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. Introduction Artificial lift represents a major cost in the operation of oil fields, and this cost becomes more severe as the production environment becomes more challenging, as is the case of offshore developments and remote areas. This paper evaluates the alternative of using the energy of the reservoir by controlling remotely the amount of gas that is produced, in what is called ‘natural gas-lift’. Many fields have gas zones that are part of the produced reservoir or separate gas or high GOR zones. Reservoirs with thin oil zones between a gas-cap and a bottom aquifer are natural candidates for this application, but depleted oil reservoirs with or without adjacent gas zones (provided that some source of gas is accessible from the well) are also good candidates. This paper is divided into three sections. The first section addresses the behavior of an oil reservoir with a gas-cap and a bottom aquifer. For this case, a conceptual reservoir model was built to study the drivers for natural gas-lift applications. The next section deals with the operational experience gained in an actual field where the gas from the gas-cap is used to produce the oil zone. Finally, the third section covers an example of the use of a non-contiguous gas zone (lower than the oil zone) that is used as a source of gas to enable oil production. Natural Gas-Lift from a Contiguous Gas Zone This process applies mainly to reservoirs where the production drive mechanism is dual drive (water encroachment at the bottom and gas expansion on top). Depending upon well type and completion, the water-cut is expected to increase with time, and eventually artificial lift may be necessary to support the production demand. From the recovery point of view the displacement of oil by gas is more efficient than the displacement by water. This is due to the lower residual saturation of oil when gas is the displacing phase. Horizontal wells are a very good alternative for the production of dual drive reservoirs when the oil zone thickness is relatively small (compared to the gas and water zones), because they provide higher flexibility in well placement within the oil zone to satisfy the production strategy. In fact the well placement and the production rates play a major role defining what would be the dominating production drive. Numerical and analytical studies, as well as field experience, have shown that better results are achieved by drilling the horizontal section as far as possible from the gas-oil contact. Some studies have even addressed the placement of the wells in the water zone.1