The Strait of Hormuz – the narrow waterway through which between 20% and 25% of the world’s seaborne oil normally passes – has been effectively closed for just over two months.
As tensions have escalated, Iran has restricted passage through the Strait, while the US has imposed a naval blockade on Iranian shipping, sharply limiting Tehran’s ability to export crude. On May 3, the US president Donald Trump announced Project Freedom, by which US warships would escort vessels from countries not involved in the conflict through the Strait. But some reports have suggested that Iran has since fired on several ships attempting to transit and the waterway remains effectively closed.
The immediate consequences are tankers stranded, prices surging, and Iran rapidly running out of places to store its oil. Analysts now warn that storage could fill within weeks, forcing producers to shut wells altogether.
But the deeper story lies far below the surface. Oil wells are not designed to be switched off and on at will. And when they are, the damage can linger long after the crisis has passed.
To understand why, it helps to ditch the idea of oil fields as underground lakes. In reality, oil sits trapped inside microscopic pores in rock, typically a hundredth of a millimeter wide, held there by pressure, temperature, and a delicate balance between oil, gas and water.
Shutting them down, especially abruptly and for long periods, can alter their internal balance in ways that are difficult, sometimes impossible, to reverse. Production works because the system is in motion. When a well is open, pressure differences drive oil toward the wellbore (a drilled channel connecting the oil reservoir to the surface). Over time, that pressure naturally declines, which is why operators use techniques such as water or gas injection to maintain flow.
The key point is that reservoirs are dynamic. They depend on continuous management to remain productive.
Shut the well and the movement of the oil stops. The consequences begin almost immediately. One of the first changes occurs in pressure distribution. While shutting down a well can temporarily allow pressure to build back up near the wellbore, the broader reservoir may experience uneven redistribution.
In fields that rely on carefully managed injection, where water or gas is pumped in to push oil out, halting operations disrupts that system. The injected fluids can migrate unpredictably, sometimes bypassing oil-rich zones entirely when production resumes. The fluid can chose a different path for movement so it may no longer push the oil out of the reservoir.
Then there is the chemistry. Crude oil is not a uniform substance; it contains heavier components such as waxes and asphaltenes — long-chain hydrocarbons and dense, complex molecules that can solidify or precipitate out under changing conditions. Under stable flow conditions, these remain dissolved. But when flow stops and temperatures or pressures change, these components can essentially clog the tiny pores in the rock or the well itself. Once deposited, these materials can restrict flow unless expensive – and not always successful – techniques are used to repair the damage.
Water adds another layer of complexity. All reservoirs contain formation water (the naturally occurring water trapped in the rock alongside oil and gas), and in some cases injected seawater. When a well is shut in, water can intrude into zones that previously produced mostly oil. Over time, this “water invasion” can become entrenched, meaning that when production resumes, the well produces far more water and far less oil. Separating and disposing of that water is costly, and in some cases the oil production becomes uneconomic.
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There are also mechanical risks. The well itself is lined with steel casing and cement, and is designed to operate under certain conditions. Long shutdowns can lead to corrosion, scaling (mineral build-up), or even structural integrity issues. In extreme cases, restarting a well can require significant reworking, akin to reopening a mine that has partially collapsed.
Perhaps the most misunderstood aspect is what happens at the scale of the whole oil reservoir over longer periods. Some reservoirs are highly sensitive to pressure changes. If pressure drops too low or fluctuates unpredictably, the rock structure can compact. This compaction reduces the pores available to store and transmit fluids, permanently lowering the field’s production potential.
Gas behaviour also matters. In many reservoirs, gas is dissolved in oil under high pressure. When pressure falls below a certain threshold, gas comes out of solution, which forms bubbles that can block flow pathways . If this happens unevenly during a shutdown, it can leave behind pockets of oil that are effectively stranded.
All of this helps explain why operators are cautious about shutting in production unless they have to. It is not just a matter of lost revenue during downtime – it’s the risk of losing future production capacity altogether. That said, not all wells suffer equally. Some reservoirs are more resilient.
In many cases, particularly in large conventional fields, production can be restored relatively quickly after a shutdown, as seen in past disruptions. But this doesn’t mean the reservoir is unaffected – even when output returns, subtle changes can reduce efficiency, increase costs, or leave some oil permanently unrecovered. In practice, this can mean a reduction in how much oil is ultimately recoverable. Some pockets may become harder to access or uneconomic to produce under normal conditions, even if they remain physically in place. That does not imply the oil is lost forever, but it can shift part of it beyond reach with current technology or prices, effectively lowering the field’s long-term yield.
There are environmental risks too. Closure of wells may cut emissions in the short term, but pressure instability can increase methane leakage. Restarting wells often involves flaring and venting, adding further emissions. Over time, water intrusion and reservoir damage can raise the environmental cost per barrel, as more energy is needed to extract less oil.
Modern engineering can mitigate some risks through careful planning maintaining minimal circulation, managing pressure, or using chemical treatments. But these measures require time, coordination, and resources, which may not be available in a sudden geopolitical crisis.
The broader lesson is that oil production is not easily paused and resumed like a factory assembly line. It is a continuous interaction with a complex natural system. Interruptions especially abrupt, large-scale ones can leave lasting scars beneath the surface, long after the valves are reopened.
