YANG Li，ZHANG Chunmei，MEI Kaiyuan，LI Shangdong，CHENG Xiaowei，ZHONG Ziqin^a ，LI Xinyu^a ，WU Qimei^a

a. School of New Energy and Materials， b. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation，Southwest Petroleum University，Chengdu 610500， China

Abstract

Objective Due to the continued deepening of oilfield exploration and development， most of China's oilfields have entered the late stage of high water cut development. Long-term water injection and layered well distribution in these oilfield areas cause problems such as formation pressure disorder， active formation water， and the formation of multi-pressure systems where high-pressure layers and low-pressure layers coexist. These issues may result in water infiltration during cementing operations in older oilfields. Currently， various admixtures are used in China to enhance conventional cement slurry systems， providing early strength and rapid solidification characteristics. The primary focus is on improving the anti-channeling performance of the cement slurry after solidification. However， during the cementing process， unbalanced pressure between the annular liquid column and the formation causes formation water to invade the cement slurry through 'dissolution migration' and 'mass exchange'.This dilutes and disperses the cement slurry structure， making it difficult to cure and reducing its strength. This paper evaluated the water dispersion resistance and mechanism of a single polymer containing hydrophobic groups in the oil well cement slurry system. It was found that water invasion has a more significant impact than gas channeling. Water invasion can lead to waste of oil and gas resources and damage the cement sheath structure，causing serious consequences on cementing quality. Additionally，this can result in severe accidents such as ground environmental pollution， blowouts，and wellbore collapse.Methods The polymer KSQ-Z was synthesized via aqueous solution free radical polymerization using 2-acrylamido-2-methylpro⁃panesulfonic acid，acrylamide，α-methacrylic acid，and stearyl methacrylate as the primary raw materials. Sodium dodecyl sulfate was used as the stabilizer，and ammonium persulfate was used as the initiator. The polymer KSQ-Z was characterized using infrared spectroscopy （FTIR），thermal analysis （TG），and scanning electron microscopy （SEM）.The study evaluated the anti-water dispersion effect of polymer KSQ-Z through hydrostatic and shock action. It also analyzed the impact of different dosages of KSQ-Z on the performance of slurry and the compressive strength of cement slurry. The anti-dispersion mechanism of KSQ-Z was explored through comparative analysis and microscopic testing of cement stone mixed with polymer materials using XRD and SEM.

Results and Discussion The polymer synthesized through aqueous solution free radical polymerization， which contains an ester group， can be used in service environments below 164 ℃. Different dosages of polymer KSQ-Z were added to cement， and the anti-dispersion of the cement slurry was evaluated using shock and hydrostatic forces. Increasing the dosage of KSQ-Z had a negative impact on the rheological properties of the cement slurry， but significantly improved its anti-dispersion properties.After 7 d of curing， the compressive strength of the 1. 2% KSQ-Z cement slurry was （33. 96±1. 37） MPa，（28. 99±1. 11） MPa and （26. 98±1. 07） MPa， which were 41. 1%，26. 2%， and 21. 8% higher than that of pure cement， respectively. XRD data indicated， that polymer KSQ-Z could expedite the cement hydration process in the presence of water intrusion， ensuring normal hydration of the cement slurry and reducing the impact of water invasion. Calcium ion loss data， following water invasion， confirmed that polymer KSQ-Z could mitigate calcium ion loss and ensure the presence of key hydration ions in the cement slurry.SEM data revealed， that the polymer KSQ-Z enhanced the compactness of the cement stone and improved its stability by creating a network structure.

Conclusion In recent years， researchers have extensively studied water invasion in cementing， focusing on the formation characteristics of different oilfields and the use of various admixtures to modify cement slurry performance. This improves the setting speed of the cement slurry and reduces the duration of water invasion. Recently， there has been a surge in studies aimed at enhancing the ability of conventional cement slurry to resist water invasion by improving the microstructure of the cement itself.This paper presented the preparation and performance evaluation of a single hydrophobic group anti-dispersion polymer， KSQ-Z. The properties of cement slurry were improved by using hydrophobically associating polymer. The hydrophobically associating polymer created a reversible supramolecular network structure by associating hydrophobic groups at a critical concentration.This resulted in the cement slurry having good shear resistance， low water loss， and low volume shrinkage. The results indicated that the polymer KSQ-Z can significantly enhance the water invasion resistance of cement slurry prior to solidification by improving the cohesion of the slurry and preventing the impact of formation water on the dilution and ion loss of the cement slurry.

Keywords：oil well cement； water intrusion； anti-dispersion； turbidity

程小伟（1977—），男，教授，博士，四川省学术和技术带头人后备人选，博士生导师，研究方向为先进胶凝材料及其在固井中的应用。E-mail：chengxw@swpu. edu. cn.

DOI：10.13732/j.issn.1008-5548.2024.05.013