| In most gas wells,fluid accumulation at the bottom of the well often occurs in the later stages of production,resulting in a decrease in production and,in severe cases,even a shutdown.Due to the complex borehole trajectory,the two-phase gas-liquid flow pattern and the liquid-carrying pattern are even more complicated.Therefore,it is necessary to understand the pressure drop variation in the wellbore and calculate the critical liquid-carrying flow rate to maintain the normal production of gas wells.This paper firstly introduces the common flow patterns of gas-liquid two-phase flow in horizontal,vertical and inclined pipelines and the flow pattern prediction methods,and summarizes the main factors affecting the flow pattern variation.Secondly,from the gas-liquid two-phase flow in the wellbore,the three conservation theorems of momentum,energy and mass are used to establish a pressure drop model for the confluence section of single-and double-layer double-branch horizontal gas wells and calculate the model error,which is found to be within 14% with certain accuracy.The pressure drop in the confluence section of the wellbore decreases with the increase of the wellbore diameter and increases with the increase of the wellbore in-flow angle.Two typical two-branch horizontal wells were selected and the full wellbore pressure distribution was calculated based on the existing horizontal,vertical and inclined section models and the established confluence section model for a block with a double-branch horizontal well as an example.The calculations show that the pressure drop loss in the confluence section is relatively small,generally between 0.1 and 0.5 MPa.The wellhead pressure calculated from the bottom of the well to the wellhead in both wells was within 8% error of the actual measured pressure at the wellhead.Based on the existing production data of a block of double-branch horizontal wells,a method was established to use the decreasing production curve to discern the time of starting liquid accumulation in double-branch horizontal gas wells and to estimate the depth of liquid accumulation based on the flow pressure at the bottom of the well.The coefficients of the required fluid-carrying model were determined based on the structural characteristics of the well body combined with theoretical analysis,and a method for determining the location of fluid accumulation in a double-branch horizontal well was proposed.The calculation results show that some typical gas wells in the block start to accumulate fluid after 45-50 months,and the volume of fluid accumulation can reach several tens of meters,while some gas wells are in good production condition.It is verified that double-branch horizontal wells generally accumulate fluid at the bottom of the inclined section.Finally,using the actual gas production of a block of double-branch horizontal wells,several commonly used critical fluid-carrying models of the inclined section are compared and verified.For a block of double-branch horizontal gas wells,the Fernando model predicts better accuracy.Further analysis of the factors influencing the critical fluid-carrying model shows that the critical fluid-carrying flow rate increases with the increase of wellbore diameter,wellhead temperature and pressure,and well slope angle. |
