基于地层水蒸发的气井井筒结盐理论模型及实践--以文23储气库为例

Theoretical model and practices of wellbore salt deposition in gas wells based on formation water evaporation in Wen 23 underground gas storage

地下储气库是保障季节调峰和平稳供气最经济、最有效的手段。为了解决高矿化度地层水气藏型储气库结盐问题,尤其是井筒结盐预测问题,保障文23储气库的安全和正常运行,通过室内实验测试了不同压力和温度条件下地层水蒸发规律,确定了地层水结盐条件;并结合地层水盐析临界条件,首次建立了将气水相态与井筒流体质量守恒、能量守恒、动量守恒、井筒流体传热进行耦合的井筒结盐理论预测模型;应用模型分析了井筒结盐机理,计算了地层水矿化度和水气比对气井结盐的影响规律。研究结果表明:①随着温度增加和压力降低,地层水蒸发速度呈指数增加,地层水结盐的临界矿化度为350 g/L;②气井结盐主要机理是从井底到井口,在井筒温度和压力变化影响下烃水平衡使得地层水蒸发加剧,地层水矿化度达到结盐临界矿化度;③气井中水气比越小(低于0.15 m^(3)·10^(-4) m^(-3)),矿化度越大(高于260 g/L),井筒结盐风险越高,结盐位置在离井口1500 m左右。结论认为,建立的储气库井筒结盐理论预测模型与矿场监测到的气井结盐规律基本吻合,有助于高矿化度地层水气藏型储气库结盐防控,对保障储气库安全高效运行具有重要意义,同时该模型可为其他储气库气井井筒结盐预测、预防提供参考。

Underground gas storage(UGS)is the most economical and effective means to guarantee seasonal peak shaving and stable gas supply.In order to solve the problem of salt deposition in the UGSs rebuilt from the gas reservoirs with high-salinity formation water,especially the problem of wellbore salt deposition prediction,and guarantee the safe and normal operation of Wen 23 underground gas storage,this paper tests the evaporation laws of formation water under different pressure and temperature conditions and determines the salt deposition conditions of formation water by means of laboratory experiment.Then,the theoretical prediction model of salt deposition in wellbore which couples the phase state of gas and water with mass conservation,energy conservation,momentum conservation and heat transfer of wellbore fluid is established for the first time based on the critical salt precipitation conditions of formation water.Finally,the model is applied to analyze the mechanisms of salt deposition in wellbore and calculate the influence laws of formation water salinity and water/gas ratio on salt deposition in gas wells.And the following research results are obtained.First,with the increase of temperature and the decrease of pressure,the evaporation rate of formation water increases exponentially,and the critical salinity of formation water for salt deposition is 350 g/L.Second,the main mechanism of salt deposition in gas wells is that the evaporation of formation water is intensified from the bottom hole to the wellhead due to the hydrocarbon-water balance with the change of wellbore temperature and pressure until the salinity of formation water reaches the critical value for salt deposition.Third,the lower the water/gas ratio(less than 0.15 m^(3)·10^(-4) m^(-3)))and the higher the salinity of formation water(more than 260 g/L),the greater the risk of salt deposition in wellbore.In conclusion,the salt deposition laws in gas wells predicted by the proposed theoretical prediction model of salt deposition in the wellbore of UGS gas wells are basically accordant with the field observation results.This model is beneficial to salt deposition control in the UGSs rebuilt from the gas reservoirs with high-salinity formation water and greatly significant inguaranteeing the safe and efficient operation of UGSs.The research results provide a reference for the prediction and prevention of salt deposition in the wellbore of other UGS gas wells.