Methods for Handling Drilled Gas in Oil-Based Drilling Fluids

Abstract

This paper presents calculation methods for predicting the behavior of drilled-gas contamination of oil-based drilling fluids. The methods are verified by experiments conducted in a 6, OOO-ft [1828.8-m] test well. This paper also presents field-handling procedures developed with the calculation methods. IntroductionDuring drilling operations, the bit routinely drills through gasbearing formations. Although proper drilling-fluid density selection will prevent gas from flowing from these formations into the borehole, the gas contained in the pore space of the rock destroyed by the bit will always become mixed with the drilling fluid. This gas is often called "drilled gas." If an oil-based drilling fluid is used, drilled gas will normally dissolve completely in the drilling fluid at bottomhole conditions because the volume of gas is small compared with the volume of drilling fluid in which it is mixed (Fig. I). When this gas/drilling-fluid mixture is pumped near the surface, however, gas will come out of solution and expand rapidly because of the greatly reduced pressure. This gas evolution will often begin when the gas/drilling-fluid mixture is within a few hundred feet of the surface, providing little time for the rig crew to react before gas reaches the surface. Hazardous situations have been reported where significant amounts of gas have been released on the rig floor and drilling fluid has been spewed over the crown block while the annular blowout preventers (BOP's) are being closed.These problems occur because the equipment and procedures sometimes used are not adequate for some of the well conditions experienced. To design a system properly for safe handling of drilled gas in oil-based drilling fluids, a calculation procedure is needed for predicting well behavior under various field conditions. This type of calculation procedure would also permit evaluation of existing rig equipment for the purpose of imposing operationallimitations, such as a maximum safe penetration rate (ROP). This paper presents techniques for estimating the amount of drilled gas entering an oil-based drilling fluid and for predicting the behavior of the gas/drilling-fluid mixture in the annulus as it is circulated to the surface. With the methods presented, the gas/ drilling-fluid bubblepoint depth, the maximum loss in bottomhole pressure (BHP), and an approximate annular pressure profile can be calculated. The volume of drilling fluid that would be expelled from the well and the associated gas rate can also be estimated. The calculation procedures presented were verified by experiments conducted in a 6,000-ft [1828.8-m] test well.Several alternative methods for handling drilled gas in oil-based drilling fluids were explored with the calculation procedure. Special considerations for deepwater floating drilling operations are also discussed. Calculation ProcedureThe calculation procedure developed requires completion of three steps: (I) determination of the concentration and total volume of drilled gas entering the drilling fluid at the...

Dynamic Simulation of Gas Migration in Marine Risers

“…Flatabø et al (2015) carried out experiments to understand gas absorption in petroleum fluids at HPHT conditions. A general method to handle drilled gas which may develop as a kick over time if unnoticed is provided by O'Bryan, Bourgoyne et al (1989).…”