The use of novel chemistry to minimize the interaction of the water-based fluid with water sensitive shales is the key to achieving targets of greater levels of wellbore stability, hole gauge, reduced dilution rates and improved drilling

The more stringent environmental demands together with an increasing focus on drilling performance and economics has created a push towards a greater usage of new and more highly inhibitive water-based drilling fluids, often to replace invert emulsion fluids. The use of novel chemistry to minimize the interaction of the water-based fluid with water sensitive shales is the key to achieving targets of greater levels of wellbore stability, hole gauge, reduced dilution rates and improved drilling economics. One unfortunate result of the use of many of these water based drilling fluids has been an increase in the incidence of bit balling and cuttings accretion, which has resulted in lost time, reduced drilling rates and often expensive remediation operations. This paper reviews recent developments in the areas of both the remediation and prevention of bit balling and shale accretion in water-based drilling fluids. The paper describes the mechanism of bit balling and cuttings accretion, the influence of clay type, fluid chemistry, and drilling parameters and also gives guidelines for minimization of bit balling potential. In addition to discussing testing protocols for evaluation of bit balling potential, field results obtained from usage of a modern day bit balling prevention additive are presented. INTRODUCTION When using oil-based or synthetic-based drilling fluids, there are often associated costs for cuttings treatment, waste stream processing and compliance testing, all of which must be taken into consideration, as well as the typically higher cost per unit volume of these fluids. However the higher operational costs associated with the use of invert emulsion fluids can sometimes be offset by higher rates of penetration (ROP) and a lower risk of operational problems compared with the majority of water-based drilling fluids Over the past few years, introduction of new water-based fluid systems and specific additives have been successful in greatly reducing the incidence of wellbore stability problems. The main causes of non-productive time observed with the latest water-based fluids are issues related to bit balling, agglomeration and accretion of drilled cuttings. Slow ROP, high torque and drag, high overpull on tripping, and potential for wellbore packoff and lost circulation are the most immediate problems. Shale cuttings profiles can be evaluated in terms of soil mechanics. From initial stages as a relatively dry claycontaining material, shales will adsorb water (liquids) in an attempt to relive their internal stresses. As water is adsorbed, the shales reach a plastic limit where the sticking tendency of shale is very high therefore leading to all the problems described above. The primary differences between invert emulsion fluids and water-based fluids that decrease any tendency for bit balling and accretion in the former are the very high capillary pressures in shales that prevent uptake of base fluid, this inhibiting shale sticking, plus the wetting of shale and steel surfaces with a non-aqueous film that will hinder any shale sticking. BACKGROUND Theories based on the plasticity of the clays explains the accretion that occurred. The clay plasticity concept proposes that the rate of hydration of a shale is slowed down such that the cuttings remain in a plastic state over a longer period of time. This plasticity state is believed to contribute towards cuttings becoming molded onto the steel parts of the bottomhole drilling assembly (BHA) and being plastered onto the walls of the wellbore. By using a much less inhibitive drilling fluid, shale cuttings would normally hydrate quickly and tend to be less sticky as they continue to adsorb water thus quickly passing through the plastic stage and into a liquid stage where the shale has little cohesive strength, and readily disperses. The Attenberg limits of a clay are defined as the liquid limit, plastic limit, and plastic index and are fully described in published literature. The liquid limit (LL) is the moisture content, expressed as a percentage by weight of the oven-dry clay, at which the material will just begin to flow when jarred slightly. The plastic limit (PL) is the lowest moisture content, expressed as a percentage by weight of the oven-dry clay, at which the soil can be rolled into threads one eighth of an inch in diameter without breaking into pieces. The plastic index (PI) is the difference between the liquid limit and the plastic limit. It gives the moisture content range through which a soil is considered to be plastic. Through extensive studies on a number of clay types, it was determined that for clays that are montmorillonite in nature, the structure of the clay mineral is the most important factor in determining the properties of these clays in contact with aqueous media. The cation associated with the AADE-11-NTCE-28 The Prevention and Cure of Bit Balling in Water-Based Drilling Fluids Sashikumar Mettath, Emanuel Stamatakis, Steven Young, and Guido De Stefano, M-I SWACO 2 Sashikumar Mettath Emanuel Stamatakis, Steven Young and Guido De Stefano AADE-11-NTCE-28 montmorillonite can alter the structure established by the clay mineral, but is of secondary importance in the hydration behavior. The liquid limit of montmorillonite clays will increase with an increase in the hydrated radius of the predominant clay cation. For kaolinitic clays, the electrical attractive forces and particle orientation in the fabric play a prominent role. Thus kaolinite clays with a high degree of flocculation will have larger void spaces and exhibit higher liquid limit values, whereas those with a low degree of particle flocculation will have smaller void spaces and exhibit lower liquid limit values. Depending on the mineralogical makeup of a shale, the behavior in the presence of a water-based drilling fluid can be entirely different depending on whether the primary clay mineral is kaolinite or montmorillonite. The conclusion of many studies into clay and shale behavior can be summarized by stating that both the liquid limit and plastic limit of a shale are primarily a function of the percent clay fraction, the clay mineral type and the type of associated cations present. The ratio of PL/LL was seen to increase according to clay type: (Na-montmorillonite < Illite < Kaolinite), and to the valency and hydrated ionic radius of the associated cation: (Na < K 更多