factor is calculated,and the total skin factor of drilling fluid invasion considering the heterogeneity and anisotropy of the reservoir is derived by the differential method.The influence of anisotropy factor,borehole diameter and ROP on the total skin factor of drilling fluid invasion is analyzed.(4)Use the interpretation results of on-site well testing to verify the above model results.The results show that the drilling fluid invasion skin factor calculated by this method is not much different from the drilling fluid invasion skin factor obtained from the well test interpretation and decomposition results,and can be used to evaluate reservoir damage.The model can also be used to optimize the drilling fluid system suitable for the reservoir,calculate the yield ratio,and provide practical and effective guidance for acidification operations. |
