通过研究温度、压力、pH值等因素对CO2、CaCO3垢溶解度的影响,分析高含CO2天然气藏储集层的结垢机理.CO2的溶解度随温度升高而降低,随压力升高而升高,随地层水矿化度的升高而降低；CaCO3在水中的溶解度随温度升高而降低,随压力的升高而升高,溶液pH值升高会促进CaCO3的生成.结合吉林油田典型含CO2天然气藏储集层特征进行了结垢预测,认为钻井液滤液侵入储集层后,储集层近井地带存在结CaCO3垢的可能.针对气藏储集层埋藏深、井底温度高、地层压力系数低、裂缝发育等特点,优选了水包油及甲酸盐无固相两种欠平衡钻井液体系,CO2侵入、动态污染实验评价表明,两种钻井液均具有很好的抗CO2污染能力,储集层保护效果显著.两种欠平衡钻井液体系已分别在龙深2井和龙深3井进行了现场应用,效果良好,现场钻井液的岩心渗透率恢复值均在85％以上,满足欠平衡钻井的要求.

Calcium carbonate scaling generally plays an important role in the reservoir permeability impairment in gas reservoirs of high CO2 content.Quantitative analyses were done on the factors(such as the solubility product of CaCO3 at certain condition,temperature,pressure,salinity of solutions,and pH etc.) affecting the solubility of CO2,and studies were conducted on the scaling mechanism of the gas reservoir in Jilin Oilfield(high CO2 content).Under high temperature/high pressure at bottom hole,pressure plays a more important role in the solubility of CO2,other factors such as the salinity of formation water,and pH also affect the solubility of CO2 to some extent.No scaling was generated in the reservoir before the invasion of the drilling fluid filtrate.Scaling(CaCO3) began to take place when the filtrate of drilling fluids having pH equal to or greater than 9 invaded the reservoir;thus,the pH of the filtrate should be as low as possible.

Theoretical analysis and field application suggest that the invasion of liquid and solid of drilling fluids into fractures are the main reason for damage to fracture reservoirs.Extending "ideal packing theory" to fracture type reservoir protection and optimizing the ideal packing agent and film-forming agent,a carefully designed ultra low osmosis water based drilling fluid by synergistic interaction was developed.Laboratory experiments show that this drilling fluid has better rheological property,higher temperature resistance and slicker mud cake which can form a homogeneous and compact seal coat on borehole.The API filter loss could be controlled under 2.3mL,and HTHP dynamic filtration was less than 0.3mL.Lab tests demonstrate that this system can effectively improve fractured gas reservoir protection.

For the purpose of reducing drilling fluid expense and solving the problems occurred in Dagang Oilfield,including formation collapse,pipe-sticking,the positive-charged polyglycol/potassium chloride drilling fluids were optimized by orthogonal test designing software-SPSS13.0 based on the analysis of the formation clay and evaluation of the treating agent.Laboratory studies show that this fluid system has better inhibitive capacity,rheology,filtration,anti-pollution and lubricity.This paper also investigates the working mechanism of the mud and shows that positive-charged polyglycol and potassium chloride have synergistic effects,which helps hole cleaning and wellbore stability.