Feasibility and Evaluation of Surfactants and Gas Lift in Combination as a Severe-Slugging-Suppression Method

Abstract

nation angle for relatively low gasand liquid-flow rates. Sarica et al. (2014) divided the severe-slugging cycle into four steps, as described in Fig. 1. The classic pipe geometry for severe slugging is a slightly downward section upstream of a riser. In Step 1, gas and liquid velocities are low enough to allow stratified flow in the downward-sloping pipe section followed by liquid bridging and accumulation at the bottom of the riser. The hydrostatic pressure of the accumulated liquid initially increases equal to or faster than the buildup of gas pressure upstream of the liquid slug (Step 2). When the gas pressure eventually exceeds the hydrostatic head of the liquid slug, the gas will begin to push the liquid slug out of the riser and start to penetrate the riser (Step 3). The pressure in the gas reduces as the liquid is removed from the riser and the gas expands, increasing the velocities in the riser. After most of the liquid and gas exit the riser, the velocity of the gas is no longer high enough to sweep the liquid upward. Liquid film not swept from the riser starts falling back down the riser (Step 4), and the accumulation of liquid starts again. Severe slugging will cause periods of no liquid and gas production in the separator followed by very high liquidand gas-flow rates. The resulting large pressure and flow-rate fluctuations are highly undesirable. Several mitigation techniques are proposed in the literature. A thorough summary of these techniques can be found in Sarica and Tengesdal (2000). Surfactant application and gas lift are typically considered to be separate methods. The combination of both can provide a better mitigation of severe slugging by complementing one another. As mentioned by Sarica and Tengesdal (2000), Yocum (1973) was the first to identify multiple severe-slugging-mitigation techniques. These are reduction of the line diameter, splitting the flow into dual or multiple streams, gas injection into the riser, the use of mixing devices at the riser base, choking, and backpressure increase. Here, we will classify severe-slugging-mitigation methods into three groups: passive, active, and hybrids (combination of both passiveand active-mitigation methods). Passive methods require energy from the system; the most relevant are given as follows: 1. Choking: One of the most common mitigation techniques is the installation of a choke valve at the top of the riser. By choking the flow, the riser operational pressure changes, stabilizing the flow. Several publications regarding choking exist in the literature, as detailed in Sarica and Tengesdal (2000). Unfortunately, because of the backpressure created by choking, production is affected, and a minimum amount of energy is required for this method to be successful. This technique can be combined with a feedback control to regulate the largest choke opening that will stabilize the flow. 2. Backpressure increase: This method requires significant pressure increases at the separator or riser head. It is not considered to be as viable an option, even for shallow-water systems, because production-capacity reduction is experienced as a result of the backpressures imposed. The reduction in Copyright © 2015 Society of Petroleum Engineers