High-Temperature Flow Properties of Water-Base Drilling Fluids
Authors: Annis Max R. 
Published: December 2007

Abstract
Drilling mud rheological and gel property changes due to elevated temperatures frequently cause problems in drilling deep wells. A laboratory investigation of the effects of time, temperatures up to 300F and mud composition on these properties was made with concentric-cylinder, rotational viscometers of the Fann type. The viscometers were sealed from the atmosphere to prevent dehydration of the sample. Gel strengths were found to be more sensitive than viscosities to changes in temperature and mud composition. The high-temperature gel strengths of bentonitic clay drilling fluids were not predictable from low-temperature measurements. At high temperatures, the clay flocculates. The flocculation temperature depends on the concentration and type of solids in the mud and the degree of chemical deflocculation. As the bentonite content of a mud increases, flocculation temperature decreases. Lignosulfonate treatment raises the flocculation temperature. To minimize adverse high-temperature effects on mud properties, the mud should be maintained at minimum solids content, properly treated with a deflocculant which is stable at the maximum temperature encountered and maintained at a pH high enough to insure activation of the deflocculant for the duration required. The Methylene Blue Test for cation exchange capacity of a clay was found to be a useful guide in determining when a mud contains a dangerously high clay content.

Introduction
In drilling a well it is desirable, for several reasons. to know the effects of time and temperature on the rheological and gel properties of the drilling fluid in the hole. Problems associated with drilling fluids normally occur down-hole, so it is reasonable to be interested in the properties of drilling fluids at the conditions which exist down-hole. It has long been observed that high pump pressures are often needed to start circulation after a trip and that the "bottom-up" mud that is circulated back to the surface is usually more viscous than it was before "cooking" at the bottom of the hole for several hours. Burkhardt showed that a knowledge of rheological properties of muds under down-hole conditions is necessary to predict pressure surges. He showed that the viscosity of a mud greatly influences the magnitude of pressure surges and the likelihood of lost returns. McLean et al. showed that the rheological properties of a mud under downhole conditions are very important to cementing operations. This paper presents data showing the effects of time and temperature on rheological and gel properties of water-base drilling fluids. Measurements of gel strengths and rheological properties were made with two different concentric-cylinder rotational viscometers of the Fann type. One was the conventional six-speed viscometer modified to operate at temperatures up to 350F and at the vapor pressure of the liquid at test temperature. The other viscometer was newly designed to operate at temperatures up to 500F and pressures up to 1,000 psi. Tests with both instruments were made under essentially the same operating conditions. The new viscometer is shown in Fig. 1.