Low Salinity Polyglycol Water-Based Drilling Fluids as Alternatives to Oil-Based Muds
Authors: Bland Ron , Smith Gerald L. , Eagark Pasook , van Oort Eric , Dharma Nyoman 
Published: December 2007

Low Salinity Polyglycol Water-Based Drilling Fluids as Alternatives to Oil-Based Muds Ron Bland and Gerald L. Smith, Baker Hughes INTEQ; Pasook Eagark, Unocal; Eric van Oort, Shell (SlEP-RTS); and Nyoman Dharma, Clyde Petroleum Indonesia LTD. (formerly Marathon)

Abstract
Both laboratory and well site data are presented on the use of high performance water-based drilling fluids containing chameleonic or TAME polyglycols. The laboratory results support a new mechanism for shale stabilization by these polyglycols and provide design criteria for optimizing their application. The well site data demonstrates an effective and efficient application of the design criteria for drilling with low salinity systems. The field results show that chameleonic or TAME polyglycols can cost effectively replace invert emulsion and high electrolyte systems without the use of large concentrations of electrolytes.

Introduction
Developing a water-based fluid with the performance characteristics of an invert emulsion drilling fluid has been the "Holy Grail" of water-based drilling fluid design for at least several decades. Regulatory constraints have forced a shift from the older straight-run petroleum distillates, such as diesel, to more highly refined distillates like "mineral oils" to finally the newer "synthetic" or "pseudo"-based inverts. While this evolution in base oils have reduced toxicity and improved biodegradability in at least some of the newer invert emulsion systems, the organic loading of the environment from the high concentration of the external phase in the area of cuttings discharge is unchanged. High performance water-based drilling fluids are an attractive alternative to oil-based fluids due to the characteristics of the aqueous phase, it's conductivity and the typically lower concentration of organics.
The organics typically used in high performance water-based systems are to reduce filtrate losses, provide suspension properties, improve lubrication and to stabilize water-sensitive formations such as shales. One of the more recent developments in stabilization is the use of polyglycols. Polyglycol chemistry, characteristics and benefits of their use in drilling have been well documented in the literature. Most of the applications have been with the use of a supplemental inorganic electrolyte, typically potassium chloride, to aid in performance. This has allowed the use of less expensive polyglycols in many cases. The use of high concentrations of chloride salts is undesirable in land-based drilling due to the disposal problems and where the high conductivity from the electrolyte interferes with the sensitivity of induction logs during exploratory drilling. Use of an alternative electrolyte, such as potassium acetate or formate, is feasible. However, these salts are often cost prohibitive and still limits induction log sensitivity. The nature of properly chosen polyglycols are flexible enough to allow the design of highly stabilizing systems.

Polyglycol Chemistry & Application
Polyglycols, as used in the drilling industry, are oligomers of polypropylene glycol or polyethoxylated and propoxylated short chain alcohol's such as butanol. The chameleonic or TAME (Thermally Activated Mud Emulsion) polyglycols are the most widely used. These polyglycols are typically miscible with water at lower temperatures but, when heated, will eventually separate into 2 distinct liquid phases as the polyglycol becomes partially insoluble in the aqueous phase. The temperature at which this occurs is defined as the "Cloud Point" or "Cloud Point Temperature" (CPT) due to the scattering of the light by fine droplets of the polyglycol-rich phase which separate. This phenomena is reversible as the separated phase re-dissolves once the temperature is lowered below the CPT.
P. 273