Visualization of Fluid-Loss Polymers in Drilling-Mud Filter Cakes

Abstract

Summary The appearance of fluid-loss polymers in freeze-dried drilling-mud filter cakes was studied with scanning-electron-microscope (SEM) photography. Three fluid-loss polymers were studied: starch, polyanionic photography. Three fluid-loss polymers were studied: starch, polyanionic cellulose (PAC), and a high-temperature-(HT)-stable, sulfonated polymer. The effects of electrolyte contamination (NaCl, CaCl2, and MgCl2) and temperature (200 to 350 degrees F) on the appearance of the fluid-loss polymers were also studied. A correlation between API filtrate and polymer polymers were also studied. A correlation between API filtrate and polymer appearance was sought. Introduction Filtration-control additives for water-based drilling fluids have been used since the early 1930's to prevent leakoff of water from the drilling fluid to the formation. Organic polymers constitute by far the largest group of filtration-control additives. Important members within this group are starch and cellulose gums like carboxymethyl cellulose or PAC. Recently developed synthetic organic polymers provide superior HT stability and electrolyte tolerance polymers provide superior HT stability and electrolyte tolerance compared with the semisynthetic starch and cellulose products. Surprisingly little research has been done to investigate the mode of action of such fluid-loss polymers despite their importance in present drilling technology. Previous studies were focused more present drilling technology. Previous studies were focused more on understanding the filter-cake structure through application of theoretical models and evaluation of fluid-loss data. Use of an SEM to study filter cakes containing fluid-loss polymers has been limited because we lack a technique to prepare dried filter cakes without changing their structure. A shock-freezing/freeze-drying method that does not alter a filter cake's basic structure during the drying process was recently developed. This method is validated by a comparison of freeze-dried and frozen-hydrated samples. The frozen-hydrated technique allows direct viewing of frozen specimens without water removal. The same characteristic "house-of-cards" arrangement (honeycomb structure) of bentonite particles was found in freeze-dried and frozen-hydrated samples, which verifies that the freeze-drying technique used in this study allows filter cakes to retain their basic structure and thus is suitable for SEM investigation. Theoretical Background Clay theory says that in an aqueous bentonite slurry, the positively charged edges of clay platelets attach themselves to the negative surfaces of other platelets. The resulting edge-to-face arrangement leads to a gel structure with a hexagonal honeycomb configuration. During an API filtration test, clay present in the hexagonal structure is pressed against the filter paper. The applied differential pres-sure of 100 psi is insufficient to disrupt the electrostatically pres-sure of 100 psi is insufficient to disrupt the electrostatically induced edge-to-face attachment between the clay platelets, so the hexagonal structure is visible in the filter cake. Filtrate volume is determined by filter-cake porosity and permeability. The dense gel structure and water-binding capability of permeability. The dense gel structure and water-binding capability of hentonite lowers the filter-cake permeability. Electrolytes disturb the electrochemically induced honeycomb structure of a bentonite slurry, resulting in higher API filtrates. This is especially the case with salts containing the divalent calcium and magnesium ions. Temperature aging also deteriorates the structure. Partial dehydration and face-to-face attachment of the bentonite particles result in losses of gel structure and thixotropy. Filter-take porosity increases with the coagulation of clay platelets, resulting in poor filtration control. Organic polymers are used to protect the bentonite from the negative effects of electrolytes and temperature. Anionic polymers are believed to attach with one end of their negative molecular chain to the positively charged edges of clay platelets, resulting in polymer chains that extend like fingers into platelets, resulting in polymer chains that extend like fingers into the cake pores. The fingers may bridge adjacent walls of a pore, depending on the polymer's molecular weight (chain length) and the pore size. One objective of this study was to investigate the validity of the theoretical models on bentonite/polymer interaction.