The Experimental Research Of Water-Blocking Damage And Removal Methods For The Ordos Tight Sandstone Gas Reservoirs

The water-blocking effect refers to the phenomenon that the reservoir permeability decreases when the working fluid invades and retains inside the reservoir,blocks the flow of oil and gas during the process of oil and gas exploitation.The water-blocking effect can seriously affect the gas production,so it is required to evaluate the water-blocking damage so as to explore tight gas reservoirs in the eastern part of the Ordos basin.Based on previous research and field data and combined with the characteristics of the reservoir in the eastern part of the Ordos basin,the paper takes experimental test as the main research method,evaluates the water-blocking damage and the removal potential in tight gas reservoirs,explores removal methods of water blocking and provides an experimental basis for prevention or removal of water blocking in the development of tight gas reservoirs.Studies have shown that water blocking is the main damage types of the tight sandstone gas reservoir in the Ordos basin.The compactness,strong hydrophilic and the high capillary force of the reservoir brings out serious gas-water interference named “Permeability Jail”,which mainly causes water-blocking damage.In addition,strong heterogeneity,developed micro-fractures and ultra-low initial water saturation are reasons that water-blocking damage happens.Both guanidine fracturing fluid and slick water,that decrease the permeability rapidly as soon as invade into the reservoir,result in a severe damage to the reservoir.Differing from the water-blocking self-removal mechanism of the shale gas reservoir,the sandstone reservoir in Ordos basin has no tendency of micro-crack extension and the fluid diffusion trend into the depths of matrix is weak,so removal methods of water blocking are mainly to inhibit the infiltration of fracturing fluid and strengthen the fracturing fluid flow back,the "evaporation" of the remaining liquid in the pore can also reduce the degree of water-blocking damage.Movable pore volume,which chiefly determines the removal potential of water-blocking,can be used as the key evaluation index for the removal potential of water-blocking in tight gas reservoir.The reservoir with large movable pore volume and high permeability has great potential to remove the water blocking through flow back of the fracturing fluid.Based on the damage and removal mechanism of the water blocking,it has proposed two ways to remove the water-blocking damage,namely hydrodynamic method and chemical method.The hydrodynamic method is to drive the fluid out of the trapped reservoir through displacement pressure difference,and it has good effect on the reservoir with large movable pore volume and high permeability.The chemical method is to reduce the capillary force by adding surfactant,reducing the surface tension and changing the wettability of the reservoir,so as to reduce the invasion of the extraneous liquid and the retention fluid inside the reservoir,the compound fluorocarbon surfactant can accelerate the discharge of retained fluid inside the reservoir,nanometer surfactant produces good wettability reversal effect.Both above methods provide reliable and important means to remove water blocking.Experimental research shows that,the water-blocking damage of tight sandstone gas reservoir in the eastern part of the Ordos basin is severe,and there is large difference between reservoirs.For the sake of reducing the water-blocking damage and improving the removal ability of water blocking in the reservoir,it is feasible to change the properties of fracturing fluid,reduce the amount of liquid inhalation,improve the efficiency of pressure difference displacement,enhance the flow back,reduce the contact time between fracturing fluid and the reservoir,or combine several measures above.