1134 EXPLORATION GEOPHYSICS 



An alternative and more rapid method requiring only one determina- 

 tion of the weight of the gas column is illustrated in curve B. As before, 

 the production rate is maintained constant and two or more measurements 

 are made of fluid levels versus casing-head pressure. Extrapolation of the 

 data to the depth of the mean producing horizon gives the theoretical 

 casing-head pressure necessary to force the fluid level down to the pro- 

 ducing horizon. The total pressure at this depth may be obtained by adding 

 the weight of the gas column at this depth to the extrapolated casing-head 

 pressure. Referring to the figure, the extrapolated casing-head column 

 was found to be 258 pounds, while the calculated weight of the gas column 

 was 38 pounds, giving a total formation pressure of 296 pounds. (In 

 this procedure, a direct determination of density cannot be obtained from 

 the slope of the graph.) 



PRODUCTIVITY DETERMINATIONS: WELLS 

 WITH LOW CASING-HEAD PRESSURE 



The "absolute potential" of a well is determined by conducting a series 

 of measurements to evaluate two factors : (a) the effects of casing-head 

 pressure and (b) the effects of rate of production. In most wells operating 

 at low casing-head pressures, or at atmospheric pressure, the casing-head 

 pressure plus the weight of the gas column is sufficiently low to have a 

 negligible effect on the potential. Measurements on such low pressure 

 wells need therefore be conducted only to determine the effects of rate 

 of flow on the fluid height or the bottom-hole pressure. 



The producing characteristics of wells with negligible casing-head 

 pressures may be determined by obtaining bottom-hole pressure or fluid- 

 level data at two or more stable production rates. If fluid-level measure- 

 ments are made, the casing-head pressure is held constant during the 

 measurements at each of the production rates. The data are then plotted 

 as shown in Figure 704. 



It will be noted that the pressure-rate curve must deviate from a 

 straight line in order to fit the static reservoir pressure value. Extrapola- 

 tion of the curve to the Q-intercept gives the theoretical maximum pro- 

 duction which the well is capable of yielding, provided flow friction, water 

 coning or other variables encountered in practice do not change the slope 

 of the curve, t 



The productivity index is determined from the slope of the pressure- 

 rate curve and is defined as "the barrels per day of gross liquid produced 

 per pound per square inch pressure drop at a specified subsurface datum." t 



Any condition tending to change the productive capacity of the well 

 is indicated by a change in the productivity index or slope of the curve. § 



t M. L. Haider, loc. cit. 



t B. P. Kantzer and E. G. Trostel, "Oil Well Performance, Discussion and Proposed Ter- 

 minology," A.P.I. , Drilling and Production Practice, 1937. 



§T. V. Moore, Proceedings Amer. Pet. Inst., 1930, 11 (4), 27. 



M. L. Haider, loc. cit. 



H. C. Miller, E. S. Burnett and R. V. Higgins, loc. cit. 



