Abstract: This study takes old oilfields of PetroChina Xinjiang Oil as examples, begins with Cierkaqov’s equation, relates to analysis of economic appraisal stage, gives computations for well spacing density at economical limit and optimal economic spacing density of an old oilfield. This method is practical implications for infill development of old fields, particularly to sporadic infill drilling of old fields with high water cut. This method allows decision maker to avoid risk from exploitation investment in a great extent.

Key words: old petroleum area, infill drilling, adjustment, computation, economy