Experimental Assessment of Fluid Velocity to Control Gas Accumulation and Removal While Drilling "Horizontal" Wells

Abstract

Abstract Horizontal wells often include sections with angles greater than 90 degrees due to steering uncertainties or the well path following the reservoir updip. If gas is present inside the well-bore either because of underbalanced drilling or taking a kick, the gas can accumulate in the resultant high spots and complicate effective well control. A series of experiments were conducted to understand accumulation and removal of gas in a flow tube simulating a fully eccentric well bore annulus at inclinations of 91.5 to 100 degrees. Water, or polymer-viscosified water, and air were circulated in a 6 inch by 2.375 inch annulus at a range of liquid velocities representative of both well control and routine drilling operations. The studies demonstrated that low flow rates favor counter-current flow in either slug or elongated bubble flow patterns that would cause accumulation at high spots. As the liquid flow rate increases, co-current flow develops, and ultimately the counter-current flow disappears. The co-current flow patterns observed were stratified, stratified wavy, and bubble. Flow maps were made to show co-current and counter-current flow windows and void fractions as a function of superficial gas and liquid velocities, fluid type, and inclination angle. Some counter-current flow was found at superficial liquid velocities below 1.2 ft/sec almost regardless of other conditions. Non-Newtonian fluids with a higher yield point were found to sometimes encourage counter-current flow and accumulation in contrast to conclusions published by other authors. Gas was effectively removed at inclination angles up to 96 degrees by a superficial liquid velocity of at least 3.3 ft/sec. Failure to continuously remove gas during underbalanced drilling can result in accumulation that ultimately "spills" into the vertical section of the well. A case history showing excessive surface pressures encountered during underbalanced drilling for gas is presented as an example of this effect.