Impact of Heat Aging on Sediment Toxicity of Ester/Olefin-Based Drilling Fluids to Leptocheirus Plumulosus

Abstract Synthetic-based drilling fluids (SBF) have been used in a variety of drilling applications and are an important component of deepwater drilling operations. One of the applications of SBF is high-temperature wells. In order to address regulatory concerns about SBF in high temperature applications, an inter-industry study was conducted to examine the effects of temperature and time on the sediment toxicity (to Leptocheirus plumulosus) of synthetic-based fluids used for high-temperature wells. Four factors were examined: (1) ester content, (2) time, (3) temperature, and (4) alkalinity (absence or presence of green cement). Three base-stock fluids were tested, a 100% internal olefin (C1618 IO) and two traditional ester/olefin blends (10%/90% and 30%/70%). The base fluids were blended into drilling fluids and subjected to temperatures ranging from 275 to 350°F and time of exposures to temperature ranging from 16 to 160 hr. Phase 1 toxicity results indicated that temperature and time parameters bracketed the region where ester/olefin drilling fluids transitioned from passing to failing the sediment toxicity limitation. Phase 2 used a statistical experimental design to identify significant factors impacting toxicity, and a statistical model was developed to predict sediment toxicity ratios (STR). 2-Ethyl hexanol, an indicator of ester hydrolysis, was positively correlated with increased STR and could be used to screen for potential sediment toxicity. The sediment toxicity tests, analytical measurements, and the statistical STR model supported the hypothesis that, for the types of muds studied, toxicity increased for ester/olefin blends under downhole conditions of increased time and temperature. The data from this study support caution in the use of drilling fluids containing the type of esters (traditional esters) used in this study when temperatures exceed a threshold of ~300°F. Above 300°F, these materials may break down and result in increased sediment toxicity for the drilling fluids.