Physical Modelling Of Vertical Multiphase Flow: Prediction Of Pressure Gradients In Oil And Gas Wells

Abstract

ABSTRACT A new method is presented for predicting pressure profiles in oil and gas wells. The method combines mechanistic low pattern transition criteria with physical models for pressure loss and liquid holdup calculations for each of the flow patterns considered. Past published methods relied heavily on empirical fit of limited field data. As a result, they are inaccurate when used outside the range of data upon which they are based. In contrast, the new method is universally applicable to all types of wells under all operating scenarios since it is based on fundamental physics rather than curve-fit of field data. Its prediction performance has been demonstrated by extensive comparison to field data from a variety of wells. Profitable production of many offshore oil and gas fields relies on accurate prediction of the multiphase well tubing flow. Economic tubing and gas-lift system designs strongly depend on reliable calculation of multiphase flow pressure gradients under a variety of operating conditions. The predictions of the proposed method have been compared to 1895 data sets from oil and gas wells including carbon dioxide source wells, carbon dioxide EOR wells, high H2S wells, and high pressure deepwater wells. The comparison results show that the proposed method outperforms eight of the most known published and proprietary methods used by the industry. Based on all data tested, the new method gave a near zero percent arithmetic error in the total tubing pressure drop and an average absolute error of only eleven percent. The proposed method is largely based on mechanistic modelling of vertical and inclined multiphase flow. It has been validated by comparison to the largest ever reported well data base. It has also outperformed eight of the most known published and proprietary methods used by the petroleum industry. INTRODUCTION Two-phase upward gas-liquid flow is of great importance to the production of oil and gas. It has received continuous and strong attention by many investigators from the early days of the invention of the air-lift pump to the present time. The application of gas-lift principles in the late 1920's stimulated further interest in the area of vertical two-phase flow and resulted in a large number of theoretical as well as experimental studies. However, while considerable progress has been made in the overall and qualitative understanding of the problem, much remains to be done from a quantitative point of view. The main difficulties in analyzing vertical gas-liquid flow is the variety of flow patterns possible and the existence of slip between the phases. Four flow patterns are usually defined for vertical upward flow: bubble, slug, churn and annular flow. In a very short pipe only one of these patterns will exist for a given pair of liquid and gas flow rates. However, in long risers and in the tubing of oil wells usually two or three flow patterns can exist at different locations along the pipe.