Study On Risk Assessment Of Formation Blockage In Geothermal Water Reinjection In Xining Basin

Geothermal waste water reinjection is an important way for safe and environmental protection utilization of geothermal water,and it is of great significance to study the risk of reinjection blockage.In the process of deep pore-type geothermal water reinjection,there are often reservoir blockage risks due to suspended solids invasion,secondary scaling,and sand-mud particle migration.The first two can be prevented by technological measures,but the risk of sand-mud particle migration is often inevitable.Especially for reservoirs with high mud content and poor consolidation.In this paper,taking the mid-low temperature geothermal reservoir in Xining Basin as an example,through on-site water and rock sampling and analysis,the risk assessment experiment of reinjected formation blockage and the mechanism analysis experiment of geochemical reaction are carried out,and the theoretical and mathematical model is established to evaluate the risk of geothermal reinjected blockage according to the 1/5-1/2 criterion,and the rock-fluid mechanism in the process of geothermal reinjection was studied,which including the impact of sand particles migration,formation water scaling,suspension particles invasion,water-rock reactions and others mechanisms on the physical properties of geothermal reservoirs.In this way,the risk of formation blockage during geothermal water reinjection is evaluated,and the major controlling factors causing formation blockage are identified,which also lays a foundation for preventing formation blockage,maintaining reinjection capacity,and realizing the effective development and utilization of geothermal resources with medium-low temperature in Xining Basin.The research results show that:（1）The typical medium and low-temperature geothermal reservoirs in Xining Basin are characterized by high salinity,high argillaceous content,weak rock cementation,low porosity,and low permeability,whose lithology is dominated by sandstone,and the interstitials are mainly clay-based.The reservoir grain size distribution is obviously bimodal,and there are a lot of movable mud-sand particles.（2）There were obvious velocity sensitivity and water sensitivity in the geothermal reservoir,in which the permeability could be temporarily increased with the increases of water flow rate.However,the core permeability would decrease rapidly when the flow rate is above 2.95-6.20 cm·min^-1,and it could also be reduced by 35%-53%in the case of long-term low injection velocity(0.31-1.64cm·min^-1).When the salinity of the reinjection water drops to 70%,it would cause significant hydration and expansion of clay minerals,and obvious particle migration and temporary blockage of the core could be observed.Compared with the impact of mud-sand particles migration on the reservoir,the short-term invasion of low-concentration scaling particles or suspended particles had little impact,and the core would be blocked only when reinjection for a long time or when there were a large amount of scaling particles or suspended particles in the reinjection water.（3）During the long-term reinjection process,there would be complex water-rock reactions between geothermal water,reinjected water,and rock minerals,which had an important influence on reservoir physical properties.（4）Besides,according to the rock grain size and the 1/5-1/2 pore throat blockage criterion,a calculation method for the content of movable mud-sand particles in weakly consolidated geothermal reservoir was established,and a blockage risk assessment model for near-well formation with geothermal water reinjection was established based on solid particle generation（starting）-migration-sedimentation-blockage pore throat mechanism.This model could comprehensively consider authigenic mud-sand particles and intrusive particles to determine the migration and distribution of solid particles in the near-well formation,and predict formation porosity,permeability,and reinjection rate.The prediction accuracy of the model could be improved by fitting the parameters of the reinjection test.