The effect of surfactants on two-phase flows in flowlines and risers

Abstract

Oil and gas production systems consist of flows in wells, in horizontal and inclined flowlines and in risers. Due to the reservoir composition and changes in pressure and temperature, the flow is often under multiphase conditions. With time, the reservoir matures and the reservoir pressure decreases. The gas rate drops below a critical gas rate, that is required to produce the liquid to the wellhead and the facilities. As a result, liquids build up down-hole in the well. This phenomenon is known as liquid loading, which may kill the well. One of the deliquification methods is to use surfactants, which are injected through a capillary at the bottom of the well. Recent field trials have shown that the injection of surfactants down-hole in a well can prevent liquid loading problems. The surfactants create a foam which forms relatively thick interfacial waves along the wall of the production tubing. The gas core gets more grip on the liquid film, making it easier to produce the liquid to the surface.

Due to the success of the application of surfactants in subsurface vertical wells, it is of interest to investigate whether surfactants can also help to overcome liquid management problems in flowline-riser systems on the surface. The objective of this work is therefore to find the effect of surfactants on two-phase (air-water) flows in flowlines and risers. Experiments were carried out in the Severe Slugging Loop at the Shell Research and Technology Centre Amsterdam. The flow loop consists of a 100 m horizontal and downward inclined flowline with an inner diameter of 0.051 m, followed by a 16.8 m riser with an inner diameter of 0.044 m. The working fluids are air and water, and operation is at atmospheric outlet pressure. Two configurations of the SSL were used. In configuration #1, both water and air are injected into the flowline. The mixture is transported through the flowline to the riser. In configuration #2, water is injected into the flowline. Air is injected directly into the riser base. The dish washing detergent Dreft is used as a surfactant to create foam. The concentration was systematically increased to find the effect on different types of slugging (in the flowline, in the riser and severe slugging at the riser base), the pressure gradient in the riser, the developing length in the riser and the liquid and foam holdup in the riser. Various measurement techniques were used: Differential Pressure Indicators (DPIs), Pressure Indicators (PIs), Distributed Acoustic Sensors (DAS), quick closing valves and flow visualization.

When operating the SSL in configuration #1 with a superficial gas and liquid velocity of u_sg= 1.21 m/s and u_sl= 0.4 m/s, slugging in the flowline is observed. The slugs are identified as growing slugs: the stratified flow builds up regularly due to a growing instability, forming a slug. The average length of the liquid body of the slug is 32 m, the average passing time of the liquid body of the slug is 51 s, and the average slug velocity is 0.66 m/s. Through DAS and pressure measurements one can see that the slugs are mitigated when the surfactant is added. The slugs completely disappear when an effective surfactant concentration of 1000 ppm is added to the air-water mixture.

When operating the SSL in configuration #1 with a superficial gas and liquid velocity of u_sg= 1.4 m/s and u_sl= 0.27 m/s, a severe slugging cycle is found. Without surfactants, the cycle has a period of 109 s in which the riser is filled entirely with the liquid body of the slug before is is pushed out by the air. Pressure drop measurements over the riser show that adding surfactants does not prevent the slugging cycle to occur. However, the creation of foam does increase the amount of gas in the riser, making the pressure build-ups more irregular. The slugging cycle reduces to 89 s when an effective surfactant concentration of 3000 ppm is added to the air-water mixture.

When operating the SSL in configuration #2 with a superficial gas and liquid velocity of u_sg= 0.37 m/s and u_sl= 0.27 m/s, slugging in the riser is observed. Differential pressure measurements were taken over a distance of 3 m at multiple locations: one at the riser base, and two at the top of the riser. The surfactant reduces the differential pressure along the riser. For a concentration of 3000 ppm, the differential pressure reduces by 5.1 % at the riser base and by 18.9 % at the top of the riser.

The pressure drop curve is used to analyze the flow behaviour for a range of superficial velocities. Two types of pressure drop curves were considered: 1) for a constant superficial liquid velocity (u_sl= 0.05 m/s), and 2) for constant gas-to-liquid ratio (GLR= 60 and GLR= 100). For the low gas flow rate region, i.e. the gravity dominated part of the curve, both methods show a decrease in pressure gradient for a surfactant concentration of 500 ppm or greater. In the high gas flow rate region, i.e. the friction dominated part of the curve, an increase in the effective concentration leads to an increase in the pressure gradient.

The developing length of the air-water mixture with and without surfactants is analyzed by means of differential pressure measurements. Measurements were taken over a distance of 3 m at multiple locations: one at the riser base, and two at the top of the riser. With an effective concentration of 3000 ppm, the differential pressure is slightly different for the two locations on top of the riser. This indicates that the flow is not fully developed at the top of the riser. The developing length is slightly increased with the addition of the surfactant.

The foam holdup is analyzed by measuring the height of the foam between two simultaneously closed quick closing valves on the riser. The experimental results are compared with the simulation results from the Shell Flow Correlations. The experimental results show a spread due to the transient behavior of the flow. Despite the spread, the experimental results follow the trend of the simulations. At low gas flow rates, surfactants decrease the foam holdup. At high gas flow rates, surfactants increase the foam holdup. However, the foam holdup decreases for increasing gas flow rates and eventually levels off to a constant holdup value.

It can be concluded from the small-scale experiments that surfactants: 1) mitigate (growing) slugs in flowlines, 2) do not remove the severe slugging cycle, 3) decrease the pressure gradient in the riser for small gas flow rates, 4) slightly increase the development length of the flow in the riser, and 5) decrease the foam holdup for low gas flow rates, and increase the foam holdup for high gas flow rates in the riser.
It is recommended to carry out the experiments on a larger scale, i.e. at higher temperatures, for larger pipe diameters, to better relate to existing flowline-riser production systems. It is also recommended to perform similar experiments to find the effect of the surfactant on other slug types, such as terrain slugs and hydrodynamic slugs, in the flowline.