Enhancing Thermal Stability of Inhibitive WBM: Fluid Design for HPHT and Geothermal Drilling

Abstract This paper aims to develop an inhibitive potassium/polymer-based drilling fluid for high-temperature applications such as geothermal drilling. Three clay types, bentonite, sepiolite, and attapulgite, and two synthetic polymers, Polymer-1 (copolymer) and Polymer-2 (terpolymer) were evaluated to control rheology and fluid loss of the fluid system. Potassium chloride (KCl) was used with two encapsulating polymers (Polymer-1 and 2) to provide shale inhibition. Three fluids were formulated: KCl/Polymer/Bentonite, KCl/Polymer/ Attapulgite, and KCl/Polymer/Sepiolite. Clays were used as rheology-modifiers and fluid-loss reducers, KCl as a hydration-suppressant, and two polymers for fluid-loss control and shale encapsulation. Samples were aged at 400°F and 400-psi for 16 hours at roller oven before testing. The best formulations were determined from post-aging rheological and filtration properties. Selected formulations were tested in a shale fluid interaction testing cell using Mancos shale core samples, simulating downhole dynamic conditions. This approach allowed us to pinpoint the most effective mud formulations for HPHT conditions through rigorous evaluations and screenings. Two drilling fluid formulations, KCl/Copolymer/Bentonite mud (Formulation-1), and KCl/Copolymer/ Attapulgite mud (Formulation-2), provided desired properties under HPHT conditions. Formulation-1 produced a higher-quality filter cake than Formulation-2. Although Formulation-3 produced the highest quality filter cakes among other fluid formulations, it was eliminated because it could not provide the desired rheological properties. Regarding fluid loss, both formulations achieved comparable low fluid loss values. Therefore, both formulations achieved this goal equally. Rheologically, Formulation-1 and Formulation-2 showed quite good stability after thermal aging. Formulation-1 is significantly more effective in enhancing rheological performance than Formulation-2. Formulation-1 provided higher plastic viscosity and yield point values before and after aging than Formulation-2. The high yield point values of Formulation-1 after aging indicate that this formulation will be more effective in hole cleaning. Formulation-1 and Formulation-2 successfully inhibited shale reactivity with Mancos shale core samples, meeting the shale inhibition target even at the lowest KCl concentration of 4%. This paper contributes novel insights by showcasing the comparative performance of various clay-polymer combinations under HPHT conditions, providing engineers with actionable design guidelines to refine drilling fluid formulations for improved thermal stability. It directly tackles the challenges faced in geothermal well drilling and supports the industry’s move towards more sustainable practices. Additionally, this research furthers operational safety by ensuring the inhibitive characteristic of the proposed fluid formulations, mitigating drilling fluid performance risks related to wellbore instabilities, and promoting safer drilling practices.