New Delhi, Nov 30 (IANS): The Great Eastern Energy Corporation (GEECL) on Friday said it would invest $2 billion over the next 10 years for shale gas exploration and development from its Raniganj block in West Bengal.In a statement, GEECL said it expects to start the initial shale gas exploration work of drilling core wells in the first half of next year. "Depending on the results obtained and analysed from the core wells, we thereafter intend to drill an optimum number of pilot production wells.