Paper: Clay-Free Aqueous Drilling Fluids

Historically, montmorillonitic clays have provided the essential rheological and filtration properties of drilling fluids; and because of their versatility, clay muds continue to this day to be used more frequently than any other type of mud. They are, however, inherently high-solids, high-viscosity muds. Even if they are initially formulated to have a low viscosity, their tendency to incorporate shales and clays encountered during drilling causes the viscosity to increase, often to undesirably high values. Unfortunately, the thinners used to combat these high viscosities increase the tendency of the drilled solids to disperse into the system, thereby creating a vicious circle. These high viscosities create handling problems; the concomitant high gel strengths increase the tendency to swab-in gas when pulling out of the hole, and cause pressure surges which may result in loss of circulation when running into the hole. But, worst of all, highviscosity, high-solids muds are slow drilling muds, and consequently increase drilling costs. Clay-free fluids were introduced to overcome these disadvantages. There are many different types but their essential features are that they contain no clay in their initial make-up, none is added during drilling, and they are treated either chemically or mechanically or both to reject virtually all drilled solids at the surface. For this reason, they are sometimes referred to as "closed circuit systems". In order to maximize drilling rate the solids content must be kept very low, and the viscosity no higher than that required to clean the hole. Furthermore, the agents used to provide filter loss properties, increase the viscosity or raise the density are those that will have minimum influence on drilling rate; for example, shear thinning polymers are used to increase viscosity and soluble salts are used to increase density. Further advantages of the polymers are that they provide excellent rheological properties for cleaning the hole at relatively low pump pressure, and that some of them have the property of inhibiting caving shales. True clay-free systems are not as versatile as clay muds and cannot be used in every well. For example, since they use soluble salts for weighting purposes, weights above 11.5 ppg cannot be obtained unless solids are added, which violates the low solids requirement. Similarly, it is difficult to drill through a thick section of montmorillonitic shale and maintain the low solids requirement. Also, the maximum permissible bottomhole temperature is 375_F. This paper describes the principal types of clayfree fluids, the principles under which they operate and the conditions to which each type is best suited.