Experimental Investigation of Shrinkage/Expansion of Oil-Well Cement

ABSTRACT: The expansion/shrinkage of oilwell cement is one of the key factors affecting cementing quality and the safe operation of oil and gas wells. Accurate measurement of cement expansion/shrinkage is crucial for ensuring cementing integrity. Existing devices can measure cement shrinkage accurately but fail to capture cement expansion. Therefore, this paper introduces a novel cement expansion/shrinkage evaluation apparatus, which converts cement expansion into radial stress, enabling effective measurement of cement expansion. Experimental studies on cement containing MgO expansion agents have validated the effectiveness of the device. The results indicate that the addition of MgO expansion agents effectively suppresses cement volume shrinkage, and the effectiveness of the MgO expansion agent is primarily influenced by temperature. The presence of overburden pressure inhibits axial cement expansion, explaining why existing devices are incapable of measuring expansion of cement. Furthermore, the water environment surrounding the cement significantly influences its expansion/shrinkage. When the cement is in conditions with excess water available, the cement with expansion agent exhibits a greater expansion magnitude. 1. INTRODUCTION Cement volume shrinkage is a major cause of gas migration and poor cementing quality in oil and gas wells (Backe et al., 1998; Li et al., 2021; Prohaska et al., 1993; Reddy et al., 2007; Zhou and Wojtanowicz, 2000). Cement shrinkage compromises the bonding quality of the cement sheath, and severe shrinkage can create annular gaps, providing channels for oil, gas, and water flow, leading to interzonal migration (Bois et al., 2011). Cement shrinkage during and after setting significantly impacts cementing quality. To address shrinkage, expansion agents are added to cement formulations to mitigate shrinkage (Watters and Sabins, 1980), but due to the varying downhole conditions, cement with expansion agents cannot accommodate all complex scenarios. Hence, laboratory experiments on cement expansion/shrinkage are critical for controlling shrinkage behavior. Researchers began recognizing the importance of cement expansion/shrinkage for cementing quality in the late 20th century and initiated laboratory measurements (Chenevert and Shrestha, 1991). Currently, the primary methods for testing cement expansion/shrinkage include length measurement and volume measurement. Length measurement involves recording axial length changes in cement, including methods like the corrugated tube method (Jensen and Hansen, 2002) and the traditional ASTM expansion test (Fig. 1.), but these methods do not allow for high pressure and temperature conditions, making them unsuitable for oil well cement. Volume measurement, which reflects shrinkage/expansion by tracking cement volume changes, is commonly used for chemical shrinkage tests (Ghanem et al., 2024).