No abstract

In oil and gas drilling operations, drilling fluids perform essential tasks such as lubricating the drill bit, providing hydrostatic pressure and removing drill cuttings. One important function of the drilling fluid is to stop compacted clay minerals, commonly encountered in drilling operations, from taking up water from the drilling fluids and consequently swelling. Such a scenario can have an adverse impact on drilling operations and may lead to significantly increased oil well construction costs. With increasingly stringent environmental guidelines determining which swelling inhibitors are available for use in the oilfield as drilling fluid additives, there is a need to fully understand the mechanisms of clay hydration in order to design new swelling inhibitors which conform to evolving regulations. Using a range of computational techniques and analysis, combined with known experimental results, we have devised a set of ''rule-based'' design criteria for clay-swelling inhibitors. To achieve this, we have formulated a hydration energy parameter, which assesses the changes in energy during the step-wise progression from mono-to bi-to trilayers of water in the clay sheet galleries. This parameter can be used to rationalise and predict the swelling profiles for clays containing both cationic and neutral clay swelling inhibitors. The rules we have devised are as follows: (i) Cationic inhibitors should be able to replace sodium ions in the interlayer. (ii) Cationic inhibitors should possess a water soluble, hydrophobic backbone. (iii) Cationic inhibitors should have primary di-amine or mono-quaternary amine functionality. (iv) Cationic inhibitors should have little alcohol functionality. (v) The hydrophobic backbone of the cationic inhibitor should be long enough to form a dense monolayer in the interlayer. (vi) For neutral inhibitors, the inhibitor should be a water soluble organic molecule of low molecular weight with well defined domains of relatively high hydrophobicity and small domains of hydrophobicity. Our ''rule-based'' criteria will facilitate the rational design of improved-and more environmentally acceptable-clay swelling inhibitors for oilfield drilling operations.

Polyols (e.g. glycols, glycerols, polyalkylene glycols) are now established as effective shale inhibitors in water-based muds (WBM). These water-soluble compounds are typically used at concentrations between 3 and 10% in WBM and are generally found to reduce wellbore problems associated with reactive shales, increase drilling rates and reduce the environmental impact of the drilling operation. The mechanisms by which these molecules provide shale inhibition are not well understood and the aim of this paper is to provide some insights into these mechanisms. The adsorption of three polyols on to a swelling clay mineral (montmorillonite) was studied. The adsorption experiments were carried out from distilled water and from potassium chloride solutions. The resulting clay complexes were characterised by X-ray diffraction and infra-red spectroscopy techniques. Shale inhibition measurements were also made and the results correlated with the adsorption of the polyols. The polyols are shown to be strongly adsorbed by the clay. During the adsorption process, water is displaced from the clay surface and ordered structures of polyol are formed. The nature of these structures and their stability in aqueous fluids is strongly controlled by the presence or absence of potassium cations:Under most conditions, a single polyol layer forms on the clay in the presence of potassium and the resulting complexes are stable in water.A complex containing 2 polyol layers is formed when potassium is absent. This complex is less stable in water. The interaction between potassium ions and polyols at the clay surface is proposed as the critical factor in the provision of shale inhibition. This interaction appears to account for all the observed inhibition in the case of two of the polyols studied. For a 3rd polyol, an additional (minor) contribution may come from interactions between polyol molecules at the clay surface. The selection of the most appropriate polyol for inhibitive muds is discussed. This is a complex issue that must consider cost and environmental acceptability as well as technical performance.

There has recently been a resurgence in the use of water based muds containing sodium silicate for drilling reactive shale sections, particularly in the North Sea. The performance of these silicate water-based muds (SWBM) has been mixed: they are extremely effective in inhibiting swelling and dispersion of claystone and chalk, however some concerns exist over the stability of the system in terms of fluid properties, long term wellbore stabilisation and lubricity. This paper details new results, previously unreported, which give a better insight into the mechanism of clay stabilisation using SWBM. Recent literature has suggested that silicate fluids are as effective as oil-based muds (OBM) in wellbore stabilisation. This paper describes research and development indicating that silicate precipitation occurs within shales to form a barrier or membrane that hinders ion movement. However, there are distinct differences between the efficiency of membranes produced by oil based and silicate water based muds. Results are included which demonstrate that the ion- exclusion membrane is not as effective with Silicate systems as it is in OBM. New core flow experiments demonstrate that a permeability reduction with silicates and a slow rate of ionic equilibration are the principal causes for the osmotic barrier seen in swelling tests. Further results indicate a change in the physicochemical nature of clays after exposure to high pH silicate fluids. It appears that redistribution of alumino-silicate occurs which may play an important role in the inhibitive nature of these fluids. These evaluations are placed in context with well data in which primary considerations are the nature of any lost time incidents and overall SWBM performance. P. 897

This paper presents the results and implications of a research project* to study, under controlled laboratory conditions, the influence of oil based mud physical properties and chemical composition on the quantity of oil retained on cuttings. Substrates, representing common rock types drilled, were exposed to different oil mud formulations in which one variable was altered as far as possible independently of other properties. The most influential parameters were found to include the use of strong oil wetting agents such as amidoamines and imidazolines (which promote oil imbibition), the percent oil in the formulation and the water activity imbalance between formation and oil mud. Less influential, but nevertheless an important consideration for oil mud formulation, is the effect of plastic viscosity and intermediate oil wetting surfactants.Mud weight and HTHP fluid loss have a minor effect.