Effective High-Density Wellbore Cleaning Fluids: Brine-Based and Solids-Free

Abstract

Abstract Displacing drilling mud with clear solids-free completion brine is a critical step during well completion. As we move into deeper waters and drill to deeper depths (greater than 25,000 feet MD), conventional methods and cleaning fluids become a limiting factor in this phase of the operation. Conventional cleaning fluids use fresh water or seawater treated with surfactants to remove wellbore solids and water wet tubulars. Using low-density cleaning fluids creates a negative differential pressure between the working kill weight fluid and the formation, casing and cement liners. In many situations, the negative differential pressure cannot be tolerated and the risk of failure at the liner top, etc. is increased, especially if the wellbore has not been pressure integrity tested. Additionally, with increasing rig / spread costs, high pump rates are necessary to decrease the time it takes to perform these operations. The pump rate is indirectly proportional to the pump pressures required. Weighted spacers decrease the overall pressure differential, which allows for higher pumping rates. In order to overcome the density limitation of these cleaning fluids, conventional techniques such as additional hydraulic horsepower, backpressure schedules, the addition of solids to lighter cleaning fluids (water, seawater), or balancing the weight of the low-density cleaning fluid with a matching higher-density fluid is utilized. However, each of these "fixes" has inherent limitations and is accompanied with reduced cleanup efficiency. Furthermore, conventional surfactants are not active or effective in high-density brines. New brine-compatible surfactant chemistry and the corresponding balanced-displacement engineering design were developed to overcome limitation of conventional displacement technology. This paper describes the field applications of new brine-based high-density solids-free cleaning fluids in balanced-displacements in deepwater and offshore shelf wells. The new high-density fluids were based on, new surfactant technology developed to ensure effective wellbore cleaning, wellbore design parameters, and displacement modeling. In addition, weighted spacers aid in reducing high pump pressures and wellbore pressure differentials. In one case history, maximum pumping pressure of more than 9,000 psi was expected for conventional water based displacement, but was reduced to a little over 3,000 psi with the new design. High-Density cleaning fluids, with densities up to and greater than 17.5 ppg, have been formulated and utilized successfully without compromising cleanup efficiency and significantly reducing differential pressures. Results from laboratory development and field applications are presented.