NEW YORK/LONDON/SINGAPORE (Reuters) - After the first OPEC oil production cut in eight years took effect in January, oil traders from Houston to Singapore started emptying millions of barrels of crude from storage tanks.

FILE PHOTO: Crude oil storage tanks are seen from above at the Cushing oil hub, appearing to run out of space to contain a historic supply glut that has hammered prices, in Cushing, Oklahoma, March 24, 2016. Picture taken March 24, 2016. REUTERS/Nick Oxford/File Photo

Investors hailed the drawdowns as the beginning of the end of a two-year supply glut - raising hopes for steadily rising per-barrel prices.

It hasn’t worked out that way.

Now, many of those same storage tanks are filling back up or draining more slowly than investors and oil firms had expected, according to global inventory estimates and more than a dozen oil traders and shipping sources who told Reuters about storage in facilities that do not make their oil volumes public.

The stalled drawdowns shed light on the broader challenge facing OPEC - the Organization of the Petroleum Exporting Countries - as it struggles to steer the industry out of the downturn caused by oversupply. With U.S. shale oil production surging, inventories remain stubbornly high and prices appear stuck in the low-$50s per-barrel range.

The market has not strengthened enough to drain many major storage facilities around the globe - which OPEC oil ministers had hoped would be a first step toward rebalancing what has been a buyer’s market since late 2014.

Estimated inventories in industrialized nations totaled 3.025 billion barrels at the end of March - about 300 million barrels above the five-year average, according to the International Energy Agency’s latest monthly report.

Preliminary April data indicated stocks would rise further, the IEA said. Crude stocks stood at a record 1.235 billion barrels.

OPEC and other non-OPEC nations - most notably Russia - are now widely expected to extend production cuts for another nine months, through March 2018.

The ongoing struggle to thin supplies has forced economists to cut their oil price forecasts. Bank of America, for instance, last week lowered its 2017 target for Brent crude by $7 a barrel to $54.

During the two-year price war started by OPEC, about half a billion barrels of crude and refined products flowed into storage facilities as oil prices hit lows of less than $30 a barrel in early 2016.

Much of the inventory build-up came as traders started using storage to make easy money on the widening spread between rock-bottom spot oil prices and substantially higher prices for contracts to deliver the oil in future months.

That price spread - a market structure known as contango - allowed traders to profit even after they paid for expensive storage in facilities such as the Louisiana Offshore Oil Port (LOOP) - the only deep-water U.S. oil port and a major conduit for crude imports - or supertankers parked offshore in Singapore.

Although the storage trade has been less profitable since the OPEC production cuts, much of that oil remains in tanks, said Chris Bake, an executive committee member at Vitol, the world’s largest independent trader, during an industry conference last week in London.

“This 550 million barrel-plus inventory build of crude and products that started in 2014 is still very much there,” he said. “How much is going to come out? That is an ongoing debate among all of us.”

“CLOGGED” WITH OIL

From the Malacca Straits in Asia to the ports of Northern Europe and the Gulf of Mexico, drawdowns of global inventories have slowed or even reversed.

In the Amsterdam-Rotterdam-Antwerp (ARA) region – one of the most expensive areas in Europe to store oil and the benchmark pricing point for fuel - crude is starting to flow back into storage because refiners are “clogged” with oil, an industry source handling deals in that region told Reuters.

Refined fuel inventories have also jumped suddenly, with gasoil in tanks in the ARA hub rising to an eight-month high earlier this month, according to Dutch consultancy PJK International. Gasoil includes jet fuel, diesel and heating oil.

At one of the world’s largest oil storage facilities - on the shores of Saldanha Bay in South Africa - millions of barrels were sold in recent months, traders told Reuters.

But more cargoes are flowing right back into its tanks, which can hold 45 million barrels, as sellers struggle to find refiners to buy freshly loaded oil, the traders said.

In the Houston region, stored oil stocks touched record levels at the end of March, according to energy information provider Genscape.

The state of inventories appears more mixed in Asia.

In China, the world’s second-largest oil consumer behind the United States, commercial crude stocks hit their lowest level in four years in March, according to the government-controlled Xinhua News Agency. But in nearby South Korea, inventories were near a record, according to the Korea National Oil Corp.

SLOW PROGRESS

While global inventories remain bloated, there are some signs that the OPEC cuts have dented supplies.

Recent data from the U.S. Energy Information Administration showed that nationwide stocks started draining in April this year - the first decrease for that month since 1999.

Declining costs for storage is another indication that traders and oil companies are putting less oil in storage than at the height of the price war.

At the largest U.S. storage facility at Cushing, Oklahoma, storage tanks costs about 35 cents a barrel per month, traders say, compared nearly 50 cents a year ago.

Parking oil in a supertanker off the shore of Singapore, Asia’s refining hub, costs anywhere from 30 to 40 cents a barrel per month, down from 50 to 80 cents just a few months ago.

The futures contract for oil storage at the LOOP, off Louisiana’s coast, dropped to about 24 cents per barrel recently, one of the lowest prices this year.

Still, the patchy evidence of draining storage has fallen far short of what investors expected after OPEC and non-OPEC nations agreed on production cuts last November.

“People were impatient and thought we’d start drawing 10 million barrels a day since the first week of January,” said Amrita Sen, chief oil analyst at Energy Aspects. “We’re still in excess, and there’s lots of inventory around.”