THE INFLUENCE OF DRILLING FLUID RHEOLOGICAL PROPERTIES ON PRIMARY SOLIDS CONTROL

Throughout the last decades, the design and performance of the primary solid control devices have changed significantly. Some five decades ago, the circular motion shakers dominated the marked. These shakers operated by sending the drilling fluid downhill a vibrating screen. Thereafter appeared the elliptical motion or linear motion shakers where the cuttings particles were vibrated upwards a tilted screen. Onto these shakers, the use of double screen decks and finally triple screen decks became common. Within the last years also the vacuum devices appeared. Throughout the last two decades, there has been an effort to increase the g-forces on these shakers and the industry seems to have preferred the high g-force devices recently. Laboratory studies, however, has indicated that the very high g-forces are not necessary to perform proper solids control. Instead, different vibration modes interacts with the gel structure of the drilling fluid and remove yield stresses. Hence, the fluid becomes mobile for flow through the screen. Flow through screens is strongly dependent on the extensional properties within the drilling fluid rheology. Drilling fluids with high extensional viscosity seldom has a very strong gel structure, and are generally not affected equally much by vibrations. This explains why solids control is more difficult using a KCl/polymer water based drilling fluid than if using an oil based drilling fluid. This article focuses on describing how the drilling fluid rheological properties alter during primary solids control. It is based on theoretical analysis, theological studies in the laboratory and finally on practical applications in two recent exploration drilling operations. The solids control efficiency resulting from using different screen configurations is outside the scope of this article, as this topic requires a higher focus on separation technology.