Three candidate asphaltene inhibitors have been laboratory tested for their effectiveness on a Canadian crude. One inhibitor, an oil-soluble polymeric dispersant developed by Shell Chemicals, showed superior behaviour compared to the others: flocculation titrations with n-heptane resulted in an optimum concentration of 1300 ppm. PVT calculations, however, indicated that the prevailing conditions downhole can be quite favourable with respect to the amount of effective inhibitor compared to the atmospheric laboratory titrations which appear to be quite severe tests. Therefore, lower initial concentrations were recommended for a field trial. The chemical could be continuously injected through a capillary string, thereby avoiding the lost oil production associated with solvent cleaning operations. It has proved to be very effective at concentrations as low as 66 ppm, resulting in both a technically and an economically successful trial. Introduction Precipitation of asphaltenes in reservoirs, wells and facilities has a severe detrimental impact on the economics of oil production because of a reduction well productivity and/or clogging of the production facilities. The nature and behaviour of asphaltenes in crude oils is complex. Asphaltenes are heterocyclic macromolecules mainly consisting of carbon and hydrogen and minor components such as sulphur, nitrogen, and oxygen. It is generally accepted that resins and maltenes (these structures are comparable to asphaltenes but with a much lower molecular weight) are responsible for keeping the asphaltene particles in dispersion. The asphaltenes are surrounded by the polar head groups of the resins and maltenes while the nonpolar alkyl tails interact with the oil phase. So, crudes with a high ratio of resins to asphaltenes are less subject to asphaltene deposition whereas crudes with large amounts of non-polar saturates compared to aromatics are more prone to exhibit asphaltene precipitation problems. At "normal" reservoir conditions the asphaltenes, resins, maltenes and oil phase are in thermodynamic equilibrium. This equilibrium can be disturbed by a number of factors: decline of the reservoir pressure towards the bubble point, change in temperature or addition of a miscible solvent to the oil as applied in various EOR techniques. Much research has been focused on modelling the deposition behaviour of asphaltenes in reservoir crudes upon changes in pressure, temperature or composition. These models are based on the Flory-Huggins theory for colloidal systems and calculate the chemical potential for large molecules in the various phases. They require solubility parameters as input. In addition, the industry has made a substantial effort to develop pragmatic solutions to the problem of asphaltene deposition in producing wells.

A new method is discussed to improve the HPHT stability of conventional rheology modifiers and fluid loss polymers used in water-based drilling fluids. The method exploits the interactions of polysaccharides (e.g. xanthan gum, scleroglucan), cellulosics (e.g. CMC, PAC) and starches with polyglycols. Polymer and polyglycols were found to associate by intermolecular hydrogen bonding and hydrophobic interactions. This association / complexation was found to stabilize the polymers at higher temperatures. Our laboratory findings are validated by field observations in the Kakap field in Indonesia that show improved HPHT stability when adding polyglycols to water-based drilling fluid formulations. Introduction During the history of oil- and gas exploration drilling fluids have evolved from simple clay-based "muds" to highly sophisticated drilling fluids that are specially engineered to meet demanding tasks. Chemicals are intentionally added to perform very specific functions, such as rheology modification, fluid loss control, shale stabilization etc. Until now, relatively little attention has been given to the mutual interactions that may occur unintentionally between drilling fluid components and their consequences for the properties of these complex fluids. Polyglycols are well-known in the drilling fluid industry for their use in controlling troublesome shales. Excellent control over rheology and fluid loss, low dilution rates and overall ease of maintenance are some of the characteristics of polyglycol systems frequently reported by mud engineers handling them in the field. In this paper, it is shown that there actually exists a scientific basis for these subjective observations. In a drilling operation it is the task of the mud engineer to let a drilling fluid system perform at its best within the given limitations of that particular system. Such limitations are clearly posed by the degradation of polymers at high temperatures, which leads to loss of rheology and fluid loss control. There may be significant consequences for drilling progress and ultimate recovery if these properties cannot be maintained. Loss of rheology can result in hole cleaning problems, leading in turn to stuck pipe and loss of hole. Loss of fluid loss control can lead to unnecessary loss of fluids to formations. Note that invasion of formation-incompatible filtrates may cause reservoir impairment and reduced recovery of hydrocarbons. In the last decade there has been a growing interest in the association of anionic, cationic and nonionic surfactants with water-soluble polymers because these complexes are of great importance to cosmetic products, paints, coatings and in enhanced oil recovery. Here we show that similar complexes can also be utilized in the drilling fluid industry to extend the HPHT stability of polymers used for fluid loss control and rheology modification. Introduction to Polyglycols The generic group of glycols is strictly limited to diols which contain two hydroxyl (-OH) endgroups. Polyglycols are formed when these groups are condensated between glycols to form ether links (i.e. C-O-C bonds). A significant number of commercial additives also known as polyglycols in the oilfield, however, are in fact initiated from alcohols (e.g. butanol), fatty acids or fatty amines which are condensated with ethylene oxide (EO) or propylene oxide (PO). Note that the additives thus formed contain only one hydroxyl endgroup and are therefore not true diols/glycols. Most of them are non-ionic surfactants which combine in a single molecule a hydrophobic group (e.g. the alkyl-chain that originated from the initiator) and a hydrophilic group (the EO- or EO/PO chain). In conformance with oil-field convention, these molecules will still be referred to as polyglycols below. Main focus in this paper will be on three polyglycol types. Polyglycol A is a polyalkylene glycol with a 50/50 EO/PO distribution. Polyglycol B is a simple polyethylene glycol (PEG). Polyglycol C is a fatty amine ethoxylate. All have a molecular weight in the 500–600 a.w.u. range. P. 249^