W.B. Fair Jr., Interamerican Petroleum Consultants
Pressure buildup analysis has been used for many years to evaluate the performance of wells and estimate the properties of reservoirs. Usually, the application of pressure transient theory to the evaluation of well tests requires that accurate bottomhole pressure measurements be made. In the case of a well producing by beam pumping, the direct measurement of bottomhole pressures is impossible, since the presence of the pump in the tubing physically precludes the use of wireline pressure gages and permanent downhole pressure installations are usually not economically attractive. For this reason, indirect bottomhole pressure measurements are usually made by measuring the annulus pressure at the surface along with the fluid level in the annulus. From these measurements and the fluid densities, the bottomhole pressure may be estimated with an accuracy depending on the accuracy of the estimated fluid properties and the measurement accuracy. This procedure has been criticized, since the pressures being analyzed are not directly measured and the accuracy of the data and estimated fluid properties limits the analysis precision. This report documents an improved evaluation method for wells producing on beam pumps. Instead of indirectly calculating the bottomhole pressure, the wellbore is modeled and the analysis is performed directly on the measured data. Since both casing pressure and fluid level are measured, the number of data points is essentially doubled over standard well testing and the accuracy of the analysis can be judged by comparison with the actual measured data. In addition, since the casing pressure appears to be mainly affected by wellbore effects, the evaluation accuracy is improved even more than would be expected from statistical considerations alone.