Oil-Based Drilling Mud as a Gas-Hydrates Inhibitor

Abstract

Summary Gas-hydrates formation must be considered when petroleum reservoirs are developed in arctic regions and deepwater environments. This paper demonstrates that gas hydrates can form in oil-based muds, but that paper demonstrates that gas hydrates can form in oil-based muds, but that two major components-oil and dissolved solids in the aqueous phase- significantly inhibit this formation. This work identifies two major components in oil-based drilling mud that affect gas-hydrates formation. The temperature and extent of gas-hydrates formation both can be inhibited significantly, but not necessarily prevented, in oil-based drilling muds. A system that contained 20-vol% water and had an oil-continuous phase inhibited gas-hydrates formation 5 to 10 degrees F. Dissolved solids in a 19.22-wt% calcium chloride (CaCl2) brine inhibited gas-hydrates formation 20 to 25 degrees F and significantly reduced the extent of formation. Gas-hydrates formation in an oil-based drilling mud, prepared with 20-vol%, 19.22-wt% brine, was inhibited more than 30 degrees F over the pressure range studied, 500 to 4,500 psig. In most cases, oil-based pressure range studied, 500 to 4,500 psig. In most cases, oil-based mud can be prepared with sufficient concentrations of dissolved solids to prevent gas-hydrates formation under downhole conditions. Mud samples prevent gas-hydrates formation under downhole conditions. Mud samples should be tested to determine the temperature of gas-hydrates formation before field use. Introduction Gas hydrates have been known and studied for more than 160 years. The petroleum industry first noted them in the 1930's when they were found to be the cause of plugging in natural gas lines. Until recently, most research has been related to natural gas transportation. More recently, gas hydrates have been found in situ in both arctic and deepwater drilling. As the industry moves into deeper water, the conditions (higher pressures and lower temperatures) become more favorable for the formation of gas hydrates in systems containing water and hydrocarbon gases. In fact, such occurrences in water-based muds and their detrimental effects have been noted. In this paper, we do not review the properties or structure of gas hydrates because these are well documented. It suffices to say that many gases (carbon dioxide, nitrogen, hydrogen sulfide, methane, ethane, propane, and isobutane) that are common in the presence of water under pressure induce the formation of a solid presence of water under pressure induce the formation of a solid phase at temperatures above the freezing point of the aqueous phase at temperatures above the freezing point of the aqueous solution. The formation of gas hydrates in drilling mud can result in blockage of flowlines and valves, with potentially serious safety and economic consequences. In deepwater drilling, the drilling fluid may pass through regions during circulation where the temperature is pass through regions during circulation where the temperature is low enough and the pressure high enough to form gas hydrates. In 1986, Sloan** tested an oil-based mud and detected no gas hydrates. The mud was 20-vol%, 30-wt%, CaCl2 brine. The question of whether oil-based muds could be used without concern for the formation of gas hydrates arose. In general, oil-based muds are not water-free and are as much as 30-vol% aqueous phase. We chose 20 vol% for the aqueous phase. In each oil-based mud, oil was the continuous phase. It was believed that this would prevent gas-hydrates formation. Another known gas-hydrates-formation inhibitor is dissolved salts in the aqueous phase-in this work, CaCl2. A series of tests was proposed. This series was made up of four basic groups: pure water as the baseline, brine, oil-based mud prepared with pure water, and oil-based mud prepared with brine. prepared with pure water, and oil-based mud prepared with brine. Each system is discussed in detail. Experimental Equipment and Method Figs. 1 and 2 shows schematics of the two experimental apparatus. Fig. 1 shows the blind cell, which was a 0.07-gal glass-lined autoclave reaction vessel rated to 5,400 psi. The system volume included the volume of the cell and tubing connected to the pressure transducer and pressure relief devices. The autoclave was pressure transducer and pressure relief devices. The autoclave was fitted with a magnetically driven impeller to mix the gas and liquid. Mixing was essential because gas-hydrates formation is a surface phenomenon. If not disturbed, a gas-hydrates crust will form at the phenomenon. If not disturbed, a gas-hydrates crust will form at the gas/liquid surface, retarding further formation of gas hydrates.