Study On The Mechanism Of Deep Conformance Control By Using Chemical Technology In The Fractured Tight Reservoir

During water plugging operation in the fractured tight reservoirs,one of the most common failure phenomena is that the gelant cannot be injected into the small fractures,and the other is that the gelant cannot plug the big fractures after gelation,so that oil well is still in high water cut during the subsequent water flooding.In order to effectively plug the fracture channeling-path and provide an important theoretical basis for the practical application of the polymer gel in the deep conformance control field,the characteristics of transportation of the HPAM/Cr³⁺system in the micro-fracture and the method and principle of plugging of the polymer gel in the large fracture are systematically studied by using viscometry,ultraviolet-visible absorption spectrophotometry,displacement experiments with the visual models and the fractured cores.During the transportation of HPAM solution in micro-fracture,there is an appropriate corresponding range between the viscosity of HPAM solution and fracture permeability,which can provide guidance for screening out the appropriate molecular weight or concentration to obey the permeability.The flow resistance of HPAM solution in the micro-fracture is lower than that in porous media under the same water phase permeability,and the characteristic can be quantitatively revealed according to the dynamic mechanics of HPAM solution in the porous media.In formation water,the process and outcome of the cross-linking reaction between HPAM and chromium acetate is significantly affected by salinity.The experiments quantify a matching relationship between cations in the formation water and the HPAM/Cr³⁺system.When the salinity is too low,chromium acetate cannot cross-link HPAM.With an increase of salinity,the cross-linking reaction occurs,and the degree of the reaction increases to a balance point.When the cross-linking reaction occurs,two different types of the reaction appear under different salinity.The intramolecular crosslinking reaction is the first to happen,whether under high salinity or under low salinity.In this stage,the cross-linking reaction is a first-order reaction and plays a greater role in the whole process of the reaction due to its higher reaction rate and higher reaction degree.Next,the intramolecular cross-linking continues to occur if the formation water is in the low salinity range,but the intermolecular cross-linking reaction occurs if the formation water is in the high salinity range.In this stage,the cross-linking reaction is a multistage reaction.A high-M_w HPAM is advantageous to the intramolecular cross-linking reaction of the HPAM/Cr³⁺system but disadvantageous to the transportation of the HPAM/Cr³⁺system in micro-fractures.At the intramolecular cross-linking stage,the injection pressure of the HPAM/Cr³⁺system in micro-fractures is almost equal to that of the HPAM solution,which undergoes no change with the degree of the cross-linking reaction.The higher the HPAM M_w,the earlier the intramolecular cross-linking ends(thus,the intermolecular cross-linking reaction of HPAM/Cr³⁺occurs earlier,which leads to an earlier increase in the injection pressure of the HPAM/Cr³⁺system).A relationship between the fracture aperture and the HPAM/Cr³⁺system is matched to minimize the chromatographic separation when the HPAM/Cr³⁺system transports in the micro-fracture.For the conformance control of a fractured tight oil reservoir,an HPAM/Cr³⁺system with low-M_w HPAM can easily enter the deep reservoir to expand the swept volume on a larger scale.However,the system with a high-M_w HPAM can form a gel with a higher viscosity to produce a higher plugging strength.For plugging the large fracture,the combination of the polymer gel and the particle gel can be achieved a good effect.In the premise of the particle gel injection in the large fracture,a large size particle gel is beneficial for plugging.When the size of the particle gel and the injection rate are both in a moderate range,the particle gel can be uniformly and compactly distributed in the fracture channeling-path.After the particle gel injection,a suitable HPAM/Cr³⁺system can be optimized by calculating the changes of the pressure before and after plugging by using the particle gel.The method of plugging the vertical fracture is the same with that plugging the horizontal fracture by using the polymer gel and the particle gel.After conformance control,the valid of increasing oil production and decreasing water cut is determined by the stabilization stage of creep deformation of the gel under the effect of water injection pressure.A viscoelastic mechanical model is established through the mix of Maxwell model and Kelvin model,which can calculate the time of creep rupture and forecast the validity of conformance control when the point of the gel damaged strain is obtained under high stress and the mechanical formula of the creep deformation of the gel is measured under low stress by dynamic mechanical analyzer.The experimental results can be used to effectively plug the fracture channeling-path in the high water cut fractured tight reservoir.