Investigating Improved Rheological and Fluid Loss Performance of Sepiolite Muds Under Elevated Temperatures

Abstract

The most common commercial clay used in drilling fluids is so-called Wyoming bentonite. Since the salinity of water greatly affects the hydration ability of commercial bentonite, a fibrous clay mineral called attapulgite may be used when the water salinity is too high for the use of bentonite. However, inadequate performance of attapulgite based drilling fluids in high temperature geothermal environments requires the search for substitute clays. Therefore, sepiolite, a magnesium silicate clay mineral with fibrous texture, has been proposed as the attapulgite replacement for both the high temperature and the high salinity environment. Although there could be temperature dependent minor changes in crystalline structure, sepiolite is stable at temperatures up to 260oC. Additionally, the basic structure of sepiolite is known to be firm in saturated saline-water phase.
This study is an attempt to better characterize both rheological and fluid loss behavior of water-based drilling fluid prepared with sepiolite clay. The effects of grain size, mixing time and mixing speed on elevated temperatures have been investigated. Substantial differences on rheological properties as a function of mixing time and mixing speed have been observed. No additives other than salt have been used while formulating sepiolite muds to determine the rheological and filtration properties. The results have indicated that the sepiolite based drilling fluid is superior to the bentonite and/or attapulgite based drilling fluids in terms of both rheological and fluid loss properties under elevated temperature conditions, particularly at high salt concentrations.