Experimental Research on Fluid Sensitivity of Carbonate Gas Reservoirs with High Content of Acidic Gas

The fluid sensitivities of the carbonate gas reservoirs with high content of acidic gas,hydrogen sulfide and carbon dioxide,in Leikoupo,Jialingjiang,Feixianguan,and Changxing Formations in Northeast Sichuan Basin are investigated experimentally.The experimental results show that the velocity sensitivity of the gas reservoirs is weak to strong,the water and the salinity sensitivities—middle minus to very strong and the alkali sensitivily—extremely strong.The pertological analyses indicate that there exist massive potentially damaging minerals such as illite,microcrystalline(MC) quartz,dolomite,calcite,and calcium sulfide in the gas reservoirs.Drilling-in fluids invaded can disturb the dynamic balance among the original reservoir materials and cause the reservoir formation to be demaged.The main mechanisms involved in formation damage are summerized as follows: ① destabilization of MC quartz and clay minerals owing to increased pH values of the formation fluids;② creation of more demaging new mineral facies such as silicates,kaolinite,hydrophyllite,and others in reaction of alkali lye with MC quartz,feldspar,and dolomite;③ reduction in binding forces between illite particles owing to changes in formation fluid salinity;④ hydrolysis of calcium sulfide to give massive OH-,combination of excessive OH-with Ca2+ and precipitation of Ca(OH)2 thus formed; ⑤ desorption and redeposition in farther zones of pyrobitumen owing to changes in zonal water sturation and foramation fluid salinity;⑥ hydration,dispersion and migration of anhydrite.Based on the formation demaging mechanisms,it is suggested that the reservoir formation damage in the carbonate gas reservoirs sensitive to formation fluids can be well controlled through creation of high quality filter cake by a temporarily shielding drilling-in fluid.