PRODUCTION PROBLEMS 



1135 



For oil which is fully saturated with gas at initial conditions of reservoir 

 pressure and temperature, the productivity index will decrease as the 

 reservoir is being depleted, because of the corresponding decrease of the 

 oil saturation in the reservoir rock and the fact that the viscosity of the 

 reservoir liquid increases as the reservoir pressure diminishes. Other 

 secondary effects may contribute to productivity index decline. Some 



-t 



-ill 



Production- Barrels per* Day 



Fig. 704. — Determining productivity in wells making small quantities 

 of gas from fluid-level or pressure-gauge data. (Jakosky, A.I.M.E. Pet- 

 roleum Technology, Tech. Pub. 1058, 1939.) 



commonly-observed effects are filling of the liner by sand, and plugging 

 of the perforations of the sandface by calcareous or waxy deposits or fine 

 silts. In some instances the productivity index is found to increase. This 

 condition occurs when the reservoir fluid is undersaturated at initial condi- 

 tions of reservoir pressure and temperature, since, then, the liquid viscosity 

 decreases as the reservoir pressure declines to the saturation point. Other 

 factors which may contribute to productivity index increases are the elim- 

 ination of completion effects and the establishment of drainage channels. 

 Periodic productivity index measurements are thus a vital necessity in 

 keeping a close check on a well's performance. The decline of a well's 

 potential and its pumping performance can be intelligently followed, and 

 in many cases controlled, by the use of bottom-hole pressure or fluid-level 

 data. 



