Visualization and quantification of pore structure of oil-well cement slurry in liquid-solid transition stage using high-resolution computed tomography

Understanding the pore structure of oil-well cement slurry during early hydration is key to resolving short-term gas migration in natural gas wells. This study evaluated the three-dimensional microstructure and pore structure of oil-well cement slurry during early hydration using in situ high-resolution X-ray computed tomography and image reconstruction techniques. Experimental results showed that in the initial hydration stage the microstructure of the oil-well cement slurry comprised ‘dispersed particles’. When the hydration rate increased, some hydration products formed between the dispersed particles, and the microstructure of the slurry changed from a ‘dispersed particles’ structure to an ‘agglomerated skeleton’ structure. Furthermore, when the hydration time increased from 180 to 600 min, the porosity of the slurry decreased from 11.46% to 10.14% (a 11.52% decrease), the porosity of pores with volumes of 201 or more voxels reduced from 2.96% to 1.81% (a 38.83% decrease), and the connected porosity decreased from 4.47% to 2.26% (a 49.47% decrease). These results indicate that during early hydration, the formation of hydration products reduces the pore volume and cuts off some connected pores to produce many pores with small volumes. Based on the Kelvin−Laplace equation, the reduction in pore size increases the capillary pressure of the pore solution and shifts the weight of the pore solution acting on the agglomerated skeleton to reduce the hydrostatic pressure of the slurry. This study further proves that the pore structure of oil-well cement slurry is a key factor in reducing hydrostatic pressures and enabling short-term gas migration.