Abstract: Based on available approaches in analysis and economical evaluation,a new method for calculating reasonable and ultimate well spacing density in an old oilfield of Yumen that has been developed for many years. It takes into account both interest from investment and interests from overhead expenses and crude-selling income year after year. This method can be applied to verify the feasibility of infill drilling in an old oilfield or reservoir, getting rid of taking a risk for uncertain investments.

Key words: mature field, well spacing density, recovery factor, economic evaluation