Abstract

A continuous-flow gas-lift installation design for injection-pressure-operated gas-lift valves is outlined on the basis of tubing capacity and daily injection-gas rate. the maximum daily production rate by gas lift decreases with a required increase in the depth of lift. The reservoir inflow performance of the producing formation is not considered in the installation design procedure. This valve spacing method is suited for gas-lift installations in wells with unknown productivity and/or changing reservoir pressure.

Introduction

A variable production rate gas-lift (GL) installation design is recommended when the static reservoir bottomhole pressure (pws) and deliverability are unknown and/or changing during the life of the GL operations1. Wells with little or no production history include newly drilled wells and wells that have been plugged back and re-completed in new formations. Formation liquid rates, water cuts, and formation gas production can change significantly as the result of response from secondary recovery methods and EOR reservoir management projects. A variable rate GL design illustrates the flexibility of GL as compared to most other methods of artificial lift.

As the required depth of lift increases, the maximum daily liquid production rate by GL decreases for a given operating surface injection-gas pressure (pio). The gas-lift valve (GLV) mandrels can be located on a predictable decreasing production rate and increasing total gas/liquid ratio (TGLR) with depth for a given tubing size and pio. Then the port sizes in the GLVs can be based on the liquid production rate and injection gas/liquid ratio (IGLR) for each lower valve depth (Dv). The concept of GLV depths and port sizes on the basis of a decreasing liquid rate and increasing TGLR with depth is more logical than assuming a high daily production rate for spacing the upper GLVs and an arbitrary constant distance of 200 to 400 ft between the lower GLV mandrel depths.

A varying production rate installation design need not be considered if the available pio will unload a well at the maximum design production rate to the deepest possible Dv, such as immediately above a packer. These conditions will occur when a high pio is available for lifting shallow wells.

A continuous-flow GL installation cannot compete with an electrical submersible pump (ESP) in terms of a maximum daily production rate from wells with a high productivity and a low bottomhole flowing pressure (BHFP). An ESP can add work to the flowing fluids to supplement the lift process, but continuous-flow GL can only decrease the flowing fluid density by adding injection gas to the flowing fluid mixture. Additional pump stages can be added to a pump for increasing the flowing tubing pressure at the pump discharge while ensuring a minimum BHFP at the pump intake.

Flowing-Production Pressure at Depth Traverses

The flowing-production pressure at depth (ppfD) traverses in Fig. 1 are based on a daily injection-gas rate (Iqgsc) of 500 Mscf/D and a formation gas/liquid ratio (FGLR) of 200 scf/STB. As the liquid rate decreases, the IGLR and TGLR increase for the same Iqgsc. The injection-gas pressure at depth (pioD) curve is based on a kick-off and operating pio of 1000 psig for spacing the top two GLVs. The pio is decreased for spacing the third and lower injection-pressure-operated (IPO) GLVs to ensure closure after the depth of gas injection transfers to a lower GLV. The injection-gas specific gravity and the temperatures at the surface and at depth are the same as the well data given in Table 1.

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