Thermal Stability Of Drilling Fluids Prepared From Saudi Palygorskite
Authors: Dahab A.S. 
Published: March 2010

Abstract
High temperature in geothermal and deep wells is a major reason for the instability of drilling fluids. As a result of changes in their chemical and physical properties, problems such as gellation and formation damage can occur. Formation damage may result from high API fluid losses and reaction with formation fluid salts and hydroxides.
In this paper the effect of elevated temperatures on drilling fluids prepared using Saudi palygorskite is investigated. Geothermal drilling fluids have been formulated by adding palygorskite to a combination of polymers and copolymers in sea water to reduce fluid loss and maintaining good rheology while subjected to heat. Several mixtures exhibited satisfactory physio-chemical properties, however, the best results were obtained using polyanionic cellulose polymer and bacterially produced biopolymer.
Umm er Rahduma clays have proven to be suitable for drilling operations because of their thermal stability and ability to yield in high electrolytic environment.

Introduction
Clay based muds are the most common drilling fluid used in drilling operations because clays are commercially available, less toxic and cheaper than other types of muds. The composition of the clay based muds depends upon the requirements of the particular drilling operation. Different types of clay based muds can be used to drill through many different formations.
Several costly problems may occur while drilling deep hot wells with clay based fluids, because the rheological properties of many drilling fluids become unpredictable under bottomhole conditions. At high temperatures, mud may become thinner or thicker than it appears at the surface. Some drilling fluids react with formation fluid salts and hydroxides(1), while others form thick gels. Viscosity may also become unstable, thus affecting the penetration rates(2,3). Filtration rates usually increase at elevated temperatures. Thick filter cake may form around the wall of the well leading to tight hole and increasing the possibilities of mechanical and differential sticking. Some chemical additives also become ineffective at high temperatures(4–6).
Palygorskite clay mineral is found in Umm er Radhuma formation near Khurays, Saudi Arabia(7,8). Preliminary rheological tests at atmospheric conditions revealed that this local clay is a promising raw material for drilling fluids applications(9,10). The use of Saudi palygorskite as a basic ingredient in drilling fluids may save money that otherwise would be spent on purchasing clays from outside the area.
The objective of this work is to study the effect of elevated temperature on drilling fluids containing palygorskite as a basic ingredient and to formulate geothermal drilling fluids capable of attaining good rheological properties at high temperatures and tolerating electrolyte influx.

Experimental Set Up
A capillary viscometer was used to measure the mud effective viscosity at high shear rates equivalent to those accuring inside the drill pipe or at the nozzles of the bit. It consists of a pressure vessel, from which the mud is discharged into a capillary tube with a specified length and diameter. Discharge rates were measured at several differential pressures and the corresponding shear rate and shear stress determined. A high pressure - high temperature fluid loss cell was used to measure filtration properties.