Abstract

The development of synthetic-based drilling fluids (SBMs) has generated the need for new analytical techniques to identify oil contamination and to distinguish SBMs from oil-base muds (OBMs). In response to this need, laboratory techniques using gas chromatography/mass spectrometry (GC/MS) and field techniques using portable gas chromatography with a flame ionization detector (GC/FID) have been developed and successfully applied to SBM in the North Sea, Gulf of Mexico, and other global locations. This paper describes the analytical techniques and equipment used to monitor SBMs and reviews field case histories where contamination of SBM was identified.

Introduction

In the early 1990's, synthetic-based drilling fluids were developed as environmentally safe alternatives to oil-base muds. Offshore guidelines have prohibited the discharge of OBMs and OBM cuttings in US waters for some time. North Sea operators have been required to perform detailed chemical and biological site assessments of locations that use OBM. Development of SBMs made it possible to have a drilling fluid with all the drilling benefits of OBMs and improved environmental benefits.

When SBMs were first introduced, it was necessary to develop analytical tools to monitor the quality of the base fluids and the resulting drilling fluids. These tools would allow the determination the initial composition of the base fluid that was to be used in the drilling fluid; identify any changes have occurred due to contamination; and determine if there was any change in composition of the base fluid received from the supplier. Potential sources of contamination are:

  1. Crude oil contamination while drilling, influx of formation fluid or kick.

  2. Diesel oil contamination from contact with OBM during shipping to and from the well, at the mixing plant, or during storage between jobs.

  3. Mineral oil contamination due to contact with mineral oil-based muds.

  4. Other contaminants such as surfactants, detergents, lubricating oils, hydraulic oils, etc. that are found at the well site, in the mixing plants, or in the boats that transport the muds.

In addition to possible contamination, occasionally surface phenomena will be seen in the waters around the drilling site. This might be small drops to large patterns. To satisfy the MMS, EPA, and US Coast Guard as to the chemical Composition of the surface phenomena and whether it is related to the SBM used at the site, it is necessary to have a means of collecting samples of the material on the water and testing the composition of the surface film.

Analytical test procedures have been developed for both onsite monitoring and offsite laboratory testing of SBMs, SBM cuttings, surface film samples, and seafloor sludge samples.

Chemistry of SBMs and OBMs

The EPA definition for SBM, as outlined in the Final Effluent Limitations Guidelines, is a drilling fluid that has a "synthetic material" as the continuous phase with water as the dispersed phase. Synthetic material is defined as material produced by the reaction of specific purified chemical feedstock, as opposed to the traditional base fluids such as diesel and mineral oil which are derived from crude oil solely through physical separation processes and/or chemical reactions. Since synthetic materials are synthesized by the reaction of purified compounds, they are typically free of polycyclic aromatic hydrocarbons (PAHs).

Typical base fluids that have been used, or are currently being used, in SBMs are polyalphaolefins (PAOs), linear alphaolefins (LAOs), internal/isomerized olefins (IOs), esters, and acetals. P. 179^

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