[CCoE Notice] PhD Dissertation Defense

[Knudsen, Rachel W]

The Petroleum Engineering Department

MS Thesis Defense
of

Veerabhadra Sasikanth Denduluri



Topic: Riser Gas Behavior in Deepwater Wells – An Experimental Study of Taylor Bubble Rising in a Stagnant Column of Liquid with Drilling Mud Properties



Committee Chair: Dr. George K. Wong

Date: Monday, December 02, 2019

 Location: Technology Bridge (Formerly ERP) Building 9, Room 123

Time: 9:00 am



 ABSTRACT

The severe effects of the free-bubble gas formation in deep water drilling riser is a major concern for offshore Oil and Gas industry. Understanding the comprehensive assessment of risks and consequences of the hydrocarbons entering the drilling riser is very important to control and prevent future well related accidents.

This study designed and conducted a series of experiments with air slug or Taylor bubble rising in a vertical tube and in a stagnant column of different liquids open to atmospheric pressure. The objective is to gain insights on the mechanisms of gas slug rising and expanding near the top of the drilling riser and provide experimental data to help calibrate computational models with liquid properties similar to drilling muds. The bubble rising test apparatus, representing the drilling riser, consists of 3 acrylic tubes for a total of 18 ft of height and 6.5 inches of internal diameter. We generated a Taylor bubble by pressurizing a fixed volume of air trapped in the lower 6 ft section of the apparatus. This bubble is then introduced into the test apparatus by opening and closing a ball-valve. We record, track, and calculate the bubble rising velocity and expansion using twelve pressure transducers and cameras placed 1 ft apart and movable cameras traveling along the height of the riser. We conducted tests in three different fluids: water, viscous shear-thinning gel made with Xanthan Gum, and slurry of proppant with gel. We selected Xanthan Gum gel and the slurry to emulate the shear thinning fluid properties and effects of solids in drilling mud, respectively.

Experimental results showed, as the bubble rises in the riser tube, the bubble expands due to decreasing hydrostatic pressure in the column. Air bubble-water tests showed the bubble rising velocity and length were comparable to literature data. Values interpreted from video cameras and pressure gauges, measured along the height of the riser tube, were consistent and complementary. From these results, we established and verified a method to calculate bubble rising velocity and bubble expansion using only pressure data.  Air bubble-viscous gel tests calculated slower bubble rising velocity and longer bubble length because of higher gel viscosity and thicker liquid film, respectively. Air bubble-slurry tests calculated slower bubble rising velocity and similar bubble length to that of air bubble-water case. We did not find data of Taylor bubble in slurry in the literature for comparison. Results from this study provided new insights on the mechanisms of slug or Taylor bubble movement in drilling riser with comparable drilling mud rheology.

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