Sand Transportability in Pipe

Sand transportation in multiphase pipelines depends on several factors. Some of these factors are:

Flow regime, hold up, fluid properties (such as viscosity), inclination of the pipe in hilly terrains, particle size distribution, relation between the superficial velocity of the liquid phase (Vsl) and the superficial velocity of the gas phase (Vsg), pipeline diameter, friction factors, etc. For example, a change of inclination implies a change in the flow pattern, and therefore, a change in the sand transportation and sand behavior. The same happens for the viscosity. If the viscosity of the liquid phase changes, the energy distribution of the gas and liquid phases is going to change, conditioning the geometrical distribution of both phases in the pipeline, which means changing the flow pattern.

Angelsen in 1989 found that sand transport in horizontal pipelines has four main patterns depending on the fluid flow rate (Salama, 1998). Basically, four flow regimes can be identified for the solid-liquid slurry flow in horizontal pipe; those are saltation static bed (sand bed), saltation moving bed (moving dunes), heterogeneous flow (scouring), and homogeneous flow (dispersed) (Chen, 1994).

Figure 3.7 Flow pattern in slurry flow

However, sand transportation in pipe concludes of more complex fluid composition: water, oil, and gas. The flow regime determined before only told about the gas-liquid phase distribution, so that the behavior and flow pattern map is a combination from gas-liquid and slurry flow.

Figure 3.8 Multiphase flow regimes consist of liquid, gas and solid

In annular flow, the sand particles can be transported in the water firm and in the gas core. In this flow regime, since the velocities are high, the main concern is not the sand accumulation but the erosion rate produced by the aggressive sand particles movement.

In low hold up wavy flow, the liquid is transported in a thin film on the bottom of the line, where the sand concentration may be high, enhancing the creation of a settled sand bed.

In plug flow, gas pockets move along the top of the pipe having little effect upon the solid behavior.

As long as the gas velocity is increased, the gas pocket gets depth and the fluctuating velocities affect the sand transportation similar to described at next in slug flow. Under this flow regime, for upwardly inclined pipes, it can be seen that either the sand is transported in the plug body and in the film region, or the sand particles settle in the gas plug zone (film region), and are only transported into the plug body, or clusters of collided sand particles are formed, moving backwards in the gas pocket (film region) and only moving forwards in the liquid plug body.

In slug flow, the sand particles behavior is complicated since the solids may be settled during the film region and transported in the slug body; the sand movement is always intermittent and gas pockets moving along the pipe have high effect upon the solid behavior. There can be a large diameter effect as the depth of the film varies and shields the pipe bottom from the turbulence of the slug.

Moreover, the slug frequency is an important factor in sand transportation (Ruano, 2008).

Figure 3.9 Schematic sand behaviors in slug with low gas superficial velocity

Ruano in 2008 came with his observation about sand behavior in multiphase horizontal and near horizontal (+5^o) pipelines for his magister thesis. He tried to find the correlation between sand behavior and flow pattern and vice versa. Flow regime analysis is conducted through the measured hold up by capacitance instrumentation, for its comparison with the visual observation, and a relation between flow pattern and sand transportation is pointed up. The real sand transportation in multiphase oil pipelines is studied here by using water/air flows which contain different loads of

sand, by means of conducting sand settling experiments in the 4” (0.1 m) facility loop of Process and System Engineering Department at Cranfield University, for a liquid superficial velocity interval from 0.55 to 0.15 m/s, for a gas superficial velocity range from 2.5 to 0.02m/s and for three sand production rates: 0.04275, 0.57 and 1.425kg/m3.

It has been found that the sand transportation strongly depends upon the flow regime and, however, upon each and every parameter which affects the flow pattern, such as inclination or, even, sand production. It has been seen that the flow regime observed mainly depends upon the inclination, showing big differences between horizontal and near horizontal (+5^o). Therefore, the sand behavior observed in horizontal pipe is completely different that in the upwardly inclined pipe.

First, Ruano identified the flow regime without any sand load to study how sand concentration affects the flow pattern. For a certain value of Vsl, Vsg values are varied until all the regimes are concluded. Then he replied those methods with different sand production rates; a recorded video from the bottom of the pipe is conducted.

a. Smooth Stratified Flow

 No obvious sand particles movements in liquid film zone

 Sand settled in the bottom, sand dune formation in higher sand concentration

Figure 3.10 Sand behaviors in smooth stratified regime

Figure 3.11 Sand dune formation behaviors

b. Stratified-Wavy Flow

 Formation of the big waves

 Sand are seen to settle along the flow direction

 Enough energy for sand to be transported

Figure 3.12 Sand behaviors in stratified-wavy regime c. Plug Flow

 High value of Vsl and low Vsg

 Sand is not transported to barely move sliding in a plug body and settle in the film zone

 Sand is encountered to be rolling or creeping

Figure 3.13 Sand behaviors in plug regime d. Slug Flow

 Facilitate sand transportation

 As soon as the turbulent energy reaches the sand settled on the pipe well, the sand will be carried and lifted into the slug body

Figure 3.14 Sand behaviors in slug regime

3.3 Critical Flow Velocity in Sand Transport