Effects of Mud Filtrate Invasion on Well Log Measurements

Abstract

Abstract Invasion of mud filtrate into surrounding formation affects most data acquired by logging tools, which impedes accurate formation evaluation. The invasion process may result in formation damage and reduction in well producibility. The objectives of this work are to determine the effects of invasion on induction well logs, identify the invasion profile present, and to determine the factors influencing invasion. Therefore, a realistic model depicting the actual wellbore conditions is required in order to accurately analyze the invasion process, and its effects on well logs. In this work, a numerical model is developed. The physics of mud filtrate invasion in overbalanced drilling is numerically simulated using a 3D multiphase fluid flow simulator. Sensitivity analysis is done to quantify the influence of several petrophysical parameters on the spatial distribution of mud filtrate away from the wellbore, and the corresponding effects on induction logging tools. Result showed that the distribution of salt concentration due to the invasion of fresh water-base mud into a formation containing highly saline formation water affects resistivity measurements. Based on the plot of resistivity against radial distance of invasion from the wellbore, annulus invasion profile is found to be present. Also, sensitivity analysis showed that formation parameters such as porosity and permeability, in-situ fluid saturation and mudcake permeability are important factors to consider when determining the extent of filtrate invasion. A plot of formation resistivity versus formation porosity shows that a reduction of formation porosity from 0.25 to 0.1 increased the radial distance of invasion by about 70ft, and also shifted the annulus deeper into the formation. Result also showed that both medium and deep induction logs can be affected by invasion, and this depends on the total time of invasion, the formation rock and fluid properties and mudcake properties.