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* Oil sands project continues to lag design rates
* Opti shares unchanged, Nexen up 1 pct
CALGARY, Alberta, Jan 12 (Reuters) - Production at Nexen Inc's NXY.TO Long Lake oil sands project averaged 29,100 barrels per day of bitumen in December, junior partner Opti Canada Inc OPC.TO said on Wednesday, up 11 percent from the prior month but still well below the facility's capacity.
The C$6.1 billion ($6.16 billion) oil sands project hasn't been able to reach its 72,000 barrel per day capacity since it started up nearly three years ago.
The thermal project, where steam is pumped into the ground to liquefy tarry bitumen deposits so they can flow to the surface, has struggled with problems at its processing plant and has had to boost its ability to generate steam.
Nexen said in November it expects output at Long Lake to average between 38,000 and 45,000 bpd in 2011. The project produced 26,200 bpd in November.
Nexen has a 65 percent stake in the project while Opti owns the remainder.
Opti shares were unchanged at 65 Canadian cents while Nexen rose 17 Canadian cents to C$23.18 by midmorning on the Toronto Stock Exchange.
$1=$0.99 Canadian Reporting by Scott Haggett; editing by Rob Wilson