Abstract: this paper uses prediction model of grey system and LOGISTIC to establish the relationships between cumulative
oil preduction and time, wellnumbers and time, water cut and time respectively, to study the tendency of produc-
tion costs with water cut, to establish mathematics model between profit and total well numbers by analysis of
break even, and to get out economic limit of retional well numbers. One tract of some field was used to test the
numbers of economic rational well and numbers of economic limit well under diffent price system this method can
provide important information for oilfield development adjusting and desiging later, and it is simple and uesful to
oilfield, and possesses the value of popularization and application.

Key words: mathmatical model, lost checking, well spacing density, pattern modification, economic evaluation