73
processing software ImageJ. Pressure and temperature are monitored to ensure
reproducible experimental conditions. Again, LSC oil droplets are generated using
a capillary at the bottom of the cell. While the evolution of the droplet volume and
the respective equivalent diameter is measurable, the rise velocity is not directly
accessible in this setup. When pressure is released from an initial value of 15.1 MPa
to ambient pressure (0.1 MPa) with a decompression rate of 1 MPa/min at a constant temperature of 20 °C, the single droplets are shrinking by 3–5% of their initial
volume-equivalent diameter. This shrinking is accounted for by mass transfer or
dissolution of fairly water-soluble components into the surrounding seawater (Pesch
et al. 2018).
5.5 How Reservoir and Deep-Sea Conditions Change
Everything: Rise Behavior of Live Oil Droplets
During deep submarine blowouts like in the case of the DWH accident, the oil exits
a reservoir with a very high pressure into the deep sea at a still fairly high pressure
level. From there, pressure is steadily decreasing along the rise path of ascending oil
droplets. As detailed in Chap. 3, tremendous amounts of the present natural gas dissolve in the crude oil under reservoir conditions. As pressure decreases during the
blowout, the amount of gas that can stay dissolved in the oil decreases, too. The
initially gas-saturated, so-called live oil becomes supersaturated and starts to degas.
The majority of the gas components is nonpolar and therefore tends to stay inside
the oil, forming a second, gaseous phase: The pressure decline and supersaturation
of the oil cause formation and growth of tiny bubbles inside the liquid oil droplets,
giving rise to the term “internal degassing.” A simplified modeling approach by
Pesch et al. (2018) uses methane as model component, like has been done for the
experimental determination of live oil properties in Chap. 3. The internal degassing
leads to three complementary effects. First, the droplet volume and therefore its
volume-equivalent diameter d p increase as the internal gas holdup ϵ increases.
Second, the mean density ρ p of the composite bubble/droplet decreases with an
increasing amount of free gas. And third, the methane density is decreasing tremendously with decreasing pressure. This triad of changes in the droplet’s properties
causes an increment of buoyancy and hence an acceleration of the droplet rise.
For the calculation of the droplet rise velocity u p , measured physical properties
for all involved fluids as detailed in Chap. 3 and the correlation of Grace et al. (1976)
are applied. For instance, the rise of a gas-saturated LSC oil droplet with an initial
diameter d p of 1 mm, rising from a depth of 1.5 km (rise height z, 0 m; 15.1 MPa) to
the sea surface (rise height z, 1500 m; 0.1 MPa) takes approximately 6.08 hours at a
mean rise velocity u p of 0.069 m/s. Notably, this is less than half of the rise time of
a 1 mm dead LSC oil droplet. The calculated curves of the gas void fraction, the
droplet diameter d p and the mean droplet density ρ p , depicted in a dimensionless
form by dividing them by the respective initial values, are displayed in Fig. 5.3a.
The resulting rise velocity u p of a live oil droplet with an initial diameter d p, 0 of
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…
processing software ImageJ. Pressure and temperature are monitored to ensure
reproducible experimental conditions. Again, LSC oil droplets are generated using
a capillary at the bottom of the cell. While the evolution of the droplet volume and
the respective equivalent diameter is measurable, the rise velocity is not directly
accessible in this setup. When pressure is released from an initial value of 15.1 MPa
to ambient pressure (0.1 MPa) with a decompression rate of 1 MPa/min at a constant temperature of 20 °C, the single droplets are shrinking by 3–5% of their initial
volume-equivalent diameter. This shrinking is accounted for by mass transfer or
dissolution of fairly water-soluble components into the surrounding seawater (Pesch
et al. 2018).
5.5 How Reservoir and Deep-Sea Conditions Change
Everything: Rise Behavior of Live Oil Droplets
During deep submarine blowouts like in the case of the DWH accident, the oil exits
a reservoir with a very high pressure into the deep sea at a still fairly high pressure
level. From there, pressure is steadily decreasing along the rise path of ascending oil
droplets. As detailed in Chap. 3, tremendous amounts of the present natural gas dissolve in the crude oil under reservoir conditions. As pressure decreases during the
blowout, the amount of gas that can stay dissolved in the oil decreases, too. The
initially gas-saturated, so-called live oil becomes supersaturated and starts to degas.
The majority of the gas components is nonpolar and therefore tends to stay inside
the oil, forming a second, gaseous phase: The pressure decline and supersaturation
of the oil cause formation and growth of tiny bubbles inside the liquid oil droplets,
giving rise to the term “internal degassing.” A simplified modeling approach by
Pesch et al. (2018) uses methane as model component, like has been done for the
experimental determination of live oil properties in Chap. 3. The internal degassing
leads to three complementary effects. First, the droplet volume and therefore its
volume-equivalent diameter d p increase as the internal gas holdup ϵ increases.
Second, the mean density ρ p of the composite bubble/droplet decreases with an
increasing amount of free gas. And third, the methane density is decreasing tremendously with decreasing pressure. This triad of changes in the droplet’s properties
causes an increment of buoyancy and hence an acceleration of the droplet rise.
For the calculation of the droplet rise velocity u p , measured physical properties
for all involved fluids as detailed in Chap. 3 and the correlation of Grace et al. (1976)
are applied. For instance, the rise of a gas-saturated LSC oil droplet with an initial
diameter d p of 1 mm, rising from a depth of 1.5 km (rise height z, 0 m; 15.1 MPa) to
the sea surface (rise height z, 1500 m; 0.1 MPa) takes approximately 6.08 hours at a
mean rise velocity u p of 0.069 m/s. Notably, this is less than half of the rise time of
a 1 mm dead LSC oil droplet. The calculated curves of the gas void fraction, the
droplet diameter d p and the mean droplet density ρ p , depicted in a dimensionless
form by dividing them by the respective initial values, are displayed in Fig. 5.3a.
The resulting rise velocity u p of a live oil droplet with an initial diameter d p, 0 of
5 Behavior of Rising Droplets and Bubbles: Impact on the Physics of Deep-Sea…