| Popis: |
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. |
