Filtrate and Mudcake Characterization: Implications for Formation-Damage Control
Authors: Wilson Adam 
Published: January 2017

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 174273, “Study of Filtrate and Mudcake Characterization in High Pressure/High Temperature: Implications for Formation-Damage Control,” by Saeed Salehi, SPE, University of Louisiana at Lafayette; Ali Ghalambor, SPE, Oil Center Research International; Fatemeh K. Saleh, University of Louisiana at Lafayette; Hadi Jabbari, SPE, University of North Dakota; and Stefanie Hussmann, University of Louisiana at Lafayette, prepared for the 2015 SPE European Formation Damage Conference, Budapest, Hungary, 3–5 June. The paper has not been peer reviewed.
This paper focuses on experimental methods quantifying water-based muds and investigating effects on particle bridging, filtrate invasion, and permeability. To show the particle-bridging effect, high-pressure/high-temperature (HP/HT) filtration tests were conducted on sandstone cores with permeability ranging from 10 md to more than 1100 md. Analytical models were used to calculate mudcake permeability for the tests using different mix designs. The results from this study can be applied to designing wellbore-strengthening fluids to mitigate formation damage.
Introduction
In an overbalance situation, the fluid phase of the mud, called filtrate, invades the formation, whereas solid particles of the mud build up mudcake. Characteristics of filtrate and mudcake are strongly controlled by mud particle type, size, and concentration. It is often desirable for a mud to leave a thin, low- permeability cake that helps with near-wellbore stability and strengthens the wellbore. The goal is to reduce the amount of whole mud flowing into the formation and to prevent loss of circulation, which causes many drilling-related problems.
Drilling-fluid particles present different sizes; the larger particles form the first layer of the filter cake, and the smaller particles deposit within the cake formed by the larger particles. At the same time, the filter cake is undergoing compaction by the effect of the fluid drag as the smaller drilling-fluid particles are flowing through the filter cake. As a result of the deposition and compaction undergone by the filter cake, the thickness of the cake and its porosity and permeability will vary, thus affecting the performance of the filtration. During filtration, new particles are deposited on the surface of the cake and, over time, the thickness of the cake increases until filtration subsides.
Filtrate behavior of the drilling fluid affects the permeability of the filter cake. Thin mudcakes, which have low permeability, strengthen the wellbore; however, thick mudcakes can cause operational problems such as stuck pipe, excessive torque, drag, and high swab and surge pressures. Lost-circulation material, such as properly sized calcium carbonate, can prevent leaks of whole mud into the formation, which, in turn, prevents leaks of the base fluid such as oil or water. These two chain processes significantly increase wellbore strength to prevent fractures or fracture propagation.