Gas-lifted wells have been dominated by injection pressure operated (IPO) valves over the last half- century.
The technology is well known and offers great benefits over other forms of artificial lift in some wells and fields. Advances in electronics, manufacturing processes, and technology are delivering breakthroughs that offer more intelligence and functionality downhole. Surface-controlled gas lift is one of the most recent of these breakthroughs.
Surface-controlled gas lift allows real-time control of downhole gas lift valves from the surface. This provides some very predictable benefits such as increased production due to deeper injection. Continuous optimisation with intervention is another benefit that is accepted but much harder to put a value on. However, many unanticipated benefits that provide the ‘icing’ on the ‘cake’ of increased production are less obvious.
IPO gas lift vs surface-controlled gas lift
Gas lift uses gas injected into the production string to reduce the hydrostatic column, allowing the reservoir pressure to push oil out of the formation. Traditionally, IPO valves have been used to allow gas to flow from the annulus to the tubing based on the pressure in the annulus. They are placed along the length of the production string and, in general, close as the annulus pressure drops. Each IPO station acts autonomously based roughly on the pressure in the annulus. This system provides a degree of oil production, but there are shortcomings.
The pressure and temperature-related opening and closing pressure of each IPO valve must be set before installation. This requires a good understanding of the well’s behaviour. Optimisation relies on the assumption that the well will be stable over time, the reservoir characteristics are completely understood before completion, and the infrastructure will not change. However, none of these assumptions are valid, especially in unconventional fields. Correction for poor misassumptions or dynamic factors requires intervention.
The design of a well to utilise IPOs also requires concessions. One that affects production is the pressure drop required in the design of each IPO station. A good rule of thumb is a 20 - 50 psi pressure drop for each additional station. If 10 stations are required to get to the desired injection depth, 500 psi is lost in the design, which could be used to inject even deeper, increase the drawdown, and increase production.
Surface-controlled gas lift offers absolute control over where gas is injected and at what rate at any time in the life of the well. It also offers instant adjustment of gas injection without any intervention.
Three main components are required for surface- controlled gas lift: downhole flow control devices, a way to communicate with the downhole flow control devices, and the surface system to communicate with the downhole devices (Figure 1). It includes the downhole surface-controlled gas lift valves, a communication line to the surface, and a surface control system to relay communications. This control system is typically communicably attached to the operator’s surface control and data acquisition (SCADA) system.
Advantages of surface-controlled gas lift
Surface-controlled gas lift systems, such as Silverwell Energy’s Digital Intelligent Artificial Lift (DIAL) production optimisation system, allow deeper gas injection because the available pressure is not lost to guarantee the operation of IPO valves. This leads to more drawdown at the formation and increased production (Figure 2).1
1. FAUX, S., GLENDAY, T., ‘The Benefits of Introducing Intelligent Gas Lift Management to Create a Smart Field’.
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