Study On Damage And Cleanup Of Water Blocks In Low Permeability Sandstone Reservoirs In Su Dong Block

Low permeability sandstone gas reservoir has abundant reserves,but it is easy to cause water lock damage and seriously reduce oil and gas production because of its small pore throat,complex structure and high capillary pressure.Based on the experimental evaluation of water lock damage in the Sudong block of Ordos Basin,this paper studies the micro-mechanism of water lock damage by mathematical fitting and experimental methods,and explores the influence of different factors on water lock damage.After clarifying influence factor,the appropriate characteristic index is selected and the gray correlation method is used to predict the damage of water lock.By optimizing the components,a synergistic water-unlocking agent was developed to evaluate its water-unlocking performance and to be applied to different well-entry fluids.The pore throat of cores in Sudong block is small,and the permeability of reservoirs is poor.The core components of the block are mainly quartz,which has strong hydrophilicity on the surface of core and serious potential water lock damage.The study of water lock damage mechanism shows that the water lock damage mechanism includes capillary spontaneous imbibition and liquid phase retention.The pore throat radius controls the spontaneous imbibition rate in the rapid spontaneous imbibition stage;the results of comprehensive nuclear magnetic resonance T₂ test and core displacement show that under saturated water condition,the less movable water is in the reservoir core,the more serious the water lock is;the nuclear magnetic resonance relaxation time is converted to pore size by taking the value of nuclear magnetic resonance T_2cutoff as the boundary,and the small pore and core displacement are investigated.The results show that the proportion of pore water film thickness to pore diameter is much higher than that of pore water film thickness.The smaller the radius of core main throat is,the more complex the pore structure is,and the smaller the recovery value of permeability after gas flooding.When the initial water saturation of gas reservoir is less than the irreducible water saturation,the greater the difference between the two,the easier the water lock damage will be caused,but the influence will be weakened as the water saturation reaches the irreducible water saturation.It is difficult to eliminate the damage of liquid phase when the permeability gradually recovers and the pressure difference increases to a certain value.Grey correlation method has high reliability in predicting water lock damage.The deviation between model predicted results and experimental measured values is about 10%.The results show that cationic surfactants can maximize the contact angle of strong hydrophilic surface to 84.76°,and fluorine-containing surfactants have the best effect on reducing surface tension.The combination of anionic and cationic surfactants has obvious synergistic effect.The introduction of EO group in anionic surfactants can effectively alleviate the precipitation of the system.The synergistic effect of the combination of cationic and fluorine-containing nonionic surfactants is obvious.The surface tension of the system can be reduced to less than 20 m N/m at low concentration,and the contact angle can be maintained at about 70°,and the stability of the system is good.Two kinds of water-unlocking agents WBR-1 and WBR-2 were developed.The evaluation results show that WBR-1 can reduce adhesion work to 7.1626.Both WBR-1 and WBR-2 can significantly reduce capillary pressure and water-lock damage,and the effect is better than commercial water-unlocking agents SHK and OP-10.The application of WBR-1 in drilling,completion and fracturing fluids shows that the degree of reservoir water lock damage can be significantly reduced on the premise of ensuring the stable performance of the inflow fluids themselves.