| The Shishen 100 of Xianhe Oil Production Plant in Shengli Oilfield belongs to ultra-low permeability sandstone reservoir.The reservoir adopts water injection development mode,but there are some problems such as small pore-throat radius,high starting pressure of water injection,and low level of formation pressure,which leads to prominent contradictions between injection and production as a whole and low development index.Matrix acidizing technology was used to improve the water absorption capacity of the injection well.However,conventional acidizing adopts the four-step method of “acidizing”,“washing”,“soaking” and “draining”.Although it is effective,with more rounds of treatment,the construction effect gradually becomes poor.Moreover,during the construction process,the conventional acidizing treatment with the advancement of the dissolution radius,the fluid is affected by the high temperature of the reservoir and the acidic environment is reduced,resulting in serious secondary precipitation,and it is difficult to effectively remove the plugging of the reservoir with poor physical properties and deep pollution.For this reason,the multi-stage corrosion technology was put forward in the field,and a breakthrough was made in the construction.However,at present,the multi-stage corrosion technology lacks the theoretical guidance for the field application.In this paper,the core of ultra-low permeability reservoir in Shishen 100 block was taken as the research object.The high-pressure mercury intrusion experiment,casting thin section experiment,and scanning electron microscope experiment were carried out to clarify the petrology characteristics of the reservoir and analyze the factors affecting the reservoir permeability.Aiming at the problem that traditional experimental methods can not deeply evaluate the acidification effect from the rock pore microstructure,through the acid rock displacement flow simulation experiment,combined with NMR technology and scanning electron microscope,the change law of core pore throat during the acidification process was revealed.Finally,using COMSOL software,a scale model of acidified sandstone cores was established to simulate and evaluate the effects of different injection times and injection rates on the pore changes of cores.The results show that diagenesis has a great influence on reservoir permeability in Shishen100 block.The compaction is the strongest,which leads to the non-development of primary reservoir pores.The existence of interstitial material makes the seepage channel more complex and increases the seepage resistance.The dissolution resulted in the development of intergranular solution pores,ingranular solution pores,and a few mold holes,which increased the rock storage space.Nuclear magnetic resonance(NMR)results show that both acid systems can significantly improve the porosity and permeability properties of cores,but at the same time enhance the heterogeneity of cores.The effect of the main acid decreases with the increase of the injection distance,and even produces blockage at someplace in the core.The timely supplement of the post-acid is beneficial to reduce the damage of pore volume caused by the secondary precipitation,and the effect of pore expansion is obvious.Scanning electron microscope results showed that there were unreacted residual minerals in the pore throats under the action of two kinds of acid liquids,and these residual minerals would fall off due to the fluctuation of water flow,migrate and accumulate in the small throat,and block the seepage channel.The content of quick-reaction minerals,acid injection time and acid injection rate have great influence on the acidification effect of sandstone matrix.Numerical model experiments show that increasing the rate of acid injection can enlarge the sweep volume of acid solution and increase the effective action distance of acid solution. |
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