Investigation of New Tool to Unload Liquids from Stripper-Gas Wells

Abstract

Abstract Liquid loading in low production gas wells is a common problem faced in many producing regions around the world. The techniques available to remove liquids from the wellbore impose significant capital and operational costs. This study investigates a new method for unloading and restoring continuous production of low rate (stripper) gas wells. The performance of a patented vortex flow modifier tool was examined using a 125-ft vertical flow loop of 2-inch diameter clear PVC. The vortex device was found to alter the basic flow structure in the pipe resulting in improved liquid flow. The tool was observed to reduce tubing pressure loss by up to 17 precent and lower the minimum gas velocity required to lift liquids up the tubing string. Introduction The production of natural gas is usually accompanied by the production of brine and/or hydrocarbon liquids. These liquids are transported to the surface as small droplets by the natural gas. However as the reservoir pressure declines, the drag force exerted by the gas is no longer sufficient to carry these liquids to the surface and they are instead held up in the wellbore. Accumulation imposes a backpressure on the formation that can significantly reduce the production capacity, and can eventually kill the well. A minimum or critical gas flowrate must therefore be maintained to prevent the onset of liquid load-up. Numerous authors1–3 have offered predictions for determining the critical velocity. Turner1et. al's correlations are the most widely used. It is based on determining the velocity of the gas that would exert a drag force sufficient to balance the gravitational force of a liquid droplet. That is,Equation 1 and the expression for the critical gas flowrate is,Equation 2 It is evident from these equations that liquid unloading can be achieved by,Increasing the gas rate.Reducing the area for flow.Reducing the surface tension or density of the liquid phase. A number of techniques4–8, such as the use of soap sticks, plungers. rod pumps, or swabbing, are available as corrective action to return the well to production. The external interference due to these methods comes at the expense of additional capital and operating costs. In additional to the methods listed, unloading can also be achieved by reducing the pressure drop in the tubing string. This would increase the value of CD, which would translate into more efficient use of the existing reservoir energy. As a result unloading would occur at lower gas rates. Mingaleeva9 studied the lowering of pressure drop in self-twisting helical flow. He observed the mechanism from an energy standpoint, and concluded that the liquids and gases will flow through a path of least resistance. Also the power spent to overcome the hydraulic drag for raising an air column in a helical trajectory, was compared to the motion and rising of an equivalent air mass at the same velocities by a straight column, was significantly lower. Therefore he concluded that the helical path was more favorable from an energy-use viewpoint. As a result the air column suffered a lower pressure drop when is moved in a helical path. This paper examines the use of a flow-modifying device that creates a helical flow to unload liquids. Laboratory experiments were conducted using a 125-ft vertical flow loop on 2-in. diameter clear PVC. In these experiments, the effects of gas and water flow rates on the flow-modifying device were considered and compared with the behavior in normal pipe flow.