Iran is about to get a refresher course in the capricious nature of the oil market and the durable nature of economic sanctions.
When United States and other international sanctions were tightened in 2012 and took nearly 700,000 barrels a day of Iranian crude oil off world markets, the price of an average barrel of Opec oil that year was US$109.45 ($169.41).
But with the easing of those sanctions, Iran is poised to boost its sales of oil in the middle of a massive glut, with the Opec benchmark average barrel selling for just US$25, less than a quarter of the 2012 level.
The result will be sharply lower revenues for Iran than its leaders anticipated two years ago when they began negotiations to end sanctions linked to the country's nuclear programme.
The glut will force a slower ramping up of Iran's oil fields and exports than Iran had planned, and it could make international oil companies more wary and tight-fisted about making new investments.
"In many respects, this could not come at a worse time for Iran, because oil is at 11-year lows and the International Monetary Fund has recently offered quite dismal remarks about Iran's banking system, economic growth prospects and tepid recovery," said Elizabeth Rosenberg, director of the energy, economics and security programme at the Centre for a New American Security.
Moreover, the lifting of sanctions on Iran could heighten tension over re-establishing production quotas in Opec - especially between Iran and Saudi Arabia, the cartel's co-founders and longtime rivals. Eager to protect its market share, Saudi Arabia has been pumping at high levels despite calls by some Opec members that the kingdom rein in output to prop up falling prices.
Riyadh's relationship with Tehran was further strained this month when Saudi Arabia executed a prominent Shiite cleric and, after Iran's condemnation, cut diplomatic ties.
The return of production in Iran would further fuel simmering tensions. The Iranians hinted that they might hold back.
"We don't want to start a sort of a price war," Mohsen Qamsari, director general for international affairs at the National Iranian Oil Company, told Reuters on January 6.
"We will be more subtle in our approach and may gradually increase output," Qamsari said. "I have to say that there is no room to push prices down any further, given the level where they are."
Iranian officials earlier pledged to add a half-million barrels a day within six months and 1 million barrels a day in a year.
But Bhushan Bahree, a senior director and Opec expert at IHS Energy, said the consulting firm is forecasting an rise of only 400,000 barrels a day during the next year.
"I think they'll go in a little gradually, both because it is in their interest to do so for price reasons and for its relations with others in Opec, like Saudi Arabia," he said.
For companies around the world, implementation day is like flicking the switch on Iran's economic relations.
Iran will be able to conduct banking transactions through the essential Society for Worldwide Interbank Financial Telecommunication, or Swift.
Roughly US$100 billion in frozen bank accounts will be released, though about half of that will go to pay debts or other commitments.
In addition, the US Treasury Department, as agreed to under the nuclear accord with Iran, will lift sanctions that restricted foreign companies doing business in finance and banking, insurance, energy and petrochemical sectors, shipping and shipbuilding sectors, gold and precious metals, and more.
Many of those companies have been visiting Iran. But for most American companies, the lifting of sanctions linked to Iran's nuclear programme will mean little. Many American citizens, companies and banks are barred from doing business with Iran under other sanctions, legislation and regulations tied to human rights, ballistic missiles and terrorism.
There are exceptions for those involved in aviation, health-care or medical devices.
Health-care and medical devices were exempted for humanitarian reasons. Aviation sales are designed to bolster the safety of Iran's ageing fleet of Boeing passenger planes.
As of Friday, the Treasury Department was completing regulations for the implementation of remaining sanctions. One item in particular - whether foreign subsidiaries of US firms can conduct business with Iran - was being awaited by company executives to see whether it would be a minor exception or a loophole big enough for an oil tanker to sail through.
The Treasury Department is supposed to issue guidelines for "activities with Iran" that are "consistent with" the nuclear agreement. It must also say how it would handle deals conducted abroad in dollars, which must be cleared through American financial institutions.
Even US, European or Asian companies seeking to do business in Iran face hurdles, however.
One obstacle is that investors and traders will be prohibited from doing business with more than 200 Iranian or Iran-related individuals and entities who will remain on the US Specially Designated Nationals List.
Moreover, the country is plagued by high inflation, subsidies, difficulty getting permits and nonperforming loans.
Worse yet, the Iranian economy is dominated by opaque companies linked to the Revolutionary Guard Corps or the clerical establishment, notes Robert Hormats, vice-chairman of Kissinger Associates. Many of their executives are on the Treasury's list of prohibited individuals.
A system of "bonyads", tax-exempt charitable institutions controlled by the clerics, was established after the 1979 revolution with assets seized from the royal family and wealthy individuals.
"It's an alluring and attractive market because of the talent of the people, its diversity and its raw materials," Hormats said, "but it is complicated because of the structure and because there are so many influential groups that have such power over individual companies and sectors."
Ben Rhodes, US President Barack Obama's deputy national security adviser, predicted that foreign companies would take a wait-and-see approach.
"It's not going to be a flood. And frankly, the Iranians know that because there's still a tangled web of other sanctions," Rhodes said at a Bloomberg News lunch on Friday.
• Iran has 38 million barrels in floating reserves ready to enter the market, according to the International Energy Agency.
• More money coming into the country will allow it to repair oil and gas fields to boost its production.
• Iran has the world's fourth-largest proven reserve of crude oil and ranks second in proven natural gas reserves behind Russia.