The drilling fluid used for managed pressure drilling (MPD) operations is of a lower density than that required for normal overbalanced drilling. This means that the hydrostatic pressure from the drilling fluid is often lower than the formation pore pressure. Because of this, the drilling fluid is "at balance" or even "underbalanced" when static. A hydrostatic overbalance is required when tripping to avoid the influx of formation fluids, which may result in a kick. One common way the hydrostatic overbalance is accomplished is by using the mud cap method, where a high-density fluid is placed on top of the underbalanced drilling fluid, resulting in the desired total bottomhole hydrostatic pressure. The major shortcoming of the mud cap design is the risk of comingling of the mud cap with the lower-density drilling fluid beneath it.
An attempt to viscosify the high-density drilling fluid had been made to effectively address this shortcoming; however, several issues were encountered, including more difficult movement and storage of the tripping fluid, along with the risk of increasing the well's bottomhole pressure while pumping the large, viscous pill into place. A better solution to the comingling issue is to place a physical barrier between the two fluids to prevent their mixing. Although barrier pills have been used in the past, they were either limited by not being viscous enough to effectively separate the mud cap from the lower-density drilling fluid or failed as a result of routine drillsite contamination.
An improved barrier pill was developed to offer superior performance in fluid separation as well as to simplify tripping in MPD operations. The new pill involves novel synthetic materials, rheology engineering, and extensive testing. This fluid exhibits a very high degree of thixotropy, far beyond conventional clay or polymer suspensions. When the barrier is static, the progressive gel develops exceptional strength capable of resisting pressure and buoyancy forces. The shear-thinning behavior ensures that agitation and flow make the mixture mobile and pumpable. Shear-thinning properties permit placement of the pill as well as the passage of pipe, coiled tubing, and wireline through the resultant gel without hindrance. With a relatively short coverage interval, the gel can be broken without generating high ECD values. The improved barrier pill will maintain separation and prevent comingling of muds with different densities. Lab testing has shown that the pill works with a variety of fluid system, resists contamination, and is able to maintain integrity with a density differential greater than 6 lb/gal.
The developments of this improved barrier fluid are the subjects of this paper.