Operational Limits Of Synthetic Drilling Fluids
Authors: Growcock F.B. , Frederick T.P. 
Published: October 2008

In environmentally sensitive areas like the North Sea, synthetic-based drilling fluids are becoming increasingly popular as substitutes for conventional oil-based muds. In many cases, they appear to perform every bit as well as oil-based muds. As currently formulated, synthetic-based muds are limited to lower temperatures and mud densities and higher O/W (oil/water) ratios than oil-based muds. Limits of these properties can be extended by utilizing high concentrations of surfactants and low-shear rheology modifiers.

INTRODUCTION

Synthetic-based muds (SBM'S), also known as pseudo-oil-based muds or inert drilling fluids, have promise as environmentally acceptable alternatives to conventional oil?based muds (OBM'S), because their base fluids (non-petroleum organic compounds) are thought to be readily biodegradable, Laboratory and simulated seabed studies indicate that, in most cases, SBM'S do indeed degrade in seawater faster than do OBM'S.1,2 At the same time, SBM'S generally appear to perform as well as OBM'S in the field. However, the unique chemistry of synthetic fluids imparts special properties to SBM'S that can lead to some operational differences.

Two key properties that can affect the upper limits of temperature, mud density and OM/ ratio are viscosity and emulsion stability.3 SBM'S have higher viscosity than OBM'S at low to moderate temperatures, but observations in the field indicate they also thin more rapidly with increasing temperature. Emulsion stability of SBM'S versus OBM'S is understood even less. especially at elevated temperatures. Since SBM'S, like OBM'S, are generally formulated as invert emulsion(water-in-"oil") muds, it may be more difficult to achieve high stability of SBM'S at high temperatures if the enhanced degradability of SBM'S is somehow associated with their increased dispersibility in seawater. This raises the question of whether it is possible to formulate SBM'S that have acceptable emulsion stability and can adequately suspend cuttings and weighting material at elevated temperatures, yet are pumpable and degradable in seawater at low temperatures.

SYNTHETIC FLUIDS

The base fluids in SBM'S are synthesized organic compounds which are essentially immiscible with water. The most common of these synthetic fluids are similar in average molecular weight to LTMO'S (Low-Toxicity Mineral oils):Ester - this was the first synthetic fluid utilized to formulate a drilling fluid; it is made by the reaction of an alcohol with fatty acids from palm kernel oil, ..[synthetic esters are purer and more stable than natural esters like vegetable and fish oils]Ether - made by condensation and partial oxidation of alcohols; the current version, a di-ether, is more biodegradable than its mono-ether predecessor.,. [lower molecular weight di-ethers are used as solvents, cosmetics, perfumes and flavoring agents]Poly-?-Olefin (PAO)5 - a straight-chain non-aromatic hydrocarbon made by polymerization of ethylene; there are at least two versions - one is an olefin (has a double bond in the middle) and one is a paraffin... [PAO's resemble "white mineral oils" but have a narrower molecular weight distribution and are not branched]