Study On The Formation Damage Mechanisms And Protective Techniques Of Tight Sandstone Gas Reservoirs In The Center Of Sichu An Basin

With the reducing of conventional gas reservoirs, people are paying attention to the exploration and exploitation of tight sandstone gas reservoirs. Tight sandstone gas resource in China is very rich and has huge development potential. There is estimated as much as 12×l0¹²m³, which take 40 percent of the total production of the natural gas. Most of the tight gas resource exists in Sichuan, Ordos, Songliao, Tarim, Zhunger, and Chidam basin. The upper of Triassic is deep 2100³600m, it is thick about 450¹000m, the lithological character is gray, gray-white feldspathic quartzite sandstone and mesograin lithic feldspathic quartzite sandstone in the formation of XiangSi and XiangEr. The range of the porosity in Xiangxiqun is 1.39% to 12.22%, the average of the porosity is 5.71%, and the range of the permeability in Xiangxiqun is 0.001×10^-3μm² to 0.953×10^-3μm² , the average of the permeability is 0.064×10^-3μm3. According to the intrusive mercury curve, the maximal radiuses of pore throats range from 0.77μm to 2.42μm pore throat radiuses of saturation median pressure is from 0.06μm to 0.387μm, about 50% of the pore throat radiuses below 0.25μm.This paper studies Xiangxiqun tight sandstone gas reservoirs in the center of Sichuan basin. Through studying the primordial data and observing the core, the characters of tight sandstone gas reservoirs are investigated. Compared with conventional oil/gas reservoirs, tight sandstone gas reservoir is characteristic of low porosity, low permeability, and high clay mineral content, high effective stress, and high capillary power, natural fractures developed. These increase the difficulty of exportation and exploitation. The damageable characters of tight sandstone make them more difficult to develop. Tight sandstone gas reservoir is easy to damage during the process of drilling and completion, it results in very low natural production of gas wells.Potential damage factors and mechanisms of tight sandstone gas reservoirs were studied by experiments. The results show that there are serious water phase trapping, serious alkali sensibility, serious acidulous sensibility, middle to serious stress sensibility in this area. Formation damage is studied by the impact on the associable multi-factors. Laboratorialevaluation the stress sensitivity impact on effective stress-time, the permeability is decrease with the time increase. Evaluation the tight sandstone gas reservoirs damaged by working fluid which strengthen the stress sensitivity. The result shows that the stress sensitivity of the core samples which damaged by working fluid is obviously strengthened. Improving the laboratorial evaluation methods, the permeability change which caused by gradual increase or decrease of the fluid pH is synthetically evaluated. It is proved that the micro-structure of clay mineral is damaged by the high or low fluid pH. It makes illite and chlorite dissolution, dispersion, migration and thus causes permeability decrease. The water phase trapping tendency is studied by the way of the spontaneous imbibition's method to initial water saturation, and the damage mechanisms of the water phase trapping are synthesized.At last, the necessity of using shield temporary plug-up technology is identified. The prior mud should be added temporary plugging agent though the way of assay the prior mud capability. Then the shield temporary plug-up drilling fluids formulation is expounded. The results of experiments at laboratory prove that shield temporary plug-up drilling fluids can reduce the invasion of solid particles and filtrates. In the end, perforating fluids is studied. Based on the reservoir characteristic and the results of experiment, the temporary plug-up perforating fluids is recommended in this area.