VIENNA —The Organization of the Petroleum Exporting Countries (OPEC) Friday trimmed its forecasts of demand for its own crude over economic concerns and rising output by rival producers, a move that makes it likely to cut production significantly next year. The downgrade, the second such revision in as many days, suggests Gulf producers will be pressured to tighten their spigots when the group meets in Vienna on Dec. 12. In its monthly oil market report, OPEC, whose members extract more than one in three barrels of oil consumed each day across the world, said demand for its crude in 2013 will be about 100,000 barrels a day less than previously expected. It now forecasts demand for its own oil next year to be 29.7 million barrels a day — 400,000 barrels a day less than this year. The difference is almost equivalent to the daily output of its lowest-producing member, Ecuador. The organization also cut its forecast for global oil demand for 2013 by about 20,000 barrels a day to 89.57 million barrels a day, though this still represents 1 percent growth from this year. “The economy is placing a considerable amount of uncertainty on the world oil demand forecast,” the group warned. The organization also cut its global demand forecast for 2035, the end of the period covered by the report, by 2 million barrels to 107.3 million a day, as higher prices curb consumption and cars and ships use fuel more efficiently. OPEC will need to bolster output by a “modest” 5 million barrels a day to meet world requirements by 2035, it said. OPEC still expects its share of global oil will remain “approximately constant” at 32 percent through to 2035 as it boosts output of natural gas liquids, or NGLs, and supplies from gas-to-liquid projects. The group raised its price assumptions, projecting that OPEC's basket of crudes, a benchmark composed of blends from each member, will average $100 a barrel through to 2016 and then climb to $155 by 2035 because of higher costs to bolster production. The basket averaged $110 this year. On Thursday, OPEC trimmed a separate demand forecast for its crude by 1.6 million barrels a day through 2015, as part of a broader downgrade by 1 million barrels a day for global oil demand in the medium term. The revisions highlight the rise in competition that OPEC is facing. The organization also upgraded non-OPEC production growth estimates for next year, mostly due to revised expectations for Australian output. The price of oil slid back to near $84 a barrel Friday as traders weighed evidence of ample US supplies against signs of recovery in China's economy. By early afternoon in Europe, benchmark crude for December delivery was down 80 cents at $84.29 a barrel in electronic trading on the New York Mercantile Exchange. The contract Thursday rose 65 cents to close at $85.09 in New York. Brent crude, which is used to price international varieties of oil, was down 59 cents at $106.66 a barrel on the ICE Futures exchange in London. A stronger dollar, which makes crude more expensive and a less attractive investment for traders using other currencies, also weighed on oil prices. On Friday, the euro was down to $1.2693 from $1.2745 late Thursday in New York. — SG/Agencies