Page 325 - Geology and Geochemistry of Oil and Gas

286 APPENDIX A

Fig. A.14. Gas bubble (or globule; bubble is smaller than globule) floating up through water in a channel
where the diameter of the bubble/globule is smaller than that of the channel. W g is the weight of bubble/
globule, B is the buoyant force, D is the frictional drag force, and v is the upward velocity of the bubble/
globule.

A.4.1. Sample problem

At laminar flow, calculate the rise velocity of a hydrocarbon gas bubble 0.1 mm in
diameter through fresh water.
Given: gas bubble diameter, d ¼ 0.1 mm (1 ft/304.8 mm) ¼ 3.28 10 4 ft; specific
gravity of water, SG ¼ 0.998; specific weight of water, g w ¼ (0.998 62.4 lb/
3
2
3
ft ) ¼ 62.27 lb/ft ; water viscosity, m w ¼ 2.089 10 5 lb-s/ft (1 P ¼ 2.089 10 3 lb-s/
2
3
ft ); specific weight of hydrocarbon gas, g g ¼ 0.0422 lb/ft ; specific weight of air at 591F
3
and 14.7 psia ¼ 0.07651 lb/ft ; molecular weight of hydrocarbon gas ¼ 16; molecular
weight of air ¼ 29.
The velocity of buoyant rise of the gas bubble is equal to
5
2
4 2
n ¼ d (g w –g g )/18m w ¼ (3.28 10 ) (62.27–0.0422)/18(2.089 10 ) ¼ 1.78 10 2 ft/s
or 0.54cm/s.
A.4.2. Gas Migration
The gas migration occurs when the upward force generated by the height of the
gas column is greater than the capillary-force resistance of the rock through which
the gas migrates. Some oil/gas reservoirs are composed of thin layers of alternating
shale and sandstone. Only in gas-wet rocks, the gas migrates upward from layer to
layer until it reaches the top of the water table, and then diffuses to the surface.
As the gas moves upward through the water in a capillary, several forces act on this
gas column of height h: (1) weight of gas column (e.g., in lb) acting downward,
2
W g ¼ ðpr hÞg , where r is the radius of capillary (e.g., in ft) and g is the specific weight
g
g
3
2
of gas (lb/ft ); (2) upward force of water on gas, B ¼ ðpr hÞg , where g is the specific
w w
weight of water; and (3) capillary force, F c , pulling the gas downward (in water-wet