Research Of Injection Characteristics And Plugging Mechanism In Insufficient Polymer Injection Well

In case of the short of injection and high injection pressure in polymer injection block in Henan Oilfield, we have studied injection characteristics of the polymer and plugging mechanisms. And have also evaluated the plug removal effect of APS blocking removers.Determining the main components of plugs by adopting the Instrument analysis and chemical analysis methods to analyze the return to spit objects of the polymer injection in Henan oilfield; Determining whether the molecular size of polymer matches the permeability of reservoir and the medium value of pore radius by adopting the dynamic light scattering technology to do research on the dynamic radius of three kinds of polymer (offered by oilfield) in aqueous solution. We have studied the characteristics of the polymer injection through core flow test, which includes:the impact of injection speed, the polymer concentration and the permeability on the injectivity as well as the change rules of the pressure gradient and the viscosity of the polymer in the process of polymer injection. The plug removal effect evaluation of APS blocking removers consists of static plug removal effect and core flow test about the blocking caused by polymer solution and polymer gel. And we have carried out the feasibility study of injection stimulation and increasing of pressure.The results of instrument analysis and chemical analysis show that the plugs of polymer injection well contain polymer and gel, oil, hydrotrope, suspended solids (SS) etc. as well as inorganic carbonate scale. Polymer and gel is the main ingredient, its content is90.98%. The higher content of the return to spit objects of the polymer injection is iron and aluminum ion, the content of iron ion reaches up to0.06%, which can cause polymer flocculation and obvious blocking; the content of aluminum ion is0.60%, Its source is mainly from that:the higher injected polymer solution viscosity of the polymer injection well make the particles have a strong whirlwind, which caused the particle migration of the formation pore. So, we have concluded that clay and mechanical impurities are also the main blocking components.We have measured the dynamic radius of the positive force II type,1630and3630s polymer, They are respectively0.151μm、0.149μm、0.145μm. The area with the permeability greater than10×10-3μm2will not cause a stable block; the Blocked parts are mainly in the low and ultra-low permeability zone. The polymer with relative molecular mass equal to or less than3.5×10-7can not block the formation with the permeability greater than0.010μm2heavily.With the increase of injection polymer rate and injection concentration, the degree of the polymer solution blocking core increases, the impact degree of injection rate on the formation plugging is small; the polymer concentration is the main factor. The greater the injection concentration is, the more serious the blocking degree of core is. If the polymer concentration is too high, the probability of overlapping and entanglement between the molecules increases, which can easily cause the chain adsorption and retention, take3630s polymer for example, the injection concentration of1200mg/1, Pbr is95.24%,the formation belongs to slight block. When the concentration goes up to2200mg/1, Pbr falls for67.02%, the block degree increases, the formation belongs to slightly intermediate state block. The injection concentration of the3630s and1630two polymers should not be greater than2000mg/L, the injection concentration of positive force Ⅱ polymer is not greater than1800mg/L.Rock permeability affects the injection of the polymer solution and the formation communication ability. The core with a smaller permeability (0.1-0.5μm2), the mechanical trapping and adsorption interaction make the blocking occurs mainly in the middle parts; with the permeability increases (>0.5μm2), polymer solution can effectively penetrate, and only the slight blockage caused by adsorption. the pressure gradient in the rear portions of core is the highest in the process of polymer flooding, first increased and stabilized, wherein the rear of the pressure gradient, which indicates that the clogged parts occurs mainly in the back. The viscosity loss of the polymer increased with the increasing injection speed and the injection concentration, when the permeability of the core reduces, the viscosity loss of polymer solution will crease.The laboratory experiment results of the core simulation have shown that APS has a good effect on de-plugging caused by the polymer solution. When permeability is nearly1μm2, the recovery rate of permeability can reach of71.72%; but for the blockage caused by polymer gel, the plug removal effect is less than ideal, when permeability is1.012μm2, the recovery rate of permeability of57.00%, effective plugging requirements can not be reached. Increasing the concentration of the APS has a good effect on plug removal of the polymer solution, the recovery rate of permeability increased by6.63%; but for the plug removal effect of the gel, it can only have an increase of2.92%. After the plug removal of the chemical, improving the injection pressure can increase the recovery rate of the core permeability, improve de-plugging. For the blocking caused by polymer solution, when injection pressure rise respectively by0.075Map,0.230Map,0.450MPa,the recovery rate of permeability will increase from64.80%to81.00%、91.27%、92.57%, the recovery rate are respectively16.2%,6.47%,7.77%; For the blocking caused by polymer gel, when injection pressure rise respectively by0.132Map、0.393MPa、0.692MPa, the recovery rate of permeability will increase from40.78%to53.85%、58.46%、62.11%, the recovery rate are respectively13.07%、17.68%、21.33%The greater the increase degree of the in injection pressure, the better the recovery rate of the core permeability. But the injection pressure can not be increased unboundedly; the injection pressure should be less than the formation fracture pressure.