An invaded zone forms when mud filters into a permeable formation during and after drilling. The mud filtration process is affected by mudcake buildup, well pressure overbalance, and mud properties. Mud invasion changes the spatial distribution of resistivity in the near-wellbore zone – data that could be estimated from inverting the results of resistivity logging data. Invasion simulation can be used to predict invasion depth, mudcake thickness, distribution of resistivity, and formation permeability.
This paper presents an invasion simulation algorithm developed from the Buckley–Leverett filtration model, which takes into account mudcake buildup. This algorithm is used to calculate the near-wellbore water saturation and salinity distributions. These characteristics are used for resistivity profile calculations by an Archie-type equation.
The main goal of this paper is to propose a method of mudcake data interpretation based on a mud filtrate invasion simulation algorithm. This method yields the mud filtrate volume, water saturation and salinity distribution. To verify the results, the values obtained were compared with the results of the induction log and laterolog inversion from several vertical wells. This approach increases the significance of the mudcake thickness data and illustrates the practicality of the joint interpretation of resistivity log data and the mudcake thickness (the former being highly sensitive to resistivity changes within the invaded zone).
In addition, this paper suggests a technique for evaluating formation permeability from mudcake thickness. This technique is applicable if the mudcake was not damaged during the drilling operation. The algorithm was tested and verified by the log and core data obtained at several vertical wells.