The Impact of Regular Well Testing on the Accuracy of Allocation Calculations

Although the interest in using multi-phase flow meters in the oil and gas industry has recently increased, there remain some oil and gas fields in which the flow rate of their individual wells is measured by occasional well tests. In such fields, the commingled production streams from all the wells are transferred to the separation unit and the total production rates are subsequently measured by single phase flow meters. As a consequence, although continuous flow measurement data of the total production is available for the whole field, the production data of the individual wells within it is intermittent. In the absence of measured data between two well tests, well flow rates are estimated by allocation factors which are calculated based on the well test data. While allocation factors are normally assumed to be constant between two consequent well tests, fluctuations in the well production typically change values over time. Long time intervals between well tests can therefore create large uncertainties in the allocation results. In this research, the effect of increasing the regularity of well tests on the uncertainty in the allocation process has been studied. Fluctuations in the production of three actual wells have been statistically analysed and quantified using their relative standard deviation. The same fluctuations have then been applied to the production streams of a simulated oil field with 36 wells to generate three different cases. Allocation and hydrocarbon accounting calculations have subsequently been undertaken for one to four well tests conducted per month. The pattern of fluctuations has been generated using a Matlab code. Each calculation has been repeated 100 times with different patterns and the results have been subsequently averaged and reported. The results show that increasing the regularity of flow tests can considerably reduce the allocation error. The most significant reductions were observed when the number of flow tests was increased from one to two per month. In this case, the average allocation error of the three investigated cases was reduced by 0.43%, 0.45%, and 1.11% which is equivalent to an $18.2M (Million), 18.9M, and $46.6M decrease, respectively, in the cost of allocation uncertainty for the three cases based on hydrocarbon accounting calculations. The results of the case studies suggest that the cost of allocation uncertainty can be reduced by $27.1M, $29M, and $80.1M, respectively, for the three cases if well tests are undertaken weekly instead of monthly.