Global demand for Opec's crude will be lower in 2020 than next year as supply from rivals proves more resilient than expected, potentially fuelling a debate on the merits of its strategy to let prices fall to hurt other producers.

The Organization of the Petroleum Exporting Countries, which a year ago refused to cut supply to retain market share against higher-cost rivals, in its 2015 World Oil Outlook raised its global supply forecasts for tight oil, which includes shale, despite a collapse in prices.

Demand for Opec crude will reach 30.70 million barrels per day (bpd) in 2020, Opec said, lower than 30.90 million bpd next year. The expected demand from Opec in 2020 is about 1 million bpd less than it is currently producing.

Oil has more than halved its price in 18 months and sank to an 11-year low of $36.04 a barrel this week. The drop has helped to boost oil's medium-term use, although Opec said the demand stimulus of low crude prices will fade over time.

"The impact of the recent oil price decline on demand is most visible in the short term," Opec Secretary-General Abdullah al-Badri wrote in the foreword to the report. "It then drops away over the medium term."

Opec is increasingly divided over the merits of the 2014 shift to a market-share strategy, which was led by Saudi Arabia and its Gulf allies, and at a Dec. 4 meeting failed to agree a production ceiling for the first time in decades.

Nonetheless, the report shows that the medium-term outlook - from Opec's point of view as the supplier of a third of the world's oil - has improved. In the 2014 edition, demand for Opec crude was expected to fall to 29.0 million bpd by 2020.

Opec said it stopped modelling work on the report in mid-year, since when it has updated its forecast of 2016 non-Opec supply to a decline. Opec figures in the report do not include Indonesia, which rejoined in December.

The main figures in the report showing Opec medium-term market share under pressure are unchanged from those in a confidential Opec report Reuters obtained in November.

RESILIENT SHALE

Opec initially downplayed the impact of shale oil, although its annual outlook in 2012 acknowledged for the first time that the effect could be "significant".

Years of high prices - supported by Opec's former policy of cutting supply - helped make non-conventional oil such as shale viable. In a change of tack from previous reports, Opec now says many projects work at lower prices too.

"The most prolific zones within some plays can break even at levels below the prices observed in 2015, and are thus likely to see continued production growth," the report said.

Global tight oil output will reach 5.19 million bpd by 2020, peak at 5.61 million bpd in 2030 and ease to 5.18 million bpd in 2040, the report said, as Argentina and Russia join North America as producers.

Last year's estimates were 4.50 million bpd by 2020 and 4 million bpd by 2040.

Under another, upside supply scenario, tight oil production could spread to Mexico and China and bring supply to almost 8 million bpd by 2040, Opec said. As recently as 2013, Opec assumed tight oil would have no impact outside North America.

The report supports the view that Opec's market share will rise in the long run as rival supply growth fades. Opec crude demand is expected to reach 40.70 million bpd in 2040, amounting to 37 percent of world supply, up from 33 percent in 2015.

Opec nudged up its medium-term world oil demand forecast, expecting oil use to reach 97.40 million bpd by 2020, 500,000 bpd more than in last year's report.

But factors including slower economic growth, the limited share of the crude cost in pump prices and the falling value of some domestic currencies against the dollar will limit the demand response to lower crude prices, Opec said.

By 2040, Opec expects demand to reach 109.80 million bpd, 1.3 million bpd lower than a year ago, reduced by energy efficiency and climate-change mitigation efforts.

Only a gentle recovery in oil prices is seen. Opec's basket of crude oils is assumed in the report at $55 in 2015 and to rise by $5 a year to reach $80 by 2020. – Reuters