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.
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 and Mudcake Characterization: Implications for Formation-Damage Control
01 February 2016