Small-Diameter Gas Lift Systems-A Potential Technical Solution for Transport of Fluids From Low-Pressure Reservoirs

Abstract

Abstract Continuous or intermittent artificial lifting technology is competing today with electrical submersible progressive cavity (PC) pumping and sucker rod pumping for producing fluids from lowpressure reservoirs. If the shut-in fluid level is less than 45% of the depth of the well, finding a suitable and economic artificial lifting technology is a challenging task. There are thousands of dormant gas wells where bottom water accumulations of 50 m or less impede gas production. Similar conditions are often found in coalbed methane reservoirs. Due to variable (and shallow) water levels and gas presence, rod pumping cannot be used and submersible electric pumps often pose operational problems. Depending on local conditions and economics, gas lifting, alone or associated with other artificial lifting technologies, can be used for producing such reservoirs. Within a limited range of gas-liquid flow rates, depth, and reservoir pressure, the use of small-diameter pipes for gas lifting technology can become a viable technology. Laboratory investigations dedicated to small-diameter gas lifting operations have been so far limited to fluid transfer operations requiring a maximum of 10 - 20 m. This study uses mechanistic modelling approaches to respond to the industry's need for a better evaluation of depth/diameter flow rate limitations in view of assessing potential field applications of gas lifting for low reservoir pressures and relatively small liquid flow rates. Laboratory tests were conducted in a specially designed rig. Experimental results were used to evaluate the accuracy of the existing model predictions and for assessing the effect of injected gas flow rates, reservoir pressure, and liquid interfacial tension on the liquid production rates. To improve predictions of existing mechanistic models, particularly for small-diameter tubings and low pressure reservoir conditions, a new model is proposed and compared first with experimental results. The new model is then used as a scaling tool for assessing critical field depth conditions. Introduction Gas lifting or air lift has been used to remove water from flooded mines since 1782(1, 2). Today, natural gas lifting is commonly used for oil wells where gas and liquid are produced together. Conventional gas lifting uses tubing (or ducts) with a diameter greater than 2.54 cm. Vertical upward transport of gas and liquid for such conditions is well investigated and both empirical(3) and mechanistic models are available(4–6). During the last 20 years, mechanistic models are gaining more acceptance, replacing empirical models. Development of a mechanistic model involves:extensive visual observations of field and laboratory-scale models in view of assessing specific boundaries of flow patterns (e.g., bubble, slug, annular, stratified, etc.);assessment of the main gas-liquid features (e.g., bubbles, liquid film, etc.) and of the phases interface aspect (e.g., smooth, wavy, etc);estimation of local gas and liquid velocities, including the slip;estimations of local void fraction and of static and dynamic pressures;computer-assisted integration of "local" features to include the pipe pressure-volume-temperature (axial) profile; and,field validation. Flow pattern mapping and the drift-flux model(7) are essential tools used to evaluate the gas-liquid relative velocities and local void fraction.