Study On Degradation Of Oilwell Cement And Anticorrosion Technology Under CO₂ Geological Storage Conditions

Carbon capture and storage(CCS)is an essential technology to tackle climate change,achieve the goal of lowering emission and fulfill the international promise for which is the only solution that can significantly reduce CO2 emission currently.The corrosion integrity of wellbore cement sheath is the key factor for whether CCS can be applied successfully and CO2 can be safely sealed for hundreds of years.Therefore,a correct and scientific understanding of corrosion mechanisms of oilwell cement and the reliable predicting and anti-corrosion methods are quite important to the corrosion integrity of CCS wells.This dissertation conducted a series of studies on the corrosion mechanism,degradation rule,prediction model,and anti-corrosion method.Some research finding are as follows:Firstly,the material characteristics of oilwell cement were analyzed.The results indicate that the high alkalinity and porosity make oilwell cement is vulnerable to an acid C02 medium.A comprehensive discussion on the effects of CO2 corrosion such as carbonation,neutralization,shrinkage,and Ca-leaching was conducted.It is demonstrated by the thermodynamic analysis that all hydration products of cement can react with C02 spontaneously.In addition,the results of thermodynamic analysis revealed that Ca(OH)2 has the highest possibility of being carbonated and the hydration products with a higher Si/Ca ratio exhibit a lower possibility of being carbonated.The evolutions of phase composition,microstructure,pore structure,and meso-mechanic property after carbonation of oilwell cement were analyzed.The experimental results revealed that,in both time and space dimensions,there are differences in the microstructure and micro-performance of corroded cement.In time dimension,the cement of the same position suffered carbonation effect and leaching effect successively.In space dimension,C02 corroded the cement gradually from the surface to core.And then a layered structure with a porous Ca-leached layer,a dense carbonated layer,a Ca(OH)2-dissolved layer,and the unreacted layer was formed.Each layer owns a different composition,structure,and performance.Experiments were carried out to study the effects of water to solid ratio,temperature,CO2 pressure,and aqueous environment on corrosion rate and the strength degradation law of cement.Moreover,the extents of all factors were analyzed using the grey correlation analysis method and the analysis results revealed that the descending order of factors that affecting the corrosion rate was the water to solid ratio,temperature,aqueous environment,and C02 pressure.Based on the experimental results,the prediction model of corrosion rate was studied and there is a linear relationship between the corrosion depth and the square root of corrosion time which followed Fick’s diffusion law.The modeling results indicated that the built multi-factor corrosion rate prediction model could predict the corrosion rate well.In addition,a mechanical damage prediction model of corroded cement was established based on damage mechanic.There was a quadratic function relationship between strength damage degree and relative corrosion depth.When the thickness of cement sheath was certain,the relationship accorded with the function D=Nt+Mt1/2A testing apparatus was designed and manufactured to study the stress-corrosion of oilwell cement.The effects of external tensile stress and compressive stress in the levels of 25%,50%,and 75%on CO2 corrosion damage of oilwell cement were studied respectively.The mechanisms of stress state and stress level were analyzed.The research results revealed that the formation and development of microcracks were the primary effect of external stress on the corrosion process.The influencing factor was proposed to quantify the effects and time-varying rules of external stresses.The analysis results revealed that a linear functional relation existed between the influencing factor of 25%tensile stress and exposure time.As for tensile stresses with the level of 50%and 75%,the influencing factors increased sharply with the exposure time and the relationship between them can be expressed by exponential growth functions.Quartic polynomial functions were used to express the relationship between external compressive stresses in all levels and exposure time.Finally,the strength damage model for stress-corrosion was established by combining with the time-varying rules influencing factor and the strength damage model and showed high correlation with the experimental results.According to the results of corrosion mechanism and damage rules,the Si/Ca ratio,the transporting properties of cement,the content of alkaline phase,and intrinsic damage degree and initial strength are determined as the main factors that can affect the corrosion resistant ability of oilwell cement greatly.Two technological approaches including silica-rich materials and fiber with excellent dispersibility and affinity were proposed to improve the corrosion resistance of oilwell cement.Furthermore,nano-silica and UF cellulose fiber were selected out and the optimum dosage of them was determined.Finally,a cement system with engineering performance and corrosion resistance was formed.This dissertation can offer some theoretical references and technical support to the wellbore integrity of CO2 geological storage wells.Moreover,the results can also offer useful information to similar projects such as CO2-EOR,exploitation of reservoirs with high CO2 content,and fracturing with CO2.Therefore,this study is of theoretical significance and social value.