Investigation of Multiphase Flow Behavior in Horizontal Wells
Introduction
Multiphase flow is inevitably encountered in both gas and oil wells: Gas wells will have water and condensate below dew point as the liquid phase while oil wells will have vapor phase as the pressure goes below the bubble point pressure. The available knowledge of multiphase flow in horizontal wells is very limited.
Gas Wells
The multiphase knowledge in gas wells becomes important and relevant when the wells start to load-up with liquids hindering the production. Unloading of vertical and deviated wells has been studied a lot and understood to a certain comfort level. The classical work of Turner et al.4 and studies after Turner such as Coleman et al.5 describe the mechanisms of the liquid loading and provide guidelines for the prediction of loading. There have primarily been two mechanisms to keep the wells unloaded: 1) keeping the liquid film moving in an upward direction; 2) keeping the entrained liquid droplets from falling.
Mechanisms of horizontal well liquid loading have not been studied very much. The Turner and Coleman models are not applicable for horizontal wells. On the other hand, an appreciable amount of knowledge on low liquid loading flows in pipelines has been accumulated over the last two decades (Sarica et al.6, Meng et al.7, Fan et al.8, Dong et al.9, and Zhang and Sarica10). These studies can be classified into high and low gas flow rates. At high gas flow rates, the liquids are continuously produced and no accumulation or loading of the liquids is observed. While the low gas flow rate studies indicated accumulation of the liquids and irregular slugging.
Oil Wells
The interaction between main flow and side entry from the formation has been studied for single-phase oil flow in a perfectly horizontal pipe by Yuan et al.11, 12 and Aziz13. Very limited studies for two-phase flow of liquid and gas were conducted for perfectly horizontal wells. There are no studies of oil dominated multiphase flow in undulating horizontal wells with multiple entries. Multiphase flow is inevitable especially when the pressures are depleted below the bubble point which will be encountered when artificial lift is considered. A wide range of Gas-Oil-Ratios (GORs) are expected to be experienced for oil wells. The performance and selection of the artificial lift methods will be dependent on the flow behavior.
Well Geometry
Most horizontal wells are not perfectly horizontal and can have undulations. Moreover, horizontal wells can be drilled toe-up or toe-down. The flow characteristics of an undulating horizontal well are more complicated than that in a pipeline considering the multiple fluid entry locations along the well. During multiphase flow, the phases may separate, and liquid and gas may accumulate at the lower and higher spots of the horizontal well, respectively, causing irregular slugging or back-flow into the reservoir that may significantly reduce the production capacity of a formation. Hydrodynamic behavior of two or multi phases (flow pattern determination and the predictions of pressure drop and holdup) for undulating horizontal well flow are crucial.