LIU Tao¹，ZHANG Hongbo ² ，BAI Yunfei ³ ，MEI Kaiyuan¹ ，ZHANG Chunmei¹ ，CHENG Xiaowei ¹

1. Southwest Petroleum University， a. School of New Energy and Materials，b. State Key Laboratory of Oil and Gas Reservoir Geology and Exploration， Chengdu 610500， China；2. The First Cementing Company， Bohai Drilling Engineering Company Limited， CNPC， Tianjin 062552， China；3. No. 4 Oil Production Plant， PetroChina Huabei Oilfield Company， Langfang 065000， China

Abstract

Objective In oil drilling， shrinkage and fracture issues in cement slurry are particularly prominent. Due to the high temperature and pressure during the cementing process， significant shrinkage occurs. The volume shrinkage can create micro-cracks in cement sheath， compromising its bonding strength with the borehole wall and casing， and as well as the integrity of the cement sheath. These issues can cause annular channeling，resulting in pollution and loss of oil and gas resources. The use of expansion agents is an important means to avoid cracksin cement sheath.

Methods The studyused magnesite from Hebei Province as raw material. Magnesite powder was calcined in an electric furnace to obtain different active magnesia expansive agent （MEA）. MEA activity and microstructure were tested to establisha relationship between them. A comprehensive performance evaluation of cement slurry methods with different MEA activitieswas carried out，measuring linear volume expansion rate， compressive strength， and permeability of the cement stone.

Results and Discussion During calcination， magnesite initially decomposed into MEA grains. With the growth of MEA grains，their arrangement became more regular， reducing the crystal defects and limiting their deformation，thereby decreasing activity. With increasing calcination temperatures and extended holding times， the diffraction peaks of MEA graduallysharpened and the diffraction intensity increased， indicating a more complete structure. At the same calcination temperature， the longer the holding time， the more sufficient the decomposition of magnesite. As the temperature reached above 1 000 ℃， grain size increased，resulting in a denser MEA structure with lower lattice distortion. Increasing the temperature from 900 ℃ to 1 200 ℃， the grain size increased from 43. 92 nm to 126. 32 nm. At 1 000 ℃，extending the holding time from 30 min to 90 minincreased the grain size from 54. 11 nm to 115. 57 nm. When magnesite was calcined at 900 ℃ for 30 min， the resulting magnesite exhibitedsigns of insufficient calcination. Although the obtained MEA had high activity， its expansion performance was weak， with a linear volume expansion rate of only 5. 37 ‱ after 14 days. When calcined at 1 200 ℃ for 90 min， the activity of MEA was low， with a longer hydration induction time and minimal expansion， and its linear volume expansion rate was only 3. 26 ‱ after 14 days. In the 14-day cement stone， two strength development trends were observed：（Ⅰ） continually increasing；（Ⅱ） increasing first and then decreasing. The 14-day cement stone calcined at 1 100 ℃ for 90 minachieved a compressive strength of 43. 20 MPa.

Conclusion The paper shows thata single MEA particle is composed of many aggregated MEA grains， sintered at high temperatures. Calcination temperature and time influence the microstructure of MEA， affecting grain size， lattice distortion， and specific surface area. Higher calcination temperatures and longer calcination timespromote the growth of MEA grains，leading to a more regular grain arrangement， reduced lattice defects， increased grain sizes， decreased lattice distortion， and ultimately reduced hydration activity. Although the MEA activity is the highest when calcinedat 900 ℃ for 30 min， the expansion rate of MEA cement paste is the lowest，due to the insufficient decomposition of magnesite and weak expansion performance. In contrast， at 1 200 ℃， magnesite decomposition is complete， with larger grain sizes， smaller lattice distortion， and greatly reduced activity. Although higher activity MEA can compensate for the larger cement shrinkage by providing greater expansion in a short time， the excessive expansion force can break the internal structure of the cement stone， thus affecting its mechanical properties.

Keywords：oil well cement； magnesium oxide； calcination method， microstructure， comprehensive property