Experimental Analysis Of Sandstone Formation Damage With Acidizing Treatment In Keshen Area(Tarim Oilfield)

Deep tight gas reservoirs,characterized with high temperature and high pressure,are vulnerable to the formation damage during the processes of acidizing treatment and acid fracturing stimulation.Based on the preliminary field study for the ultra-deep and ultra-high-pressure gas reservoirs in the Keshen Block?Tarim Basin?,it is demonstrated that the unsatisfied well performance after the acidizing treatment is resulted from the decrease of reservoir permeability,which is contributed by the blocking effects in pore throats during the process of the acid-rock chemical reaction.Challenges exist and few studies have been performed for the understanding of the acidizing damage mechanism of this type of reservoirs featuring high temperature,huge thickness and tight formation properties.This study aims at investigating the causes for the formation damage of tight sandstone reservoirs during acidizing treatments with high reservoir temperature.Experimental apparatuses,including the multi-functional core flow simulation system,the diversion acid evaluation experimental system,the laser particle size analyzer,the environmental scanning electron microscope,the optical microscope and the inductively coupled plasma emission spectrometry,are used for the physical and chemical tests of rock core samples obtained from the Tarim Oilfield.The investigation of reservoir characteristics and potential damage factors were initially performed,followed by the reservoir sensitivity evaluation,positive and negative direction core-flooding tests before and after the acid injection,acidizing physical simulation tests and evaluation tests of injecting water with acid solution.Experimental results indicate that most of illite and illite/montmorillonite minerals in the reservoir are distributed at pore throats and occupy the main seepage paths in the porous media.The injection of hydrochloric acid with high temperature into the core sample weakens the degree of cementation between rock particles.Hydrodynamic effects facilitate the migration of quartz and aluminosilicate minerals,blocking the pore throats and thus reducing the conductivity in the formation.Further,complex physical behaviors and chemical reactions take place between the mud acid and the tight sandstone reservoirs with high temperature.K₂SiF₆,Na₂SiF₆,Si?OH?₄ and other solid particles are generated during the main-treatment stage.These generated fines migrate under strong hydrodynamics,which also contributes to the blockage effects.The formation damage rate is only 10.7%when the salinity of the liquid water injected is similar to that of the formation water,and the suspended matter content is less than 6.2mg/L.These conclusions provide solid foundations for the development of new low-damage fluid systems and the design of reservoir protection schemes for the deep-seated reservoirs.