Study of the PVT Properties of Gas—Synthetic-Drilling-Fluid Mixtures Applied to Well Control

Abstract

Summary The study of the interaction between the formation gas and the drilling fluid during offshore operations as well as in high-pres- sure/high-temperature environments is essential to drill the pay zone of the well safely and economically. The environmental regulatory issues and the peculiar technical aspects involved in deep and ultradeep waters require low-toxicity synthetic drilling fluids. The main objective of this study was to understand the pressure/ volume/temperature (PVT) behavior of those fluids by the experimental determination and modeling of properties such as solubility, specific gravity, and formation volume factor of the fluid. Those properties have a direct impact on kick detection and circulation out of the well and should be addressed in well-control planning and execution. The experimental work was conducted in a PVT laboratory, using mixtures of methane and n-paraffin-based emulsions as unweighted drilling fluids, tested at temperatures of 158, 194, and 302°F. The pressure, temperature, and fluid-composition effects on those gas/liquid mixture properties were analyzed, and the experimental data for solubility and formation volume factor were compared to predictions considering the additivity hypothesis and mathematical fittings based on the experimental data. A model for the methane/n-paraffin system based on literature data and the Peng-Robinson equation of state has also been fitted to the data to discuss the application of the additivity hypothesis for the emulsions. Pit gain evaluation based on the correlated experimental data for the synthetic drilling fluid has been performed and compared to the values expected for water-based muds (WBMs).