Simulation Of Oil-based Mud Invasion Process And Its Characteristics Analysis

During overbalanced drilling,the drilling mud penetrates the permeable formation,which leads to changes of the physical properties of the rock and the corresponding logging response and the difficulty of reservoir logging evaluation.With the development of exploration and exploitation,Oil-based mud is applied more and more extensively.However,the understanding of the invasion mechanism and characteristics of oil-based mud is still insufficient at the present.Therefore,conducting relevant research is of great significance to further understand the invasion mechanism of drilling mud and improve logging response correction method.In this paper,the characteristics of oil-based mud invasion are studied combining numerical simulation and physical simulation simultaneously.In terms of numerical simulation,firstly,based on the oil-water two-phase seepage equation and the formation mechanism of mud cake,the mathematical model of drilling mud invasion concerning dynamic growth of mud cake was established.Then the finite difference discrete is performed in the cylindrical coordinate system and the pursuit method was used to solve the differential linear equations.Therefor the finite difference numerical simulation of oil-based mud was realized.In terms of physical simulation,using a self-developed multi-functional physical simulation system as the experimental platform and large-sized formation module as the experimental object,The physical simulations of oil-based mud invasion under various formation properties,pressure differences and invasion time are realized.The invasion characteristics of oil-based mud are studied by analyzing the dynamic data such as the resistivity and filtration velocity recorded during the experiments.Finally,the experimental laws are used to calibrate the corresponding numerical simulation results and expand the results to the formation scales.According to the methods above,the radial resistivity of the sandstone module increases gradually away from the wellbore,which is consistent with the characteristics of increased resistance invasion during oil-based mud invasion.During the whole invasion,the filtration velocity of mud filtrate and the invasion depth change rate in the low porosity and low permeability formation is higher than that in the medium-high porosity and permeability formation,which is because the formation velocity of mud-cake is slower in the low porosity and low permeability formation.Compared with water-based drilling fluid,the invasion depth of oil-based mud is shallower,which is mainly determined by high viscosity of oil-based mud.