Mechanistic Model For Two-Phase Flow In Liquid-Cut Gas Wells

In China, the development stages of many gas fields have entered the mid-to late period. Most wells produce natural gas with liquid present, which increases the bottom-hole back pressure, aggravates the gas-liquid slip, and sharply reduces the gas rate. To avoid these, engineers have to analyze wellbore dynamic and design deliquification technology. Both of them depend on the gas-liquid two phase flow model in wellbore. However, most current two phase flow models are developed using empirical fitting methods, based on the specific laboratory experiments or well tests. When applying to gas well conditions, most of them show poor performance. Especially for low-rate gas wells, the calculated pressure drop errors are usually large. In addition, their application ranges are usually narrow. When the gas reservoir or the gas well condition changes, one have to reselect models or even fit new models. In this paper, a new mechanistic modeling method was adopted. It has broken through the traditional modeling method and focused on the physical phenomena. The flow mechanisms causing two phase flow to occur and the similar transport rules for each flow pattern were defined and modeled mathematically. Finally, a new comprehensive mechanistic model for two phase flow in liquid-cut gas wells was proposed. The specific works include:(1) Two experimental loops with30mm-ID plexiglass tube and50mm-ID stainless steel tube were designed and established. Flow pattern experiments for air/water two-phase flow were conducted with different tube inner diameters and different pressures (0.12,0.60MPa). The flow configurations of bubble flow, slug flow, churn flow and annular flow were reappeared with extremely slow speed using high speed photography system. Then, three experimental flow pattern maps were obtained. The effects of tube diameter and pressure on the flow pattern transitions were also compared. These works provide experimental data for researching new mechanistic model.(2) A new liquid film extractor was specially designed for annular flow. The droplet entrainment was tested with different gas and liquid velocities. The relationship between annular flow emergence and droplet entrainment was analyzed, which revealed the formation mechanism of annular flow. The variation of droplet entrainment with different gas and liquid velocities was also analyzed. A new correlation for predicting droplet entrainment was proposed.(3) Based on the experimental observations, the transition mechanisms between each two flow patterns were defined and modeled mathematically, including annular flow formation, slug-churn flow transition and bubble-slug flow transition. Then the transition equations between each two flow patterns were derived based on the models. New models can not only predict the flow patterns, but also explain why and how the flow patterns transfer.(4) Based on the fractional flow assumption, the geometric relationships between gas core and liquid film in annular flow were derived. By introducing droplet entrainment, dimensionless film thikness and gas/liquid continuity equations, the liquid holdup was expressed as the function of droplet entrainment and film thickness. Combining momentum conservation equation, a new mechanistic model for annular flow was proposed. New model can predict the gas core velocity, the droplet entrainment, and the film thickness, et al.(5) Based on the gas segment-liquid slug unit-cell assumption, the gas core and falling film in gas segment of slug/churn flow was simplified as countercurrent annular flow pattern. Combining the unit-cell mass conservation equation and the gas core-liquid film momentum combination equations, considering the interfacial shear stress in gas segment and the bubble swarm in liquid slug, a new mechanistic model for slug and churn flow was proposed. New model can predict the falling film velocity, the liquid holdup in slug, and the slug length.(6) The bubble velocity profile was studied according to the experimental observation. Based on the drift flow model, a mechanistic model for bubble flow was developed. It considers the slip effect of bubble and the inhomogeneity of bubble velocity.(7) Combining the mechanistic models of annular flow formation, slug-churn flow transition and bubble-slug flow transition, considering the pressure drop mechanistic models of annular flow, slug/churn flow, bubble flow, a new comprehensive mechanistic model for two phase flow in liquid-cut gas wells is developed. It can predict the flow pattern and pressure drop for gas wells, describe the transition mechanisms between different flow patterns, and predict the characteristic parameters for each flow pattern.(8) The pressure prediction errors of the new model was evaluated using167gas well pressure test data, which includes the gas condensate wells and the water-cut gas wells. The performance of the model was also evaluated by considering other three aspects, including the regularity of the tubing performance curve, the validity of the pressure drop models for each flow pattern and the rationality of selecting fluid property method.The proposed comprehensive model in this paper provides the theory and method for wellbore dynamic analysis and deliquification technology design in liquid-cut gas wells. It also broke through the traditional modeling method, which is of great importance to enhance the theory research level of two phase flow.