Evaluation of Three New Methods For Predicting Pressure Losses In Vertical Oilwell Tubing

Abstract

The reliability and relative merits have been estimated for three recently published multiphase-flow pressure-drop prediction correlations applicable to vertical tubing. Data from 726 tests, embracing broad ranges of flow rate, pipe size, API gravity, gas/liquid ratio, and water/oil ratio were used in the evaluation. It was found that no single correlation consistently performed best in every range. Introduction Six of the several correlations available for predicting pressure losses during simultaneous, predicting pressure losses during simultaneous, continuous, steady-state flow of oil, water, and gas in vertical pipes were evaluated statistically by Lawson and Brill. Three more promising correlations have recently been published - those of Beggs and Brill Aziz et al. and Chierici et al. The purpose of this paper is to extend the work of Lawson and Brill to cover the new correlations.Details of the three new methods can be found in the original sources listed in the references. The same flowing pressure surveys and related data for 726 field and experimental wells were used as were reported by Lawson and Brill. Fluid physical properties were estimated using the same correlations as employed by Lawson and Brill.Although every attempt was made by Lawson and Brill to screen data in the data bank, no doubt some questionable data have been included. In a study such as this, the quality of the data is critical. It has been found, for example, that small errors in measured gas volumes and oil formation volume factors can significantly affect the calculated results. A valid criticism of the bank is that inadequate use was made of what limited measured PVT data were available. Another possible criticism is that many of the data used were gas-water data with low gas/water ratios, a situation seldom found in practice. No claim was made that the gas-water data were meant to represent gas well production. Rather, these data are extremely valuable because of the absence of mass transfer between phases and the resulting improved accuracy of, for phases and the resulting improved accuracy of, for example, predicted in-situ phase velocities.All three of the correlations were developed from two-phase flow data, one of the phases being gas. A third phase (water) can be included if we assume that its presence does not change the physical phenomena of two-phase flow. Possible changes that could invalidate results are slippage between oil and water, formation of emulsions, and the influence of water on gas bubble coalescence and the formation of gas slugs. The degree to which such changes are present in the three-phase data is unknown.We applied the three correlations to all data, knowing full well that many of the data were beyond the range of variables used to develop the correlations. However, correlations are frequently used indiscriminately beyond their stated ranges of validity. Therefore a test of their performance over a broad range of data is of value. Validation of Programming Pressure losses calculated by a computer program of Pressure losses calculated by a computer program of each method were compared with the corresponding pressure losses reported by each author for identical pressure losses reported by each author for identical well cases. This comparision indicated the degree of agreement between the results of the two programs. JPT P. 829