Natural Gas-Lift: Theory and Practice

Abstract

Abstract This paper examines the concept of "natural gas-lift" or production of oil by in situ gas. This process involves the commingled production of an oil reservoir and a contiguous or non-contiguous gas zone in a controlled mode as an alternative to artificial gas-lift. Besides the normal conception of gas-lift as a remedy to high water-cut flow, lift assistance may be required at startup to commission production, and can also be required at low to moderate water-cut, when the wellhead pressure requirements are stringent. The latter is typical of subsea installations and platform installations with high-pressure processing. Contiguous gas-lift is a more complex process, due to the interaction of the oil column with the overriding gas-cap, but can be envisaged more easily than non-contiguous gas-lift which requires the presence of a suitable gas zone or depleted oil zone. We present results of numerical modeling of the contiguous gas-lift process for horizontal wells, for the case of a conceptual reservoir model with characteristics similar to certain North Sea provinces. Results show the applicability of natural gas-lift dependent upon standoff (with respect to the initial gas-oil and water-oil contacts) and target production rate. We also address design considerations for natural gas-lift applications and report the operational experience gained in the Troll field with contiguous or gas-cap gas-lift applications. Finally we examine a gas-lift application of the non-contiguous type.