Study On Regulating The Properties Of Water-Based Drilling Fluid Mud Cake And The Technique For Enhancing Strength At Cement-Formation Interface

In the process of well drilling and well completion,cementing operation is one of the important working procedures.The poor cementing quality will result in the isolation failure of stratigraphic trap fluid effectively.The studies in recent years have found that the mud cake formed at cement-formation interface during drilling operation,hinders the direct contact between borehole wall and cement sheath,and it is vulnerable to form micro cracks.So the mud cake at cement-formation interface become the weak link of isolation systems between borehole wall and cement sheath that will result in the poor cementing quality in the cementing operation and become one of the main causes of interlayer channeling and pipe out of the interlayer.Although the problems above have received considerable attention in recent past and many corresponding techniques of enhancing cementing quality have been researched,the proper and effective methods of reducing the formation of microcracks have not been found.Following the development of oil field exploitation constantly,the complex formations appear more and more which reduce the cementing quality largely.The poor cementing quality caused by the cracks in cement-formation interface has seriously restricted the effect of petroleum exploration and development and urgently needs to be resolved.In order to improve the shearing strength at cement-formation interface and increase the cementing quality,a vast numbers of study have been carried out using mud cake curing experiment,interface bonding test,drilling fluid performance test,et al.Firstly the influences of drilling fluid,cement slurry and various kinds of additives added in the drilling fluid and cement slurry on mud cake properties were studied in the paper.And then the mechanism and methods of improving mud cake properties including strength,thickness and toughness were summarized according to the experiments results above.Based on the above,a new enhancing-thinning addtive named EA that was added in water-based drilling fluid was researched in the lab at last,aiming at increasing cementing quality.The study experimentally prove the EA could increse the cementing quality greatly by improving the mud cake properties.The application of EA in Daqing Oilfield has gained significant effect that the cementing qualities of 431 oil wells are all qualified.The main conclusions in the paper are as follows:1.The effects of drilling fluid compositions on the mud cake properties were studied by the measurement of mud cake properties and microstructure analysis with SEM and XRD.The results show that there are two main factors influencing mud cake strength and thickness which contain the dispersion state of bentonite and the solid content in drilling fluid.The bentonite dispersion depends on the content of bentonite and alkali,the kinds and content of drilling fluid additives.When the bentonite to water ratio is 5%,the mud cake is the most strongest and thinnest.The effects of Na2CO3 and NaOH on mud cake strength are more observably compared with mud cake thickness.Among of them NaOH with strongly alkaline give more powerful ability to improve the mud cake properties,especially the toughness.On the study of the relations between drilling fluid additives and mud cake properties,it is found that Na2O·nSiO2 and filtrate reducer could increase the mud cake strength and decrease the mud cake thickness,but the thinners can decrease the mud cake thickness only.Adding the heavyweight additives with lower density and particle size in the drilling fluid will make the mud cake more enhanced but little thickened.In conclusion,one of the methods of forming harder mud cake is adding macromolecular substances in drilling fluid to provide structural support and make the bentonite particles connect with each other from end to face but disperse well in the drilling fluid.And the other is adding the heavyweight additives with lower density and particle size in the drilling fluid.The mud cake toughness could be improved by increasing the OHcontent in the drilling fluid properly or filling the gaps between bentonite particles with gels.Decreasing the viscosity of drilling fluid and make the bentonite disperse well with lower particle size will be beneficial for decreasing the mud cake thickness.2.The compositions of the cement filtrate were analysed firstly by many quantitative analysis methods such as atomic absorption spectrometry,complex titration analysis and so on.Then the mud cake curing experiments were carried out and the change of mud cake after impregnating in cement filtrate and its mechanism were researched.The results show that the mud cake properties are also influenced by ions in cement filtrate which contains Ca²⁺,Mg²⁺,Al³⁺,OH-,CO₃^2-and SiO₃^2-,and the influence degree depends on the ion species and pH of the cement filtrate.The detailed influences of the ions in cement filtrate on mud cake properties are summarized as follows: The anions including OH-,CO₃^2-or SiO₃^2-could not obviously enhance the mud cake at lower pH,but the enhancing function become dramatically as pH increases that the mud cake strength could be enhanced to 5.73 times by SiO₃^2-when pH is 14.Besides,OH-and SiO₃^2-are beneficial to improve the mud cake interity and toughness.The mud cake thickness would not change greatly as the anions and filtrate pH vary.Comparing with above anions,cations containing Ca²⁺,Mg²⁺,Al³⁺ or Fe³⁺ give more strong enhancing abilities as pH is 14 and the degree of enhancing mud cake is Ca²⁺>Al³⁺>Mg²⁺>Fe³⁺,in which Ca²⁺ can make the mud cake strength enhance to 706 times.The four cations could also decrease the mud cake thickness.The higher pH the more obviously thinning effect of the cations in which Al³⁺ could decrease the mud cake thickness by 50.5% showing the best reduction function.After the mud cake were impregnated in the solution containing Ca?OH?2 with different content,the mud cake strength is increased in different degrees.But more higher content of Ca?OH?2 is bad for decreasing mud cake thickness and results in generation of cracks in the mud cake.In cement additives,accelerators,retarders and dispersants could decrease the mud cake thickness only.But the accelerators without Cl could both increase mud cake strength and decrease mud cake thickness in which the sodium silicate is the best one.The mud cake strength is increased while the thickness is decreased after the mud cake is impregnated in cement filtrate at high pH.The mud cake properties are improved mainly by OH-,Al³⁺,Ca²⁺ and SiO₃^2-,and the mechanisms are as follows: Ca²⁺,Al³⁺ and SiO₃^2-react with each other and then generate calcium silicates and alumino silicates in mud cake one side.On the other side,the bonds Si-O and Al-O in bentonite are broken by OH-coming from the cement filtrate and then the active SiO₃^2-and AlO₃^2-are released that would also react with Ca²⁺ or Al³⁺ to generate calcium silicates or alumino silicates.These silicates are then hydrated further to generate silicate hydrates gels because of which the mud cake is hardened greatly.Moreover the gap between particles in mud cake would be filled by the silicate hydrate gels that would be also beneficial for increasing the bond strength between particles and then the mud cake thickness is decreased accordingly.3.The composition and using method of EA were studied in the simulation test by measuring the mud cake strength,thickness,toughness and made references to the change of the basic properties of the water-based drilling fluid.Then the effect of EA using in high temperature was researched.At last EA was applied in oil wells at very complex conditions in Daqing Oilfield and the results prove that EA is very benefical to increase the cementing quality.All the detailed results are as follows:EA is composed of mud cake thickness reducer JB-3,mud cake enhancers ZQ-1,ZQ-5 and mud cake toughing agent ZR-1 at the proper quality ratio of 3:7:1.2:2.The optimal content of EA adding in potassium polymer drilling fluid is 1.0%,at which the mud cake strength is increased by 36.0 times and the thickness is decreased by 50.6%.Besides,EA greatly improved the interface cementing strength and thus the cementing strengths between cement and wellbore casing?the first interface?or cement and borehole wall?the second interface?are improved by 15.0 and 13.9 times respectively at its optimal dosage.Similarly,when EA is added in silica-based drilling fluid at the optimal amount of 0.9%,the mud cake strength is increased by 34.0 times and the thickness is decreased by 53.7%,the cementing strengths at the first interface and the second interface are increased by 16.1 and 14.8 times respectively.All that shows EA could significantly improve the cementing strength at cement-formation interface.The changes of the basic properties of the water-based drilling fluid show that EA can not affect the main basic properties at its proper content that could not affect the application of drilling fluid.Change the temperature from 45°C to 85°C,the effect of EA became more and more greatly that showed EA could adapt the temperature variation in the oil well.In order to test the effect of EA in oilfield,it was used in the process of well cementing containing 431 oil wells in Daqing Oilfield.The results prove that the all oil wells have proper cementing quality and the average cementing perfection rate was increased by 29% after EA is applied.It is verified that EA has very good application effect and is really contributed to the increasing of oil well cementing quality.