Rheological property measurement of drilling ¯uids used in geothermal wells E. Santoyo a, *, S. Santoyo-GutieÂrrez b , A. GarcõÂa b , G. Espinosa c , S.L. Moya b a Dept. de Sistemas Energe Âticos, Centro de Investigacio Ân en EnergõÂa, Universidad Nacional Auto Ânoma de MeÂxico, Apdo. Postal 34, Temixco, Mor. 62580, Mexico b Instituto de Investigaciones EleÂctricas, Unidad Geotermia, Av. Reforma 113, Col. Palmira, Temixco, Mor. 62490, Mexico c Universidad Auto Ânoma Metropolitana, Av. Michoaca Ân y La Purisima S/N, Col. Vicentina 09340, Mexico DF Received 27 August 1999; accepted 10 December 1999 Abstract With the goal of increasing understanding of drilling ¯uid temperature behaviour during geothermal well drilling operations, and for providing a database for a better development of numerical wellbore simulators, an experimental work based on a rheological evaluation of drilling ¯uids was carried out. High-temperature drilling ¯uid systems (HTDFS) which are most commonly used in the Mexican geothermal well drilling industry were selected and evaluated. Eleven water-based drilling ¯uids were formulated and chemically characterised in order to carry out the rheological evaluation. Dynamic experimental tests considering the non-Newtonian behaviour of such HTDFS were performed using a coaxial cylinder-type viscometer (Fann 50C). Drilling ¯uid viscosities in a programmed temperature range of 25±1808C (at a constant reservoir pressure and a shear rate of 3448.2 kPa and 170 s 1 , respectively) were measured. These dynamic viscosity measurements were then ®tted to derive correlating equations for determining the drilling ¯uid viscosity as a function of temperature. Details of the experimental rheological tests including a complete description of the equipment and all the HTDFS used as well as the derived viscosity±temperature equations are presented. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Dynamic viscosity; Viscometer; Non-Newtonian ¯uid; Bentonite; Geothermal energy; Well drilling Applied Thermal Engineering 21 (2001) 283±302 1359-4311/01/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1359-4311(00)00003-X www.elsevier.com/locate/apthermeng * Corresponding author. Tel.: +52-562-297-74; fax: +52-562-297-42. E-mail address: esg@mazatl.cie.unam.mx (E. Santoyo).