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Offshore Drilling
Published in Sukumar Laik, Offshore Petroleum Drilling and Production, 2018
Underbalanced drilling (UBD) is a procedure used to drill oil and gas wells where the pressure in the wellbore is kept lower than fluid pressure in the formation being drilled. So, fluids in the reservoir flow toward the well while drilling, which is the opposite of what happens with conventional overbalanced drilling methods.
Ventilation and methane, dust, and noise controls
Published in Syd S. Peng, Longwall Mining, 2019
In surface horizontal hole-drilling technology, the hydrostatic load of a column of straight water drilling fluid, in most cases, is greater than the internal pressure of the reservoir such that it damages the potential of methane gas production. The pressure may force drilling fluid and cuttings into the coal cleat structures and plug the methane flow paths. To correct this problem, Zupanick (2005) described a low pressure or an “underbalanced” dual well drilling system that had been practiced in the gassy Pocahontas #3 seam in southern West Virginia. In this system, a vertical “cavity” well, 8.75 in. (222.25 mm) in diameter, is pre-drilled down through the coal seam of interest (Fig. 11.3.12A). The hole is cased with three concentric casing strings between which two concrete layers are poured to strengthen the borehole lining. The casings extend all the way down to 50 ft (15 m) above the coal seam of interest, leaving an open hole above and below the targeted coal seam. The wellbore around the coal seam is then enlarged to around 6 ft (1.8 m) in diameter by an under-reaming tool. The coordinates of this enlarged wellbore are then carefully surveyed and recorded for interception by the second surface borehole. The second borehole is located about 300 ft (91 m) from the first hole and drilled vertically down to within 200 ft (61 m) of the seam of interest. From that point directional drilling techniques are applied to articulate the wellbore from vertical to horizontal so that the horizontal section will be located in the coal seam before it intersect the first well at the enlarged section. The path of the horizontal wellbore is then carefully directed toward the surveyed coordinates of the enlarged section of the first wellbore. Once the articulated second well has intercepted the first well, underbalanced horizontal drilling begins. Compressed air is then introduced to the cavity well. The air and fluid mixture will then return to the fluid junction, thereby reducing the density of the vertical fluid column, creating and maintaining an “underbalanced” drilling condition.
Numerical investigation of a double-circulation system for cuttings transport in CBM well drilling using a CFD-DEM coupled model
Published in Engineering Applications of Computational Fluid Mechanics, 2020
Bing Shao, Yifei Yan, Xiaomei Wang, Fujin Liao, Xiangzhen Yan
The use of gas in underbalanced drilling (UBD) has become common over the last decade, the engineering purpose of which is to promote the return of drilling fluid from the well bottom while keeping the wellbore pressure lower than the formation pressure (Mykytiw, Davidson, Shell UBD Global Implementation Team [SUGIT], Frink, & Blade Energy Partners [BEP], 2003; Paredes et al., 2010; Suryanarayana, Hasan, & Hughes, 2004). UBD with gas injection has received much focus and been widely applied around the world due to its many merits, one of which is that the injection of gas can offer assistance with cuttings transport. However, due to the high compressibility of gas compared with liquid, gas injection is more volatile – especially when local collapses occur in the well wall or when the content of the cuttings in the drilling fluid fluctuates, both of which are almost inevitable in coalbed methane (CBM) drilling due to the fragility of coal rock. Worse still, uncontrolled gas pressure may induce pressure jumps at the bottom of the well, which can potentially aggravate the collapse of the well wall (Akbarian et al., 2018; Ghalandari, Koohshahi, Mohamadian, Shamshirband, & Chau, 2019; Guo, Yao, & Ai, 2008; Nguyen, Somerville, & Smart, 2009; Zhu, Liu, & Tong, 2011).