木质素磺酸钙对固井水泥石变形破坏特性的影响及其改性机制

Effect of calcium lignosulfonate on the deformation and failure characteristics of cementing stone and its modification mechanism

在采动区地面井煤层气抽采工程中,工作面的回采会对上覆岩层产生较大的扰动,进而导致地面井变形破坏,不能有效发挥卸压煤层气地面井的抽采效能。固井技术能够有效延长采动作用下卸压煤层气地面井的寿命,水泥作为影响地面井固井质量的关键因素,其早期强度需满足较高的固井要求,水泥固结后形成水泥石,水泥石的力学性能对维持卸压煤层气地面井的稳定性至关重要。通过添加分散剂提高水泥石的力学性能是改善固井效果的有效途径。针对目前采动作用下木质素磺酸钙改性水泥石变形破坏特性的研究较少,通过开展循环荷载下不同质量分数的木质素磺酸钙改性水泥石单轴压缩试验,探究木质素磺酸钙对卸压煤层气地面井水泥石变形破坏特性的影响及其改性机制。研究结果表明:水泥石纵波波速和峰值应力随木质素磺酸钙质量分数增加呈现出先增加后降低的趋势,而水泥石声发射总振铃计数随木质素磺酸钙质量分数增加呈现出先减小后增大的趋势。随着单轴循环梯级的增加,水泥石的变形模量呈现出强化现象,且初始加卸载循环对变形模量的强化作用最为显著。随着木质素磺酸钙的加入,改性水泥石的破坏模式呈现出拉伸主导→剪切主导→拉伸主导的拉剪组合破坏模式,同时改性水泥石的分形维数呈现出先减小后增大的趋势,表明适量木质素磺酸钙的加入能够有效提高单轴循环加卸载下水泥石的抗破坏能力。随着木质素磺酸钙质量分数的增加,水泥石水化产物之间孔隙呈现出先减少后增多的趋势。适量木质素磺酸钙的加入会促进水泥石中C−S−H凝胶和钙矾石的大量生成,沉淀在水泥颗粒表面相互交织,可显著提高水泥石的峰值应力,对于提高水泥石的力学性能具有积极作用,同时也使得水泥石的孔隙率降低,进而导致水泥石纵波波速增加、循环荷载过程中水泥石累计声发射振铃计数减小。而木质素磺酸钙过量加入时,木质素磺酸钙的引气作用和电性相斥作用在水化过程中占据主导作用,会引入较多气泡,造成水泥颗粒之间间隙的产生,同时抑制了C−S−H凝胶和钙矾石的早期生成,对水泥石的力学性能造成消极影响。此外,会造成水泥石孔隙率增加进而导致水泥石纵波波速减小、循环荷载过程中水泥石累计声发射振铃计数增加。因此,木质素磺酸钙对水泥石的力学性能的影响具有双重效应。

In the coalbed methane extraction of surface wells in the mining area,the mining of coal seam in working face will cause a significant disturbance to overlying rock layer,which in turn will lead to the deformation and damage of sur-face wells.The coalbed methane extraction of surface wells in the mining-disturbed areas cannot be effectively performed.Cementing technology can effectively elongate the life span of mining-disturbed coalbed methane surface wells.As a crit-ical factor of the cementing quality of surface wells,the early strength of the cement needs to meet the high cementing re-quirements.Cement stone is formed after cement consolidation.The mechanical properties of cement stone are of great importance to maintain the stability of mining-disturbed coalbed methane surface wells.It is an effective way to improve the cementing effect by adding dispersants to enhance the mechanical properties of cement stone.There are few re-searches on the deformation and failure characteristics of calcium lignosulfonate modified cement under the influence of mining disturbance.Uniaxial compression tests of calcium lignosulfonate modified cement with different mass fractions under cyclic loading were carried out to investigate the effect of calcium lignosulfonate on the deformation and failure characteristics of cement stone in mining-disturbed coalbed methane surface wells and its modification mechanism.The results show that the P-wave velocity and peak stress of cement stone increase first and then decrease with the increase of calcium lignosulfonate mass fraction,while the total acoustic emission ringing counts of cement decreases first and then increases with the increase of calcium lignosulfonate mass fraction.With the increase of the uniaxial cycle steps,the de-formation modulus of cement stone shows a strengthening phenomenon,and the initial loading and unloading cycle has the most significant strengthening effect on the deformation modulus.With the addition of calcium lignosulfonate,the modified cement stone shows a tensile-dominated→shear-dominated→tensile-dominated combined damage modes.Be-sides,the fractal dimension of modified cement stone shows a trend of decrease before increase,indicating that the addi-tion of the appropriate amount of calcium lignosulfonate can effectively improve the damage resistance of cement stone under uniaxial cyclic loading.With the increase of the mass fraction of calcium lignosulfonate,the pores between cement hydration products show a trend of decrease first and then increase.The addition of an appropriate amount of calcium lignosulfonate can promote the formation of abundant C−S−H gel and ettringite in cement,and the precipitates interweave on the surface of cement particles,which can significantly improve the peak stress of cement stone,and play a positive role in improving the mechanical properties of cement stone.Additionally,the porosity of cement stone decreases,which leads to the increase in the longitudinal wave velocity of cement stone and the decrease of cumulative acoustic emission ringing counts of the cement stone during cyclic loading.However,when the calcium lignosulfonate is excessively added,the air entraining and electrical repulsion of calcium lignosulfonate play a dominant role in the hydration process,which will introduce more bubbles,resulting in the occurrence of the gap between the cement particles,and the inhibition of the early formation of C−S−H gel and ettringite,which has a negative impact on the mechanical properties of cement stone.Furthermore,the porosity of cement stone increases,which leads to the decrease of the longitudinal wave velocity of ce-ment stone and the increase of the cumulative acoustic emission ringing count of cement stone in the process of cyclic loading.Therefore,the influence of calcium lignosulfonate on the mechanical properties of cement stone has a double ef-fect.