Drilling Further with Water-Based Fluids - Selecting the Right Lubricant

Abstract

Abstract This paper summarises the results of extensive laboratory work carried out in recent years to determine the performance of a number of experimental and commercially available lubricants for reservoir applications. The laboratory work included a series of tests to determine formation damage potential in both completion brines and low-solids, water-based reservoir drilling fluids, as well as reduction in friction coefficient. The testing to evaluate formation damage potential included brine miscibility, greasing and emulsion formation potential, as well as return permeability testing on outcrop sandstone. Introduction In recent years, improvements in equipment and fluids have allowed extended reach wells to be drilled to and beyond limits that were unthinkable previously. Environmental and technical requirements can make water-based reservoir drilling fluids the preferred option. Good drilling practices and the inherently thin filter cake and lubricious nature of the polymer additives of low-solids, brine-based reservoir drilling fluids can reduce the torque and drag values associated with water based fluids.1 However, in many cases, an additional chemical lubricant is required to drill these extended reach wells to total depth. The choice of the correct lubricant for water-based reservoir drilling and clear brine completion fluids is primarily driven by technical performance and environmental restrictions. In the last few years, increasingly strict environmental legislation imposed in many parts of the world has changed the choice of chemistries utilised for water-based fluid lubricants. Whereas hydrocarbons and fatty acids previously constituted the majority of effective additives, there has been a move towards more environmentally acceptable alternatives, such as esters and naturally occurring vegetable oils. These chemical families are a source of highly lubricious materials that can significantly reduce metal-to-metal and metal-to-rock coefficients of friction in water-based fluid environments, in some cases by as much as 70% in laboratory tests. Typically, the most effective additives have a relatively high degree of surface activity, which improves their adhesion to materials (i.e. metal casing or drilling mud solids), and so enhances the lubricity of the surface. However, this surface activity makes them more capable of reacting with other components of the fluid - whether deliberately added, or present as a contaminant. For example, many effective water-based fluid lubricants may act as an emulsifier in the presence of even small quantities of oil. Under moderate shear conditions, the combination of oil, lubricant and brine can produce an ultra low oil:water ratio invert emulsion with the consistency of cottage cheese. This highly viscous material is at best a drilling hazard, stripping lubricant from the fluid and possibly blinding shaker screens. At worst, the ‘cheese’ may damage the production zone or plug the completion assembly, particularly where sand screens are applied. Another potential problem is the reaction between the lubricant and divalent ions, resulting in the formation of a grease-like precipitate. Depending on the chemical nature of the lubricant, this ‘grease’ may be formed with relatively low concentrations of calcium or magnesium ions, in some cases with concentrations as low as 1000 mg/L, which may easily be encountered in freshwater or monovalent salt fluids while drilling. The potential consequences of the formation of this grease are similar to those of the creation of the ‘cheese’-like emulsion. These issues need to be addressed when selecting suitable lubricants for water-based drilling fluids - most importantly those used for drilling reservoir in order to prevent fluid related formation damage.2 In addition, the effect of the lubricant on the reservoir fluids needs to be evaluated, in case of any interaction with lubricant in the filtrate.