Forecasting Oil Production in Waterflooded Reservoirs

Abstract

Abstract A three-parameter model relating the decline of the oil cut with the fractional oil recovery for waterflooded petroleum fields or reservoirs is described. The model is based on observation of the behavior of several Brazilian oil fields, in which the oil displacement occurs either by water injection or by natural aquifer influx, or both. The model is used to derive equations to forecast the oil and water production rates in waterflooded systems. These equations may also be used to design a waterflood expansion in any phase of a field's life. An example of the application of the model equations to forecast the behavior of a waterflood in a mature heavy oil field is presented. Because the results of the analysis indicated that the field will still be profitable at the end of the concession period, the model equations are applied to design and forecast an augmented waterflood for the field. The model equation may be used to estimate the oil reserves at any stage of maturity of an oil field. The model parameters are obtained by fitting the model equation to historical field data by means of a least square algorithm. A type-curve procedure is applied to obtain the initial guesses of the model parameters required by the mathematical algorithm. A relation between the oil cut decline model and the classical rate-time decline is derived for the case in which the total liquid flow rate is constant. Examples of oil cut decline of several Brazilian fields are presented and the decline model is used to determine the ultimate oil recoveries of these fields. The ultimate oil recovery is proportional to the amount of water produced during the field's life, which is related to the water-oil ratio at abandonment conditions. When the operating conditions of a field change, so does the oil-cut decline trend. An example of the change in the oil-cut decline trend is presented for a heavy oil field submitted to a late steam drive.