Abstract:

The post-frac flowback and production of the alkali-salt layers of Fengcheng Formation in Mahu Sag suffer from wellbore salt deposition，but the salt deposition law remains unclear. Therefore，through static and dynamic salt precipitation evaluation experiments，the reservoir conditions affecting the salt precipitation rate and the impacts of flow rate and gas production on wellbore salt deposition in the production process are investigated to clarify the wellbore salt deposition law. The results show that the three typical salts，with their solubility from most to least sensitive to temperature，are sodium carbonate，sodium bicarbonate and sodium chloride. Moreover，the boundary conditions （total salinity and ion composition） and salt crystal types of salt crystal precipitation are defined for composite salt solutions with different compositions. Under the same salinity，the precipitation degrees of composite salts with different compositions are between those of the individual salt，and the priority orders of salt precipitation are sodium bicarbonate，sodium carbonate and sodium chloride，which is consistent with the change of solubility of various salts with temperature. With the increasing flow rate and gas production，the salt deposition first grows and then declines. The salt deposition peaks in the case of the daily liquid production of 40 m³/L and the gas-liquid ratio of 200 m³/L.The study on the salt deposition law in alkali-salt wells helps to set reasonable drainage parameters，delay the salt deposition during flowback，ensure the smooth completion of formation testing after fracturing，and provide a theoretical basis for on-site salt prevention measures in the later production from Fengcheng Formation alkali-salt layers.

Key words:

Mahu Sag, Fengcheng Formation, wellbore salt deposition, sodium carbonate, sodium bicarbonate, post-frac flowback

摘要：

玛湖凹陷风城组碱盐地层压裂后排采过程中存在井筒结盐的问题，但结盐规律尚不明确。通过静态和动态结盐评价实验研究了结盐率的油藏条件影响因素以及生产过程中流速和产气量对井筒结盐的影响，明确了井筒结盐规律。结果表明：温度对三种典型盐溶解度影响程度由强至弱依次为碳酸钠、碳酸氢钠、氯化钠；明确了不同组成复合盐溶液析出盐晶的边界条件（总盐度、离子配比）及对应盐晶类型；在相同盐度下，不同组成复合盐析出程度介于单独盐之间，析出盐优先顺序依次为碳酸氢钠、碳酸钠、氯化钠，与各种盐溶解度随温度变化规律一致；随着流速、产气量的增加，结盐量均先增加后减少，当日产液量为40 m³/d、气液比为200 m³/L时，结盐量最大。对碱盐井筒结盐规律的研究可指导生产过程排采参数的合理设置，在一定程度上延缓返排阶段井筒结盐问题，保障压后顺利试油，为后期生产过程中风城组碱盐地层防盐措施的制定提供理论依据。

关键词:

玛湖凹陷, 风城组, 井筒结盐, 碳酸钠, 碳酸氢钠, 压裂返排