Abstract A modification of the standard material-balance equation for undersaturated reservoirs is presented. The effect of water influx is accounted for by the term, which is the ratio of the aquifer to reservoir pore volumes.For an approximately circular reservoir, values of, may be obtained quite easily by reiteration from the internal-cylinder solution of the diffusivity equation. The technique has been applied to a Colombian field, with acceptable results. Introduction Weak water drives that might have a negligible effect below the bubble point may exert a very appreciable influence above the bubble point. Under these conditions, use of the standard material-balance equation (as presented by Hawkins, and by Hobson and Mrosovsky) may lead to considerable error. This paper presents a modification to the standard equation; a field example indicates that the technique yields acceptable results. The method is quite straight-forward, its solution requiring only a desk calculator and a book of six-figure tables. Derivation of the Modified Equation Let the reservoir pore volume be Vpo. At any time t, let the effective pore volume of the aquifer that has been influenced by the pressure drop in the reservoir be Vpa.At a pressure drop p, the total pore space will be reduced to . The initial water volume of the reservoir is increased fromto . Hence,=. Originally, and Hence, Sorting into a more conventional form, .....(1) Evaluation of Ritchie and Sakakura have presented a solution to the diffusivity equation for a uniform system of infinite extent, bounded internally by a cylinder of radius rw. It seems likely that the majority of field engineers may be unable to obtain copies of Ref. 3. However, Mortada offers quite a thorough discussion of this solution and its modification for nonuniform aquifers.In oilfield units (and ignoring third- and higher-order terms), the solution reduces to ......................(2) and JPT P. 391^