Research On Mechanism Of Efficient Removal Of Filter Cake To Improve Cement Zonal Isolation

Cement sheath has been used to provide long-term well integrity through zonal isolation in oil&gas wells,and therefore to ensure reservoir development benefit.But the integrity on cement-formation interface.which the key elements in well barrier&isolation system,is easily compromised due to the existing of filter cake forms during the drilling process.Therefore,removing filter cake during the cementing process is crucial for cement-formation interface zonal isolation.To achieve the goal,serials works were conducted on filter cake physical parameter,particle interaction and stress analysis,and a method to removing filter cake during the cementing process was established based on advanced oxidation process(AOPs)in this dissertation.The main findings are as follows:1.The distribution of effective stress on filter cake with different compressive coefficient was established by analyzing porosity,permeability,specific resistance and compressive coefficient based on previous studies.Removal of filter cake was supposed to accumulation of single particle initial motion and transportation.Based on the assumption,forces acting on a single particle was analyzed and torque balance equation was established.Extended Derjaguin-Landau-Verwey-Overbeek(XDLVO)theory was taken to describe the interaction forces between particles.exposure degree was adopted to describe the positional relationship of particles in adjacent layer and moment arm of forces.2.Based on the torque balance equation,critical velocity for particle transport with the effects of annular parameters,filter cake parameters.XDLVO forces and polymer adsorption were calculated and discussed:(1)The results indicate that particles could be transported only within a certain size range and the dominant forces highly depend on particle size.Effective stress.gravity and flow drag force play a dominant role for particles diameters larger than 10 ?m,and XDLVO forces and flow drag force dominate particle removal for diameters less than 10 ?m.It was found that the threshold range of carried particles size increased with improved plastic viscosity but decreased with enhanced yield value in Bingham fluid.In power law fluid,critical velocity decreased but threshold range of carried particles size kept constant as consistency index K increased,more particles could be removed by enhancing flow behavior index n.In addition,high static hydraulic pressure,high friction pressure caused by viscosity,thin thickness and low compressive coefficient of filter cake would limit particle removal.Moreover,particles with radius smaller than 10?m exhibited a quadratic function relationship between exposure degree and critical velocity.(2)Particles interaction calculation based on XDLVO theory manifest that van der Waals force and acid-base force(A-B force)among particles act over short distances and electrostatic force act over relatively long distances.Particle migration from filter cake need to overcome the total force at secondary minimum on XDLVO curve.Van der Waals attraction forces between barite particles was higher than bentonite but A-B attraction force was lower due to its higher Hamaker constant and lower surface tension component.The calculation results also reveal that keeping pH value of filter cake at 9 to 11 and decrease ion concentration(Na+?K+?Cl-)was essential for particle removal due to improved electrostatic repulsive force.(3)Due to the decrease of Van der waals force and A-B attraction force.slight decline of XDLVO forces was obtained after adsorbed polymers.But the reduction was eliminated by attractive bridging force and depletion force of polymers.The calculation results obviously demonstrated that the attractive bridging force could be 100 times higher than van der Waals force and the depletion force could be 37.88 times higher than van der Waals.Furthermore,both attractive bridging force and depletion force show a direct proportional to polymer concentration.molecular weight and chain length.Decreasing polymers content and molecular weight was benefit for particle removal.3.A filter cake removal method based on AOPs was established.Experiments on high molecular weight polymer degradation including polyacrylamide(PAM).Carboxymethylcellulose sodium salt(Na-CMC)and Potassium Polyacrylamide(KPAM)by alkaline/heat activated persulfate and chelating agent-Fe2+/alkaline/heat activated persulfate systems were carried out.Higher degradation efficiency and persulfate utilization was observed in alkaline/heat activated system with alkaline-persulfate molar ratio of 4 which compared to the lower ratio.A further improvement on degradation efficiency was obtained in chelating agent-Fe2+/alkaline/heat activated system with chelating agent-ferrous-persulfate molar ratio of 2/1/10 and initial pH value of 10.The chelating effect greatly restricts the concentration of free Fe2+in initial system that made the product of free Fe2+and OH-less than solubility product Ksp.Therefore.precipitates of Fe(OH)2 and Fe(OH)3 weren't observed even in system with pH of 10.Diethylenetriaminepentaacetic acid(DTPA).Ethylenediaminetetraacetic acid disodium salt(EDTA-2Na)and Citric acid(CIA)were studied and DTPA revealed well chelating effect and pH controllability.In addition,filter cake removal experiments were conducted to verify the efficiency of persulfate oxidation systems.Obtained data indicated an ideal filter cake removal efficiency could higher than 80%even PS system reacted 50 min prior to react with filter cake.Shear strength on cement-rock interface was distinctly improved by filter cake removing method.Peak force-quantitative nanoscale mechanical characterization(PF-QNM)was used to evaluate the elastic modulus of cement which 10?m off the interface.Results shown that persulfate oxidation systems exhibited as positive effect on cement elastic modulus which the improvements can up to 25%.4.Numerous measurements were carried out to identify the structure and elements variation of polymers degraded by persulfate oxidation system and mechanisms was proposed.Free radicals including HO·,SO4·-and O2·-was detected in oxidation system.Backbone cleavage and side groups detachment happened due to the free radicals induced H-adsorption and electron transfer effect.Meanwhile,hydrolysis and peeling reaction happened in alkaline solution which caused chain cleavage as well.PAM/KPAM exhibited an obvious network-like structure break;Na-CMC shown formation of large amounts of pores on polymer films,and totally broken due to further connection of pores.Molecular weight and polymers concentration dramatically decreased and adsorption functional group eliminated which reduced the bridging and depletion force.The results in this dissertation demonstrated the filter cake removal method and the persulfate oxidation system has good potential and application prospects for improving the zonal isolation on cement-formation interface.In addition,the results could provide theoretical basis and technical support for the well cleaning and cementing parameter design and spacer developments.