The drilling fluid contamination is a major problem during oil and gas well cementation,especially when such contamination comes from cement.The treatment is usually done with sodium bicarbonate (NaHCO3)where pH control is not a problem but with sodium carbonate(Na2CO3)to increase the pH of the drilling fluids with low pH.Determining the effective concentrations of these treating agents have always been a challenge as either the drilling fluid is undertreated or over treated which could be detrimental to the entire fluid system and the drilling operations.Therefore,there is a need to determine the effective concentrations of the chemical agents for the treatment of cement contamination.During cementation,the calcium hydroxide[Ca(OH)2]released by the cement decomposes and disperses calcium ions in the drilling fluids which could lead to flocculation of the fluid system and consequently increases the rheological and the filtration properties.When this occurs,the contaminated fluid system may be treated with some chemical agents to precipitate the calcium ions formed depending on the severity.The challenge remained on the determination of the effective concentrations of the chemical agents that will optimally treat the fluid system without causing harm. This paper investigates the effects of cement contamination on the rheological and filtration properties of a typical KCL/Polymer Water Based Drilling Fluids and also determines the effective concentrations of the treating agents(Sodium Bicarbonate and Sodium Carbonate)in the treatment of the cement contamination.A fresh KCl/Polymer fluid was formulated to serve as a control standard and was then contaminated with certain amount of class-G cement at varying proportions.The contaminated fluids were treated with different amount of Sodium Bicarbonate and Sodium Carbonate respectively to determine the effective concentrations required for the treatment of the different levels of the contamination.The drilling fluids properties such as plastic viscosity,yield point,gel strength,pH and fluid loss were adversely affected when contaminated with the cement.Experimental data were generated and linear correlations were developed to determine the effective concentrations of the sodium bicarbonate and sodium carbonate required for optimal treatment of specific amount of the cement contamination during cementation.This will act as a proactive step during oil and gas well cementation to optimally treat cement contamination, thereby reducing drilling time and minimizing overall well cost.
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