Computation of Mud Filtrate Invasion Profiles
Authors: Phelps G.D. 
Published: March 2010

Abstract
The invasion of permeable formation by mud filtrate plays a central role in the log interpretation process. Logging tools measure near wellbore properties, so tool response and interpretation are strongly influenced by invasion and just as importantly, by the log analyst's perception of invasion.
This paper considers the physical mechanisms underlying the invasion process and shows how they govern the formation and evolution of saturation, pressure and resistivity profiles.
One dimensional (radial) and two dimensional (radial-vertical) numerical models have been developed in order to simulate the coupled fluid flow and filtercake growth models. Results obtained indicate that the relationship between fluid loss rate and formation permeability is a complex one, depending as much on the filtercake properties and drilling history as on formation permeability. Other results presented include the behaviour of filtrate in layered systems, in which filtrate crossflows from low to high permeability layers, and the accumulation of filtrate at oil-water contacts and permeability barriers due to gravity segregation.
The invasion profile is intimately linked with the growth and' nature of the filtercake. The development of a suitable model for the growth of the filtercake under both dynamic and static filtration conditions is a key element of the paper. Factors controlling' the cake properties under different filtration regimes are discussed and numerical simulation is used to show how these factors influence the invasion profile. The affect on invasion profiles of cake removing operations such as reaming tripping or logging are also investigated, and it is shown that there is a; threshold above which cake removal has a significant effect on the invasion profile.

Introduction
An unavoidable consequence of current drilling practice is the invasion of permeable formation by drilling fluid filtrate. This invasion results in a redistribution of formation fluids and alteration of pressure, saturation and resistivity profiles around the wellbore. Knowledge of these changes, and by implication determination of the original uninvaded state, is the prime goal of the log analyst.
Considerable advances in logging tool design and interpretation methodologies have not been matched by similar advances in the understanding of the invasion process. This is largely due to the apparent complexity of the process, a combination of different types of filtration mechanism along with multicomponent flow in the formation.
Work on invasion has tended to be either directly related to log interpretation or has taken the form of experimental mud fluid loss studies. Early examples of the former can be found in the work of Dumanoir et al.(1), Cambell and Martin(2) and Gondouin and Heim(3). Examples of the latter are numerous, Williams and Cannon(4) and Ferguson and Klotz(5) are just two of a great many early papers devoted to experimental studies of static and dynamic filtration. Vaussard et al. (6) and Fordham et a1.(7) are examples of more recent experimental work on dynamic filtration.