JACK SMITH Fort Worth-based Quicksilver Resources is selling 27.5 percent of its leasehold interests in the Alliance area in Tarrant and Denton Counties to the Italian oil company Eni for $280 million in cash. Quicksilver plans to use the proceeds to pay down debt.Quicksilver announced today that it is forming a strategic alliance with Eni to acquire and develop Barnett Shale natural gas properties in an area covering approximately 270,000 acres surrounding Quicksilvers Alliance properties. Quicksilver will be the operator for the properties.Quicksilver last year paid $1.3 billion for the properties, much of it borrowed.